characterisation of adhesive systems used to produce multi ...€¦ · figure 29 - double...

Characterisation of adhesive systems used to produce

multi-layered composites made of natural materials

Madalena Barata Garcia

Thesis to obtain the Master’s Degree in

Materials Engineering

Supervisors

Prof. Pedro Miguel Gomes Abrunhosa Amaral

Prof. Virgínia Isabel Monteiro Nabais Infante

Examination Committee

Chairperson: Prof. José Paulo Sequeira Farinha

Supervisor: Prof. Alberto Eduardo Morão Cabral Ferro

Member of the Committee: Prof. Pedro Miguel Gomes Abrunhosa Amaral

November 2017

Aos meus pais,

Isabel e José

i

Acknowledgments

I would like to thank my supervisors, Pedro Amaral and Virgínia Infante for the knowledge

and the close mentoring given in these last few months; Hugo Cardoso and Ilaria Andreazza for

being my companions in this adventure; everyone at Frontwave especially Joel Pinheiro, Ricardo

Azevedo and Paula Rebola; everyone involved at IST, especially Prof. Luís Santos, Prof. Luís Alves

and Pedro Teixeira, and finally my whole family and friends – your support led me here. This work

was supported by FCT, through IDMEC, under LAETA project UID/EMS/50022/2013.

iii

Abstract

Sandwich composite materials have been a growing subject considering that their properties and

characteristics can be design by joining two or more materials into one. The most used cores are

rigid polymeric foams and aramid or aluminium honeycombs. Only recently asymmetric designs have

been employed.

This work outlines a methodology for evaluating the adhesive system of an asymmetric sandwich

composite of natural stone, cork agglomerate and glass fibre reinforced epoxy. This class of

structural composites is designed to have high stiffness/strength-to-weight ratios. To achieve these

properties, this work will focus on the understanding of the mechanical behaviour of each component

and the type of interaction between them.

Being aware that delamination is known to be the principal mode of failure of the designed panels,

several tests and standard procedures are used to determine the debond fracture toughness of a

composite. No public literature available includes asymmetric composites, neither composites made

of more than two constituents (the typical structures are symmetrical usually including a core and

two fibre-reinforced skins). The procedures employed represent an approach to determine the

delamination causes of the composite and, consequently, its limitations concerning practical use.

Results demonstrated that stone surface characteristics in contact with the first fibre layer (roughness

and porosity) influence the results of both the delamination and stiffness tests, more than that of the

employed cork agglomerates.

Keywords: Asymmetric composite, sandwich composite, cork agglomerate, stone, delamination.

v

Resumo

Os materiais compósitos são cada vez mais utilizados em várias indústrias, uma vez que as suas

propriedades e características podem ser projetadas ao juntar dois ou mais materiais. Os núcleos

mais utilizados em painéis sanduíche são espumas poliméricas rígidas e favos de mel de aramida

ou alumínio. Apenas recentemente foram desenvolvidos materiais compósitos assimétricos.

Este trabalho apresenta uma metodologia para avaliação do sistema adesivo de um compósito

sanduíche assimétrico, composto por pedra, aglomerado de cortiça e epóxi reforçada com fibras de

vidro. Esta classe de compósitos estruturais é projetada para ter uma alta relação de rigidez/força e

baixo peso e, para garanti-la, a metodologia caracteriza o comportamento mecânico de cada

componente e a interação resultante entre eles.

Dado que o principal modo de falha destes materiais é delaminação, vários procedimentos e

normas internacionais têm sido desenvolvidos para determinar a força de adesão entre camadas.

No entanto, foi verificado durante a revisão bibliográfica que nenhum inclui a utilização de

compósitos assimétricos com mais de dois constituintes (as estruturas habituais são uma sanduíche

simples composta por um núcleo e duas peles de plástico reforçado com fibras). Os testes

desenvolvidos são uma abordagem para determinar as causas de delaminação para este tipo de

painéis e, consequentemente, determinar as suas limitações relativamente a aplicações em

engenharia.

Os resultados demonstram que as características superficiais da pedra em contacto com a

primeira camada de fibras (rugosidade e porosidade) influenciam os resultados dos testes de

delaminação e rigidez, ao contrário das características dos aglomerados de cortiça utilizados.

Palavras-chave: compósito assimétrico, compósito sanduíche, aglomerado de cortiça, pedra,

delaminação.

vii

Contents

1. INTRODUCTION ...................................................................................................................... 1

2. STATE OF THE ART .................................................................................................................. 3

2.1 INDUSTRIAL FRAMEWORK............................................................................................................ 3

2.2 COMPOSITE MATERIALS .............................................................................................................. 4

2.3 CORK AGGLOMERATE AS PART OF A COMPOSITE MATERIAL ................................................................. 6

2.4 NATURAL STONE ........................................................................................................................ 9

2.5 EPOXY RESINS ......................................................................................................................... 11

2.6 GLASS FIBRES .......................................................................................................................... 13

2.7 TESTING OF ADHESIVES ............................................................................................................. 13

2.7.1 Failure Modes in Composites .......................................................................................... 16

3. CHARACTERIZATION OF THE CONSTITUENT MATERIALS ....................................................... 19

3.1 STONE SURFACE ROUGHNESS..................................................................................................... 19

3.1.1 Principles ......................................................................................................................... 19

3.1.2 Methodology .................................................................................................................. 19

3.1.3 Results and Discussion .................................................................................................... 20

3.2 DIFFERENTIAL SCANNING CALORIMETRY ANALYSIS ON EPOXY ........................................................... 20

3.2.1 Principles ......................................................................................................................... 20

3.2.2 Methodology .................................................................................................................. 21


3.3 THEORETICAL APPROACH TO EVALUATE THE COMPOSITE LOAD DISTRIBUTION ..................................... 22

3.3.1 Transformed Section Method ......................................................................................... 23

3.3.2 Classical Lamination Theory ........................................................................................... 24

4. EXPERIMENTAL TESTS ON THE COMPOSITE .......................................................................... 29

4.1 SPECIMEN PRODUCTION ........................................................................................................... 29

4.2 BENDING STIFFNESS TESTS ........................................................................................................ 32

4.2.1 Principles ......................................................................................................................... 32

4.2.2 Specimen Dimensions ..................................................................................................... 33

4.2.3 Methodology .................................................................................................................. 33


4.3 DELAMINATION TESTS .............................................................................................................. 36

4.3.1 Principles ......................................................................................................................... 36


4.3.3 Preliminary Tests ............................................................................................................ 37

4.3.4 Methodology .................................................................................................................. 42

viii


4.4 WEIBULL STATISTICAL ANALYSIS OF LOAD AT BREAK ....................................................................... 49

4.4.1 Principles ......................................................................................................................... 49


4.4.3 Methodology .................................................................................................................. 51


4.5 SCANNING ELECTRON MICROSCOPY ANALYSIS ............................................................................... 58

4.5.1 Methodology .................................................................................................................. 58


5. CONCLUSIONS ...................................................................................................................... 63

5.1 FUTURE STUDIES ..................................................................................................................... 64

6. REFERENCES ......................................................................................................................... 65

ix

List of Figures

Figure 1 – Expanded view of the sandwich panel ......................................................................... 1

Figure 2 - U.S. cladding market revenue by raw material, 2014 - 2024 (USD Million). Source:

Grand View Research ........................................................................................................... 4

Figure 3 - Classification scheme of composite materials types (adapted from Callister [9]) ........ 5

Figure 4 – Stone and cork agglomerate panels ............................................................................ 5

Figure 5 - Micrographs of cork sections observed with an optical microscope in the tangential,

transverse and radial sections (from left to right) [20]. ......................................................... 6

Figure 6 – Compressive and tensile stress-strain curves for a) an elastic-plastic honeycomb b)

cork (R-radial, NR – non-radial, T-tangential, A-axial directions) [22] .................................. 7

Figure 7 - Classification scheme of cork agglomerates based on the bonding process [20] ........ 8

Figure 8 – Stress-strain curve for several stone types [30] ......................................................... 10

Figure 9 - Mohs hardness classification of several stone types [30] ........................................... 10

Figure 10 - Adhesion test-types in ASTM specifications [39] ...................................................... 14

Figure 11 – Peel stress failure of CA [45] ................................................................................... 15

Figure 12 - Single cantilever beam test proposed to test adhesion in sandwich composites [46]

............................................................................................................................................ 15

Figure 13 - Failure modes in composites [1] [48] ........................................................................ 16

Figure 14 - Roughness average, Ra ............................................................................................ 19

Figure 15 - Third Maximum Peak-to-Valley Height, R3z .............................................................. 19

Figure 16 - Surfcorder SE1200 used to measure surface roughness ......................................... 20

Figure 17 - Typical DSC results .................................................................................................. 21

Figure 18 - DSC result for the upper layer epoxy resin ............................................................... 22

Figure 19 - DSC result for the down layer epoxy resin ............................................................... 22

Figure 20 – Representation of the distances of the middle surface of the material under study 25

Figure 21 – Strain and Stress profiles from TS and CLT methods ............................................. 27

Figure 22 – Specimen production a) Hand lay-up method; b) Hot press .................................... 31

Figure 23 - Specimen for the delamination test .......................................................................... 31

Figure 24 - Four-point loading configuration according to ASTM-C393...................................... 32

Figure 25 - Produced specimens for the bending stiffness test. ................................................. 33

Figure 26 - Four-point bending configuration .............................................................................. 33

Figure 27 - Average stiffness values for each specimen configuration....................................... 34

Figure 28 – Stiffness vs thickness of tested specimens ............................................................. 36

Figure 29 - Double cantilever beam specimen with piano hinges from ASTM D3807 ................ 37

Figure 30 - Composite layout: 1 - stone; 2 - cork agglomerate (CA); 3 - glass fiber-reinforced

polymer skins ...................................................................................................................... 37

Figure 31 - Test setup: Clamping and Force Applied zones ....................................................... 38

Figure 32 - Second test specimen............................................................................................... 38

Figure 33 - New delamination test setup ..................................................................................... 39

x

Figure 34 – a) Third and b) fourth test apparatus (the last with delamination occurring) ........... 39

Figure 35 - a) Crack in the stone layer of the specimen used in the fourth test; b) Crack on the

resin plus fibres layer of the specimen tested in the fourth test. ........................................ 39

Figure 36 - Load (N) displacement (mm) curve obtained from the fourth test ............................ 40

Figure 37 - Delamination zone between the stone and cork agglomerate layers ....................... 41

Figure 38 - Test apparatus in IST................................................................................................ 41

Figure 39 - a) Specimen with a metal sheet glued on top of the stone; b) final delamination test

apparatus (delamination occurring) .................................................................................... 41

Figure 40 - Methodology for delamination tests: a) gluing the metallic plate to the composite

specimen; b) tightening the screws to secure the specimen; c) positioning the microscope

to record delamination front. ............................................................................................... 42

Figure 41 - Load displacement curve for BAS specimens .......................................................... 43

Figure 42 - Load displacement curves for BGS specimens ........................................................ 44

Figure 43 - Load displacement curves for BGA specimens ........................................................ 44

Figure 44 - Blue stone specimens’ delamination a) delamination between the stone and GFRE

layers b) stone failure ......................................................................................................... 45

Figure 45 - Load displacement curves for WAS specimens ....................................................... 46

Figure 46 - Load displacement curves for WAA specimens ....................................................... 46

Figure 47 - Load displacement curves for WGS specimens ....................................................... 47

Figure 48 - Load displacement curves for WGA specimens ....................................................... 47

Figure 49 - Delamination a) in between the GFRE layer in specimen WAA4, b) between GFRE

layer and CA layer in specimen WGA5, c) Mixed delamination in specimen WAA6, d) Stone

failure in WGS5 ................................................................................................................... 48

Figure 50 - White stone specimens’ regression lines.................................................................. 52

Figure 51 - Blue stone specimens’ regression lines.................................................................... 53

Figure 52 - Cork and stone regression lines ............................................................................... 53

Figure 53 – Reliability a) white stone specimens; b) blue stone specimens; c) cork specimens d)

stone specimens ................................................................................................................. 55

Figure 54 - Frequency of each failure mode ............................................................................... 57

Figure 55 – Average compressive load (N) versus average thickness (mm) of the several

specimen configurations ..................................................................................................... 58

Figure 56 - Examined specimens on SEM .................................................................................. 58

Figure 57 - Areas examined by SEM: zone 1, the adhesive layer between the stone and the CA,

zone 2, the two CA and zone 3, the epoxy layer that is in contact with air ........................ 59

Figure 58 - SEM images: a) WAS specimen b) WAS specimen c) WGS specimen d) WGS

specimen e) BAS specimen f) BAS specimen.................................................................... 60

Figure 59 - SEM image of cork agglomerate a) CA-A b) CA-G .................................................. 61

Figure 60 - SEM images: WAS specimen a) and b); WGS specimen c) and d); BAS specimen e)

and f) ................................................................................................................................... 61

xii

List of Tables

Table 1 - International trade of stone materials (imports-exports). Source: Global Trade Atlas,

processing by IMM. NB: “AUV” stands for “average unit atlas”. Marble, granite and other

stones are taken into account. .............................................................................................. 3

Table 2 - Market share (first 10 countries 2015). Source: Global Trade Atlas, processing by IMM.

NB: table shows the first 10 countries in terms of export value in 2015. They account for

92% of the world export value in 2015. ................................................................................ 3

Table 3 - Failure modes, adapted from [49] ................................................................................ 17

Table 4 - Average values of Ra and R3z ...................................................................................... 20

Table 6 - Material constants used in the calculations ................................................................. 23

Table 7 - Test matrix ................................................................................................................... 29

Table 8 - Properties of CA (A and G) .......................................................................................... 30

Table 9 - Properties of the two natural stones used in the work ................................................. 30

Table 10 - Properties of the two glass fibres used in the work ................................................... 31

Table 11 - Stiffness values (MPa) ............................................................................................... 34

Table 12 - Comparison of the stiffness values considering the finish of the stone layer ............ 35

Table 13 - Comparison of the stiffness values considering the stone configuration ................... 35

Table 14 - Comparison of the stiffness values considering the CA configuration ....................... 35

Table 15 - Average maximum delamination load values (N) for each specimen configuration .. 43

Table 16 - Delamination type occurring in each blue stone specimen........................................ 45

Table 17 - Delamination type occurring in each white stone specimen ...................................... 48

Table 18 - GIC values for the white stone specimens .................................................................. 49

Table 19 - Load at failure (N) obtained in the four-point bending test ......................................... 51

Table 20 - Values v and u of Weibull distribution for each specimen configuration .................... 54

Table 21 - Load (N) at failure for R=0,8 of each specimen configuration ................................... 55

Table 22 - Failure modes during the four-point bending test ...................................................... 56

Table 23 - Failure mode for every specimen of each configuration ............................................ 57

xiv

List of Symbols

A – Section Area of the Sandwich

𝑎 – Delamination Length

𝑏 – Specimen Width

D – Bending Stiffness

E – Young’s Modulus

ed. - Edition

G – Shear Modulus

𝐺𝐼𝐶 – Interlaminar Fracture Toughness

I – Momentum of Inertia

l – Sandwich Total Length

L – Load Span

M – Moment

n – Modular Ratio (at TS model)

P – Applied Load

𝑞 – Location Parameter (of Weibull model)

Ra – Arithmetical Mean Roughness

R3z – Third Maximum Peak-to-Valley Height Roughness

S – Support Span

StdDev – Standard Deviation

t – Sandwich Thickness

Tc – Cure Temperature

Tg – Glass Transition Temperature

U – Shear Stiffness

𝑢 – Scale Parameter (of Weibull model)

𝑣 – Shape Parameter (of Weibull model)

yi – Distance from Top to Middle Point of the i-th Layer

∆ – Mid-Span Deflection

δ – Load Point Displacement

ε – Strain

𝜈 – Poisson’s Ratio

σ – Stress

xvi

List of Acronyms

ASME – American Society of Mechanical Engineers

ASTM – American Society for Testing and Materials

BAA – Blue, CA-A, Smooth specimens

BAS – Blue, CA-A, Sawn specimens

BGA - Blue, CA-G, Smooth specimens

BGS – Blue, CA-G, Sawn specimens

CA – Cork Agglomerate

CA-A – Cork agglomerate type A

CA-G – Cork agglomerate type B

CLT – Classical Lamination Theory

DSC – Differential Scanning Calorimetry

EDS – Energy Dispersive Spectroscopy

EN – European Standard

FEG-SEM – Field Emission Gun Scanning Electron Microscope

GFRE – Glass Fibre Reinforced Epoxy

ICTA – International Confederation for Thermal Analysis

INE – Instituto Nacional de Estatística

IUPAC - Union of Pure and Applied Chemistry

ISO – International Organization for Standardization

MBT – Modified Beam Theory

PAN - Polyacrylonitrile

PMC – Polymer-Matrix Composites

SEM – Scanning Electron Microscope

TSM – Transformed Section Method

WAA – White, CA-A, Smooth specimens

WAS – White, CA-A, Sawn specimens

WGA – White, CA-G, Smooth specimens

WGS – White, CA-G, Sawn specimens

1

1. Introduction

In the last few years, composite materials have been a growing subject because their properties

and characteristics can be designed by joining two or more materials into one. These “new”

properties have drawn a lot of attention, especially in the high-performance application industries,

such as aeronautic and automotive. When developing these materials, one of the challenges is to

understand the type of interaction between the constituent components to further predict the final

mechanical behaviour of the composite.

When comparing with stone panels, the addition of cork agglomerate and two layers of glass fibre

reinforced epoxy minimises the damage risks due to the final improved bending stiffness and

decrease on weight per unit area. This solves some of the transport and installation problems related

with stone’s high density and brittle behaviour, but keeping the aesthetics and the natural feel of the

final product. Other advantageous characteristics like thermal and noise insulation are improved.

The work developed in this thesis is a sequential study on an asymmetric composite made by a

core of cork agglomerate with low specific weight, two fibre-reinforced epoxy skins with low thickness

and high stiffness and a stone layer on top, as shown in Figure 1.

Figure 1 – Expanded view of the sandwich panel

Previous work addressed the mechanical behaviour of this composite by performing bending,

stiffness and fatigue tests and by developing a multivariable study of the constituents as well as the

production process characterization and optimization [1]–[4].

The main challenge dealing with this new solution is to ensure that the constituent layers don’t

delaminate because of the differences in material properties. Delamination is viewed as the principal

mode of failure of composite materials, consisting on the failure of the interfaces mainly between

different oriented layers, or, in this case, layers with different materials. In fact, two adjacent laminae

having different characteristics induce extensional and bending stiffness mismatch which, combined

with the low ductility of the matrix materials, make composite materials very sensitive to delamination

at those interfaces. The fracture usually starts at the thick skin/core interface and extends onto the

thin skin and, if the thin skin is reached, the rupture rapidly moves across the skin/core interface

Stone

Skin of glass fibre reinforced epoxy (GFRE1)

Skin of glass fibre reinforced epoxy (GFRE2)

Cork Agglomerate (CA)

2

which can lead to its catastrophic failure [5]. Moreover, delamination is an internal damage

phenomenon and is not easily detected, which increases the associated risks of structural collapse.

The knowledge of a laminated composite material’s resistance to interlaminar fracture is essential

for product development and material selection [6].

Testing is the only valid way to evaluate not only the inherent strength of the adhesive, but also

the bonding technique, surface cleanliness, effectiveness of surface treatments, application and

coverage of the adhesive, and the curing cycle [7]. The existent standards to test the adhesion

between layers of composites do not include composites with an asymmetric design and with stone

as one of the constituents. Therefore, a new test was developed specifically for this purpose.

Considering this outline, the main objectives of this thesis are:

- To characterize the constituent materials regarding the following parameters: surface

roughness of the stone that is in contact with the skin and glass transition temperature

(Tg) of the epoxy resin. Additionally, application of two theoretical methods to the

composite (Transformed Section Method and Classical Lamination Theory) as a

preliminary study on its mechanical behaviour;

- To characterize the adhesion between layers of the composite through a delamination

test; to determine the composite stiffness through a four-point bending test; to measure

the failure load and the failure modes of the composite, also in a four-point bending test.

to evaluate the layers’ interface through Scanning Electron Microscopy (SEM) analysis;

- To relate the constituent properties and experimental results, concluding which of the

material properties (of the different constituents) contributes the most for the final

mechanical behaviour of the composite.

This thesis is divided into 5 sections. The present introduction is section 1. Section 2: State of the

Art, is where literature on the market of stone panels is presented, followed by a review on the use

each constituent as a part of a composite material and the failure modes. Section 3: Characterization

of the Constituent Materials, presents the experimental methods and techniques for the tests carried

out in stone and epoxy, as well as the application of theoretical methods to evaluate the load

distribution and the location of the neutral line. Section 4: Experimental Tests on the Composite,

present the Principles, Methodology and Results and Discussion for each Experimental Test: i)

Bending Stiffness Test, ii) Delamination Test and iii) Weibull Statistical Analysis of Load at Break. At

the end, the Scanning Electron Microscopy (SEM) analysis results are also presented and discussed.

Section 5: Conclusions, presents a summary of the results and the final conclusions, along with some

considerations on further developments.

3

2. State of the Art

In this chapter, a literature review is presented, starting with the industrial framework of the stone

industry and a review on composite materials, focusing on the asymmetric ones. Following is a review

on each of the constituent materials, cork agglomerates, natural stone, epoxy resins and glass fibres,

focused on the function of the constituent in the composite (in the case of stones, focusing on

limestone since this was the stone type used for this thesis). The type of resin was not a variable in

this study because it has been previously tested [1], presenting optimal mechanical response. Finally,

there is a review on the standards used to test adhesives in composites along with a presentation of

the failure modes.

2.1 Industrial Framework

According to INE (Instituto Nacional de Estatística), exports in the stone sector had a growth of

43% from 2000 to 2015, reaching a turnover of 995 million euros in 2015. The size of the overall

market, described in Table 1, is growing every year. Portugal is in the top 10 countries in the market

share in terms of export value in 2015 (Table 2).

Table 1 - International trade of stone materials (imports-exports). Source: Global Trade Atlas, processing by

IMM. NB: “AUV” stands for “average unit atlas”. Marble, granite and other stones are taken into account.

2013 2014 2015 Var% 2015/2014

Value (euro) € 22 447 253 570 € 22 886 277 219 € 25 726 470 711 12.41

Quantity (tons) 79 959 092 86 163 478 77 838 605 -9.66

AUV (euro per ton) 280.7 265.6 330.5 24.43

Table 2 - Market share (first 10 countries 2015). Source: Global Trade Atlas, processing by IMM. NB: table

shows the first 10 countries in terms of export value in 2015. They account for 92% of the world export value in

2015.

Countries 2013 2014 2015

China 34.2 35.7 42.4

Italy 13.6 13.5 12.4

Turkey 12.9 12.1 11.2

India 10.3 10.8 9.8

Brazil 7.2 7.0 6.8

Spain 4.8 4.4 3.7

Greece 1.9 2.0 1.8

Egypt 2.1 1.8 1.4

Portugal 1.6 1.6 1.4

Iran 1.1 1.8 1.0

World exports (mil euro) 12 725.4 12 882.0 15 069.2

4

In a report by Grand View Research in 2016 [8], this industry is characterized by growing

preference for lightweight materials in major construction applications such as residential and office

buildings. Technological innovations have allowed for high performance, durable cladding systems

that allow the creation of unique facades and panels. The shift in trend towards protection of exterior

walls to extend the lifespan of existing buildings is fostering the development of attractive, yet

functional systems. Ease of raw material procurement and low installation costs are key aspects

driving investments by numerous multinational companies to develop new and innovative wall

systems incorporating high-tech materials.

Additionally, growing consumer awareness regarding greater energy efficiency in buildings, by

lowering the amount of resources required for heating, ventilation, or air conditioning are other factors

driving industry growth.

In Figure 2, an estimation of the U.S. cladding market revenue by raw material, from 2014 to

2024, is presented, with composite materials being one of the solutions to substitute the classic stone

cladding systems. A visible growth is expected, following the trend of the last few years.

Figure 2 - U.S. cladding market revenue by raw material, 2014 - 2024 (USD Million). Source: Grand View

Research

2.2 Composite materials

A composite is considered to be any multiphase material that exhibits a significant proportion of

the properties of both constituent phases such that a better combination of properties is obtained.

[9].

5

Figure 3 - Classification scheme of composite materials types (adapted from Callister [9])

Sandwich panels are a class of structural composites and they are designed to have high

stiffness/strength-to-weight ratios. A sandwich panel consists of two outer sheets, or skins, that are

separated by and adhesively bonded to a thicker core, as it is represented in Figure 4. The skins

support the bending loads and are made of a relatively stiff and strong material and the core carries

the shear loads and is designed to be lightweight, with a low Young’s modulus and must have

sufficient shear strength to withstand transverse shear stresses. The most used cores are rigid

polymeric foams (i.e., phenolic, epoxy, polyurethane), and aramid or aluminium honeycombs. Other

materials have been used as substitutes for cores, such as syntactic foams [3-4], polypropylene

foams [12] and cork agglomerates [13]. The skins are usually fibre-reinforced polymers, essentially

carbon or glass fibres in epoxy or polyester matrices.

Figure 4 – Stone and cork agglomerate panels

These sandwich composite materials can be further classified as symmetric or asymmetric,

according to its final geometry. The asymmetric sandwiches usually present the top and bottom faces

that differ in materials, thicknesses or types of skins used. The interest in these materials is mostly

Composites

Particle-reinforced

Large particles

Dispersion-strengthened

Fibre-reinforced

Continuous (aligned)

Discontinuous (short)

Aligned

Randomly oriented

Structural

LaminatesSandwich

panels

Symmetric

Asymmetric

6

based on having more choices for the design of the structure that result in a broader range of

mechanical performance [14].

Several studies in different fields of application have been made since this asymmetry has proved

to be advantageous [15]. Zhang et al. [16] studied the failure behaviour of sandwiches with closed

aluminium alloy foam cores and aluminium sheet faces with different thicknesses. They observed

several types of failure modes that resulted from using faces and cores with different thicknesses.

Bella et al. [5] studied the effects on production procedure under static load conditions of an

asymmetric composite with two different natural fibres and epoxy as faces and cork agglomerate as

core. They concluded that the vacuum bag method results in specimens with better results than the

hand lay-up method. Rion et al. [17] designed and tested an asymmetric photovoltaic sandwich

structure with carbon reinforced polymer as plies and a honeycomb core, resulting in 20% weight

savings compared to traditional symmetric designs, with similar stiffness values.

The asymmetry in the composite in study is resultant of the addition of the natural stone layer as

well as the two different density glass fibres used to reinforce the epoxy layers.

2.3 Cork agglomerate as part of a composite material

Cork is a natural material, with a cellular structure at microscopic level (Figure 5) that results in

distinctive mechanical properties [18]. This cellular structure can be compared with the aluminium

honeycomb structure that has been used in composite materials in the last decades, making cork

one of the possible substitutes to be used as core in sandwich composites [19].

Figure 5 - Micrographs of cork sections observed with an optical microscope in the tangential, transverse

and radial sections (from left to right) [20].

Honeycomb structures were first used in 1845 in a tubular railroad bridge built in Wales, but the

real breakthrough came in 1945 with the development of better adhesives for the attachment of

facings to the cores [21]. Both honeycomb and cork are cellular materials, with a structure made of

corrugated aluminium sheets in the first case and naturally formed during cork cells growth in the

second one. It is important to understand the behaviour of these materials to further understand its

7

contribution as cores in the final composite material. Figure 6 shows the compressive and tensile

characteristic stress-strain curves are represented for these two materials.

Figure 6 – Compressive and tensile stress-strain curves for a) an elastic-plastic honeycomb b) cork (R-

radial, NR – non-radial, T-tangential, A-axial directions) [22]

In compression, both materials behave as elasto-plastic materials in the sense that they go

through three different stages, the first one being an elastic deformation (viscoelastic in the case of

cork), a second one due to cell folding in honeycombs and cell collapse in cork and a third one where

the cells are compressed against each other and the stress increases significantly, reaching at the

limit the behaviour of a non-cellular solid.

In tension, the behaviour is again similar amongst the two materials and also with three-staged

compression, although the second stage is not so well defined on cork because of the natural

waviness of the cork cells. Initially the cell walls bend in a linear-elastic deformation and after the

plateau correspondent to the cell folding (that is continuous in cork) an increase in stress is observed

due to densification.

As cork is an expensive material due to its long growth cycle, cork agglomerates are produced

with by-products from the processing line of cork’s number one product, bottle stoppers. Cork itself

is considered an environmental friendly material and about 85% [20] of all cork production can be

used to make agglomerates, having a low environmental impact.

A classification concerning the bonding process of the cork granules can be made, and three

different classes are defined: the adhesive bound agglomerates (joined with an adhesive and hot

pressed), the pure expanded agglomerates (bounded with adhesives at high temperatures) and the

cork and rubber composites (Figure 7). These types of agglomerates are intended to be used for

different applications. The first ones as agglomerated cork stoppers and production of boards and

a) b)

8

sheets mostly for surfacing; the expanded cork agglomerates as insulation materials and the

agglomerate and rubber composites as vibration and shock absorbers in surfacing and industrial

equipment [20].

Figure 7 - Classification scheme of cork agglomerates based on the bonding process [20]

The general production process of cork agglomerates (CA) starts with milling the cork using a

hammer mill and subsequently with knife or disc grinders to achieve different particle sizes, from 0.2–

10 mm [20]. The adhesives that are used include thermosetting polymers, (such as urea–

formaldehyde, melamine or phenolic adhesives for flooring agglomerates), or thermoplastic polymers

(such as polyurethanes for CA stoppers and softer surfacing materials) [23]. The exact quantity of

the mixture of these two components is then measured and placed in metal moulds (the content of

adhesive is usually in the range of 3–8% [20]). Pressure and temperature are applied and the moulds

containing the pressed mixture are placed in curing chambers for the polymerization of the binding

agents.

The size of the agglomerates and adhesive content will determine the density of the agglomerate,

according to its final application. It is important to notice that at the cellular structural level, the

compression of the cork particles during agglomeration causes the partial densification of cells at the

grain boundary, with cell collapse and corrugation. The distribution of cell collapse is not uniform and

increases with the compression extent, which also translates into higher density values of the

materials [20].

To adapt these materials to the core of sandwich composites, optimized CA have been produced

and studied and have proved superior ability to substitute others [24]. For example, when comparing

cork with other core material usually used, like Nomex honeycomb or Rohacell foam, cork-epoxy

agglomerates present a significantly better core shear stress limit, which reduces the crack

propagation region [25].

Reis and Silva [13] studied the mechanical behaviour of the CA through shear and three-point

bending tests and concluded that cork-based cores are suitable as sandwich core materials when

compared with the honeycomb-based cores. Fernandes et al. [26] carried out different

characterization tests to CA boards with different formulations and an excellent recovery capacity

Cork Agglomerates

Adhesive bound agglomerates

Expanded cork agglomerates

Cork and rubber composites

9

was verified by displacement curves as an intrinsic characteristic of the material. Furthermore, they

observed a high-energy absorption capacity with minimum damage occurrence but referred the need

of reinforcement strategies to reach higher stiffness. Sadeghian et al. [27] studied a sandwich

composite with a CA core with natural fibres in the skins and compared them with the typical

honeycomb and glass fibre combination. The most common failure mode of the specimens was the

shear failure of the core (both honeycomb and CA cores) and the structural performance between

the two formulations was comparable. All these results validate CA as a core for sandwich

composites.

2.4 Natural stone

Natural stone has been used for a long time mainly due to its structural characteristics (more in

the early stages of construction technology) and high aesthetic value. However, considering the

advances of technology, new ways of processing this brittle material have been developed. This

allows, for example, to produce thin slabs (bellow 10 mm) and, consequently, new applications can

be designed.

The physical and chemical properties of stones vary widely due to time and the volubility of the

environmental conditions during the formation process.

Limestone is a sedimentary stone with at least 50% by weight calcium carbonate (CaCO3)

content, in the form of calcite (rhombohedral structure) and aragonite (rhombic structure) [28].

Limestone is composed of grains or fragments of marine origin, composed of micro-sized fossils of

marine invertebrate organisms. The colour of the stone reflects the impurities present – the white

ones are usually purer and the darker ones can have carbonaceous material and/or iron sulphide

inside. In general, limestone is crystalline with average grain sizes ranging from less than 4µm to

about 1000µm. This affects the so called “texture” of limestone, along with the arrangement of the

constituent minerals [29].

As a layer of the composite that is in contact with resin, another important characteristic to

consider is the water absorption that is, of course, related with porosity, the distribution of pore sizes

and also, in the case of limestone, with the level of carbonaceous matter. This value can go from

0.4% to 20% of water by weight [29].

Some of the most relevant mechanical properties to study a material are the compressive

strength, the tensile strength, Young’s modulus and hardness.

10

Stone mechanical properties are governed by the reaction of the stone to the forces acting on it;

forces induce a state of stress which results in deformation, i.e., a state of strain (Figure 8). When

microcracks start to develop, the strain increases rapidly leading to the growth of cracks to the size

of the grains. Stone fracture starts at the grain boundaries, which loosen and become partially

detached as tension continues and fracture approaches. Catastrophic failure is the final stage of

compression or tension [30].

The hardness of a mineral or an aggregate of minerals (stone) is its resistance to permanent

deformation and therefore an important factor for evaluating the workability of a stone in quarry and

mill and its resistance to mechanical wear [30]. There are several tests to determine hardness, being

one of them the Mohs hardness (Figure 9).

Figure 9 - Mohs hardness classification of several stone types [30]

The use of a stone layer as a reinforcement on top of the sandwich generates complexities on

the mechanical behaviour of the composite. The sandwich faces Young’s modulus, whose analytical

determination is not trivial due to the asymmetry of the sandwich implying that the usual beams

theories and the current flexure testing standards (like ASTM-C393 [14] or ASTM-D7250 [15]) cannot

be applied.

Figure 8 – Stress-strain curve for several stone types [30]

11

2.5 Epoxy resins

There are several methods of attaching the skin to the core in a composite material, such as

adhesive bonding, self-adhesive prepregs, brazing, and fusion methods. In adhesive bonding, the

adhesive is selected based on economic and weight considerations for a given sandwich structure

design. Their primary purpose is to structurally attach the skin to the core, transmitting shear and

peeling forces between the skin and the core, and enabling the materials to work together as a

system [31]. The epoxy resin in the composite in study is both used as an adhesive and as matrix

for the glass fibres constituting one of the composite layers.

Epoxy resin is a thermosetting polymer, defined by IUPAC as a prepolymer in a soft solid or

viscous state that changes irreversibly into an infusible, insoluble polymer network by curing.

2.5.1 Epoxy resins as adhesives

The overall load-bearing capacity of a sandwich component is often limited not by the strength of

the skin or core material, but by the strength of the adhesive bond between the two components [32].

In the case of honeycomb cores, the cell walls provide a relatively small area for bonding. Structural

strength relies on the formation of a fillet of adhesive at the interface between cell wall and skin, to

effect load transfer between the two components of the sandwich [32].

To explain the phenomenon of adhesion, four main theories have been proposed: mechanical

interlocking, electrostatic, diffusion and the adsorption theory [33]. The first one proposes that

adhesion between an adhesive and substrate is primarily dependent upon mechanical keying of the

adhesive into substrate surface irregularities. The second is based on the existence of an electrical

double layer across an interface. The third one states that adhesion is a direct result of intermixing

of two contact substrates at the molecular level. It is now generally recognised that none of these

effects has significant importance in the bonding of structural materials such as metals and

composites with epoxies. The forth, the adsorption theory, however, defends that adhesion occurs

due to surface force interactions between the atoms in the two contacting materials. These forces

can be van der Waals' forces, permanent dipoles, hydrogen bonds and even covalent bonds in some

cases. It’s thought that the excellent adhesive properties exhibited by epoxies is due to the reaction

of its hydroxyl groups with other groups from the contacting material, as for example the hydroxyl

groups found on most metal oxide surfaces. This is the most accepted theory within the adhesion

community [33].

Another influence on the strength of an adhesive joint is the ability of the adhesive to wet and

spread spontaneously on the substrate surface. This occurs when the surface free energy of the

substrate is greater than that of the intended adhesive and there are surface treatments designed to

increase surface free energy of the substrate to make sure a good wetting is achieved.

12

Epoxy resins are extensively used as adhesives, mainly between layers of laminate composites.

Although the processing or curing of epoxies is often slower than polyester resins and the cost is

higher, the final composite usually presents improved mechanical properties [31]. Some of the

advantages of using epoxy as an adhesive are excellent adhesion capability to various substrate

types, processing without the necessity of applying high pressures, cure by reaction mechanisms

which do not result in the generation of volatile by-products (e.g. water) and low shrinkage in the end

of the cure process [33]. The main disadvantages of epoxy adhesives are the mixing requirements

(two-part systems), limited pot life, moisture absorption and brittleness [33].

Epoxy resins, being predominantly organic in nature, are prone to water absorption and its

properties can alter with continued exposure to moisture or temperature variations. Moisture

absorption decreases the modulus and the glass transition temperature (Tg) of an epoxy resin, and,

since significant loss of epoxy properties occurs at Tg, it is usually defined as the upper usable

temperature limit of the composite. To avoid subjecting the resins to temperatures equal to or higher

than this so-called wet Tg (the wet Tg is the Tg measured after the polymer matrix has been exposed

to a specified humid environment and allowed to absorb moisture until it reaches equilibrium), epoxy

resins are limited to a maximum service temperature of about 120 °C for highly loaded, long-term

applications and even lower temperatures (80 to 105 °C) for toughened epoxy resins. The assurance

of a full cure is generally obtained by measuring both the glass transition temperature and the

residual heat of reaction (ΔH) [34].

2.5.2 Epoxy resins as matrix for glass fibres

Amongst all resins used to produce polymer-matrix composites (PMCs), epoxy resins are used

far more than all other matrices for structural aerospace applications [34]. For successful application

of epoxy resins, it is necessary to select a suitable hardener and then cure the resin to attain a

controlled network structure. After mixing the two components, there is a reaction between epoxy

and hardener reactive groups and larger molecules are formed, until the gel point is reached and the

branched structures extend throughout the sample. Cure must go beyond this point for all the resin-

hardener mixture to become elastic solid, meaning that all the gel part of the structure has been

transformed. The Tg of the curing resin increases as cure proceeds – if the cure temperature, Tc >>

Tg the rate of reaction between the epoxy and hardener reactive groups is chemically kinetically

controlled, as desired. If Tc ≈ Tg, the curing reactions become diffusion controlled, and will eventually

become very slow and finally stop [33].

As part of the PMC, the epoxy matrix is responsible for transmitting and distributing the externally

applied stress to the fibres [9]. This effect and consequent synergy occurs due to the difference in

the elastic modulus between the two components (one order of magnitude or more) and the

alignment of the fibres so they support the major external loads, resulting in very interesting

properties and behaviour. Bringing together its processing flexibility and relatively low cost (when

13

compared with other composite materials) makes the PMC’s the suitable choice for several

industries.

2.6 Glass fibres

Glass fibres are the most widely used type of reinforcement in PMC [35]. Glass is a non-crystalline

amorphous solid mainly composed of silica (SiO2) that can be transformed into very fine filaments

with desirable properties, such as mechanical strength and flexibility. They are available in a variety

of forms such as chopped strand, continuous yarn, roving, and fabric.

Glass fibres fall into two categories: low cost fibres for general applications and special application

fibres. More than 90% of all glass fibres have applications in general purpose products and these

are composed of E-glass [35]. This category is preferred by the composite industries due to its cost

and processing advantages. Originally this type was developed for electrical applications due to its

better properties of electrical insulation, hence the use of the letter "E".

For this composite, there are two layers of woven glass fibre-reinforced epoxy with different

weights (300 and 600 g/m3) that have been previously studied and optimized [1]. Woven fabrics are

produced by the interlacing of warp (0°) fibres and weft (90°) fibres in a regular pattern to ensure the

desirable mechanical properties in both directions.

2.7 Testing of Adhesives

In literature, it is generally accepted that one must resort to testing to determine the strength of a

joint. This particular composite does not fit in the existent classes or standards to evaluate adhesion.

One of the goals for this work is to develop a delamination test where different characteristics of the

specimens can be shown in the result of the test, that is, ideally, a calculated value.

Adhesion tests are used for comparison of properties (tensile, shear, cleavage strength, fatigue,

etc.), quality control to determine whether the adhesives are up to standard and determination of

parameters useful in predicting performance (cure conditions, drying conditions, bond-line thickness,

etc.) [7]. As there are many factors contributing to the results of this type of tests, it is of utmost

importance that not only the proper test is selected (or designed) and the correspondent procedure

is followed, but also to interpret results properly [36].

Since several adhesives have already been tested for this product [1], [37], there is more interest

in studying the interface behaviour directly in the application of this asymmetric sandwich other than

to study the behaviour of the epoxy resin as an adhesive.

14

There are several tests and methodologies to measure adhesion between different materials.

These tests can be divided into shear, compression, tensile, cleavage and peel tests (see Figure 10).

The peel strength or peeling force [MPa] is the first peak force per unit width of bond line required to

produce progressive separation, whereas the peel energy is the amount of energy per unit bonding

area associated with a crack opening. The stress intensity factor [MPa. m1/2] is another way of

expressing adhesion characteristics with stress intensity at the crack tip [38]. To determine

comparative rather than fundamental measurements of adhesion, peeling load values of different

specimens are measured.

Figure 10 - Adhesion test-types in ASTM specifications [39]

Most ASTM tests are developed to determine the above described properties of the adhesive

itself, but some studies have already been made to test the composites with adhered layers. Some

of them are based on bending test, and are referred as delamination or skin-core debonding tests.

Core-to-skin strength of a composite sandwich was first studied by Cantwell et al., where

structures were evaluated using a fracture mechanics approach based on the application of load via

a single cantilever beam [40]. Testing was undertaken on a woven glass fibre polyester/balsa

sandwich structure and a carbon fibre-epoxy/honeycomb structure. The fracture energy associated

with the carbon fibre/epoxy composite was higher than that measured on the glass/balsa and the

importance of the analysis whether the crack had propagated at the skin-core interface was

highlighted.

Lee et al. tested a glass/epoxy face and polyurethane foam core composite with and without resin

impregnation of the resin on the foam. The specific peel strength of the resin impregnated core

resulted in a 70% higher strength than those without surface resin impregnation on the foam [38].

Grove et al. [41] studied the effect of processing parameters on the quality of the skin-to-core

bond of Nomex honeycomb core/carbon fibre-epoxy prepreg skin sandwich panels, measured

quantitatively by the climbing-drum peel test. High peel strengths have been associated with the

largest and most regular adhesive fillets between sandwich skin and honeycomb core, emphasizing

that the results are specific to the particular combination of materials used.

Okada and Kortschot correlated the formation of the resin fillet between honeycomb core cell

walls and skin with the delamination resistance. They showed that a bigger resin fillet absorbs a

15

considerable amount of energy upon fracture, causing the critical strain energy release rate to

increase [42]. Furthermore, Rion et al. [43] developed a model to predict the shape and size of the

resin fillet based on the contact angles between honeycomb core cell walls and skin, presenting a

way of predicting the microscopic failure mechanisms of the specimens. The setback of these tests

are the challenging methodologies to guarantee that a certain amount of resin is present between

layers, and again they are specific for bonding two particular materials.

More recently, Teixeira de Freitas et al. presented a new type of peel tests dedicated to composite

bonding inspired on the standard floating roller peel test. Although their Composite Peel Tests are

suitable to assess interface adhesion of composite bonding, the test can only be used as a quality

indicator of interface adhesion if using exactly the same type of flexible peeling-off member, and it

also has to be flexible [44].

Amongst all the tests presented, none suits the asymmetry and the triple composite that is of

interest in this study. On top of that, when applied to this composite, most of these tests result in the

failure of the CA (between the grains of the corks, as represented in Figure 11) other than

delamination between the stone and cork layers.

Figure 11 – Peel stress failure of CA [45]

Ratcliffe and Reeder have highlighted the clear motivation that exists to establish a standardized

test procedure to characterize skin-core debonding, capable of producing reliable values of debond

toughness [46].They presented a method (Figure 12) as a draft testing protocol to be developed into

an ASTM standard.

Figure 12 - Single cantilever beam test proposed to test adhesion in sandwich composites [46]

16

2.7.1 Failure Modes in Composites

It is important to define the failure modes and what they represent in terms of criteria for

comparison between specimens. Failure modes of composite sandwich beams depend on the type

of loading, constituent material properties and geometrical dimensions [47]. Figure 13 presents a

classification concerning different causes of failure: adhesive failure, also referred as skin-core

debonding, cohesive failure, related with the core or skin failure, indentation failure due of the

presence of a brittle upper layer and multi-mode failure (a combination of the preceding failure

modes).

Figure 13 - Failure modes in composites [1] [48]

Adhesive failure indicates a bad adhesion between the adhesive and the adherent, since the

failure is at the interface between those two materials. Cohesive failure indicates good adhesion,

since the failure not at the interface [44]. Indentation failure can also occur when a brittle component

is in contact with the loading bearing arm, which in this case, is the reinforcement stone layer. In this

scenario, delamination may occur after stone failure, giving also an insight about the quality of

adhesion.

Relating the failure modes with the bending test, it is important to understand the stresses and

moments generated in the beam to further define the failure modes. The deflection of a sandwich

panel is made up from bending and shear components. The bending deflection is dependent on the

relative tensile and compressive moduli of the skin materials. The shear deflection is dependent on

the shear modulus of the core. Total Deflection can be defined as the sum of the Bending Deflection

and Shear Deflection [49]. Table 3 describes the failure modes, related with the property of the

material in cause.

Failure Modes

Adhesive failure Cohesive failure Identation failure Multi-mode

17

Table 3 - Failure modes, adapted from [49]

Strength

The skin and core materials should be able to

withstand the tensile, compressive and shear

stresses induced by the design load. The skin to

core adhesive must transfer the shear stresses

between skin and core.

Stiffness

The sandwich panel should have sufficient

bending and shear stiffness to prevent excessive

deflection.

Panel

buckling

The core thickness and shear modulus must be

adequate to prevent the panel from buckling

under end compression loads.

Shear

crimping

The core thickness and shear modulus must be

adequate to prevent the core from prematurely

failing in shear under end compression loads.

Skin

wrinkling

The compressive modulus of the facing skin and

the core compression strength must both be high

enough to prevent a skin wrinkling failure.

Intra cell

buckling

For a given skin material, the core cell size must

be small enough to prevent intra cell buckling.

Local

compression

The core compressive strength must be

adequate to resist local loads on the panel

surface.

19

3. Characterization of the constituent materials

3.1 Stone Surface Roughness

To understand the influence of the types of stone finish used in the adhesion between layers,

surface roughness measurements were made.

3.1.1 Principles

Two roughness parameters were chosen according to literature [50], [51] : Ra and R3z. According

to ASME B46.1 for Surface Texture, the Roughness Average, Ra, is the arithmetic average of the

absolute values of the roughness profile ordinates (Figure 14); the Third Maximum Peak-to-Valley

Height, R3z, is the mean of the third maximum peak-to-valley heights in the evaluation length (Figure

15). Consequently, it is expected for the R3z values to be higher than Ra and this complementary

information is going to be determinant to take conclusions after the mechanical tests.

Figure 14 - Roughness average, Ra

Figure 15 - Third Maximum Peak-to-Valley Height, R3z

3.1.2 Methodology

The measurements were made with KosakaLab Surfcorder SE1200 (Figure 16) in both stones,

each one tested for two different finishes. Cut-off and evaluation length were determined according

to ISO4288, and the final average values were calculated from 10 measurements.

20

Figure 16 - Surfcorder SE1200 used to measure surface roughness

3.1.3 Results and Discussion

Results are summed up in Table 4 and Error! Reference source not found.. As it is expected,

the sawn finish results in surfaces with higher Ra and R3z in both white and blue stones. Between

them, the blue stone has less variation in results for the different finishes, and the values are always

smaller than those of the white stone.

Table 4 - Average values of Ra and R3z

Ra (µm) R3z (µm)

White Smooth 1.86 ± 0.25 5.95 ± 1.06

Sawn 6.67 ± 1.28 25.79 ± 2.31

Blue Smooth 0.27 ± 0.04 0.68 ± 0.09

Sawn 0.62 ± 0.10 0.90 ± 0.27

3.2 Differential Scanning Calorimetry Analysis on Epoxy

To evaluate the cure of the epoxy resins a Differential Scanning Calorimetry (DSC) analysis was

done. The goal is to measure the Tg of the two GFRE layers, one between the stone and CA layers

and the other that is externally in contact with air.

3.2.1 Principles

The Nomenclature Committee of the International Confederation for Thermal Analysis (ICTA) has

defined DSC as a technique in which the difference in energy inputs into a substance and a reference

material is measured as a function of temperature whilst the substance and reference material are

subjected to a controlled temperature program. One of the major applications of DSC is the

measurement of Tg. In the absence of endothermic or exothermic reactions the DSC heat flow output

is proportional to the sample heat capacity, and the Tg may be determined from the characteristic

21

discontinuity in heat capacity. The Tg of a crosslinked polymer in general shows an increase with

increasing degree of crosslinking, and thus provides a useful index of the degree of cure [52]. A

typical DCS curve is presented in Figure 17, with indication of the reactions correspondent to each

peak.

Figure 17 - Typical DSC results1

The technical data sheet of the used resin indicates that Tg (DSC mid-point) is 81 ℃.

3.2.2 Methodology

Two samples were cut from a composite specimen and placed in an aluminium pan. A

temperature-scanning (dynamic) mode was used, at a rate of 10℃/min in a Microcal Differential

Scanning Calorimeter. The Tg temperatures were determined using TA universal analysis.


The obtained results seem to present a melting point, which is not expected for a fully cured epoxy

resin. This may either mean that the resin was not fully cured or that the glass fibres have a relevant

influence in the shape of the peak, although this is more unlikely to happen since the components of

the glass fibres do not suffer any alterations at these temperatures. Another hypothesis is that this is

the melting of the unreacted curing agent, as it is reported by John M. Barton [52]. In the upper layer

results, in Figure 18, there is no change is the baseline before and after the peak, but there is a

change in the baseline in Figure 19, relative to the down layer. These results are usually of difficult

interpretation and there is only one test per layer, so a further study on this matter is one of the

suggested future works, due to the importance of guarantying the cure of the resin in both layers.

1 Prof. Luís Santos class slides, Caracterização de Materiais 2016

22

Figure 18 - DSC result for the upper layer epoxy resin

Figure 19 - DSC result for the down layer epoxy resin

3.3 Theoretical Approach to Evaluate the Composite Load

Distribution

As a first view of the mechanical properties of the asymmetric composite, two theoretical models

were applied to evaluate the load distribution and the location of the neutral line during a bending

test. This approach was found to be important to further understand the failure mechanisms that

occur as well as to propose any solution to optimise the final composite structure.

In this approach, both cork and stone are assumed to be isotropic. Therefore, their properties are

fully defined with two independent constants: 𝐸 (Young’s modulus) and 𝜈 (Poisson’s ratio). The

GFRE layer is transversely isotropic, where the 1-2 plane is the plane of isotropy. Therefore, the “1”

23

and “2” subscripts on the stiffness are interchangeable. The stress-strain relations have five

independent constants: 𝐸1, 𝐸3, 𝜈12, 𝜈13, 𝐺23. In the present bending tests, the principal directions

coincide with the engineering directions (x, y, z), thus they will also be used interchangeably. The

material properties are shown in Table 5.

Table 5 - Material constants used in the calculations

Stone Cork GFRE

𝑬 (𝑮𝑷𝒂) 50 0.035 -

𝝂 0.2 0.01 -

𝑬𝟏 = 𝑬𝟐 (𝑮𝑷𝒂) - - 10

𝑬𝟑 (𝑮𝑷𝒂) - - 2

𝝂𝟏𝟐 = 𝝂𝟐𝟏 - - 0.1

𝝂𝟏𝟑 = 𝝂𝟐𝟑 - - 0.2

𝑮𝟐𝟑 (𝑮𝑷𝒂) - - 2

3.3.1 Transformed Section Method

The theory of the transformed-section method (TS) is well known and generally accepted as a

method of analysis of members loaded in bending. By imposing a strain compatibility condition,

materials making up the cross section are transformed into a fictitious homogeneous material. This

is accomplished by adjusting the geometry of each material by a ratio of its elastic modulus to that

of the base material modulus, creating a fictionalized shape of homogeneous material. The resulting

single material cross section may then be analysed in the traditional manner [53].

Several assumptions are made when using this method:

- No slippage occurs between materials;

- Small and equal deformations within the elastic stress range for all components;

- All the components of the member must deform both independently, based upon the

material properties of the component, and in a compatible manner, based upon the physical

relationship of the components. Transformed sections create a visual image of a composite

member, with distortion based on the relative strength of the materials comprising the member,

which in turn is used for its analysis.

To determine the stresses at the interface between layers, the steps descried below are followed.

1. Stone is chosen as reference layer, thus its geometry (width) remains constant.

2. For the i-th layer, the modular ratio is calculated as 𝑛𝑗𝑖 =𝐸𝑖

𝐸𝑠𝑡𝑜𝑛𝑒.

3. The thickness of the materials composing the section remains unchanged to keep the

strain profile linear. An equivalent width is calculated as 𝑏𝑖𝑒𝑞

= 𝑛𝑗𝑖 ∗ 𝑏𝑖

4. Given the equivalent geometry, the neutral axis is obtained as:

24

𝑦 =∑ 𝑏𝑖

𝑒𝑞∙ 𝑡𝑖 ∙ 𝑦𝑖

𝐴𝑡𝑜𝑡

(1)

Where 𝑦𝑖 is the distance from the top to the middle point of the i-th layer, 𝑏𝑖 the width of the

i-th layer and 𝐴𝑡𝑜𝑡 the sum of areas of all material sections with the equivalent properties.

5. The moment of inertia of the section about the neutral axis is calculated with the parallel

axis (or Huygens-Steiner) theorem.

6. The equivalent stress at distance 𝑦𝑖, assuming that all materials have the modulus of the

stone, is given by:

𝜎𝑖𝑒𝑞

= − 𝑀 ∙ 𝑦𝑖

𝐼𝑧

(2)

where 𝑀 is the Moment the 𝐼 is the Moment of Inertia.

7. The strain at 𝑦𝑖 is obtained as 휀𝑖 =𝜎𝑖

𝑒𝑞

𝐸𝑠𝑡𝑜𝑛𝑒

8. The real stresses are now 𝜎𝑖 = 휀𝑖 ∙ 𝐸𝑖 .

3.3.2 Classical Lamination Theory

Classical Lamination Theory (CLT) makes the following assumptions:

- The laminate is sufficiently thin. This means that a line originally straight and normal to

the middle surface of the laminate is assumed to remain straight and normal to the middle surface

when the laminate is deformed.

- The bonds are presumed to be infinitesimally thin as well as non-shear-deformable. This

results in continuous displacements across layer boundaries so that no layer can slip relative to

another.

- Small and equal deformations within the elastic stress range for all components.

Calculations were made with the following steps [54]:

1. Calculation of the [Q] matrix for each material. [Q] = [S]-1, and [S]:

25

Since a plane stress is assumed, the 9x9 [Q] matrix becomes 3x3:

2. Definition of the zk directed distances from the middle surface, in accordance with the

convention that the z is positive downward. zk is the directed distance to the bottom of the kth layer

and zk-1 is the directed distance to the top of the kth layer (Figure 20):

Figure 20 – Representation of the distances of the middle surface of the material under study

3. Calculation of the [Aij], [Bij], and [Dij] matrices as below, and assembly of the [ABD] matrix.

4. Calculation of the [abd] = [ABD]-1 matrix.

5. Calculation of midplane strains and curvatures.

26

6. For each layer, calculation of the layer strains at the interface.

7. For each layer, calculation of the layer stresses.

The results of the methods, inputting values shown above, are represented in Figure 21.

The neutral line of the TS method is located at 3.33 mm from the top, in the stone layer. It has

been confirmed that the strain profile is linear, whereas the stress profile has abrupt changes. The

stone above the neutral axis is in compression, while all the other layers are in tension. Cork bears

almost no tensile stress, the last layer of GFE bears the highest normal stresses, since it has the

highest modulus.

The neutral line for the CLT method is: 3.29 mm for strains in the 1- direction, 3.5 mm for strains

in the 2-direction. The position of the neutral line is almost equal for both methods, and it is shown

to be in the stiffer layer (stone). Results of previous tests in bending using a VIC (Visual Image

Correlation) software have although shown that the neutral line was located inside the cork layer, at

a much higher distance from the top. The reason for the mismatch between values can be twofold,

assuming that results from VIC are correct is that the theoretical assumptions of the used theories

are not valid (such as the isotropic behaviour).

27

It is possible that the values of the Young’s modulus and of the coupling coefficients are not the

same as those of the tested specimen, since these values have been found in literature and not

determined experimentally using our specimens. Since this is a preliminary work, its aim is not that

of a precise calculation of stresses profile, but the gaining of a perspective to be used in future

experiments, and the knowledge of the order of magnitude of stresses arising in the cross section.

This is the reason why approximated values are used.

Nevertheless, to obtain the 𝑦 value in the cork agglomerate core would mean to have values of E

of the cork (or of the stone) too high (or too low) to be true. Therefore, other causes need to be

investigated that lead to a lower neutral axis. A reason, to be later verified, could be the change in

cork stiffness due to resin impregnation in its porous surface. Concerning the second option

regarding the theoretical assumptions being too restrictive, it is not to be excluded for future analysis.

0

5

10

15

20

25

-100 -50 0 50 100

Dis

tan

ve f

rom

to

p (

mm

)

Stress Profile CTL

σxx (MPa) σyy (MPa)

0

5

10

15

20

25

-30 -20 -10 0 10 20

Dis

tan

ce f

rom

to

p (

mm

)

σ (MPa)

Stress Profile TS

Figure 21 – Strain and Stress profiles from TS and CLT methods

29

4. Experimental tests on the composite

The main goal of the experimental work is to understand the influence of the constituent materials

of the composite (stone and CA) as well as the surface finish on the stone in the adhesion between

layers. To evaluate these phenomena, a stiffness test, a delamination test and a four-point bending

test until failure were defined and carried out.

Specimens were produced with two natural stones, a white limestone “Branco do Mar” and a blue

limestone “Vidraço Azul”, two CA (labelled A and G) as well as two different stone surface finishes,

sawn and smooth. In Table 6 a test matrix is presented in with all the configurations produced, as

well as the codes used for each one.

Table 6 - Test matrix

White Limestone Blue Limestone

Finishing A Finishing B Finishing A Finishing B

CA - A WAS WAA BAS BAA2

CA - G WGS WGA BGS BGA

After production of the specimens, during cutting, the BAA specimens delaminated completely,

and so the reinforced cork and stone were tested separately. In parallel, a SEM (scanning electron

microscope) analysis was carried out on WAS, WGS, BAS and on the stone and reinforced cork.

These specimens were chosen to visually compare the effects of using different CA (by analysing

the differences between WAS and WGS specimens) and the effects of the stone (comparing WAS

and BAS specimens).

4.1 Specimen Production

As composites have a higher initial cost than other more traditional materials, it is very important

to choose the most suitable production process to produce a single part in order to ensure that the

product is economically viable. In previous studies carried out for this particular composite, two of

the most popular methods have been studied, the hand lay-up and the vacuum bag methods. Also,

some studies have proved that the calculated density for a resin infiltrated cork is about 325 kg/m3,

thus an increase of 38% when compared to the original CA panel and may also reduce water

absorption in the CA [55].

2 When cut, BAA specimens delaminated and the reinforced cork and stone were tested separately

30

Materials used to produce the specimens were:

- 8mm thick “Branco do Mar” and “Vidraço Azul” stones, obtained by a wire metal saw without

any lubricating agent;

- Two cork agglomerates with 15mm thickness (labelled A and G);

- Castropox® 1233 A/B1;

- Two biaxial fibreglass with different densities, according to previous studies [1].

As referred before, the sandwich specimens were produced by hand lay-up method, by hand

lay-up method with 68% mass fraction of resin/fibre, with temperature and humidity control. The

manufacturer properties of the CA are presented in Table 7, those of the used stones in

Table 8 and of glass fibres in Table 9.

Table 7 - Properties of CA (A and G)

Unit A G

Density Kg/m3 200 ASTM C271 210 EN672

Cork granules size mm - - 0.5 – 4

Binder - - Polyurethane

Compressive

Strength MPa 0.5 ASTM C365 - -

Compressive

Modulus MPa 6.0 ASTM C365 - -

Compression and

recuperation % - -

Comp. 45%

Rec. 75% ISO 7322

Tensile Strength MPa 0.7 ASTM C297 0.4 ISO 7322

Shear Strength MPa 0.9 ASTM C273 - -

Shear Modulus MPa 5.9 ASTM C273 - -

Thermal conductivity W/mK 0.034 ASTM C377 0.05 EN 12664

Sound absorption

coefficient - - -

Approx. 0.2

(at 2 kHz) -

Resin uptake

(per m at 1mm) g 170 - - -

Table 8 - Properties of the two natural stones used in the work

Blue White Unity

Compression strength 1650 530 kg/cm2

Bending strength 105 76 kg/cm2

Apparent density 2680 2280 kg/m2

Water absorption 0.4 6.2 %

Open porosity 0.9 13.3 %

31

Table 9 - Properties of the two glass fibres used in the work

Top layer Fibre glass Bottom layer Fibre glass Unity

Weave Plain Plain

Areal Density 612 290 g/m2

Volumetric Density 2.6 g/cm3

Filament Diameter 12 – 15 8.9 – 10.2 µm

Tensile Strength 1900 – 2400 1900 – 2400 MPa

Tensile Modulus 69 – 76 69 – 76 GPa

Elongation at break 3.4 – 4 3.5 – 4 %

Figure 22 – Specimen production a) Hand lay-up method; b) Hot press

The production process starts by drying the stone panel. The resin for the first layer is poured on

top of the stone panel, as it can be seen in Figure 22a. The amount of resin is calculated and prepared

for each layer, since the glass fibres have different densities. The curing agent and monomer are

weighed and mixed until hydrogen bubbles start to appear. The glass fibre fabric is placed on top,

and after it is soaked, the previously cut CA is also added. The second portion of resin is measured

and mixed, and it is poured on top of the CA, followed by the second glass fibre fabric. A polyethylene

sheet is placed to cover the wet epoxy and the specimen is placed in the hot press (Figure 22b). The

specimens are then post-cured and cut into the desirable dimensions by wet saw cutting. After this

it is again necessary to dry the specimens overnight.

For the delamination specimens, the plastic sheet is placed between the stone of the composite

and another stone, in order to simulate the weight and heat transfer to achieve a specimen as in

Figure 23.

Figure 23 - Specimen for the delamination test

a) b)

32

Because of the manual process and cutting, all specimens have different thicknesses. All

specimens were measured and this was considered in the calculations and discussed with the

results.

4.2 Bending Stiffness Tests

This study aims to evaluate the behaviour of the specimens under test that best mimics the

conditions of use of the composite in facades, the four-point bending test (Figure 24).

Figure 24 - Four-point loading configuration according to ASTM-C393

4.2.1 Principles

The stiffness test method procedure was based on a previous study [1]. Each specimen is tested

with the same loading type and the loading span (L) is increased as the supporting span (S) increases

(S=100mm, S=175mm, S=200mm). The test is carried out with 25% of the failure load, stopping

before achieving the onset of any permanent deformation or damage in the sandwich skin facings

and core. Deducing the mid-span deflection and replacing L=S/3 (four-point bending test with third

point distance), the mid-span deflection (∆) as a function of the applied load (P), bending (𝐷) and

shear stiffness (𝑈) can be obtained, which then can be transformed into the following equation,

similar to an equation of type 𝑦 = 𝑚𝑥 + 𝑞 (equation 3):

(∆

𝑆

1

𝑃) =

1.7

96𝐷𝑆2 +

1

6𝑈 (3)

Plotting the previous equation as a function of S2, it is possible to determine a linear regression

from the obtained results for each typology, yielding the mean results for 𝐷 from the graph slope and

for 𝑈 from the interception of the regression line:

𝐷 =1.7

96 𝑚; 𝑈 =

1

6 𝑞 (4)

33

where 𝑚 is the slope and 𝑞 the interception of the linear regression.

4.2.2 Specimen Dimensions

The specimen’s dimensions for the four-point bending test (300×50mm) were defined according

to the restrictions of the bending test machine, including the permitted spans, and the rules defined

in ASTM-C393 (l=300mm, S=250mm and S/d=16.7).

4.2.3 Methodology

The setup for the test is a four-point bending (Figure 26) according to ASTM D7249 [56]. The

compressive load is applied by an Instron 5566 universal testing machine, with the load-displacement

data being collected using a 10kN load cell. The tests were displacement controlled with a velocity

of 3mm/min. A preload of 10N was applied before each test.

The load used was 602.25N for the blue stone specimens. For the white stone a correlation was

used to relate the maximum load with the thickness of the specimen (𝑦 = 351.3𝑥 − 6132.2). This

relation was calculated for several four-point bending tests until failure. This is due to the properties

of the stone itself - as the white stone has less stiffness the grinding process results was not so

accurate as in the case of the blue stone.

Figure 26 - Four-point bending configuration

Figure 25 - Produced specimens for the bending stiffness test.

34

For the calculations described in 4.2.1, the mid-span deflection (∆) was calculated as in the

preliminary test, as the difference between the deflection value in the middle of the specimen minus

the deflection measured on top of the left roller. These deflection values were measured with

deflection gauges, as can be seen in Figure 26.

Three specimens of each configuration were tested, being each one tested two times with a

resting time of one minute, as defined in previous tests.


The stiffness values obtained are summarized in Table 10. The average values are also

represented in Figure 27. The maximum stiffness value was obtained for BAS (blue stone, CA-A

cork, sawn finish) and the minimum for BGA (blue stone, CA-G cork, smooth finish). The reinforced

cork was tested as a reference.

Table 10 - Stiffness values (MPa)

Average Maximum Minimum

BAS 151.6 ± 33.0 184.6 118.7

BGS 139.6 ± 26.7 166.2 112.9

WGS 128.9 ± 24.9 153.8 103.9

WAS 112.4 ± 14.2 126.6 98.3

WGA 94.1 ± 13.2 107.3 80.9

WAA 91.3 ± 10.7 102.0 80.7

BGA 77.4 ± 15.1 92.5 62.4

Cork 67.9 ± 22.1 90.0 45.8

Figure 27 - Average stiffness values for each specimen configuration

As there were several variables in this study, Table 10 was divided into three and the effects of

the two finishes, two stones and two CAs were analysed separately.

35

Table 11 - Comparison of the stiffness values considering the finish of the stone layer

Stiffness D (MPa)

WAS 112.4 ± 14.2 BGS 139.6 ± 26.7 WGS 128.9 ± 24.9

WAA 91.3 ± 10.7 BGA 77.4 ± 15.1 WGA 94.1 ± 13.2

In Table 11, the values corresponding to the same specimen configuration with different finishes

in the stone layer were separated in columns. It can be observed that the sawn finish resulted in

specimens with higher stiffness in the three cases.

Table 12 - Comparison of the stiffness values considering the stone configuration

Stiffness D (MPa)

BAS 151.6 ± 33.0 BGS 139.6 ± 26.7 BGA 77.4 ± 15.1

WAS 112.4 ± 14.2 WGS 128.9 ± 24.9 WGA 94.1 ± 13.2

In Table 12, the values corresponding to the same specimen configuration with different stones

were separated in columns. In this case, the first and second rows (AS and GS) the blue stone results

in higher stiffness but in the third one (GA) the opposite is observed.

Table 13 - Comparison of the stiffness values considering the CA configuration

Stiffness D (MPa)

BAS 151.6 ± 33.0 WAS 112.4 ± 14.2 WAA 91.3 ± 10.7

BGS 139.6 ± 26.7 WGS 128.9 ± 24.9 WGA 94.1 ± 13.2

In Table 13, the values corresponding to the same specimen configuration with different CA were

separated in columns. The CA-A resulted in specimens with higher values of stiffness in one case

(BS) and the CA-G in the other two cases (WS and WA).

It is important to notice that the differences in the values in each column is less in Table 13 that

in the others, meaning that this is the variable with less effect on the stiffness out of the three. Also,

the larger difference occurred between BGS and BGA and since BGA is the layout with the higher

values of stiffness, this may mean that the finish is the most relevant variable concerning stiffness.

Other than that, it was important to relate these values with the thickness of the specimens, since

this factor is not included in the calculations. A plot of these two parameters is represented in Figure

28.

36

Figure 28 – Stiffness vs thickness of tested specimens

The variation between average thickness is a result of the grinding method. As it can be seen in

Figure 28, there is an influence in stiffness, although it is not linear. Comparing the blue stone values

(BGA, BGS and BAS), the relation is exponential but comparing the white ones (WAA, WGA, WAS

and WGS) there is no visible correlation. Nonetheless, the stiffer specimens are the thicker ones and

the less stiff the thinner.

4.3 Delamination Tests

4.3.1 Principles

Considering the state of the art in testing of adhesives, and taking into account that susceptibility

to delamination is one of the major weaknesses of advanced composite structures [57], a

delamination test was designed to achieve delamination while measuring load and displacement.

The test was based on ASTM D5528 and ASTM D3807 principles, since it can be applied to

composite materials with different constituents. It serves the purpose to compare quantitatively the

relative values of the interlaminar fracture toughness, 𝐺𝐼𝐶 and to develop delamination failure criteria

for composite damage tolerance and durability analyses.

The Modified Beam Theory (MBT) expression for the strain energy release rate of a perfectly

built-in (that is, clamped at the delamination front) double cantilever beam is:

𝐺𝐼𝐶 = 3𝑃𝛿

2𝑏𝑎 (5)

where P is the load, δ the displacement, 𝑏 the specimen width and 𝑎 the delamination length. It

is important to refer that delamination growth may proceed by a slow stable extension or as a run-

arrest extension in which the delamination front jumps ahead abruptly. Only the first category of

growth is of interest in this test method because rapid delamination growth may introduce dynamic

effects in both the test specimen and in the fracture morphology.

37

Figure 29 - Double cantilever beam specimen with piano hinges from ASTM D3807

As this is not a double cantilever test (the load is only applied in one direction), the values of 𝐺𝐼𝐶

would be only calculated for comparison amongst them and should not be compared with other

values obtained by a different setup. Likewise, the machine compliance adjustment was not made,

being the same for every specimen. The results obtained for each composite formulation are

compared, taking into account its delamination mode that is registered simultaneously.


The specimens were thought for delamination to occur, and so the cork and resin had l1=230mm,

b=50mm but the stone on top had only l 2=150mm, as shown in Figure 30.

Figure 30 - Composite layout: 1 - stone; 2 - cork agglomerate (CA); 3 - glass fiber-reinforced polymer

skins

4.3.3 Preliminary Tests

The preliminary tests were carried out at Frontwave’s laboratory in an Instron 3369. The first setup

is represented in Figure 31. The clamp was placed 50mm from the composite edge and the applying

load arm 10mm from the CA edge, on the opposite site.

l 1 = 230

h2 = 5 mm

h1 =15 mm

1

2 3

l2 = 150

38

The crosshead speed used for the test was 12.7mm/min and the test stopped as soon as the

displacement arm reaches -50.8 mm (based on ASTM D3807). The load/displacement curve was

measured throughout the test and images where captured (microscope/camera);

The apparatus was prepared according to Figure 31, with one additional metallic bar between the

clamp and the stone to avoid breaking the stone when tightening the screw of the clamp. As only

one clamp was used, the load applied just rotated the specimen and no meaningful load is applied

in the interface. The test was repeated with the same specimen but the clamp was fixed in a different

and more effective way. However, as the load applied in the specimen increased, the stone fractured

in zone B (of Figure 31), as it can be seen in Figure 32.

Figure 32 - Second test specimen

To avoid the situation on the next test, the clamping distance was increased from 50mm to

100mm, as represented in Figure 33, and two clamps were used instead of one, as in Figure 34.

50 mm

10 mm

A B C

Figure 31 - Test setup: Clamping and Force Applied zones

39

The third test was prepared but as the stone fractured again in zone B, as in the previous test.

After noticing that the stone was breaking right above the metallic bars (that were placed

perpendicular to the specimen as showed in Figure 34a, the configuration was changed, placing the

two bars parallel to the specimen, one on top and the other on the bottom for the following test (Figure

34 b).

Figure 34 – a) Third and b) fourth test apparatus (the last with delamination occurring)

As the test started, the load increased more than what observed in the previous tests but the first

event was still the stone breaking (Figure 35a).

As the specimen was held between the metallic bars, the crack on the stone was confined and

so the load applied on the critical interlaminar zone kept increasing (see Figure 36). After a while,

delamination started to occur as it is shown in Figure 34b.

Figure 35 - a) Crack in the stone layer of the specimen used in the fourth test; b) Crack on the resin plus

fibres layer of the specimen tested in the fourth test.

A B C

100 mm

10 mm

Figure 33 - New delamination test setup

a)

b)

a)

b)

40

The results obtained on the fourth test were a load/displacement curve (Figure 36) with several

peaks; the first one at 100N is the stone failure in zone B of the specimen; the second, at around

150N, corresponds to the first delamination; the next four peaks (at 220N, 225N, 245N and 205N)

also correspond to delamination; The last one at 210N corresponds to the failure of the layer of resin

and fibres under the cork agglomerate, as it is shown in Figure 35b.

Figure 36 - Load (N) displacement (mm) curve obtained from the fourth test

The delamination occurs in peaks, and it is very clear when looking at the specimen during the

test; there is a value of load that induce a few millimetres to delaminate at once, producing and a

relatively loud sound. After that, the load increases until it happens again. The load always increases

until the fourth peak, the fifth and sixth have already a low load value. This means that the load

applied until that point was not affecting the material ahead that was still adhered but after reached

245N some delamination occurred even in the “adhered zone”.

The last peak occurred when the layer of resin and fibres under the cork agglomerate fractured,

and it was again above the metallic bar that was underneath the specimen. This is inevitable but it

didn’t influence the results since the maximum delamination load had been measured successfully.

The cork agglomerate layer had no crack or other visible defect in either sides, even in the zones

where the other layers fractured (stone and resin plus fibres). The stone presented pieces of both

resin and fibres, as shown in Figure 37, and the resin plus fibre that is over the cork presents voids

correspondent to the resin that appears on the stone. This demonstrates that the adhesion between

the layers is good.

Displacement (mm)

41

Figure 37 - Delamination zone between the stone and cork agglomerate layers

After the preliminary test, a new setup was set in IST’s laboratory, as shown in Figure 38.

In the first runs the expected occurred: the stone fractured and there was no delamination

occurring. For the second test, metal sheets were placed on both top and bottom of the specimen,

but the same occurred. Based on a test made on natural stone, a metal bar was used to couple the

stone to the sheet by gluing the two together and a support was designed and machined for the metal

sheet to fit into, as it is shown in Figure 39.

Figure 39 - a) Specimen with a metal sheet glued on top of the stone; b) final delamination test apparatus (delamination occurring)

Figure 38 - Test apparatus in IST

a) b)

42

One first type of glue was tested and the results were positive: delamination was observed. As

this solution was effective, another and more cheaper glue was tried out and the outcome was the

same. This setup was decided to become the final apparatus and therefore a new test methodology

was defined to be used for determining the delamination of this asymmetric composite.

4.3.4 Methodology

Specimens were prepared by gluing a previously cleaned metal sheet (90mm x 130mm) on top

of the stone with “UHU pregos” glue. According to the glue instructions, the drying time is 24h but

this time was increased to 5 days to ensure maximum fixing strength.

After mounting the setup, the specimen is placed and clamped 100mm from the composite end

and the arm is aligned 10mm from the other side. To minimize human error, the four screws were

tightened with an electric powerdrive. The compressive load is applied in an Instron 5566 universal

testing machine, with the load-displacement data being collected using a 10kN load cell. The tests

were displacement controlled with a velocity of 10 mm/min and a preload of 10N was applied before

each test. The digital microscope Dino-Lite Edge, used to record images to determine the 𝑎 value, is

turned on simultaneously as the test begins. These steps are represented in Figure 40.

Figure 40 - Methodology for delamination tests: a) gluing the metallic plate to the composite specimen; b)

tightening the screws to secure the specimen; c) positioning the microscope to record delamination front.


The obtained load displacement curves for the blue specimen configurations are represented

from Figure 41 to Figure 44 and from Figure 45 to Figure 48 for the white. The number of specimens

represented in each configuration varies since some of them were used for the preliminary tests and

a)

b)

c)

43

others were peeled off from the metallic plate instead of delamination to take place. The maximum

load values for delamination are summarized in Table 14.

Two main types of delamination were observed, a run-arrest extension in which the delamination

front jumps ahead abruptly for the blue stone specimens and a slow stable extension for the white

stone ones.

Table 14 - Average maximum delamination load values (N) for each specimen configuration

4.3.5.1 Blue stone specimens

The blue stone specimens presented a delamination in which the front jumps ahead abruptly, as

it is evidenced by the curve shape in graphics of Figure 41 to Figure 44.

Figure 41 - Load displacement curve for BAS specimens

Average Pmax (N)

BGA 95.55 ± 29.78

BAS 151.61 ± 21.10

BGS 218.25 ± 32.29

WGS 242.65 ± 22.46

WAS 304.23 ± 11.30

WAA 328.92 ± 11.13

WGA 404.56 ± 62.04

44

Figure 42 - Load displacement curves for BGS specimens

Figure 43 - Load displacement curves for BGA specimens

Looking at the three graphics for the blue stone specimens, all of them showed a similar

behaviour. The load displacement curve is linear until the energy accumulated in the interface is

enough for separation to occur. This event happens at once in this case, being represented as a

sudden decrease in load. In most specimens, delamination is observed either between the stone and

the GFRE layer, as it is represented in Figure 44a, or the stone layer fails, as it is represented in

Figure 44b. When the test pin forces the CA to bend, it induces an extension on all the composite

layers which bend in the elastic regime as much as its stiffer material, in this case, the stone layer.

Additionally, the lowest bonding energy is found between the GFRE and the stone, due to the stone

chemical stability. So, when reaching the stone’s bending limit, the accumulated energy will be

released at this interface and delamination occurs.

In Table 15 the occurrence of these two phenomena in all the tested specimens is indicated, but

the second is the one that occurred more frequently.

45

Table 15 - Delamination type occurring in each blue stone specimen

Normal Stone failure

BAS

3 ✓

4 ✓

5 ✓ ✓

6 ✓

7 ✓ ✓

BGS

3 ✓

4 ✓

5 ✓

6 ✓

7 ✓

8 ✓

In the BGA specimens, delamination occurred as in Figure 44a but there were some fibres on

both layers that did not appear in any other specimen. It is important to remember that BAA

specimens delaminated completely during cutting (the same type of BGA but with a different cork),

and this can mean that the configuration of the blue stone with the smooth finish does not result in

specimens with desirable properties. In fact, the maximum load of delamination was the lowest in

this case.

Comparing the values of load and displacement between all the graphics, it is important to notice

they vary significantly between different configurations but also between specimens of the same

configuration. This means there are other influences other than the parameters that are of study,

such as the specimen’s production method to achieve the layout for the test (Figure 30).

Looking at the average maximum delamination loads and crossing with the delamination

phenomena observed, one can conclude that the specimens that had the best adhesion were the

BGS and, for the run-arrest extension delamination, the indicator for the best adhesion is the stone

failure phenomenon. In this case the 𝐺𝐼𝐶 value was not calculated since a continuous delamination

is a pre-requisite.

Figure 44 - Blue stone specimens’ delamination a) delamination between the

stone and GFRE layers b) stone failure

a) b)

46

4.3.5.2 White stone specimens

For the white stone specimens, the observed behaviour was very different as the delamination

was continuous instead of instantaneous, as it can be seen from Figure 45 to Figure 48. There is a

linear increase followed by a maximum and the load decreased in multiple peaks, as it was previously

observed in the preliminary test (Figure 36). Nevertheless, instead of representing several millimetres

of delamination at once as in the preliminary test, in this case the peaks represented something

different. The first peak in all specimens represents in fact the start of delamination, and the linear

decrease the continuous delamination. After this peak, the load either starts to increase again or

remains constant and this happens every time delamination encounters a blockage. The different

causes for the pauses in the continuous delamination are represented in Figure 49.

Figure 45 - Load displacement curves for WAS specimens

Figure 46 - Load displacement curves for WAA specimens

47

Figure 47 - Load displacement curves for WGS specimens

Figure 48 - Load displacement curves for WGA specimens

Taking into example specimen WAA4 in Figure 45, after the linear increase in load, continuous

delamination starts and is intercalated with steps in the graphic, correspondent to the behaviour in

Figure 49a and Figure 49b. All the WAA, WAS and WGA presented behaviours like these except for

WAA6 and WAS2 where a baseline is pointed with a grey arrow, corresponding to a mixed

delamination as it is in Figure 49c.

Regarding WGS specimens, the first peak is again correspondent to the beginning of

delamination until a local minimum that is pointed with black arrows. At this point, delamination stops

and energy is locally accumulated until the stone breaks, represented by a sudden drop in load, as

is in Figure 49d. This behaviour could be a manifestation of good adhesion but since the load value

at this point is lower than in the other specimen configurations, it can point to the presence of cracks

in the stone that present an escape for the accumulated energy.

48

In this case, the stone is not only less stiff than the blue stone but it has higher porosity. The resin

is more impregnated, therefore, delamination is continuous. The separation does not always occur

between the stone and the GFRE layer, but sometimes between the epoxy and the glass fibres. The

high porosity of the stone and the bonding strength increase that arises can also generate effects

like stone failure or delamination between CA and GFRE. The first one occurs when there are small

fissures in the stone and the second because the adhesion between the cork granules and the

adhesive is weaker than all the other referred bonds.

The occurrence of each phenomena is summed up in Table 16. All specimens presented the so

called normal behaviour, and WGS presented stone failure instead of fibre failure.

Table 16 - Delamination type occurring in each white stone specimen

Normal Stone

Failure

Cork layer

delamination Fibre failure

WAS

2 ✓ ✓ ✓

6 ✓ ✓ ✓

7 ✓ ✓

WAA 4 ✓ ✓

6 ✓ ✓

WGS

5 ✓ ✓

6 ✓ ✓

8 ✓ ✓

WGA 1 ✓ ✓ ✓

a) b)

c) d)

Figure 49 - Delamination a) in between the GFRE layer in specimen WAA4, b) between GFRE layer and

CA layer in specimen WGA5, c) Mixed delamination in specimen WAA6, d) Stone failure in WGS5

49

2 ✓ ✓ ✓

3 ✓ ✓ ✓

4 ✓ ✓

5 ✓ ✓ ✓

6 ✓ ✓

8 ✓ ✓ ✓

For the white stone specimens, the 𝑎 value was measured at maximum load and equation 3 was

applied. The average calculated values of 𝐺𝐼𝐶 are presented in Table 17.

Table 17 - GIC values for the white stone specimens

Average 𝑮𝑰𝑪 (kJ/m2)

WGS 280.93 ± 56.82

WAS 633.24 ± 54.67

WGA 844.45 ± 304.6

WAA 1380.55 ± 320.64

Evaluating the average 𝐺𝐼𝐶 values, there is a clear difference between each specimen

configuration. The lowest value, and in agreement with the unusual delamination phenomenon

observed, comes from WGS specimens. The higher value was observed for the WAA specimens,

but it important to notice there are only two specimens for this configuration. The difference is smaller

between WAS and WGA specimens, however the values of standard deviation are smaller in WAS

and WGS and very similar, meaning that specimens with the sawn finish have a more consistent

behaviour. WAA and WGA have the higher interlaminar fracture toughness, meaning that smooth

finish is the best to apply to a stone with similar properties to “Branco do Mar”.

4.4 Weibull Statistical Analysis of Load at Break

A four-point bending test until failure was done in all composite specimens plus stone and

reinforced CA, and the results were analysed with the two-parameters Weibull model.

4.4.1 Principles

Weibull models have been used in many different applications to model different kind of data,

including engineering applications. They are based in the cumulative distribution function of the three

parameter Weibull model:

50

𝐹(𝑥; 𝑢, 𝑣, 𝑡) = − exp (− (𝑥−𝑞

𝑢)

𝑣), (6)

𝑞 ≥ 0, 𝑢 ≥ 0, 𝑣 ≥ 0,

where 𝑞, 𝑢, and 𝑣 are the location, scale and shape parameters, respectively. The three-

parameter Weibull distribution is suitable for situations in which an extreme value cannot have values

less than 𝑎. When 𝑎 = 0, the distribution function of the two-parameter Weibull distribution is

obtained, being the one considered in this study. The distribution function in this case can then be

written as follows:

𝐹(𝑥; 𝑢, 𝑣) = 1 − exp (− (𝑥

𝑢)

𝑣), (7)

𝑢 ≥ 0, 𝑣 ≥ 0.

In the context of this study, 𝐹(𝑥; 𝑢, 𝑣) represents the probability that the load at failure is equal to

or less than 𝑥. Using the equality 𝐹(𝑥; 𝑢, 𝑣) + 𝑅(𝑥; 𝑢, 𝑣) = 1, the reliability 𝑅(𝑥; 𝑢, 𝑣) is the probability

that the load at failure is at least 𝑥, and is defined as (Dodson 1994):

𝑅(𝑥; 𝑢, 𝑣) = exp (− (𝑥

𝑢)

𝑣),

(8)

𝑢 ≥ 0, 𝑣 ≥ 0.

The parameters 𝑢 and 𝑣 of the distribution function 𝐹(𝑥; 𝑢, 𝑣), are estimated from observations.

Several methods can be employed in this estimation, but the linear regression method is the one

used in this work. This method consists on transforming Eq. (7) into

1 − 𝐹(𝑥; 𝑢, 𝑣) = exp (− (𝑥

𝑢)

𝑣) and taking the double logarithms of both sides. Hence, a linear

regression model in the form 𝑌 = 𝑚𝑋 + 𝑟 is obtained:

ln [ln (1

1−𝐹(𝑥;𝑢,𝑣))] = 𝑣 ln(𝑥) − 𝑣 ln(𝑢), (9)

Where 𝐹(𝑥; 𝑢, 𝑣) is an unknown in Eq. 9 and, therefore, it is estimated from observed values:

order n observations, from smallest to largest, and let 𝑥(𝑖) denote the 𝑖th smallest observation (𝑖=1

corresponds to the smallest and 𝑖 = 𝑛 corresponds to the largest). Several estimators of 𝐹(𝑥(𝑖); 𝑢, 𝑣)

can be applied, being one of them the median rank of 𝑥(𝑖):

�̂�(𝑥𝑖; 𝑢, 𝑣) =𝑖−0.3

𝑛+0.4, (10)

51

Linear regression, based on least squares minimisation, is applied to the paired values (𝑋, 𝑌) =

(ln(𝑥𝑖) , ln [ln (1

1−�̂�(𝑥𝑖;𝑢,𝑣))]) for the model in Eq. 10, and the parameter estimates for u and v are

obtained. In order to compute 𝑢 and 𝑣, first, they are ordered from smallest to largest and (𝑋, 𝑌)

values are computed. Then, applying linear regression to these (𝑋, 𝑌) values, the linear regression

model with the regression line is obtained.


The specimen’s dimensions for the four-point bending test (300×50mm) were defined according

to the restrictions of the bending test machine, including the permitted spans, and the rules defined

in ASTM-C393 (l=300mm, S=250mm and S/d=16.7).

4.4.3 Methodology

The setup for the test is the same as used to make the stiffness tests, a four-point bending one

(Figure 26) according to ASTM D7249 [56]. The compressive load is applied in an Instron 5566

universal testing machine, with the load data being collected using a 10kN load cell. The tests were

displacement controlled with a velocity of 5 mm/min and a preload of 10N was applied before each

test. Load at failure and type of delamination were registered.


The values of load at failure (N) obtained are summed up and ordered from highest to lowest

(average values) in Table 18. The results from reinforced CA and stone are references, and were

also measured to emphasize the improvement of the composite when compared with its constituent

materials in separate. Other than these, the maximum average value came from BGS specimens,

2491.13N, with a significant difference from the next value of 1927.45N from BAS specimens, and

the minimum value was 1384.86N from WAA specimens.

Table 18 - Load at failure (N) obtained in the four-point bending test

Average

BGS 2491.1 ± 243.68

BAS 1927.5 ± 166.80

WGS 1638.4 ± 66.63

WGA 1619.8 ± 159.51

WAS 1590.8 ± 78.91

BGA 1503.7 ± 133.04

WAA 1384.9 ± 78.70

Reinforced CA 548.4 ± 5.12

Stone 79.1 ± 11.49

52

CA type has little influence on the results obtained, being the two highest and lowest results of

the two cork types. The stone seems to have more influence, but not as much as the finishes. Looking

at the minimum values, the four on top are the sawn and the four on bottom are the smooth, and

although the average values of WGA and WAS are commutated, this means that in fact the finish

has an influence on the composite specimens, despite the CA and the stone used. The values of the

standard deviation are high when compared with stone and reinforced CA, making clear that the

production process is resulting in non-homogeneous specimens. As they are manually produced,

two compromising parameters as fibre orientation and amount of resin are difficult to ensure and it

is hard to guarantee replication.

According to the principles described in 4.4.1, all values obtained for each configuration were

sorted from lowest to highest and calculations were made to obtain the linear regression, represented

in Figure 50 and Figure 51, Figure 52. It is important to look at the R2 value in order to consider only

results that are relevant; in this case the WGS, BAS and BGS values are lower than desirable but

since this is a preliminary study this values were still used.

Figure 50 - White stone specimens’ regression lines

y = 17,681x - 130,81R² = 0,8907

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,3 7,35 7,4 7,45

Y =

ln(ln(1

-Media

n R

ank))

)

X=ln(Load at Fracture)

WAS

y = 28,584x - 206,67R² = 0,9207

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,15 7,2 7,25 7,3

Y =

ln(ln(1

-Media

n R

ank))

)


WAA

y = 43,444x - 319,61R² = 0,9559

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,30 7,35 7,40

Y =

ln(ln(1

-Media

n R

ank))

)


WGA

y = 23,463x - 174,15R² = 0,8797

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,3 7,35 7,4 7,45 7,5

Y =

ln(ln(1

-Media

n R

ank))

)


WGS

53

Figure 51 - Blue stone specimens’ regression lines

Figure 52 - Cork and stone regression lines

Looking at the 𝑣 values summed up in Table 19, all of them are bigger than one, indicating an

increase in failure rate, or, in other words, that the material tends to fail with higher probability for

every unit increase in applied tension. The scale parameter 𝑏 measures the spread in the distribution

of data, and together they are used used to plot 𝑅(𝑥; 𝑏, 𝑐), to obtain the Weibull reliability distribution

presented in Figure 53.

y = 10,123x - 77,021R² = 0,8685

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,4 7,5 7,6 7,7Y =

ln(ln(1

-Media

n R

ank))

)


BAS

y = 11,095x - 81,619R² = 0,9381

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

7,1 7,2 7,3 7,4 7,5Y =

ln(ln(1

-Media

n R

ank))

)


BGA

y = 7,3536x - 57,948R² = 0,7691

-2

-1,5

-1

-0,5

0

0,5

1

7,6 7,7 7,8 7,9Y =

ln(ln(1

-Media

n R

ank))

)


BGS

y = 6,4394x - 28,577R² = 0,9308

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

1,5

4 4,2 4,4 4,6

Y =

ln(ln(1

-Media

n R

ank))

)


Stone

y = 80,599x - 508,79R² = 0,9145

-2

-1,5

-1

-0,5

0

0,5

1

6,29 6,3 6,31 6,32

Y =

ln(ln(1

-Media

n R

ank))

)


Cork

54

Table 19 - Values v and u of Weibull distribution for each specimen configuration

𝒗 𝒖

WAA 28.58 1380.63

BGA 11.09 1566.61

BAS 10.12 2015.79

BGS 7.35 2644.56

WAS 17.68 1633.61

WGA 43.44 1567.01

WGS 23.46 1672.89

Cork 80.60 551.49

Stone 6.44 84.59

These calculations and graphics allow to evaluate the tested batch. For example, considering the

line drawn in Figure 53 at R=0.8, the interception values can be withdrawn (Table 20) and so it can

be said there is 80% of probability of a WAA specimen to fail at 1309.58N, one of BGA at 1512,76N

and so on. When thinking about quality control, these calculations can be useful as an acceptation

criterion of the produced batch. For instance, if a certain value of load at failure is required, a batch

can be tested and it only passes the quality control test if 90% of the specimens breaks at an equal

or higher value than that required.

55

Table 20 - Load (N) at failure for R=0,8 of each specimen configuration

Load (N) @ R=0.8

WAA 1309.58

BGA 1512.76

BAS 1735.38

BGS 2165.14

WAS 1499.29

WGA 1512.76

WGS 1568.81

Cork 541.23

Stone 65.92

Other than this, the failure modes of the specimens were registered at the end of each test since

they also present a way to evaluate adhesion between layers. Six different modes were registered

and they are described and represented in Table 21.

Figure 53 – Reliability a) white stone specimens; b) blue stone specimens; c) cork specimens d) stone

specimens

a)

b)

c)

d)

56

Table 21 - Failure modes during the four-point bending test

1 Crushing of the stone under both rollers

2 Crushing of the stone under one of the

rollers

3 Crushing of the stone and core failure

(with disruption of GFRE2)

4 Delamination

(between stone and GFRE1)

5 Core failure

6

Crushing of the stone,

core failure (without disruption of GFRE) and

delamination

The first three modes were characteristic of the white stone specimens and the remaining of the

blue stone, as it is emphasized in the frequency graphic shown in Figure 54. Failure mode four was

the most common in the blue stone specimens and modes one and two in the white ones.

57

Figure 54 - Frequency of each failure mode

Comparing the observed modes shown in Table 22 with the load at failure values on each

specimen, there is no obvious relationship, especially amongst the white stone specimens since they

are similar and mainly related with failure of the stone itself. In the case of the blue stone specimens

there is no obvious difference between modes four and five but there is a variation between these

and mode six, that only happened in BGS specimens, the ones with higher values of load at break.

Consequently, this is the desirable failure mode for specimens with stones similar to “Vidraço Azul”.

Table 22 - Failure mode for every specimen of each configuration

BAS WAS WAA BGS WGS BGA WGA

9 4 1 2 6 1 5 2

10 4 2 1 5 2 4 2

11 1 2 3 6 2 4 1

12 2 2 2 6 1 4 1

13 4 2 1 - 1 4 1

14 - - 2 - 1 - 2

Finally, it was important to evaluate the influence of the specimens’ thickness in the load results,

and so a plot of average load (N) versus thickness (mm) of the several specimen configurations is

represented in Figure 55. Although there is a small influence, it is not relevant in these results, not

even amongst specimens with the same stone type, same CA or same finish.

0 2 4 6 8 10 12

1

2

3

4

5

6

Blue White

58

Figure 55 – Average compressive load (N) versus average thickness (mm) of the several specimen

configurations

4.5 Scanning Electron Microscopy analysis

A Scanning Electron Microscopy (SEM) analysis was carried out to evaluate and compare the

resin penetration on the two types of CA and stone. This study is very important to attribute causes

to the phenomena observed in the mechanical tests.

4.5.1 Methodology

An analytical FEG-SEM (Field Emission Gun Scanning Electron Microscope) JEOL 7001F was

used at a 15kV accelerating voltage. Three specimens were analysed and a blue stone and cork

agglomerate was separated to be also analysed in the contacting surface, as it is shown in Figure

56. The specimens were coated with a conductive coating.

Figure 56 - Examined specimens on SEM

1000

1200

1400

1600

1800

2000

2200

2400

2600

20 21 21 22 22 23 23 24 24

Com

pre

ssiv

e L

oad (

N)

Thickness (mm)

BAS

WAS

WAA

BGS

BGA

WGA

59


Three areas of interest were observed, as it is shown in Figure 57: zone 1, the adhesive layer

between the stone and the CA, zone 2, the CA and zone 3, the epoxy layer that is in contact with air.

The pictures obtained for each area are in Figure 58, Figure 59 and Figure 60, respectively.

Figure 57 - Areas examined by SEM: zone 1, the adhesive layer between the stone and the CA, zone 2,

the two CA and zone 3, the epoxy layer that is in contact with air

In Figure 58a, Figure 58c and Figure 58e, the stone, GFRE and CA layers are labelled, as well

as two arrows, the black pointing out the limit of the stone and starting of the GFRE layer and a white

the limit between the end of GFRE and CA. It is important to notice that all of them present penetration

of epoxy resin in the cork agglomerate, which is consistent with the delamination results – some

specimens presented detachment of the CA and the GFRE layers but the majority delaminate

between the stone and the GFRE.

In Figure 58b and Figure 58d, one void and one bubble are highlighted in the GFRE layer. In

Figure 58d, the bubble seems to be either made resin that was not in contact with either glass fibres

nor cork, resulting in a separate small body filled with air. In Figure 58b, there is a void with the same

shape of the bubble described before, even though the images are of two different specimens. In

fact, on the left side of the bubble (under the arrow), there is a piece of resin that seems to be the

rest of the resin capsule that has been opened during cutting.

In Figure 58c and Figure 58d, the glass fibres are visible and seem disconnected from the rest of

the material, being an indicator that the specimen was produced with less amount of resin that the

others, and so the fibres appear to be detached in some areas. It is however possible to identify

glass fibres in two directions, meaning that the reinforcement is applied as intended.

1

2

3

60

In Figure 59 an image of each CA-A and Ca-G, is presented. There are some visible differences

related with the cohesivity of the agglomerates. On the left image, the cork granules are well defined

and homogeneously adhered to each other; on the right image, one cannot distinguish different cork

granules and there are dark valleys throughout the sample, indicating less cohesion. That said, the

practical results of the tests previously completed showed that this difference is not determinant in

the parameters studied.

a)

c)

e)

b)

d)

f)

Stone

GFR

CA

Stone

GFR

CA

Stone

GFR

CA

Figure 58 - SEM images: a) WAS specimen b) WAS specimen c) WGS specimen d) WGS specimen e) BAS specimen f) BAS specimen

61

Figure 59 - SEM image of cork agglomerate a) CA-A b) CA-G

The images of the last zone of interest (zone 3 of Figure 57) are presented in Figure 60. Marked

by the white arrow is the limit between CA and GFRE layers. Again, there is a good adhesion

between layers and the bubbles are not significant in the adhered area.

Figure 60 - SEM images: WAS specimen a) and b); WGS specimen c) and d); BAS specimen e) and f)

a)

a)

b)

b)

a)

c)

b)

62

In Figure 60a and Figure 60b the glass fibres are visible in the GFRE layer, but the same doesn’t

happen in Figure 60c, relative to the blue stone specimen. As this is the layer that is on top during

production, the resin can be draining into the CA layer. Other than that, there is a good adhesion

between the layer, the only bubbles appearing in the CA layer, not in the interface.

The SEM images along with the EDS results allowed to verify that adhesion between layers is

good in all specimen configurations observed. A continuity through layers is observed in all images.

In the separated stone sample that was observed in the surface in contact with CA, rests of resin

were in the stone which also reveals a good adhesion. The existant bubbles were observed on the

CA layer, which is expected since cork is made of closed cells containing gas that can be released

during the specimen’s production.

63

5. Conclusions

The following conclusions are formulated according to three categories: influence of components,

methods used and the comparison of experimental results with theoretical assumptions. This will

facilitate the understanding of the relative impact of each factor on the final composite’s behaviour.

Influence of Components

In the stiffness test, specimens with the highest stiffness were composed of blue stone, type A

cork agglomerate, with smooth finish. The most relevant of the variables in study (cork agglomerate

and stone surface finish) is the stone surface finish. Thickness of the specimens had, as expected,

influence on the results but there is not a clear relation between thickness and the resultant stiffness

values.

In the delamination test, specimens that presented the best adhesion were white stone, type B

cork agglomerate and sawn finish. Looking to all the results of the delamination test, one can

conclude the stone itself is the component with more influence in the mechanical behaviour. Not only

different stones resulted in different types of delamination but also the surface finish that appears to

result in better adhesion is different for each type of stone, sawn for the blue and smooth for the

white. Open porosity, compression and bending strength of the natural stone are the most relevant

properties to evaluate when concerning delamination. The indicator for best adhesion for the run-

arrest extension delamination is the stone failure phenomenon. In the case of continuous

delamination, the indicator is delamination of the cork layer. This means that failure happened in one

of the (not brittle) components and not from the adhesion between them.

The surface roughness analysis showed that the superficial finish that is applied corresponds to

very different roughness measurements, and so no generalization about the use of a specific finish

should be made; in fact, when analysing the results of the stiffness tests, the effect of the smooth

finish in the blue stone overcame the influence of the stone type being the results from the white

stone (which was expected to be less stiff) higher.

With the SEM analysis sugests a good adhesion between layers and that the glass fibre

reinforcement is being applied as intended.

Influence of Methods Used

The measurement of span deflection was carried out with strain gauges which resulted in

standard deviation values of about 19%, in relation to the measured values, which is higher than

desirable (up to 10%).

The delamination test should be carried out with a cut between the stone and the first glass fibre

reinforced epoxy layer to avoid negative contribution of resin waste that accumulates on the

64

delamination front during production. Nonetheless, this proved to be a good method to understand

the adhesion between layers, and the desired result for this application should be continuous

delamination.

The Weibull distribution can be used as an acceptation criterion for produced batches, used in

this case for the bending load at failure.

Comparison of Experimental Results with Theoretical Assumptions

The Transformed Section Method and the Classical Lamination Theory were applied and the

results demonstrate that the stone layer is not completely in compression which is not ideal nor

desired. These results were slightly different from other experimental ones obtained in previous

studies probably because the assumptions made with the theoretical assumptions do not apply for

the material in study (such as cork and stone being isotropic).

5.1 Future Studies

A similar study with more stones should be developed to understand the influence of each of the

stone characteristics (open porosity, compressive strength, etc) and, in the end, according to the

most relevant property, one could categorise them and attribute the ideal surface treatment for a

specific range of values. This study would also be important to single out the reason for the mismatch

on the results in the application of the theoretical models. This would allow the assessment of the

true mechanical distribution of loads in the composite.

The DSC analysis uncovered some questions about the curing of the resin, and since there was

not a sufficient number of samples there is also space for future work. Also, it is known that water

absorption from either one of the layers has influence on the cure, and this can also be studied.

The span deflection on the stiffness test was measured with strain gauges instead of VIC system

(Visual Image Correlation) that has been used in previous works. The latter presented more detailed

information and most importantly with less errors associated and so a way of facilitating the analysis

of the data should be developed to make it the method of choice.

65

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characterisation of adhesive systems used to produce multi ...€¦ · figure 29 - double...

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