new composite materials_ selection, design, and application-springer international publishing (2014)

DomenicoBrigante

New Composite MaterialsSelection, Design, and Application

New Composite Materials

Domenico Brigante

New Composite Materials

Selection, Design, and Application

ISBN 978-3-319-01636-8 ISBN 978-3-319-01637-5 (eBook) DOI 10.1007/978-3-319-01637-5 Springer Cham Heidelberg New York Dordrecht London

Library of Congress Control Number: 2013952926

Springer International Publishing Switzerland 2014 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifi cally the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfi lms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifi cally for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publishers location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specifi c statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.

Printed on acid-free paper

Springer is part of Springer Science+Business Media (www.springer.com)

Domenico Brigante Olympus FRP - BRIGANTE

ENGINEERING Group Napoli , Italy

Thanks to Giusi and Francesco and Antonio, Prof. I. Crivelli Visconti and Claudio Cigliano.

Thanks to my grandparents.

Thanks to Gabriella Piscopo for the translation.

vii

Foreword

It was a real pleasure for me to fi nd out about the volume New Composite Materials: Selection, Design, and Application authored by Domenico Brigante, a clear demon-stration that the education given to our students during their university years is not always neglected nor forgotten. In the case of Dr. Brigante, the publishing of this book states the relevance and importance of spreading technical and scientifi c knowledge in the pursuit of a kind of continuity over time in the development of positive achievements, as has happened for composite materials, thus leading to an improvement in our everyday social and technical lives.

This book deals precisely with the use of composites in civil construction and architecture, a fi eld involving each and every one of us that is now, after many years of hesitation and doubts, fi nally interested in the huge potential represented by composites.

Extremely relevant and diverse from other minor technical publications dealing with building issues, this volume focuses extensively on the description of the char-acteristics of the techniques employed for buildings or for the structural restoration of specifi c monuments or simple common constructions.

This is actually a prevailing aspect in the use of composite materials, diverse from the use of traditional materials, since the fi nal properties of the products or applications strongly depend on the same way that different materials are applied that should be accurately designed and distributed following the most effective pro-cedures which designers can only choose if they have a deep and accurate knowl-edge of the techniques required. This is true for the use of both composites on wooden or steel and concrete structures whose different existing possibilities are carefully detailed in this volume.

The examples and descriptions of inspection and monitoring procedures crown the volume, making it an effective tool for designers and for fi nal users as well.

Napoli , Italy Ignazio Crivelli Visconti

ix

Contents

1 Composite Materials ............................................................................... 11.1 Composite Materials ........................................................................ 11.2 Main Properties ................................................................................ 21.3 Fibers ................................................................................................ 3

1.3.1 Glass Fibers .......................................................................... 41.3.2 Carbon Fibers ....................................................................... 61.3.3 Basalt Fibers ......................................................................... 71.3.4 Aramid Fibers ...................................................................... 81.3.5 Steel Fibers ........................................................................... 101.3.6 Hybrid Fabrics ...................................................................... 111.3.7 Natural Fibers ....................................................................... 111.3.8 Cost Aspects of Fibers ......................................................... 12

1.4 Matrices ............................................................................................ 121.5 Plastic Matrices ................................................................................ 13

1.5.1 Polyester Resins ................................................................... 151.5.2 Epoxy Resins........................................................................ 151.5.3 Phenolic Resins .................................................................... 151.5.4 Silicone Resins ..................................................................... 15

1.6 Grout-Based Matrices ...................................................................... 161.7 Other Types of Matrices ................................................................... 16

1.7.1 Metal Matrices ..................................................................... 161.7.2 Ceramic Matrices ................................................................. 16

1.8 Thermoplastic Matrices ................................................................... 17

2 Manufacturing Processes ....................................................................... 192.1 Manufacturing Technologies............................................................ 19

2.1.1 Composite Materials Production Processes ......................... 202.2 Hand Impregnation Without Pressure or Vacuum ........................... 202.3 Filament Winding ............................................................................ 20

2.3.1 Winding ................................................................................ 212.3.2 Impregnation ........................................................................ 22

x 2.3.3 The Mandrel ....................................................................... 22 2.3.4 Machines ............................................................................ 22

2.4 Pultrusion ....................................................................................... 23 2.4.1 Reinforcement Feeding ...................................................... 24 2.4.2 Impregnation ...................................................................... 25 2.4.3 Preforming ......................................................................... 26 2.4.4 Forming and Polymerization .............................................. 27

2.5 Resin Transfer Molding ................................................................. 30 2.6 Resin Infusion Under Flexible Tooling .......................................... 30 2.7 Autoclave Forming......................................................................... 32 2.8 FRP Grids....................................................................................... 34

3 Choice of the Composite System ............................................................ 35 3.1 Advantages of Composite Materials .............................................. 35 3.2 Design of the Materials .................................................................. 37 3.3 FRP: Fiber-Reinforced Polymer .................................................... 37 3.4 SRP: Steel-Reinforced Polymer ..................................................... 38 3.5 FRG: Fiber-Reinforced Grout ........................................................ 39 3.6 SRG: Steel-Reinforced Grout ........................................................ 39 3.7 Choice of the Composite System ................................................... 40 3.8 Flatness of Strengthening Structures Surfaces ............................... 40 3.9 Impact of Temperature ................................................................... 413.10 Behavior in Humidity..................................................................... 413.11 Employment of Skilled Labor ........................................................ 413.12 Employment of Individual Safety Devices .................................... 423.13 Full Deterioration of Work Tools ................................................... 423.14 Fire Resistance ............................................................................... 423.15 Resistance to UV Rays ................................................................... 433.16 Radiotransparency .......................................................................... 44

4 Strengthening of Existing Structures: Technical Standards ............... 45 4.1 International Technical Standards .................................................. 45 4.2 Main Standards .............................................................................. 47

4.2.1 CNR guidelines .................................................................. 47 4.2.2 Canadian Guidelines .......................................................... 47 4.2.3 American Guidelines.......................................................... 48 4.2.4 Fib Guidelines .................................................................... 50 4.2.5 Japanese Guidelines ........................................................... 51

4.3 Comparison of Standards About Flexural Strengthening ........... 51 4.3.1 JSCE Code ......................................................................... 51

5 Strengthening of Reinforced and Prestressed Reinforced Concrete Structures ............................................................ 55 5.1 Italian Technical Paper CNR-DT 200/2004 ................................... 55 5.2 Draft of Guidelines: Department of Italian

Civil ProtectionReLUIS ............................................................. 56

Contents

xi

5.3 Guidelines of the General Assembly of Higher Council for Public Works (CSLLPP Guidelines) ........................................ 58

5.4 Symbols.......................................................................................... 58 5.4.1 General Notations ............................................................ 59 5.4.2 Uppercase Roman Letters ................................................ 59 5.4.3 Lowercase Roman Letters ................................................ 60 5.4.4 Lowercase Greek Letters.................................................. 61

5.5 Introduction ...................................................................................... 62 5.5.1 Partial Factors................................................................... 62 5.5.2 Partial Factors m for Materials and Products .................. 62 5.5.3 Partial Factors Rd for Resistance Models ........................ 63 5.5.4 Environmental Action and Conversion Factors a ........... 63 5.5.5 Loading Mode and Conversion Factor

for Long-Term Effect l ................................................... 63 5.6 Failure Mechanisms Due to Debonding ........................................ 64

5.6.1 Verifi cation of Safety Following Debonding ................... 65 5.7 Flexural Strengthening ................................................................... 67

5.7.1 Analysis at the Ultimate State Limit ................................ 69 5.7.2 Behavioral Analysis at the Serviceability Limit State ..... 71

5.8 Shear Strengthening ....................................................................... 72 5.8.1 Design of Shear Strengthening ........................................ 74

5.9 Reinforcement of Floors in Brick and Cement .............................. 765.10 Strengthening of r.c. Columns ....................................................... 77

5.10.1 Design Axial Capacity Under Concentric and Slightly Eccentric Force of the Confi ned Member .......... 78

5.10.2 Circular Sections .............................................................. 80 5.10.3 Square and Rectangular Sections ..................................... 80 5.10.4 Ductility of FRP-Confi ned Members Under

Combined Bending and Axial Loading ........................... 815.11 Strengthening of BeamColumn Joints ......................................... 82

5.11.1 Criteria for the Localized Strengthening of Unconfi ned Joints ........................................................ 83

5.12 Choice of the Adequate Composite Material ................................. 885.13 Seismic Applications ..................................................................... 90

6 Reinforcement of Masonry Structures .................................................. 95 6.1 Introduction .................................................................................... 95 6.2 Goals and Criteria of a Reinforcement Project .............................. 96

6.2.1 Safety Assessments .......................................................... 98 6.3 Failure Mechanisms Due to Debonding ........................................ 100

6.3.1 Resistance to Debonding in the Ultimate Limit State ...... 100 6.4 Reinforcement of Masonry Panels ................................................. 102

6.4.1 Checks for Out-of-Plane Loads........................................ 102 6.4.2 Check for Simple Overturning ......................................... 103

Contents

xii

6.4.3 Check for Flexural Failure of Vertical Masonry Stretching .............................................................. 103

6.4.4 Check for In-plane Actions .................................................. 1066.4.5 Combined Compressive and Bending Stress ....................... 1066.4.6 Shear..................................................................................... 106

6.5 Reinforcement of Masonry Arches and Vaults ................................ 1076.5.1 Simple Curvature Vaults (Barrel Vaults) and Arches ........... 1086.5.2 Double Curvature Vaults ...................................................... 109

6.6 Reinforcement of Masonry Columns ............................................... 1096.6.1 Axially Loaded Confi ned Members ..................................... 1096.6.2 Confi nement of Circular Columns ....................................... 1116.6.3 Confi nement of Squared or Rectangular Columns .............. 111

6.7 Pretensioning Systems ..................................................................... 1126.7.1 Tensioning System for SRGSRP ........................................ 112

6.8 Anchor Systems ............................................................................... 1166.9 Preparation of the Substrate ............................................................. 117

7 Strengthening of Steel Structures .......................................................... 1197.1 Introduction ...................................................................................... 1197.2 Applications ..................................................................................... 1207.3 Technical Standards ......................................................................... 1217.4 Repair of Steel Pipes ........................................................................ 1227.5 Reinforcement of Steel Structures for Telecommunications ........... 124

8 Characterization and Monitoring ......................................................... 1278.1 Introduction ...................................................................................... 1278.2 Materials .......................................................................................... 128

8.2.1 Pultruded Laminates ............................................................ 1288.2.2 Laminates Produced Onsite ................................................. 1298.2.3 Production Tests ................................................................... 129

8.3 Experimental Tests ........................................................................... 1308.4 Work Inspection ............................................................................... 130

8.4.1 Destructive Tests .................................................................. 1318.4.2 Pull-off Tests ........................................................................ 1328.4.3 Shear Tearing Test ................................................................ 1338.4.4 Non-destructive Test ............................................................ 1338.4.5 Stimulated Acoustic Tests .................................................... 1338.4.6 High-Frequency Ultrasound Tests........................................ 1338.4.7 Thermography Tests ............................................................. 1338.4.8 Acoustic Emission Tests ...................................................... 1348.4.9 Failure Tests on Reinforced Members, Beams,

and Columns ........................................................................ 1358.5 Test Operators .................................................................................. 135

Contents

xiii

9 Application Techniques .......................................................................... 137 9.1 Introduction .................................................................................. 137 9.2 Reinforcement of Floors in Brick and Cement ............................ 137

9.2.1 General Principles ............................................................ 138 9.2.2 Construction Details ......................................................... 139 9.2.3 Application Procedure...................................................... 139

9.3 Reinforcement of Steel Beam Floors ........................................... 140 9.3.1 General Principles ............................................................ 140 9.3.2 Construction Details ......................................................... 141 9.3.3 Application Procedure...................................................... 141

9.4 Reinforcement of r.c. Columns .................................................... 142 9.4.1 General Principles ............................................................ 142 9.4.2 Construction Details ......................................................... 143 9.4.3 Application Procedure...................................................... 143

9.5 Shear and Bending Reinforcement of Concrete Beams ............... 144 9.5.1 General Principles ............................................................ 145 9.5.2 Construction Details ......................................................... 146 9.5.3 Application Procedure...................................................... 147

9.6 Reinforcement of Masonry Structures ......................................... 148 9.6.1 General Principles ............................................................ 149 9.6.2 Construction Details ......................................................... 149

9.7 Reinforcement of Masonry Arches and Vaults ............................ 152 9.7.1 General Principles ............................................................ 152 9.7.2 Construction Details ......................................................... 153 9.7.3 Application Procedure...................................................... 153

9.8 Reinforcement of Wooden Bearing Structures ............................ 154 9.8.1 Construction Details ......................................................... 155 9.8.2 General Principles ............................................................ 155 9.8.3 Application Procedure...................................................... 156

10 Examples of Applications ....................................................................... 157 10.1 Hotel Boscolo Exedra, Nice, France ............................................ 157 10.2 Telecoms Building, Rome, Italy .................................................. 157 10.3 Industrial Factory, Milan, Italy .................................................... 157 10.4 Albergo Reale dei Poveri (Bourbon Hospice

for the Poor), Naples, Italy ........................................................... 161 10.5 Monastery of Santa Chiara, Naples, Italy .................................... 162 10.6 Monument for Neapolitan Martyrs, Naples, Italy ........................ 164 10.7 Hotel Boscolo Exedra, Rome, Italy ............................................. 165 10.8 Laminated Wooden Structure, Crotone, Italy .............................. 167 10.9 Church of San Gaetano, Bitonto, Italy ......................................... 16810.10 Radio Station Base in Forna, Ponza, Italy ................................... 170

Bibliography .................................................................................................... 173

Index ................................................................................................................. 177

Contents

xv

Introduction

Over the last several years, the scientifi c interest towards innovative fi ber-reinforced plastic (FRP) applications for structural reinforcement on one side and the peculiar-ity of the extremely diversifi ed Italian architectural heritage on the other directed the attention of many researchers to the fi elds of structural mechanics, construction, structural reinforcement, and seismic engineering. Several scientifi c programs have resulted from this, funded by the most important research centers in the world.

Worth mentioning are, for instance, the programming regulations drafted and issued on specifi c journals by the fi b Task Group 9.3, the European founding com-mittee in 1998, or the offi cial journals of the American Concrete Institute (ACI), whose aim is to provide new guidelines for the design and construction of FRP concrete structures.

A further contribution on the topic comes from the European Committee for Standardization that published the new regulations of design and strengthening with FRP in the Eurocode 8 Design of structures for earthquake resistance Part 3 Assessment and retrofi tting of buildings, Draft N 7, January 2003.

In Italy, the decree nr. 3274 May 2005, concerning the technical regulations for the design, evaluation, and seismic adjustment of buildings, introduces the use of FRP for the seismic strengthening of reinforced concrete (r.c.) members and specifi -cally refers to the instructions of CNR-DT 200/2004 for the safety assessments.

This volume deals with several topics strictly linked to the most up-to-date appli-cations of composite materials in civil engineering, and industrial and historical or monumental buildings.

This timely volume presents a range of critical topics on the use of composite materials in civil engineering; industrial, commercial, and residential structures; and historic buildings. Structural strengthening techniques based on composite materials represent a practice employed internationally and have become an impor-tant component in the restoration of buildings impacted by natural hazards and other destructive forces.

New Composite Materials: Selection, Design, and Application stands as a highly relevant and diverse effort, distinct from other technical publications dealing with

xvi

buildings issues. The book focuses extensively on the characterization of techniques employed for structural restoration and examines in detail an assortment of materi-als such as concrete, wood, masonry, and steel.

This book

Provides engineers and architects with a lucid explanation of how to easily design an innovative system of structural reinforcement with composite materials

Presents details for readers to readily assess the feasibility of reinforcement applications

Includes a section for construction managers written to facilitate the installation of composite structural reinforcement materials with maximum effi ciency and cost benefi t

Features many examples of applications and construction details to help engineers and architects realize their projects

Offers a comparative analysis among various international technical standards

The structural strengthening techniques making use of composite materials, also known as FRP, currently represent a sound reality in national and international sce-narios, and have become a constituent part of the restoration works of buildings impacted by earthquakes that have hit many countries.

Extremely relevant and diverse from other minor technical publications dealing with buildings issues, this volume extensively focuses on the description of the characteristics of the techniques employed for buildings or for the structural restora-tion of specifi c monuments or simple common constructions.

Specifi c aspects of the implementation of r.c. structures, wood, masonry, and steel are extensively detailed both in terms of the technical design and of the execution stages, as well as the subsequent mechanical performances of the systems obtained.

This book provides a useful tool that can be applied directly to different kinds of technical documents.

The examples and descriptions of inspection and monitoring procedures crown the volume, making it an effective support for designers and for fi nal users as well.

The work results in a fundamental practical handbook for any engineer, designer, architect, or any other technician who is willing to handle this innovative technique of structural strengthening.

Napoli, Italy Domenico Brigante

Introduction

1D. Brigante, New Composite Materials: Selection, Design, and Application, DOI 10.1007/978-3-319-01637-5_1, Springer International Publishing Switzerland 2014

1.1 Composite Materials

The new technologies and greatest discoveries of science are often nothing but the study and reconstruction of what has always been before our eyes, in the most sim-ple structures of nature. Even in the case of composite materials, nature has pre-ceded thousands of years of our studies with materials such as wood, cellulose dispersed in lignin, or bones consisting of collagen and apatite.

Composite materials mainly represent an evolution of the science and technol-ogy of materials, since they blend the best properties of several materials resulting from the most up-to-date technologies, which empower them with outstanding physical and mechanical properties. The study of composites is a kind of philoso-phy of materials design aiming at enhancing both the composition of materials themselves and their structure, thus leading to a converging and interactive process. It is both a science and a technology, demanding a strict interaction between differ-ent study subjects, such as design and structural analysis, study of materials, mechanics of materials, and process engineering.

In a historical perspective, the concept of fi ber strengthening is quite an old one. Even the Bible contains references to the strengthening of masonry with straw in Ancient Egypt. Iron bars were employed to strengthen masonries during the 19th century and this gave a boost to the development of reinforced concrete. Phenolic resins strengthened with asbestos were introduced in the 20th century. The fi rst boat made out of fi berglass dates back to 1942 and, during the same period, reinforced plastics also started to be used in aeronautics and in electrical devices. Wrapped members were invented in 1946 and used for applications in the rockets sector dur-ing the 1950s. The fi rst high-resistant boron and carbon fi bers were launched during the 1960s for application in advanced composites in aeronautics components. Metal matrix composites with boron/aluminum were introduced in 1970. In 1973, DuPont developed aramidic fi bers.

Chapter 1 Composite Materials

2 During the last decades of the 1970s, applications with composites gained popularity in the aeronautics, automotive, sports items, and biomedical sectors. Successively, the 1980s saw a meaningful development of the use of high-modulus fi bers.

Today, the focus is on the development of more modern composites with cementmortar matrix or mixed matrices with mortar and epossidic resins for high- temperature applications. There are several different applications of these composites: buried pipes, containers, boats, road vehicles, aeronautics and space devices, civil engineering applications, automotive components, sports equipment, biomedical products, and many other items designed to have high mechanical performance and/or dimensional stability in different laminated and low-weight settings.

1.2 Main Properties

A composite material is defi ned as a system made out of two or more phases, whose properties and performances are designed such as the result is greater than those of the constituent materials acting independently. Usually, one of the two phases is a discontinuous one, stiffer and stronger, and is known as the strengthening, whereas the other is weaker, less stiff, and continuous, and is called the matrix. In some cases, there can be an additional phase resulting from the chemical interac-tions or other effects, known as interphase, occurring between the strengthening and the matrix.

The properties of a composite result from the properties of its constituents, and from the geometry and distribution of the phases. One of the most relevant param-eters is the volume (or weight) of the strengthening fraction or the volume ratio of the fi bers. The distribution of the strengthening conveys the system its features. The less uniform the strengthening, the more heterogeneous the material and the higher the likeliness of failure in weaker portions, whereas the geometry and orientation of the strengthening impact on the anisotropy of the system.

The composite phases play different roles and depend on the typology and appli-cation of the composite itself. In case of low- or medium-performance composites, the strengthening is usually made out of short fi bers or particles, which allow for a certain stiffness and, at the same time, strengthen the material only locally. On the other hand, the matrix is the main member responsible for load bearing and for defi ning the mechanical features of the material.

In case of high structural performance composites, they are usually made out of continuous fi bers building the frame of the material and conveying it stiffness and resistance toward the fi ber direction (Fig 1.1 ). The matrix phase conveys protection, support for fi bers, and transfer of local strains from one fi ber to the other. The inter-phase, though small in dimension, can play a very important role in controlling the failure mechanisms, the tensile strength, and, above all, the strains/stresses behavior of the material.

1 Composite Materials

31.3 Fibers

As aforementioned, due to their limited dimensions, fi bers show an outstanding structural perfection; this feature, alongside the inherent properties of constituent materials, conveys:

High breaking stress Very high tensile modulus Very low specifi c gravity Linear elastic behavior up to failure

The most popular fi bers used in composites are glass, carbon, organic, and min-eral fi bers. They can be either in composites or continuous fi bers running parallel on a plane or also chopped strands running with random orientations on a surface (mat), or, fi nally, they can be woven according to a weftwarp confi guration and applied on a surface (Table 1.1 ).

1.3.1 Glass Fibers

Glass fi bers are mostly produced in the standard type of E-glass, known mainly for its electrical applications. A higher strength fi ber is S-glass: its tensile strength is, actually, nearly 33 % greater than that of E-glass (Fig. 1.2 ).

Another type of fi ber is obtained with highly alkaline glass, called C-glass: it shows good chemical resistance, but, on the other hand, only scarce electrical fea-tures. Other types are D-glass, with excellent electrical features, and L-glass that, due to its lead content, allows for good protection against radiation and can be used as a track in the X-ray tracking of fi bers.

All glass types have a very high strengthweight ratio, although glass fi bers are among the synthetic inorganic fi bers with the highest density. Glass can preserve its mechanical properties, up to 50 % of its strength capacity under a temperature of

Fig. 1.1 Composite materials

1.3 Fibers

4375 C, and up to 25 % under a temperature of 538 C. The following are the advan-tages offered by glass fi bers and particularly by E-glass compared to other materials:

Ratio between high tensile strength and high resistance: with the equivalent weight of glass, the fi ber has twice the strength of a steel wire.

Dimensional stability: glass does not shorten or lengthen with varying environ-mental conditions. Glass fi bers show a maximum lengthening of 3 % before failure.

High thermal resistance: glass fi bers show good performance in applications under high temperatures. They preserve 50 % of their tensile strength at a tem-perature of 340 C.

Low humidity absorption: glass is an acellular material, so humidity cannot pen-etrate the fi laments surface.

Good electrical properties: glass has a low dielectric constant and good insula-tion capacity.

High fi re resistance: glass does not burn or set fi re.

Fig. 1.2 Uniaxial glass fi bers

Table 1.1 Mechanical properties for the most common types of fi bers

Fiber

Properties Stiffness E (DaN/mm 2 )

Strength max (DaN/mm 2 )

Density (g/cm 3 )

Specifi c stiffness ( E / ) (10 8 mm)

Specifi c strength ( / ) (10 6 mm)

E-glass 7,000 300 2.5 28 120160 S-glass 8,000 450 2.5 32 180 Carbon HM 38,500 200 1.9 202 105 Carbon HS 26,000 250 1.9 136 131 Carbon M 20,000 250 1.8 111 138 Boron 42,000 240 2.4 175 139 Aramid 12,000 220 1.5 80 88 Steel 21,000 250 7.8 26 32 Tungsten 35,000 250 19 18 13 Beryllium 31,500 130 1.8 175 72


5 Hereby, it represents a product combining different physical properties that could not be achieved with an organic fi ber. The strength of glass fi bers results from the conditions under which they are formed, as well as from the coating system used to treat the glass fi ber surface.

The coating stage impacts signifi cantly the strength of glass fi bers and their sur-face properties. The effect of chemical surface treatment has been proved to enhance the strength of glass fi bers by as much as 20 %.

The coating system (chemical treatment) consists of an organic coating applied directly on glass fi bers underneath the insulating coating and before stretching the fi laments together to create a single fabric. The coating is applied in order to protect glass fi bers during the successive processing and, so, to achieve the best compatibil-ity with the resins to be strengthened. The choice of ingredients used for fi ber coat-ings depends on specifi c applications: they are all patented and classifi ed under one or more categories.

The coating agent, as its name suggests, has the task of coupling glass fi bers to the matrix or to other coating ingredients, which, in turn, interact with the matrix.

Once the chemical bonding between glass fi bers and the matrix has formed, the strengthened glass composites turn into a very strong material that can be employed in engineering, due to the effective transfer of strains from a relatively weak matrix to the extremely resistant glass fi bers.

Lubricants are useful to facilitate the processing and composition: glass fi bers are made out of a brittle material and, thus, they are easily abraded when in contact with other materials, including glass itself. During manufacturing, fi laments get broken, thus resulting in a dispersion of glass fl uff. The choice of the right lubricant can reduce this phenomenon.

The fabrication of glass fi bers starts by weighing the glass components and suc-cessively mixing them into a homogeneous mass with a predefi ned composition. It is then put into a kiln at a temperature suitable to convert carbonates and sand into liquid oxides (about 1,400 C), which, in turn, should have enough viscosity and fl ux so as to convey the right homogeneity. The molten glass is then cooled to lower temperatures (1,100 C) to prepare it for the following stages: in order to produce high-quality glass with very few imperfections, excellent processing conditions are required, as well as perfectly designed kilns.

The mass of high-quality molten glass is then drawn through the holes of a plati-num plate and bushed into fi bers of the desired diameter. The electrically heated plate has a varying number of nozzles, ranging from 200 to 4,000. Immediately underneath the plate, fi laments are covered with a coat or an organic coating.

Varying numbers of fi laments can be assembled with a chaser or a collection chunk. For instance, in the case of a 400-nozzle plate and a collection chunk with two exits, the result will be two bundles of 200 fi laments each, which will be then wrapped on a chaser. The fi nal product will be a hank. In order to prevent the bundles running parallel to each other and to facilitate their disentangling, a transversal or spiral line is used to impose a zigzag movement to the ends as they approach the winch.

The obtained packs are then put into the kiln to remove the water and assure protection of the glass surface. They are then put into a rack and assembled into bundles to form a ball.

1.3 Fibers

61.3.2 Carbon Fibers

The most popular fi bers used in composites applications have long been glass fi bers. Despite their good strength and low density, they have fairly low tensile strengths. That is the reason why, nearly 25 years ago, the experimentation and conversion of organic composites into fi bers and carbon and graphite fabrics began (Fig. 1.3 ).

The high mechanical properties of carbon fi ber result from the crystal structure of graphite. The greater this crystal structure, the better are the material properties.

A graphite crystal shows a structure made out of overlapped layers of carbon atoms. The bonds between the atoms belonging to the same layer are strong (cova-lent bonding), whereas those among atoms of different layers are quite weak (Van der Waals bonding): it goes without saying that crystals are extremely anisotropic structures and, during the manufacturing process, the crystal structure will be arranged in the desired direction.

This is not an easy task of course: basically, perfect crystals and orientation accu-racy can almost never be obtained; consequently, the effective mechanical features will be lower than expected.

Carbon fi bers result from the graphitization of organic rayon or polyacrylonitrile (PAN) textile fi bers under an inert atmosphere, at more than 2,000 C. The original fi bers are known as precursors. During the process of graphitization, fi bers are put under tensile stress: the greater the stress exerted, the higher the Youngs modulus obtained.

Besides, the increase of modulus is counterbalanced by a reduction of strength. Both high-modulus carbon fi bers with reduced strength and low modulus with high strength fi bers are available on the market.

Fig. 1.3 Uniaxial carbon fi bers


7 They are known respectively as C1 and C3 type or, using the Anglo-Saxon ter-minology, HM (high modulus) and HS (high strength, that is high tensile strength) or also HR in Italian.

There are three main advantages offered by carbon fi bers compared to glass fi bers:

A very high Youngs modulus A low volume density A very low coeffi cient of thermal expansion

For these reasons, they have gained popularity in replacing glass fi bers in fi elds where, alongside low weight, high stiffness (aeronautics structures, sports equip-ment, etc.) or a quite high dimensional stability at varying temperatures (optical devices, radars, etc.) are also required.

Carbon fi bers are much more expensive to produce than glass fi bers, but their increased spread is explained by the high mechanical properties that they convey.

1.3.3 Basalt Fibers

Over recent years, research on new types of fi bers for buildings and civil engineer-ing has turned towards the study of basalt fi bers (Fig. 1.4 ).

Fig. 1.4 Uniaxial basalt fi bers

1.3 Fibers

8 They are very thin fi bers of basalt, a volcanic rock made out of plagioclases, pyroxenes, and olivines. Basalt fi bers usually have a diameter ranging from 9 to 13 m and are ideal for replacing asbestos fi bers, since their diameter is signifi cantly greater than the respiratory limit (about 5 m). Basalt fi bers are excellent thermal and acoustic insulators, and they preserve their mechanical properties even at high tem-peratures and are also highly chemically stable (in both acid and alkaline settings).

Resulting from the melting of a single raw material, basalt fi bers have higher performance than other fi bers in terms of heat protection, thermal and acoustic insu-lation, durability, and vibration resistance.

Basalt fi bers are quite cheap, despite their features, which are signifi cantly better than other similar materials employed today, such as glass fi ber. As for heat conduc-tion, items made out of basalt fi bers are three times more effective than those in asbestos and are better performing than glass and mineral fi bers. The application temperatures of basalt fi ber products are relevantly higher (from 260 to 900 C).

Thanks to their elasticity at both the micro- and macrostructure levels, basalt fi bers are resistant to vibrations compared to similar products. This is a particularly relevant feature in the fi eld of mechanical structures and civil buildings and engineering.

For instance, in the case of civil buildings close to highways, railways, or under-ground lines, the shock absorbers of mineral materials or glass fi bers would suffer damage and, eventually, failure, whereas basalt plates are resistant to vibrations and, thus, they last longer.

As for chemical properties, basalt fi bers are resistant in aggressive environments (i.e., acid or basic). Due to this, basalt fi ber pipes can be suitably used in chemical plants to transport heated acids or in sewage systems to transport liquids and aggres-sive gases, melted materials, etc.

The electrical properties of plasticbasalt composites, particularly the volumet-ric strength of basalt fi bers, is one or two times higher than that of glass fi bers.

Processing techniques used for basalt fi bers are similar to the traditional tech-niques used for glass fi bers (fabric, fi lament, staples, glass fi ber-reinforced polymer [GFRP]). Thanks to their excellent properties, basalt fi bers are employable in heat- resistant as well as alkaline-resistant products (containers, pipes, GFRP, materials for thermal insulation).

1.3.4 Aramid Fibers

Aramid fi bers are synthetic fi bers based on aromatic polyamides. Among the com-posite materials, the most important fi ber is the high modulus one introduced by DuPont at the beginning of the 1970s. At fi rst, the aramid fi bers were developed to replace steel wires in radial tires. The advantage of this was the reduction of weight combined with higher resistance and longer durability (Fig. 1.5 ).

The manufacturing process is similar to that of other synthetic fi bers: polymer-ization, extrusion, drawing. The polymer is melted in a liquid and extruded at a


9temperature of about 200 C, while the solvent is made by evaporation. The extru-sion can only take place via a solution, since the melting point of the fi ber is much higher than the decomposition temperature. At this stage, the product has only about 15 % of the strength and 2 % of the stiffness of the fi nal fi bers. The polymer has a structure made out of laminates with a low orientation with respect to the longitudi-nal axis of the fi ber. Crystallization and orientation of the structure is achieved by stretching the fi ber at a temperature of 300400 C.

Aramid belongs to the family of polyamides, but the bonding of matrix (resin) appears to be more diffi cult than on PA6 (nylon). In order to improve the behavior of the fi ber, it is treated with fi nishing substances. Nevertheless, the compressive strength reaches only 25 % of the tensile strength.

The chemical structure of Aramid

Over the years, this kind of synthetic fi ber has been the object of developments in terms of mechanical strength. Since the very beginning, it proved to be promis-ing, with a strength twice that of steel, with an equivalent mass. This was a dramatic achievement for the time and, immediately after, lighter materials were used in jack-ets for the personal protection of Vietnam soldiers and for aircraft.

Since then, even more resistant products have been developed, allowing for at least a ratio of 5:1 over steel. These performances refer to mechanical strength, but do not consider the attrition or the temperature: there exist no gears or engine

Fig. 1.5 Biaxial aramid fi bers

1.3 Fibers

10

components made out of Kevlar. The resistance to penetration of this material when used for protection is effective only against bullets, slightly less against bayonets and knives (surprising as this might be), so the jackets used nowadays usually have tita-nium patches inserted in order to protect from all kinds of danger. The combination of titanium and light alloys in both titanium and aluminum is common to many fl ying vehicles, particularly where a maximum reduction of weight is needed. For this rea-son, synthetic fi bers like Kevlar, the less renowned Nomex, carbon fi ber, light alumi-num alloys, magnesium, and titanium are the most popular for use in helicopters.

The main advantages of aramid fi bers are: high tenacity, good chemical and elec-tromagnetic inertia, low specifi c gravity, and high strength and elastic modulus.

1.3.5 Steel Fibers

Carbon steel fi bers represent a different type of fi ber used during more recent years for the production of steel-reinforced polymer (SRP) matrix composite materials or steel-reinforced grout (SRG). The high-strength steel fi ber fabrics used for struc-tural strengthening are made out of steel fi laments featuring an extremely high mechanical strength. Steel fi ber fabrics are available today on the market only with a monoaxial geometry because of the large dimensions of fi laments, which make it diffi cult to produce warp and weft fabrics made out of steel fi laments.

The following are the main characteristics of this system:

High strength Great ductility (they can be adjusted to any kind of profi le) and enhancement of the

ductility of the strengthened member Possibility to preserve the geometry of the strengthened member Reduced thickness and low weight Manageability and easy application Joints and bonding issues dramatically simplifi ed Fire resistance in case of concrete matrix Corrosion resistance and, consequently, longer durability over time

There are several advantages obtained during the construction of historical build-ings thanks to the possibility of impregnating the fabric with grout, thus resulting in a strengthening material that is fully compatible with the underlayer, easy to remove, but always with high physical and mechanical properties. Due to their tenacity and high shear strength, these materials are particularly suited for pretensioning with suitable systems and bonding through traditional systems without having to break the fi bers.

Steel fi ber fabrics are characterized by a metal coating made out of lead or a layer of galvanized zinc. Both coatings aim at assuring an excellent corrosion resistance, making it a material that is extremely durable over time. The choice of the type of coating depends on the expected exposition of the material during the service of the structure.


11

1.3.6 Hybrid Fabrics

Hybrid fabrics are used with the main purpose of achieving an optimal ratio between the performances of fabrics and costs. Within the same fabric, it is actually possible to apply varying weight fabrics as well as different chemical properties and mechan-ical features, thus designing a composite allowing for the physical and mechanical properties required for the different directions of stresses and avoiding the wasting of money. For instance, a combination of aramid and carbon fi bers in the weft and warp confi guration is possible, resulting in a composite with different elastic behav-iors in the two main directions of stress.

1.3.7 Natural Fibers

Another type of fi ber available in the arena of composite materials, particularly for civil engineering purposes, are natural fi bers, such as hemp and linen, which, though not having very prominent mechanical features, can be employed in bioengineering and for the restoration of old historical constructions.

Natural fi bers are those which are already existing in nature and obtainable with different processes, both mechanical and chemical. They can be approximately classifi ed according to their origin: vegetable fi bers, which are all those natural lig-nocelluloses fi bers, animal fi bers (wool, silk), and mineral fi bers (asbestos).

The fi rst defi nition, crucial for a better understanding of the subject, is that of vegetable fi ber. This term denotes a single cell having contributed to the growth of the plant from which it had been extracted and which has now ceased its vital func-tions (Fig. 1.6 ).

Fig. 1.6 Hemp fi bers

1.3 Fibers

12

As is widely known, fi bers used to obtain natural composites consist of macro-scopic particles (in the order of millimeters) and are obtained by technologies such as the crushing of the woody material selected from fi laments, or very long strands (in the order of meters) obtained from the rather strong and thick leaves of some plants living in the tropical regions of the world. A third possibility to obtain fi bers exists, resorting to more or less severe chemical treatments of the vegetable matrix from which they are to be extracted (Table 1.2 ).

The most commonly used natural fi bers are:

Hemp fi bers Flax fi bers Cotton fi bers Agave fi bers

1.3.8 Cost Aspects of Fibers

The cost of the fabrics depends on the fi ber used, the weight per square meter, and the distribution of the fi bers in the plane: monoaxial, biaxial, or multiaxial. The glass fi bers are the most economic, have very low costs, but not very high mechani-cal properties. Basalt fi bers have slightly higher costs compared to glass fi bers and, also, their mechanical properties are slightly higher. Aramid fi bers have costs and mechanical properties which are intermediate between the glass fi bers and carbon fi bers. The cost of the carbon fi bers strongly depends on the elastic modulus: the higher the elastic modulus, the greater the cost of the fi bers. By doubling the modu-lus of elasticity of the fi bers, the cost can also be three or four times higher.

1.4 Matrices

Fibers would not be so relevant, despite their high strength and elastic modulus values, without a stable shape of the member to be designed. This task is performed by the matrix, which incorporates the fi bers and ensures that a shape is given to the member

Table 1.2 Some characteristics of the fi bers

Elementary fi bers Industrial fi bers

Length (mm) Diameter () Length (mm) Thinness (mg) Stiffness Breaking length (km)

Hemp 28.0 20 1.02.2 200250 1.4 3565 Kenaf 8.0 18 1.21.8 100300 1.8 3035 Urena 7.0 20 1.21.8 250300 1.7 3540 Jute 1.62.0 18 1.53.0 250300 1.51.7 3040 Ramie 160.0 4055 0.20.4 2,0002,500 1.4 5560 Sisal 2.04.0 2030 0.51.0 3035 2.3 4555 Formier 4.08.0 813 1.22.5 100150 3545


13

and, at the same time, protects the fi bers from the external environment. As previously mentioned, matrices can be distinguished into plastic, metal, and ceramic matrices.

1.5 Plastic Matrices

A plastic matrix can be composed of a thermosetting or a thermoplastic resin. Thermosetting resins show such a structure that, by increasing the temperature beyond a specifi c limit, they degrade irreversibly, that is, after polymerization, they cannot be turned back into a liquid state, whereas thermoplastic resins become more liquid with increasing temperatures, but once cooled down, they recover their prop-erties and, thus, offer the advantage of the ability to be reshaped even after polym-erization. This difference in behavior is due to the structure of polymer molecules and, namely, to the spatial distribution of different types of modules and the degree of crystallinity. Thermoplastic applications are impeded by low operating tempera-tures, so they are used to produce even more complex geometries in an easy and rapid fashion, whereas thermosetting resins can be applied under a wide range of temperatures (Fig. 1.7 ).

Though many researchers think that, in the future, thermoplastic matrix compos-ites will gain popularity, there is still a long way to go before they achieve wide-spread use in the structural fi eld. The most important thermosetting plastic matrices (resins) are polyester, epoxy, phenolic, and silicone.

Thermosetting matrices usually employed in the fi eld of composites appear, prior to application, in a more or less viscous liquid state. At this stage, they have not yet undergone cross-linking and, in order to trigger this process, specifi c agents

Fig. 1.7 Resins


14

are added to the polymer, also known as catalyzers in case of polyester matrices, and hardeners otherwise. The time required for cross-linking can be adjusted by adding accelerators or inhibitors. Even with the same kind of composite, the cross- linking time is strongly infl uenced by temperature, which decreases as the tempera-ture rises.

Control over the quantity and type of catalyzers, hardeners, accelerators, and inhibitors allows for matrices with very short polymerization periods (in the order of a few minutes) even at room temperature or, vice versa, very long periods (in the order of several hours) at high temperatures, depending on the needs of the composite.

The type of matrix chosen scarcely impacts the mechanical and static properties of composites towards the direction of fi bers. Yet, the matrix is the component which is in the most direct contact with the environment in which the composite operates, thus conveying it:

Corrosion resistance Heat resistance Abrasion resistance

For all the applications (containers of corrosive liquids or food products, auto-motive components, etc.) where these properties are required, the choice of the right type of resin gains a special relevance.

The main properties of thermosetting matrices are shown in Tables 1.3 and 1.4 .

Table 1.3 Mechanical properties for the most common types of matrices

Resin

Properties

Type Density (g/cm 3 )

Youngs modulus (N/mm 2 )

Tensile strength

r (N/mm 2 ) Epoxy Thermosetting 1.11.4 2,1005,500 4085 Phenol formaldehyde Thermosetting 1.21.4 2,7004,100 3560 Polyester Thermosetting 1.11.4 1,3004,100 4085 Acetal Thermoplastic 1.4 3,500 70 Nylon Thermoplastic 1.1 1,3003,500 5590 Polycarbonate Thermoplastic 1.2 2,1003,500 5570 Polyethylene Thermoplastic 0.91.0 7001,400 2035 Polyester Thermoplastic 1.31.4 2,1002,800 5560

Table 1.4 Maximum temperatures for the use of resins

Thermosetting C Thermoplastic C Polyester 95 Nylon 66 140 Vinyl esters 95 Polyurethanes 180 Epoxy 175 Polysulfones 150 Polyamides 315 Polyamideimides 240


15

1.5.1 Polyester Resins

Polyester resins offer good properties, low costs, are easily workable, and set at room temperature. Their easy cross-linking dramatically reduces the costs of pro-duction technologies, which is the reason why these resins are widely used in the nautical and construction industries. The most common risk related to their activity in the sector of the processing of polyester resins is their exposure to styrene. They are usually combined with glass fi bers in these applications. The exposure mainly results from the inhalation of styrene exhalations. Styrene is actually a substance producing neurotoxic effects and, because of this, protective measures need to be taken in order to reduce the level of exposure as much as possible.

With open-matrix technologies such as hand rolling, chopping, spraying, and fi lament winding, the styrene concentration can easily exceed the maximum allowed quantity. With a scarce aeration of the working environment (measurable with spe-cifi c devices available today), measures for the protection of respiratory airways need to be adopted.

1.5.2 Epoxy Resins

Epoxy resins show better properties than those of polyester resins, for instance, good ultimate elongation, which is an extremely important factor for the mechanical properties of composites. On the other hand, they are more expensive and more complicated to be applied when compared to polyester. These are the reasons why epoxy resins are mainly used in technology-based fi elds such as aeronautics, aero-space, and sport. The high mechanical and bonding properties make epoxy resins the most commonly used resins for civil engineering applications.

1.5.3 Phenolic Resins

Phenolic resins mainly gained popularity for their good resistance at high tempera-tures (up to 250 C). This feature makes them widely employed in the aerospace, electronics, and automotive industries.

1.5.4 Silicone Resins

Silicone resins are made out of inorganic polymers, which allows for great strength at temperatures that would be unconceivable for organic polymers, though these show better properties at room temperature. Silicon resins have made it possible to use composites in structures at temperatures of up to 450500 C, which is why they are mainly used for electrical purposes and for components of supersonic aircraft.


16

1.6 Grout-Based Matrices

Grout can be employed as the matrix for the production of composites made out of steel fi bers; with varying chemical formulations, a composite known as SRG can be obtained. Depending on the mechanical features expected from the SRG composite, either concrete grouts or selected hydraulic binders producing baking raw materials at low temperatures (

17

1.8 Thermoplastic Matrices

The thermoplastic matrix represents the new frontier of research among the plastic matrices. These matrices are composed of thermoplastic polymers, which are linear or branched polymers that can be melted by providing them with an appropriate amount of heat. During plastifi cation, no change at the chemical level occurs. It can be forged (and reforged) into any shape using different techniques, such as injection molding and extrusion. It is obtained through heat melting of these polymers, which, subsequently, once in contact with the mold walls, solidify by cooling. The process of melting/solidifi cation of the material can be repeated over and over without mak-ing substantial changes to the performance of the resin, because they lose hardness in high-temperature regimes, so they always reacquire a plastic state at a specifi c temperature. This feature allows to reach a liquid state by heating the cured resin and passing through a glassy-rubbery stage. Thermoplastic resins can be distin-guished into crystalline, with an orderly crystalline structure and opacity to light, and amorphous, with a disordered structure and, generally, transparency to light. Their use is required when the member to be strengthened has a complicated geom-etry. Generally, the thermoplastic polymers do not crystallize easily as a result of cooling, since the polymer chains are very tangled. Even those that crystallize never form perfectly crystalline materials, but are characterized by semicrystalline and amorphous areas. The crystalline regions of these materials are characterized by a melting temperature (Tm). The amorphous resins and amorphous regions of par-tially crystalline resins are characterized by their glass transition temperature, Tg, the temperature at which they turn quite abruptly from a glassy state (very stiff) into a rubbery one (much softer). This transition coincides with the activation of certain motions of the macromolecules that make up the material. Below this temperature, the polymer chains have trouble moving and have very locked positions.

Table 1.5 Main properties of resins

Main properties Polyester Low temperature limit

Medium/low mechanical properties Low cost Production of styrene

Epoxy Low temperature limit High cost High mechanical properties

Metal matrix High temperature limit Medium cost Medium mechanical properties

Ceramic matrix High temperature limit Low cost Medium mechanical properties

1.8 Thermoplastic Matrices

19D. Brigante, New Composite Materials: Selection, Design, and Application, DOI 10.1007/978-3-319-01637-5_2, Springer International Publishing Switzerland 2014

2.1 Manufacturing Technologies

Several technologies can be used to produce composite materials: materials with very high physical and mechanical properties can be obtained, as well as very high volumetric percentages of fi bers, and it is also possible to obtain members with lower properties but dramatically reduced production costs. Manufacturing tech-nologies of members made with composite materials vary according to the shape, size, and properties required for the fi nished part.

Depending on the properties required from a member with composite materials, on whether or not there is the need to reproduce it or to perform continuous produc-tion, different types of available technologies can be distinguished: closed- or open- mold technologies, continuous or discontinuous, and manual or automated.

Even though only a limited number of technologies is normally used when deal-ing with structural strengthening systems made of composite materials, the main characteristics of well-known technologies which, despite not being currently used in this young fi eld of application, may be conducive to major future innovations will be examined later.

The term open mold indicates a mold allowing to obtain only a single surface with an accurate fi nishing of the details. As for strengthening systems in civil engi-neering, molds are made out of the same structural members that are to be rein-forced. Open-molding processes are commonly suitable for manufacturing extremely voluminous parts: in such cases, it would be impossible to make use of closed molds due to the arduous handling requirements related to their excessive weight. In civil engineering, impregnation is performed with a brush by using dry fi bers when making a detailed part and simultaneously placing the layers on the surface of the mold which, in this specifi c case, is the masonry or reinforced con-crete support. The inevitable air bubbles between the layers are eliminated by exe-cuting a rolling procedure and, possibly, if better results are required, by having recourse to a vacuum bag.

Chapter 2 Manufacturing Processes

20

However, the above operations show some disadvantages compared to more advanced manufacturing technologies: fi rst of all, hand lay-up of fi bers requires a greater quantity of resin than normally necessary. Secondly, eliminating the super-fl uous part turns out to be quite a hard operation, even by means of a vacuum infu-sion and, as a result, the composite will be of worse quality, since very mobile fi ber layers increase the diffi culty in executing a good vacuum bag.

2.1.1 Composite Materials Production Processes

There are several production processes of composite materials, among which the most widespread are the following:

1. Hand lay-up 2. Resin transfer molding (RTM) 3. Filament winding 4. Pultrusion 5. Vacuum infusion (resin infusion under fl exible tooling, RIFT) 6. Autoclave production

2.2 Hand Impregnation Without Pressure or Vacuum

This is still a largely common process for works on wide surfaces, such as swim-ming pools and boat hulls, which are typically produced in small lots and it is the most employed process in civil engineering. Reinforcements in the form of mats or fabric, in a percentage as per the design, are spread inside the mold, which, for applications in civil engineering, is the masonry or reinforced concrete support. Afterwards, the fi bers are impregnated with catalyzed resin and then laid up by hand and strengthened by using metal or plastic rolls in order to eliminate the excess resin. Polymerization usually occurs at room temperature (Fig. 2.1 ).

The typical values of V f , being the volume fraction of the fi bers (ratio between the volume of the fi bers and the overall volume of the composite), that can be obtained with this technology are 2530 %.

In a few cases, in order to improve the quality of the printed laminate, the impreg-nation of the fabrics is performed before laying them up, with ad hoc equipment so as to use the right quantity of resin for each foil, which allows to reach V f values of 3538 %.

2.3 Filament Winding

Even though this technique has been known for more than 30 years now, it has been profi tably used only recently thanks to the introduction of reliable materials and devices which facilitated production and made it less expensive. Together with

2 Manufacturing Processes

21

pultrusion, it is undoubtedly the production process which has seen major develop-ments in high-quality series at relatively low costs.

The process mainly consists in the winding of continuous fi laments coated with resin on a rotating body, called a mandrel, whose shape corresponds to the geometry of the part to be produced. Resin solidifi cation is obtained by placing the component in an oven or autoclave. The fundamental factors underlying this production tech-nology which strongly impact the properties conveyed to the fi nal composite prod-uct are:

(a) The type of winding (b) The type of impregnation (c) The type of mandrel (d) The type of machine (e) The type of polymerization process

2.3.1 Winding

The winding angle, defi ned as the angle between the direction of the fi laments and the tangent to the meridian of the mandrel, is extremely important. The fi bers are wound on the mandrel rotating around its axis at a given angular velocity, through the aid of a mechanical arm with a deposition loop which moves at a specifi c velocity

Fig. 2.1 Hand impregnation

2.3 Filament Winding

22

on an axis parallel to that of the mandrel. The ratio between the angular velocity of the mandrel and that of the arm determines, second after second, the winding angle which, as a result, can be modifi ed at will by acting on these parameters.

2.3.2 Impregnation

Two different winding technologies, wet winding and dry winding, can be used. For the fi rst method, impregnation is carried out on the fi lament just before it is wound on the mandrel after passing it through a tub containing resin. It requires a relatively low processing speed in order to guarantee the appropriate impregnation of the fi la-ment. For the second method, this is based on the use of prepeg, that is, preimpreg-nated fi laments with previously partially polymerized resins (in this case, the process is called wrapping). With this method, it is possible to reach a higher production speed, which is no longer hampered by fi ber wettability problems. Although this process requires higher initial costs, the use of prepeg offers the pos-sibility to produce high-quality products. Moreover, the use of prepegs allows to use all kinds of resins, including those resins whose viscosity does not require them to be used for a direct impregnation of the fi laments during the winding.

2.3.3 The Mandrel

This component is essential for obtaining the correct geometry of the part. It can be metallic (steel or aluminum), plastic, or even made of chalk, and it can be fi xed or removable. When it is fi xed, it stays inside the piece, becoming an integral part of it, whereas if it has to be removed, it will be possible to extract it, provided that its shape allows for this or, if it has been made with an appropriate material, it could be unwound. Irrespective of its material and shape, the mandrel has to be able to bear the stresses exerted upon it by the tension of the fi lament winding on the mandrel (this is another extremely important parameter for obtaining a high-quality product).

2.3.4 Machines

There can be two types of machine:

With a horizontal axis, for helicoidal windings With a vertical axis, for polar windings and preferred for very large size

members

Beyond its winding function, the machine has to be equipped with another arm, which cleans the part before the confi nement can be manufactured (Fig. 2.2 ).


23

2.4 Pultrusion

The meaning of the term pultrusion becomes extremely clear if we think about the technological model on which the process is based. Actually, while the extrusion of aluminum or of thermoplastics results from exerting a thrust on the material such as to force it to pass through the mold, in the case of strengthened plastics, the same shape can be obtained by pulling the fi bers, forcing them, once wet with resins, to pass through the mold. That is, the thrust action of the extrusion process is replaced by a pulling action, thus the origin of the term pultrusion.

This technology is characterized by a continuous production; when the system is equipped with an automated fl ying cutting saw, production only requires a reduced human presence, limited to system startup and control of possible power interruptions to reinforcement or to the level of resin in the impregnation basin. Only recently has this technology found relevant industrial applications, but, actu-ally, its fi rst application dates back to 1948 and the fi rst patent goes back to 1951. The fi rst products manufactured by pultrusion were high-precision bars, which are still the most popular product today.

The high tensile strength and percentage of reinforcement that can be manufac-tured, alongside other meaningful properties, such as electrical insulation, corrosion resistance, and low weight, lead to an extension of the pultruded products palette to applications such as cable-carrying columns, insulator bars, spillways for sewage treatment plants, gangways, decks and parapets, scales, connector sockets and fused isolators, guardrails, Citizens Bands (CB) aerials, structural beams, and much more. The process requires a mainly continuous fi ber strengthening and a low- viscosity resin, usually a thermosetting liquid. The most popular reinforcement is glass rov-ing; only recently, due to economic reasons, and in specifi c cases, have carbon and aramid fi ber reinforcements been applied. These kinds of reinforcements are also used for hybrid composites together with glass. The basic scheme of the process is:

(a) Reinforcement feeding (b) Impregnation

Fig. 2.2 Filament winding ( http://www.lawrietechnology.com )

2.4 Pultrusion

24

(c) Preforming (d) Forming and polymerization (e) Pulling mechanism (f) Chopping (g) Postforming (Fig. 2.3 )

2.4.1 Reinforcement Feeding

Reinforcements used in the pultrusion process have the shape of continuous and monodirectional fi laments, mats, and fabrics. Both the reinforcement and the matrix should, nevertheless, comply with specifi c technical requirements:

Having enough strength and stiffness towards the tensile direction in order to bear the stresses exerted by pulling just beyond the cross-linking area

Reporting relevant values of thermal conductivity at specifi c heats in order to calculate the heat transmission speed from the heating drawing machine to the product under polymerization. A good result can also be achieved after a number of preliminary tests

Allowing for a control of the reinforcementmatrix volumetric ratios, such as to monitor both the dilatation and the mass effect (of the thermosetting matrix) when heating

Fig. 2.3 Pultrusion


25

The number of rovings from where the glass fi lament is taken ranges from a few units up to thousands of units; actually, in the industrial fi eld, the average number is in the range of hundreds. Just out of curiosity, the record number of rovings simul-taneously working in industrial applications is 5,000, which is the number used for the production of structural box girders with ca. 25.5 -mm walls.

Roving winders are usually positioned on shelves (creel stocking for rovings) with suitable racks.

Many manufacturers preferably draw glass fi laments from inside the winder and not from outside, so as to convey a helicoidal movement to the fi lament; in any case, there is only a slight difference in the fi nal effect. The end of the winder can then be connected to the initial part of the successive one in order to allow for continuity. Glass fi laments are kept separated from each other through ad hoc rails in the shape of rings, usually made of ceramic material or steel with chromium beads, so as to prevent the abrasion action on the fi lament and to increase the durability of the rail. Cases of fi berglass rakes are also frequent, which are made from a combination of pultruded members.

Roving is combined with mats, in the shape of different width bands, wound around 90100-m-long rolls. Mat rolls are usually available in standard widths that very often do not fi t the manufacturers requirements.

That is the reason why many companies would rather order rolls in ordinary sup-ply widths so as to avoid additional costs for the cutting that they can easily perform by themselves. During cutting, the shrinkage of the material should also be taken into account.

Rovings and mats are the most widely used reinforcements in the production of fl at sections with thin walls. Very often, manufacturers insert a roving layer between two mat layers, to give a sandwich structure. The mat does not actually have enough resistance to resin impregnation without the additional support of roving.

This issue, particularly relevant in the past, is now perceived less as a problem. The end of a mat roll can be sewed onto the initial part of the successive roll, and this is done by hand. Every roll lasts from 1 to 3 h at an average speed; this means that, if many rolls are used simultaneously, the sewing operation can be required at such a frequency that it becomes very expensive. Besides, the mat should be han-dled and directed with greater care than roving.

2.4.2 Impregnation

Pultruded resins are, in the majority of cases, made of unsaturated polyester, but there can also be cases of epoxy or silicon resins for specifi c and limited applica-tions. Polyester-based matrices are made of esters between unsaturated acids and glycols, diluted in a polymerizable monomer (styrene), which allows the formation of a three-dimensional grating for polymerization.

Epoxy resins are mainly used for carbon strengthening. Alternatively, the use of a polyester matrix with this kind of strengthening would lead to a composite with dramatically lower mechanical properties. Specifi cally, it would result in a reduction

2.4 Pultrusion

26

of stress and shear strength. Another issue that should not be neglected is that of the volatility of styrene; the use of low-volatility monomers or composites should be preferred. In the range of commonly used polyesters, a special relevance is enjoyed by the so-called low-profi le formulas, that is, those containing acrylic resins allowing for a reduction of the shrinkage effect. Among the epoxy resins, bisphenol A diglycidyl ethers (DGEBA) are generally the most suited for pultrusion.

The level of fi llers, if necessary, can attain a maximum of ca. 20 %. The most commonly used are calcium carbonate, antimony trioxide, alumina trihydrate, etc. The quantity of fi llers is actually limited by resin viscosity: too high a level of fi llers may lead to problems with roving impregnation. Often, pigments are added to resin in order to obtain a properly colored fi nal product and/or a greater resistance to UV rays. The impregnating basins have varying widths and lengths, generally ranging between little less than 1 m up to nearly 2 m. The strengthening path in the basin is directed through a system of drawing machines/rolls, forcing a zigzag movement. In some cases, fi laments are drawn through a pair of rolls fl attening the fi lament and often unwrapping it, thus facilitating proper penetration of resins among the fi bers. In the case of several sheets, the pairs of sheets should be impregnated separately and simultaneously, and then matched only after impregnation. The volume of resin in the basin should be kept at a minimum level required for a good outcome of the operation and periodically topped up, in such a way as to prevent excessive resin consumption and to avoid the risk of premature polymerization, which is also due to the mass effect. The resin viscosity is kept at an average between 200 and 1,200 cP. Sometimes, the basin is heated to enhance the viscosity and improve the wettability of the fi bers, but this leads to a reduction of the resin operation time.

Alongside the method just described as an example, different impregnation sys-tems are available. In 1971, a company patented a system of impregnation based on resin injection with a drawing machine under 6 atm pressure. This system has been widely applied for the production of pipes with average mechanical properties, pigmented with a gel-coat layer.

2.4.3 Preforming

The pultruded profi le can hardly, if not ever, be formed in a single operation, but it is usually performed gradually, achieving the fi nal shape of the reinforcement pass-ing through intermediate shapes. The purpose of preforming devices is actually the facilitation of this gradual forming, allowing simultaneously a better lining up of fi bers as well as reuse of excess resin. These devices, though not indispensable for all kinds of profi les, represent, nevertheless, a very useful additional tool to improve the quality of production.

Preforming devices are not necessarily sophisticated, as the most common machines simply have plates properly laid and the resins pass through these prop-erly shaped slots. In almost all cases, the preforming rails are built in-house with the use of plastics or soft metals. Sometimes, the preforming nozzle has the same shape as the main drawing machine, but with sizes increased by 510 %.


27

2.4.4 Forming and Polymerization

The heated drawing machine is one of the most expensive parts of the system. The following is a list of the main features that the material used should satisfy:

Good workability, particularly to satisfy the need for very sharp edges close to the joints

High surface hardness, with average values around C50 Rockwell Possibility to be worked so as to achieve a surface quality at least of 0.2 m CLA Excellent resistance to chemical agents, that is, resins and cleaning agents Optimal mechanical properties at high temperatures

Usually, the mold is made of chromium steel in order to give greater durability. Sometimes, chromium is replaced by high-alloy steels, since it is particularly sensi-tive to epoxy resins. The length of a drawing machine mainly depends on the section of the product, on the type of process, and the resin used. It is increased when very thin sections are produced or when high-frequency productions are required. Obviously, with equal complexity of the system, the cost of drawing increases pro-portionally with increasing length.

Particularly cost-effective may be the design of a mold that is perfectly inter-changeable, making it possible to exchange them when the inlet, which is the part that is subjected more to wear and tear, has exceeded tolerance levels. The drawing machine should be positioned perfectly parallel to its axis and lined up with the pre- and postforming systems. The inlet should have a taper of nearly 710, whose function is to eliminate the excess resin. This trick is less useful in the case of a preforming system already equipped with a device for eliminating excess resin.

The laid up chromium layer can even double the mold durability. Generally, it can be assumed that a drawing machine should produce, on average, between 20,000 and 30,000 m of pultruded product before performing the successive chro-mium coating. In any case, this operation cannot be repeated indefi nitely, since it damages the steel underlayer of the drawing itself. Besides, the smoothness of the mold is an extremely sensitive aspect, above all in the areas where resins have gelled, though not yet attaining the fi nal expected hardness.

If the resin is scratched on the mold surface, not only the fi nal fi nishing of the product surface will be lacking, but the removed resin may be absorbed in other surface areas of the pultruded product as well, or may even clog up the mold itself. In this case, an intermittent pultrusion is required, in order to obtain consistent polymerization along the entire mold. If so, the initial part of the drawing should be cured to prevent the resin from gelling outside the mold and around it.

Usually, drawing machines are composed of two separated parts along a horizon-tal plane parallel to the pultrusion direction. In the case of simpler sections, for instance, bars, single-block molds are used, obtained by piercing it and then smooth-ing the hole that can also be chromium-coated with specifi c techniques. The fi nishing of the product surface is positively impacted by the absence of joints in the molds.

2.4 Pultrusion

28

When designing a drawing machine, the issues related to the thermal expansion of material during polymerization and its successive shrinkage should be carefully taken into account. For clarifi cation, it can be assumed that, with a composite mate-rial made of epoxy resin and carbon strengthening, the average dimension of the mold is increased by 2 % only for the part requiring it. Hereby, thermal expansions of the material will not cause dangerous shear stresses.

Within the drawing machine, the most sensitive stage of the process takes place: polymerization. It is responsible, to a great extent, for both the quality of the fi nal product and the frequency of the overall production. It should be carried out in a way to ensure that the tempo-temperature history of the matrix allows for a proper level of polymerization, before the material accesses the drawing system. Moreover, a control system should be in place so that the wet reinforcement, when passing through the drawing machine, does not cause shear stresses to the interface with the steel wall, which exceeds its strength capacity at that stage of the process.

The heating of the material within the drawing machine takes place in the major-ity of systems via conduction through the drawing machine. This, in turn, is heated through external plates or electrical resistances located inside. This second solution is, of course, more expensive than the fi rst. Other heating systems make use of radi-ating systems or hot oil transfer systems.

Nevertheless, in order to grant a uniform distribution and recurrence of the objects properties in all its parts and at every work shift, the temperature control, which ranges on average between 150 C and 170 C, should only allow for a maxi-mum tolerance of 1 C.

The polymerization speed can be increased by dielectrically preheating the mate-rial immediately after the impregnation stage. In this kind of system, the microwave device starts the cross-linking by preheating from the inside out of the resin and the fi bers, while the next heating system, usually an electrical one, accomplishes curing by heating from outside into the mass. The fi nal outcome is an extremely uniform distribution of properties and an increased speed of production. The radio frequen-cies generally range between 45 and 500 MHz. Microwaves with frequencies rang-ing between 950 and 5,200 MHz are allowed for certain thermosetting matrices, whereas frequencies ranging between 1,000 and 2,000 MHz are used with epoxy resins and between 40 and 70 MHz with polyester resins.

Postforming

Postforming devices aim to grant proper lining up of the product section at the drawing machine outlet by counterbalancing any possible side tensile stress that may lead to a bending of the profi les. They are mainly made out of pairs of pulleys or rolls, mounted perpendicular to each other and with adjustable widths. The lining up of the devices with the drawing machine and the preforming devices is of utmost im

new composite materials_ selection, design, and application-springer international publishing (2014)

Documents