overview of mechanical properties of polymer‐matrix...

Overview of mechanical

properties of polymer‐matrix

composites

Dr. Suhasini Gururaja

Assistant Professor

Aerospace Engineering, IISc, Bangalore

1 Overview of Mechanical Properties for PMCs Dr. Suhasini Gururaja, AE, IISc Bangalore

(Parts of the material for this presentation has been borrowed from lecture notes of Prof. K. Lin and Dr. Patrick Stickler at University of Washington, Seattle. Some figures have been reproduced

from open literature and used here for purely pedagogical purposes.)

History Composite materials as a scientific/engineering discipline is

approximately 60 years old.

Composites have been useful for thousands of years

◦ Animal hair was added to pottery to improve strength

◦ Straw-reinforced clay was used to make bricks (Exodus

5:7)

◦ Bitumen was embedded with papyrus reeds to build boats

◦ Achilles’s shield was a composite laminate design (Homer’s

Illiad, xviii: 468-480)

Composite materials found in nature*:

◦ Wood: cellulose fibers in a lignin mating

◦ Bone: Collagen fibers in an apatite matrix

*Mechanical Design in Organisms, Wainwright et al, 1976.


What is a composite material?

Combination of two or more chemically distinct materials on the macroscopic scale tailored to achieve improved properties that neither constituents individually possess.

Improved properties achieved include ◦ Improved specific strength, stiffness, durability,

corrosion resistance etc.

Classification of composites ◦ Polymer Matrix Composites (PMCs)

◦ Metal Matrix Composites (MMCs)

◦ Ceramic Matrix Composites (CMCs)

Introduction


High Specific Strength (strength/density)

High Specific Stiffness (modulus/elasticity)

Tailored properties in load application direction

Tailored CTE for critical components

Excellent fatigue performance

Depending on resin/matrix combination and design

Corrosion resistant

UV resistant

Good dielectric


Potential structural advantages of

advanced composites


Specific property comparison

Key Differences between Composites and Metal

Anisotropy

Tailored Properties

Fatigue and Corrosion

Lighting protection

Discontinuous stresses

Delamination

Damage Tolerance

Environmental Effects

Repairability

Reduction in parts counts


Tailored Properties Composites

◦ Properties CAN be

tailored

Properties can be tailored by

combining different

percentages of 0o, 45o, -45o

and 90o plies

◦ Optimal use of

material

Material properties can be

tailored per loading

requirements to meet

design allowables while

reducing overall weight

Metals

◦ Properties CANNOT be tailored

Properties are represented in fixed values that cannot be tailored

◦ Material CANNOT be optimized

Structural performance can only be improved through changes in geometry, such as thickness, which adds to weight


Fatigue and corrosion

Composites

◦ Better fatigue

performance than metals

in tension (relatively flat

S-N curves)

◦ Compressive fatigue

properties are not as

good as those in tension

◦ Superior corrosion

resistance for CFRP

◦ Galvanic corrosion

occurs between CFRP

and Al, Mg, Cd plate and

steel.

Metals

◦ Relatively poor fatigue

properties in both

tension and

compression (more

steep S-N curve)

◦ Poor corrosion

resistance, especially in

a cracked structure


Discontinuous Stresses In-plane strains:

◦ Continuous throughout the

laminate thickness

Constant under uniform

extensional forces

Distributed linearly under

bending

In-plane stresses:

◦ Generally discontinuous

throughout the laminate

thickness because each ply

has different stiffness values

9

The strain-based design criteria are generally used in industry

Overview of Mechanical Properties for PMCs Dr. Suhasini Gururaja, AE, IISc Bangalore

Stresses due to

stretching Stresses due to

bending

Delamination

Occurs in laminated composites

Causes local bending and buckling in compressively loaded structures

Can grow under normal and shear loads

Careful designs needed at locations prone to delamination


Damage tolerance/Repairability

Critical damage

◦ Impact, delamination

◦ Compression after Impact (CAI)

strength is a key design parameter

Damage growth

◦ Complicated by multiple damage

types and failure modes

◦ Current design is for “no damage

growth”

Damage assessment is more difficult

– surface and internal damage

Most repairs are bonded repairs –

time consuming and require highly

skilled labor

Repair materials and adhesives are

time sensitive

Critical damage

◦ Fatigue crack, stress corrosion

Damage growth

◦ Crack growth can be reasonably

well predicted using fracture

mechanics approach

◦ Refer to FAR 25.571, “Damage

tolerance and fatigue evaluation

of structures”

Damage detection techniques are

well defined and surface damage

can be easily found

Most repairs are bolted repairs –

relatively easier and cheaper

No shelf life of repair materials

Quality of repair is easier to

control 11 Overview of Mechanical Properties for PMCs Dr. Suhasini Gururaja, AE, IISc Bangalore

Composites Metals

Environmental Effects/Thermal Stresses


Composite properties are strongly affected

by moisture, temperature, sunlight,

microbes, release agents, solvents etc.

Property reduction factors (knockdown

factors) are used appropriately

Due to mismatch in CTEs, thermal

residual stresses exist in structures.

Thermal stresses must be considered in

composite tool design and manufacturing.

Continuous fibers: lengths are in effect infinite -

Unidirectional Tape, Woven or braided Fabric

13

*Prof K. Lin AA532 Notes, University of Washington


Classifications of reinforcements

Whiskers, short fibers, and continuous fibers all have very small

diameters relative to their length (high aspect ratio)

Classifications of reinforcements Advanced composites

“modern”

◦ Particulates: roughly

spherical particles with

diameters (typically 1-100

mm)

◦ Whiskers: lengths <

10mm

◦ Short (or “chopped”)

fibers: length 10 – 100mm

SMC and Preforms


Laminate construction

Ready-to-cure part

on mandrel

Very good quality

Excellent

repeatability

Stacking cut plies

into a desired

sequence

15

~8mm dia

carbon fiber

127 mm ply

thickness

Fibers appear as ovals

because they were cut

at an angle to the 0º

direction.

*Prof K. Lin AA532 Notes, University of Washington Overview of Mechanical Properties for PMCs Dr. Suhasini Gururaja, AE, IISc Bangalore


Tensile properties of Fiber, Matrix and Composite

16

0 1 2 3 4 0

100

200

300

400

500

600

Strain (%)

Te

nsile

Str

ess (

ksi)

Fiber

Composite

Matrix

Manufacturing Methods - Bag Molding Process

17

1. Mold surface covered with nonstick Teflon-coated glass fabric separator.

2. Prepreg plies laid up in desired fiber sequence and orientation.

3. Porous release cloth and a few layers of bleeder papers placed on top of prepreg stack.

4. Complete lay-up covered with another sheet of Teflon-coated glass fabric separator, caul plate, and thin,

heat-resistant vacuum bag.

5. Entire assembly placed inside autoclave where a combination of heat, external pressure, and vacuum is

applied to consolidate and densify separate plies into a solid laminate.

• Note: To prevent moisture pickup, prepreg roll on removal from cold storage should be warmed to room

temperature before use.

Overview of Mechanical Properties for PMCs, Dr. Suhasini Gururaja, AE, IISc Bangalore

Bag Molding Process Typical two-stage cure cycle for a carbon fiber-epoxy prepreg :

1. First stage

Increasing temperature up to 130°C (266°F).

Dwelling at this temperature for nearly 60 minutes until the minimum resin viscosity is reached.

During the temperature dwell, external pressure applied to prepreg stack that causes excess resin to flow out into bleeders.

2. End of temperature dwell

Autoclave temperature increased to actual curing temp. of resin.

Cure temperature and pressure maintained for 2 hours or more, until predetermined level of cure has occurred.

At end of cycle, temperature slowly reduced while laminate still under pressure.

Flow of excess resin from the prepreg is extremely important in reducing the void content in the cured laminate.

18 Overview of Mechanical Properties for PMCs, Dr. Suhasini Gururaja, AE, IISc Bangalore

Bag Molding Process

19

Typical two-stage cure cycle for a carbon fiber-epoxy prepreg

Overview of Mechanical Properties for PMCs, Dr. Suhasini Gururaja, AE, IISc Bangalore

Mechanical Properties of PMCs

“A material property” is a measurable

constant characteristic of a particular

material, which can be used to relate

disparate quantities of interest.

Key Mechanical properties include:

◦ Stress tensor to strain tensor

◦ Temperature/Moisture to strain tensor

◦ Stress (or strain) to failure/cycles to failure

◦ Crack growth to failure


Anisotropic behavior

Composites

◦ Anisotropic

Properties are dependent

upon directions

◦ Inhomogeneous

Properties are different in

different plies

◦ Mostly Brittle

Linear stress-strain relation

and low strain to failure

Metals

◦ Isotropic

Properties are the same in all

directions

◦ Homogeneous

Properties are the same in all

directions

◦ Mostly Ductile

Nonlinear stress-strain

relation with a large plastic

deformations


Anisotropic versus isotropic

22

x

y

z

Specimen 3

Specimen 1

Specimen 2

Three specimens machined at different orientations from a single “parent” block.



23

Specimen 1 (Exx) Specimen 2 (Eyy) Specimen 3 (Ezz)

Tensile tests of three individual specimens



Isotropic

Anisotropic

Anisotropic materials

◦ The value of Young’s modulus depends on the

direction within the material the modulus is

measured

◦ A similar dependence on direction can occur for

other mechanical properties (n’s, CTEs, ultimate

strengths, etc)

PMCs are anisotropic at the structural level

24

xx yy zzE E E

xx yy zzE E E


Principal Material Coordinate System


A thin uni-directional (UD) composite panel Two specimens machined

from the UD panel

Note: The 1-2-3 coordinate system is the principal material coordinate system

Principal Material Coordinate System


A thin braided composite panel Two specimens machined

from the braided panel

Note: In this case the principal material coordinate system is not aligned

with the fiber direction

Anisotropic behavior


PMCs are anisotropic at the structural level

One of the most unusual features of anisotropic materials is that they can exhibit coupling ◦ Coupling between

normal stresses and shear strains

◦ Coupling between shear stress and normal strains

Coupling exists

between sxx and gxy

Coupling of bending and stretching deformations

The extensional force can cause shear deformation for unbalanced laminates

The extensional force can induce bending curvature for asymmetric laminates

A balanced laminate can develop twisting curvature under extensional forces

Unbalanced and asymmetric laminates result in a larger bending deformation, lower natural frequencies of vibration, and lower critical buckling loads.

28 Overview of Mechanical Properties for PMCs, Dr. Suhasini Gururaja, AE, IISc Bangalore

Uniaxial tensile test

29

*Vassilopoulos and Keller, Fatigue of FRCs, Springer, 2011

ASTM 3039

- Adhesively bonded tabs

Properties

- Ultimate Tensile Strength

- Ultimate tensile strain

- Modulus of Elasticity

- Poisson’s ratio

30

Designation Title

D3039 Standard Test Method for Tensile Properties of Polymer Matrix Composite

Materials

D5450 Standard Test Method for Transverse Tensile Properties of Hoop Wound

Polymer Matrix Composite Cylinders

D695 Standard Test Method for Compressive Properties of Rigid Plastics

D3410 Standard Test Method for Compressive Properties of Polymer Matrix

Composite Materials with unsupported Gage Section by Shear Loading

D5467 Standard Test Method for Compressive Properties of Unidirectional Polymer

Matrix Composites Using a Sandwich Beam

D5449 Standard Test Method for Transverse Compressive Properties of Hoop

Wound Polymer Matrix Composite Cylinders

D3518 Standard Practice for In-Plane Shear Response of Polymer Matrix

Composite Materials by Tensile Test of a 45°Laminate

D5379 Standard Test Method for Shear Properties of Composite Materials by the V-

Notched Beam Method

D4255 Standard Test Method for In-plane Shear Properties of Polymer Matrix

Composite Materials by the Rail Shear Method

D5448 Standard Test Method for In-plane Shear Properties of Hoop Wound

Polymer Matrix Composite Cylinders Overview of Mechanical Properties for PMCs Dr. Suhasini Gururaja, AE, IISc Bangalore

Yielding and Fracture of Composites

Predicting fracture of multiangle composite laminates under general load conditions remains a challenging area of research.

Three distinct "materials" regions may be defined ◦ The fiber

◦ The matrix

◦ The fiber-matrix interphase

The mechanical properties exhibited by the polymer in the interphase region differ from bulk properties.

The initial nonlinear deformations exhibited by a PMC are therefore almost entirely initiated within the polymeric matrix.

The fracture process is initiated when one or more microcracks are formed in the matrix.



Matrix Cracks ◦ Cracks that occur in the polymeric matrix, at some

distance from the fiber/matrix interface.

◦ Matrix cracks generally occur in planes either parallel or perpendicular to the fiber direction.

Fiber-Matrix Debonding ◦ The crack has formed in the interphase region, and a

(non-planar) crack extends around the periphery of the fiber.

Fiber Cracks ◦ Cracks that occur in the fiber itself.

◦ Fiber cracks almost always occur in a plane perpendicular to the axis of the fiber, and extend across the entire width of the fiber.



Viscoelastic behavior: yielding and crack growth in polymers is a time-dependent phenomenon called “creep”.

◦ An increase in temperature and/or an increase in moisture content further accentuate the time-dependency.

◦ If a tensile stress is applied and held constant the composite may eventually fail due to slow crack growth (often called a "creep-to-rupture" failure).

Chemical aging: polymers aging occurs due to ultra violate light.


Failure of Multi-angle Composite Laminate

Multi-angle laminates are subject to failure modes that do not exist in unidirectional laminates. ◦ The initiation of delamination failures is often

attributed to free-edge stresses.

◦ Free-edge stresses occur whenever adjacent plies possess differing Poisson ratios or coefficients of mutual influence.

Pre-existing thermal and/or moisture stresses occur in multi-angle laminates. ◦ Due to a mismatch in effective thermal expansion

and moisture expansion coefficients from one ply to the next.



Additional damage mechanisms in composites include: ◦ Fiber-matrix debonding: a crack forms around the

periphery of a fiber.

Load can no longer be transferred from the matrix to the fiber. ◦ Fiber micro-buckling: fibers within a ply that

experiences compressive stresses in the fiber direction buckle.

Reduces the compressive stiffness exhibited by the ply.

Leads to failure of the fibers due induced bending stresses.



Failure modes for multiangle laminates.

◦ Matrix cracking/splitting (microcracks).

◦ Delamination.

◦ Fiber fracture.

◦ Fiber/matrix debond.

◦ Fiber “kinking”(microbuckling).

◦ Global laminate buckling.

Failure should be verified experimentally.



Experimental observations of the evolution of damage in a quasi-isotropic laminate (monotonically increasing uniaxial load, Nxx)



The 90o plies yield as Nxx increases to a critical level.

Cracks begin to form in the 90o plies at load levels above the first-ply failure stress.

As the effective stress is further increased, cracks eventually begin to form within the ±45 plies

As the effective stress is increased further, delaminations begin to develop.

Matrix cracks begin to form between plies, and these new matrix cracks lie within planes that are parallel to the x-y plane



The delaminated regions grow in size as the stress is increased and eventually coalesce, such that a delaminated region may extend across the entire width of the specimen.

At still higher effective stress levels matrix cracks begin to form within the 0ºplies (often referred to as “splitting”).

These cracks lie within a plane perpendicular to the x-y plane.

Final laminate fracture is precipitated by fiber failures within the 0ºplies.

The effective stress level at which final fracture occurs is often called the last-ply failure stress.

◦ At final fracture the laminate fractures into fragments.

◦ Extensive and pre-existing matrix cracks and delamination that occurred at lower stress levels.

◦ Large amount of energy release associated with fiber failure.



Reifsnider et al studied damage progression of multi-angle composite laminates under fatigue loading. ◦ Material:

graphite/epoxy.

◦ Layup: [0/±45/90]s

◦ Tension-tension fatigue spectrum.

σmax= 0.62 σult

σmin= 0.062 σult

R = (σmin/ σmax) = 0.1


Nxx

0

s

Ds

smax

smin

sa sm



Experimental observations of damage

sequence under tension-tension fatigue

load spectrum.

◦ Matrix cracks in 90ºplies.

◦ Matrix cracks in ±45ºplies.

◦ Delaminations.

◦ Matrix cracks 0ºplies (splitting).

◦ Fiber failure.

◦ Final fracture.

Development of

characteristic damage state

Significant reduction in

stiffness.



Tension-tension fatigue Transverse Matrix Cracks

Constant Life Diagrams

43

0o 45o

90o

*Vassilopoulos and Keller, Fatigue of FRCs, Springer, 2011

ASTM E739-91

Case Study*: Tension-tension fatigue

properties of chopped GFRPs

44

SMC R27

SMC R37

Preform R25

Preform R40

Fatigue behavior of these four

Compression molded composites

shall be presented


*Results from T.Briggs and M. Ramulu, "An Experimental Characterization of the

Failure Mechanisms Activated in GFRP Composites" IMECE07, Seattle

Composite Material Composition

45

Material Component SMC-R27 SMC-R37

Resin base: PG Maleate/PVA low profile Polyester Polyester

Filler material Calcium Carbonate

(MgO thickeners)

Calcium Carbonate

(MgO thickeners)

Glass content by weight 27% 37%

Material Component Preform–R25 Preform-R40

Resin base w/LPA of thermoplastic Polyester Vinyl ester

Filler material Clay Calcium Carbonate

Glass content by weight. 25% 40%

Fine glass veil 0.76 mm thick 0.76 mm thick


Compression Molding Process

46

SMC Preform


Burn out Virgin Sample

47

x

y

SMC R-27 SMC R-37

PreForm - R25 PreForm – R40


UTS and tension-tension Fatigue Test Setup

48

Ambient air thermocouple Fatigue specimen

thermocouple

MTS 89-KN static tensile and tension-tension fatigue load frame


Stress vs. Strain to failure


Knee

Specimens after UTS Experiments

50

30 mm

SMC-R27

SMC-R37

Preform-R25

Preform-R40


Temperature versus Frequency

51

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40 45 50 55 60 65

De

g C

Minutes

10 HZ

5 HZ


Stress life plots

52

y = -5.045ln(x) + 130.73

y = -5.495ln(x) + 125.98

y = -3.501ln(x) + 82.985

y = -4.676ln(x) + 101.27

0

20

40

60

80

100

120

140

160

0.1 10 1000 100000

Max

imu

m S

tre

ss (

MP

a)

Log Cycles

R = 0.05, Room Temp

SMC-R37 Preform-R40 SMC-R27 Preform-R25


Tensile Modulus degradation

53

100

102

104

106

5000

10000

15000

(a) SMC-R27Ten

sile

Mo

du

lus

(MP

a)

Log Cycles

70% UTS

60% UTS

50% UTS

40% UTS

30% UTS

100

102

104

106

5000

10000

15000

(b) SMC-R37

Ten

sile

Mo

du

lus

(MP

a)

Log Cycles

70% UTS

60% UTS

50% UTS

40% UTS

30% UTS

100

102

104

106

5000

10000

15000

(c) Preform-R25Ten

sile

Mo

du

lus

(MP

a)

Log Cycles

70% UTS

60% UTS

50% UTS

40% UTS

30% UTS

100

102

104

106

5,000

10,000

15000

(d) Preform-R40

Te

nsile

Mo

dulu

s (

MP

a)

Log Cycles

70% UTS

60% UTS

50% UTS

40% UTS

30% UTS


54

SMC-R27

SMC-R37

Preform-R25

Preform-R40

Fiber bundle

Pull-out

Matrix fracture

surface

Fiber

fracture

Fiber

bundle

damage

Fractography - 70% UTS fatigue test (R = 0.05)


55

Low-Velocity Impact

Effect of LVI on residual compression and tensile strengths of HTA/913 and HTA/982 CFRP laminates and E-glass/913

(normalized wrt undamaged material). [(±45,02)2]s (Courtesy: Brian Harris, Fatigue in Composites, CRC Press, 2003.)


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