Composites Module 4a. Spring 2001 ISAT 430 Dr. Ken Lewis2 An aside: Stress – Strain Tension test...

Composites

Module 4a

Spring 2001 ISAT 430 Dr. Ken Lewis 2Module 4a

An aside: Stress – Strain

Tension test Used to determine mechanical properties such as

Strength Ductility Toughness Elastic modulus Elongation to break stiffness

Relative mechanical Properties

Strength Hardness Toughness Stiffness Strength/Density

Glass Fibers Diamond Ductile Metals Diamond Reinforced Plas.

Graphite Fiber Cubic BN Reinforced Plas. Carbides Titanium

Kevlar Fiber Carbides Thermoplastics Tungsten Steel

Carbides Hardened steels Wood Steel Aluminum

Molybdenum Titanium Thermosets Copper Magnesium

Steels Cast Irons Ceramics Titanium Beryllium

Tantalum Copper Glass Aluminum Copper

Titanium Thermosets Ceramics Tantalum

Copper Magnesium Reinforced Plas.

Reinforced TS Thermoplastics Wood

Reinforced TP Tin Thermosets

Thermoplastics Lead Thermoplastics

Lead Lycra®

Spring 2001 ISAT 430 Dr. Ken Lewis

Stress - Strain

There is a standard specimen for each type of material having a known initial gage length (l0) and diameter (cross sectional area (A0)

Metals l0 =~ 50 mm ( 2 in) A0 dia =~ 12.5 mm (0.5in)

Fibers l0 =~ 25 mm ( 10 in) A0 dia =~ the fiber

Stress - Strain

Specimen is mounted in the jaws of a tensile testing machine.

Usually can be tested at various rates of extension and temperature.

Produces the important Stress – Strain curve.

StressA

Strainl

Elastic Plastic

Fracture

YieldStress

Ultimate TensileStress

Stress - Strain

In the elastic region, stress and strain are proportional

The ratio or slope of the line in the elastic region is called

Modulus of elasticity, E, Young’s modulus

The linear relationship is known as Hooke’s Law

StressA

Strainl

Elastic Plastic

Fracture

YieldStress

T. Young (1773 – 1829)

R. Hooke (1635 – 1703)

Stress - Strain

The higher the E value The higher the load required to

stretch a specimen to the same extent

The stiffer the material

StressA

Strainl

Elastic Plastic

Fracture

YieldStress

Note Strain is dimensionlesstherefore

E has units of stress F/A.

Stress - Strain

Brittle Materials Glass Most un-reinforced ceramics

Elastic to the end.

Stress - Strain

Materials that strain harden Some steel alloys

Material is ductile until grain boundaries intersect

Movement stops More stress can then be borne.

Stress - Strain

Elastomers Lycra® Rubbers

Small truly elastic region Large elongation with little

increase in stress At the end, crystallization occurs

and stress is borne until rupture.

Stress – Strain - Plastics

Stress - strain &Temperature

Cellulose Acetate Note

the large drop in strength The large increase in ductility

Composites

A way of combining the benefits of different materials to influence the bulk properties

Make plastics competitive with steel Allow building a single piece boat hull Increase the elastic modulus and strength of light metals.

An important class of engineered materials.

Composites

Definition Combination of two or more chemically distinct and insoluble

phases Properties and structural performance superior to those of the

constituents acting independently.

Common classes Reinforced Plastics Metal Matrix Composites (MMC) Ceramic Matrix Composites (CMC)

Early examples

Brick reinforced with straw ( 3000 BCE) Concrete reinforced with iron rods (1800’s)

In truth,Concrete is itself a

Composite(sand, cement, gravel)

In both cases, the reinforcing material

(straw and iron)provide needed tensile

strength.

Mechanical Properties

Material Ultimate Tensile Strength (Mpa)

Young’s Modulus

E (Gpa)

Elongation to break

Acetal 55 - 70 1.4 – 3.5 75-25

Acetal Reinforced 135 10 --

Epoxy 35-140 3.7-17 10-1

Epoxy Reinforced 70-1400 21-52 4-2

Polycarbonate 55-70 2.5-3 125-10

Polycarbonate reinforced 110 6 6-4

Polyester 55 2 300-5

Polyester reinforced 110 8.3-12 3-1

Nylon 55-83 1.4-2.8 200-60

Nylon reinforced 70-210 2-10 10-1

Composite Classification

Matrix The material that surrounds the other component Provides the bulk form of the part Hold the reinforcing phase in place

Reinforcing phase The embedded material may be metal, ceramic, plastic Usually provides the strength

In terms of its effect, the shape of the reinforcing material is of particular importance.

Matrix Function

To support the fibers in place Transfer stresses to them Let them carry the tensile load

To protect the fibers Physical damage Environment

Reduce crack propagation Greater matrix ductility (sort of, usually)

Reinforcing phase

Fibers Continuous

Very long Discontinuous

L/D about 100 Whiskers

Hair like single crystals with diameters down to about 40 x 10-6 in. Very strong.

Fibers of choice

Glass – cheapest and most widely used Glass fiber reinforced plastic (GFRP) Made by drawing molten glass through a platinum spinneret. Types

E – glass (calcium aluminosilicate glass) S – glass (magnesia aluminosilicate glass)

Fibers of choice

Graphite – more expensive High strength, stiffness and low density Carbon fiber reinforced plastic (CFRP) Made by pyrolysis of organic precursors – usually polyacrylonitrile

(PAN) or pitch Two kinds of fibers

Carbon (80 – 95% carbon) Lower modulus and strength

Graphite (> 99% carbon) Crystalline and very high modulus and strength

Fibers of choice

Aramids (among the toughest fiber) Kevlar® is the best example Have some elongation before rupture (~3%) so are very tough.

Mechanism used in the bullet proof vest! Absorb moisture which can degrade properties.

Fibers of choice

Boron Formed by chemical vapor

deposition onto tungsten fibers. Very strong and stiff both in

tension and compression However, very heavy because of

the tungsten ( = 19.3 g/cm3)

Tungsten

Particles and Flakes

Orientation in the matrix is usually quite random Properties therefore are isotropic Called fillers

Crack stoppers

Effect of Fiber Type on Properties

Mechanical and physical properties depend on the reinforcing mediums

Kind Shape Orientation

Short fibers are less effective than long fibers

Fiber Matrix Bond - Plastics

Strength of the fiber matrix bond is critical The load is transmitted through the fiber – matrix interface

Weak bonding can cause Fiber pullout delamination

Strength as a function of fiber direction and content

In general… The highest stiffness and strength

is obtained when the fibers are aligned in the direction of the tension force.

Cool…. But, there is a caveat

This makes the composite very anisotropic.

Strength as a function of fiber direction and content

Result of anisotropy… Other properties are anisotropic

Stiffness Creep Thermal & electrical conductivity Thermal expansion

Example – fiber reinforced packaging tape

Strong in the fiber direction Easily pulled apart in the width

direction

Reinforcing Fibers

Fiber Density g/cm3) Young’s Modulus (GPa)

Tensile Strength (GPa)

Steel 7.83 210 2.1

Tungsten 19.3 350 4.2

Beryllium 1.84 300 1.3

E-Glass 2.48 75 3.5

S-Glass 2.54 85 4.6

Alumina 3.15 320 2.1

SiC 3.0 400 2.8

Boron 2.6 420 4.0

High Modulus C 1.9 390 2.1

High Strength C 1.9 240 4.0

Kevlar® 29 1.44 83 2.8

Kevlar® 49 1.44 130 3.2

The Rule of Mixturesc = composite

r = reinforcing phasem = matrix

The mass of the composite body is: c r mm m m

Equivalently: c c r r m mV V V

r r m mc

If we let: the volume fraction of the reinforcing materialr

sincec r mV V V 1m

The Rule of Mixtures

We get: (1 )c r mf f

This is the Rule of Mixtures

Each component contributes to the propertiesOf the composite in proportion

To its VOLUME fraction

Fiber reinforcement

When the filler is in the form of thin fibers strongly bonded to the matrix

Properties depend on the fiber and the amount present There is a critical minimum volume fraction There is a critical length

Fiber should be continuous of If chopped long enough to accept stresses transferred from the

matrix

Fiber reinforcement

If we use the criteria It takes more force to shear the matrix at the fiber boundary (pull out

the fiber) than to break the fiber

We get an approximate critical length lcr.

D = fiber diameterT = fiber tensile strengthi = interfacial shear strength

Tensile Strength

Unidirectional Composites Complex

Upon loading a single fiber breaks first The aim is to neutralize the effect of local failure.

Matrix should be ductile enough to not propagate a crack Matrix should be able to carry the shear stress on the fiber matrix

interface. The rule of mixtures is helpful but actual design is made using

actual statistical properties.

Long Fiber Composites

Rule of mixtures works in the longitudinal direction.

E 1c r r m m

r r r m

E f E f E

f E f E

Perpendicular to the

longitudinal direction

' m rc

m r r m

f E f E

Example

Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7

GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the longitudinal direction?

(1 )Lc f mE f E f E

0.26 75 0.74 2.7LcE GPa GPa

21.498LcE GPa

Example

Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7

GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the transverse direction?

(1 )f m

f E f E

75 2.7

0.26 75 0.74 2.7Tc

GPa GPaE

GPa GPa

3.6TcE GPa

Recall

21.498LcE GPa

Other Composite Types

A) conventional laminar structure

B) sandwich with a foam core C) sandwich structure using a

honeycomb

B and C gain stiffness using an increase in the moment of inertia

Applications of Reinforced Plastics

1907 - First application was for an acid resistant tank made of a phenolic resin and asbestos.

1920’s – Formica, used for counter tops 1930’s – Advent of epoxy as a reinforcing material 1940’s – fiberglass/epoxy boats, some aircraft, sporting

goods 1970’s – beginning of “Advanced Composites” using hybrid

plastics and carbon fibers.

Applications of Reinforced Plastics

Aircraft (DC-10, L-1011, 727, 757, 767, 777) The Boeing 777 is about 9% (weight) composites

Floor beams and panels Most of the vertical and horizontal tail

The Lear Fan 2100 passenger aircraft structure is almost all graphite/epoxy.

90% of the world circling Voyager is plastic composites The stealth bomber is made of carbon and glass fibers, epoxy resin

matrices, high temperature polyimides (and other neat stuff).

Composite Sailboard

K. Easterling, Tomorrow’s Materials, p.133, Institute of Metals, 1990

Metal Matrix Composites

The matrix is usually a low density metal, primarily Aluminum Aluminum - lithium Magnesium Copper titanium

The reinforcement is often SiC, Al2O3, or carbon

MMCsFiber Matrix Applications

Graphite Aluminum

Magnesium

Copper

Satellite, missile, and helicopter structures

Space and satellite structures

Storage- battery plates

Electrical contacts and bearings

Boron Aluminum

Magnesium

Titanium

Compressor blades and structural supports

Antenna structures

Jet-engine fan blades

Alumina Aluminum

Magnesium

Superconductor restraints in fission power reactors

Storage-battery plates

Helicopter transmission structures

Silicon Carbide Aluminum

Super alloy

High-temperature structures

High-temperature engine components

Molybdenum

Tungsten

Superalloy High-temperature engine components

Ceramic Matrix Composites

In polymer matrix composites the reinforcement is always stronger and of much higher elastic modulus than the matrix

Thus a significant increase in strength can be had by transferring stresses from the matrix to the fiber through a strong interface.

This is also true of MMC (for low elastic moduli material such as Al, Mg)

Ceramic already have a high elastic modulus (except glasses) so the purpose of a CMC is to increase toughness.

MonolithicCeramics

Fail completelyIn

BrittleCatastrophic

Ceramic Matrix Composites

Matrix materials Silicon carbide Silicon nitride Aluminum oxide Mullite (aluminum, silicon oxides)

Ceramics are strong and stiff, retained at high temperatures but are brittle.

CMC Toughness

(A) Crack Deflection A crack meeting the

reinforcement is deflected along the interface where energy is used to effect separation

(B) Crack Propagation Barrier Reinforcement can force the

crack to bow out, increasing the stress necessary for propagation

CMC Toughness

(C) Fiber Bridging Sometimes fibers bear the load

across the crack, putting the crack in compression

(D) Fiber Pull-out Most important, energy is used

in pulling the fiber out. The interfacial bond strength

must not be so high that the fiber breaks rather than pulling out.

Composites Module 4a. Spring 2001 ISAT 430 Dr. Ken Lewis2 An aside: Stress – Strain Tension test...

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