Kuliah_12_kxex 1110

CHAPTER

12 Composite

8-1

Introduction

• Every material is composite at one or the other level. • A composite material is a material system, a mixture or combination of two or more micro or macroconstituents that differ in form and composition and do not form a solution. • Properties of composite materials can be superior to its individual components. • Examples: Fiber reinforced plastics, concrete, asphalt, wood etc.

8-2

C o m p o s it e s P a r t ic le - r e in fo r c e d L a rg e p a r t ic le

D is p e r s io n s tre n g th e n e d

F ib e r - r e in fo r c e d C o n t in u o u s ( a lig n e d )

S tru c tu ra l

D is c o n t in u o u s (s h o rt) A lig n e d

R a n d o m ly o r ie n t e d

L a m in a t e s

S a n d w ic h p a n e ls

Terminology/Classification • Composites: -- Multiphase material w/significant proportions of each phase.

woven fibers

• Matrix:

-- The continuous phase -- Purpose is to: - transfer stress to other phases - protect phases from environment

-- Classification: metal

MMC, CMC, PMC

ceramic

0.5mm cross section view

polymer

• Dispersed phase: -- Purpose: enhance matrix properties. MMC: increase σy, TS, creep resist. CMC: increase Kc PMC: increase E, σy, TS, creep resist.

-- Classification: Particle, fiber, structural

0.5mm

Particle-reinforced • Examples: - Spheroidite matrix: ferrite (α) steel

Fiber-reinforced

(ductile)

60 µm

- WC/Co cemented carbide

matrix: cobalt (ductile) Vm : 10-15 vol%!

Structural particles: cementite (Fe3 C) (brittle)

Adapted from Fig. 10.19, Callister 7e. (Fig. 10.19 is copyright United States Steel Corporation, 1971.)

particles: WC (brittle, hard)

Adapted from Fig. 16.4, Callister 7e. (Fig. 16.4 is courtesy Carboloy Systems, Department, General Electric Company.)

600 µm

- Automobile matrix: rubber tires

particles: C (stiffer)

(compliant) 0.75 µm

Adapted from Fig. 16.5, Callister 7e. (Fig. 16.5 is courtesy Goodyear Tire and Rubber Company.)

Concrete – gravel + sand + cement - Why sand and gravel?

Sand packs into gravel voids

Reinforced concrete - Reinforce with steel rerod or remesh - increases strength - even if cement matrix is cracked

Pre-stressed concrete - remesh under tension during setting of concrete. Tension release puts concrete under compressive force - Concrete much stronger under compression. - Applied tension must exceed compressive force

Post tensioning – tighten nuts to put under tension nut

threaded rod

• Elastic modulus, Ec, of composites: -- two approaches. E(GPa) 350 Data: Cu matrix 30 0 w/tungsten 250 particles 20 0 150 0

upper limit: “rule of mixtures” Ec = VmEm + VpEp

(Cu)

lower limit: 1 Vm Vp = + Ec Em E p 20 40 60 80

10 0 vol% tungsten

(W)

• Application to other properties: -- Electrical conductivity, σe: Replace E in equations with σe. -- Thermal conductivity, k: Replace E in equations with k.

Particle-reinforced

Fiber-reinforced

Structural

• Fibers very strong – Provide significant strength improvement to material – Ex: fiber-glass • Continuous glass filaments in a polymer matrix • Strength due to fibers • Polymer simply holds them in place

• Fiber Materials – Whiskers - Thin single crystals - large length to diameter ratio • graphite, SiN, SiC • high crystal perfection – extremely strong, strongest known • very expensive – Fibers • polycrystalline or amorphous • generally polymers or ceramics • Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE – Wires • Metal – steel, Mo, W

Fiber Alignment

aligned continuous

aligned random discontinuous

• Aligned Continuous fibers • Examples: -- Metal: γ'(Ni3Al)-α(Mo)

-- Ceramic: Glass w/SiC fibers formed by glass slurry

by eutectic solidification.

matrix: α (Mo) (ductile)

Eglass = 76 GPa; ESiC = 400 GPa.

(a)

2 µm

fibers: γ ’ (Ni3Al) (brittle)

(b)

fracture surface From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig. 11.20, p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Used with permission of CRC Press, Boca Raton, FL.

• Discontinuous, random 2D fibers • Example: Carbon-Carbon -- process: fiber/pitch, then burn out at up to 2500ºC. -- uses: disk brakes, gas turbine exhaust flaps, nose cones.

(b)

(a)

• Other variations: -- Discontinuous, random 3D -- Discontinuous, 1D

C fibers: very stiff very strong C matrix: less stiff view onto plane less strong fibers lie in plane

• Critical fiber length for effective stiffening & strengthening: fiber strength in tension

σf d fiber length > 15 τc

fiber diameter shear strength of fiber-matrix interface

• Ex: For fiberglass, fiber length > 15 mm needed • Why? Longer fibers carry stress more efficiently! Shorter, thicker fiber:

σd fiber length < 15 f τc σ(x)

Longer, thinner fiber:

fiber length > 15

σf d τc

σ(x)

Adapted from Fig. 16.7, Callister 7e.

Poorer fiber efficiency

Better fiber efficiency

Composite Strength: Longitudinal Loading

Continuous fibers - Estimate fiber-reinforced composite strength for long continuous fibers in a matrix • Longitudinal deformation

∴

σ c = σ mVm + σ fVf

but

Ece = Em volume Vm + E fVf fraction

longitudinal (extensional) isostrain modulus

Ff EfVf = Fm E mVm

εc = εm = εf

f = fiber m = matrix

Composite Strength: Transverse Loading

• In transverse loading the fibers carry less of the load - isostress σc = σm = σf = σ ∴

1 Vm Vf = + Ect E m Ef

ε c= ε mVm + ε fVf transverse modulus

Equation for Elastic Modulus of Lamellar Composite •

Isostrain condition: Stress on composite causes uniform strain on all composite layers. Pc = Pf + Pm Pc = Load on composite Pf = Load on fibers Pm = load on matrix

σ = P/A σcAc = σfAf + σmAm

Figure 11.14 Since length of layers are equal, σcVc = σfVf + σmVm Where σ Vc, Vfσand V are volume V m σ V f f c m m = + fractions (Vc =1) εc εf εm Since strains εc = εf = εm, Rule of mixture of binary composites Ec = EfVf + EmVm

Loads on Fiber and Matrix Regions •

Since σ = Eε and εf = εm

Pf Pm

=

σ f Af σ m Am

=

E f ε f Af E m ε m Am

=

E f Af E m Am

=

EfVf E mVm

Pc = Pf + Pm •

From above two equations, load on each of fiber and

Isostress Condition •

Stress on the composite structure produces an equal stress condition on all the layers. σc = σf + σm εc = εf + εm

Assuming no change in area and assuming unit length of the composite εc = εfVf + εmVm But Therefore

σ σ σ εc = ,ε f = ,ε m = Ec Ef Em

σ σV f σVm = + Ec Ef Em

Figure 11.15

Elastic Modulus for Isostress Condition •

We know that

σ σV f σVm = + Ec Ef Em

• Dividing by σ V f Vm 1 = + Ec E f Em V f E m Vm E f 1 = + Ec E f Em Em E f

Ec =

E f Em V f E m + Vm E f

Figure 11.16

• Higher modulus values are obtained with isostrain loading for equal volume of fibers

• Estimate of Ec and TS for discontinuous fibers: σf d -- valid when fiber length > 15 τc

-- Elastic modulus in fiber direction:

Ec = EmVm + KEfVf efficiency factor: -- aligned 1D: K = 1 (aligned ) -- aligned 1D: K = 0 (aligned ) -- random 2D: K = 3/8 (2D isotropy) -- random 3D: K = 1/5 (3D isotropy)

-- TS in fiber direction:

(TS)c = (TS)mVm + (TS)fVf

(aligned 1D)

Particle-reinforced

Fiber-reinforced

• Stacked and bonded fiber-reinforced sheets -- stacking sequence: e.g., 0º/90º -- benefit: balanced, in-plane stiffness

• Sandwich panels -- low density, honeycomb core -- benefit: small weight, large bending stiffness face sheet adhesive layer honeycomb Adapted from Fig. 16.18, Callister 7e. (Fig. 16.18 is from Engineered Materials Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.)

Structural

Polymer Matrix Composite (PMC)

• Glass fiber reinforced plastic composite materials have high strength-weight ratio, good dimensional stability, good temperature and corrosion resistance and low cost.  ‘E’ Glass : 52-56% SiO2, + 12-16% Al2O3, 16-25% CaO + 8-13% B2O3  Tensile strength = 3.44 GPa, E = 72.3 GPa

 ‘S” Glass : Used for military and aerospace application.  65% SiO2 + 25% Al2O3 + 10% MgO  Tensile strength = 4.48 GPa, E = 85.4 GPa

• Produced by drawing monofilaments from a furnace and gathering them to form a strand. • Strands are held together with resinous binder. • Properties: Density and strength are lower than carbon and aramid fibers. • Higher elongation. • Low cost and hence commonly used.

Fiber Reinforced-Plastic Composite Materials

• Fiberglass-reinforced polyester resins:  Higher the wt% of glass, stronger the reinforced plastic is.  Nonparallel alignment of glass fibers reduces strength.

• Carbon fiber reinforced epoxy resins:  Carbon fiber contributes to rigidity and strength while epoxy matrix contributes to impact strength.  Polyimides, polyphenylene sulfides are also used.  Exceptional fatigue properties.  Carbon fiber epoxy material is laminated to meet strength requirements.

Properties of Fiber Reinforced Plastics Fiberglass polyester

(Carbon fibers and epoxy)

Carbon Fibers for Reinforced Plastics • • • •

Light weight, very high strength and high stiffness. 7-10 micrometer in diameter. Produced from polyacrylonitrile (PAN) and pitch. Steps:  Stabilization: PAN fibers are stretched and oxidised in air at about 2000C.  Carbonization: Stabilized carbon fibers are heated in inert atmosphere at 1000-15000C which results in elimination of O,H and N resulting in increase of strength.  Graphitization: Carried out at 18000C and increases modulus of elasticity at the expense of strength

•

Tensile strength = 3.1-4.45 GPa, E = 193-241 GPa, density = 1-7-2.1 g/cc.

Aramid Fibers for Reinforcing Plastic Resins • •

Aramid = aromatic polyamide fibers. Trade name is Kevlar  Kevlar 29:- Low density, high strength, and used for ropes and cables.  Kevlar 49:- Low density, high strength and modulus and used for aerospace and auto applications. Table 11.1

• •

Hydrogen bonds bond fiber together. Used where resistance to fatigue, high strength and light weight is important.

Fatigue Characteristics of Fiber Reinforced Plastics

Lamination

Ceramic-Matrix Composites (CMCs) •

•

Continuous fiber reinforced CMCs:  SiC fibers are woven into mat and SiC is impregnated into fibrous mat by chemical vapor deposition.  SiC fibers can be encapsulated by a glass ceramic.  Used in heat exchanger tube and thermal protection system. Discontinuous and particulate reinforced CMCs:  Fracture toughness is significantly increased.  Fabricated by common process such as hot isolatic pressing.

Portland Cement •

Production: Lime (CaO), Silica (SiO2), alumina(Al2O3) and iron oxide (Fe2O3) are raw materials.

•

Raw materials are crushed, ground and proportional for desired composition and blended. • Mixture is fed into rotary kiln and heated to 1400-16500C and then cooled and pulverized. • Chemical Composition:

Types of Portland Cement • •

• • • •

Types of Portland cement differ by composition. Type I: Used when high sulfate attack from soil and water, and high temperature are absent.  Examples: Sidewalks, buildings, bridges. Type II: Used in case of moderate sulfate attack as in case of drainage. Type III: Early strength type for quick use. Type IV: Low heat of hydration type and used when rate and heat generated must be minimized. Type V: Used for heavy sulfate attack as in case of groundwater.

Hardening of Portland Cement • Tricalcium silicate and dicalcium silicate constitute 75% of portland cement. • Hydration reactions: 2C3S + H2O

C3S2.3H2O + 3Ca(OH)2

2C2S + 4H2O

C3S2.3H2O + Ca(OH)2

Tricalcium silicate hydrate • C3S is responsible for early strength. • Most of compressive strength is developed in 28 days. • Strengthening might continue for years

Water Aggregate and Air •

•

•

Drinking and Non-Drinking water can be used.  Non-drinking water should be tested for level of impurities. Aggregates make up 60-80% of concrete volume.  Fine aggregates are of sand particles and coarse aggregates are rocks. Air entraining agents are sometimes added.  They increase resistance to freezing and thawing and improved workability.

Compressive Strength •

Compressive strength is higher than tensile strength and depends up on settled time. • High water content reduces compressive strength. • Air entrainment improves workability and hence water content can be reduced. Air Bubbles

Toughening Mechanisms in CMCs • Toughening is due to fibers interfering with crack propagation.  Crack deflection: Up on encountering reinforcement, crack is deflected making propagation more meandering.  Crack bridging: Fibers bridge the crack and help to keep the cracks together.  Fiber pullout: Friction caused by pulling out the fiber from matrix results in higher toughness.

Metal Matrix Composites (MMCs) •

Continuous fiber reinforced MMCs: Continuous fibers are reinforced in metal matrix – used in aerospace, auto industry and sports equipments. • Example:- Aluminum alloy – Boron fiber composite  Boron fiber is made by depositing boron vapor on tungsten substrate.  Boron fibers are hotpressed between aluminum foils.  Tensile strength of Al6061 increases from 310 to 1417GPa and E increases from 69 to 231 GPa Tungsten filament

Boron

Discontinuous fiber and particulate reinforced MMCs •

Particulate reinforced MMCs: Irregular shaped alumina and silicon carbide particulate are used.  Particulate is mixed into molten aluminum and cast into ingots or billets.  Al 6061 + 20% SiC

•

Tensile strength increased to 496 MPa ‘E’ increased to 103 GPa

Discontinuous fiber reinforced MMcs: Needle like SiC whiskers (1-3 micron diameter, 20-200 micron in length) are mixed with metal powder.  Mixture is consolidated by hot pressing and then forged or extruded.  Tensile strength of Al 6061 increases to 480 MPa and ‘E’ increases to 115 GPa

Composite Benefits • CMCs: Increased toughness Force

103

particle-reinf

1

un-reinf

10 -4 6061 Al εss (s-1) 10 -6

10 -8 10 -10

metal/ metal alloys

.1 G=3E/8 polymers .01 K=E .1 .3 1 3 10 30 Density, ρ [mg/m3]

Bend displacement

Increased creep resistance

ceramics

E(GPa) PMCs 102 10

fiber-reinf

• MMCs:

• PMCs: Increased E/ρ

6061 Al w/SiC whiskers

20 30 50

Adapted from T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp. 139-146, 1984. Used with permission.

σ(MPa) 100 200

Open Mold Process for Fiber Reinforced Plastics •

Hand lay-up process:  Gel coat is applied to open mold.  Fiberglass reinforcement is placed in the mold.  Base resin mixed with catalysts is applied by pouring brushing or spraying.

•

Spray-up process: Continuous strand roving is fed by chopper and spray gun and chopped roving and catalyst resin is deposited in the mold.

Vacuum Bag-Autoclave and Winding Vacuum Bag-Autoclave andFilament Filament Winding •

Vacuum bag-autoclave process:  Long thin sheet or prepeg carbon-fiber epoxy material is laid on the table.  The sheet is cut and laminate is constructed.  Laminate is put in vacuum bag to remove entrapped air and cured in autoclave.

•

Filament winding:  Fiber reinforcement is fed through resin bath and wound around suitable mandrel.  Mandrel is cured and mold part is stripped from mandrel.

Closed Mold Process • Compression and injection molding:  Same as in polymers except that the fiber reinforcement is mixed with resin.

•

Sheet molding compound process:  Highly automated continuous molding process.  Continuous strand fiberglass roving is chopped and deposited on a layer of resin-filler paste.  Another layer of paste is deposited on first layer.  Sandwich is compacted and rolled into rolls.

Sheet Molding • • • • •

The rolled up sheet is stored in a maturation room for 1-4 days. The sheets are cut into proper size and pressed in hot mold (1490C) to form final product. Efficient, quick, good quality and uniformity. Continuous protrusion: Continuous strand fibers are impregnated in resin bath, fed into heated die and drawn. Used to produce beams, channels, and pipes.