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Composites

Instructor: Joshua U. OtaigbeIowa State University

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Goals this Unit

• Survey composite materials– Fiber reinforced materials

» Natural» Synthetic

– Large aggregate composites– Several special types

• Understand properties of composites– Averaging schemes– Mechanical properties

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Definition/Overview

• Materials which involve some combination of two or more components, often from different materials classes

• Usually combine attractive properties from each component to make a product superior to any single component

• We’ll discuss microscopic (not macroscopic) composites

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Classification of composites

(macro)

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Wood - Natural Fiber Composite

• Most composites are patterned after natural structures– (e.g. bone, wood, etc.)

• Wood is the most common structural material (weight used each year exceeds concrete and steel)

• Organic fibers reinforcing natural polymeric matrix

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Wood structure

S h a c k e l f o r d 1 0 -5

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Anisotropy of properties is common in composites

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Fiber configurations

• Continuous fibers– (not necessarily straight)

• Discrete (chopped)• Woven fabric

(Note that the properties will(Note that the properties willtend to be tend to be anisotropicanisotropic becausebecauseof fiber alignment)of fiber alignment)

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Synthetic Fiber-Reinforced Composites

• One of the most common composite types– Micron scale reinforcing fibers (stronger/stiffer)

» Glass fibers

» Higher moduli fibers (“advanced composites”)

– Matrix material is frequently a polymer (weaker/softer/less brittle/inexpensive)

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Fiberglass Fibers

» Glass fiber-reinforced polymer

» Different compositions of glass (consider electrical, thermal, chemical and mechanical properties)

S h a c k e l f o r d 1 0 -1 , 2

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“Fiberglass” Matrix Phase

• The phase surrounding the fibers• Thermosetting polymers

– Epoxies, polyesters, silicones

• Thermoplastic– Nylon, polystyrene, polycarbonate

• Vital that there be a strong interfacial bond between fibers and matrix to transfer loads effectively between phases

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Advanced composites

• Higher moduli fibers than glass• Matrix can be polymer, ceramic or metal

– PMCs, CMCs, MMCs• Many materials were developed for military

applications (e.g. “stealth” structures)• Conversion to other markets

– Marine, aviation, civil engineering structures, automotive components, sporting goods (costs still quite high)

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Property Averaging

• Properties of composite represent some average of the constituent properties

• The “average” is extremely sensitive to geometry– parallel to fibers– perpendicular to fibers– in a uniformly dispersed aggregate

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Composite properties are intermediate between two materials

continuous/aligned E glass in epoxy

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Fiber reinforcement sizes

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Loading/alignment nomenclature

continuous/aligned

discontinuous/

alignedrandom

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Isostrain condition

Subscripts:

0 = composite

m = matrix

f = fiber

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Isostrain Loading

• Continuous and aligned fiber reinforcement

• Young’s modulus (E)

• Ratio of loads supported

fffm0 VE)V(EE +−= 1

( )( )mm

ff

m

f

VEVE

PP

=

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Effect of fiber length

• There is a critical minimum length of fibers necessary to impart effective strengthening and stiffening to the composite in longitudinal (isostrain)loading

strength) bondmatrix -fiber as (takenmatrix of strengthshear

diameterfiber d

fiber of strength (fracture) tensile

dl

m

f

m

fc

=τ

=

=σ

τ

σ=

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Effect of fiber length

• If l (fiber length) < lc, fiber imparts little benefit• If lc < l < 15 lc, fiber is said to be “short” or

“discontinuous”, but benefit of fiber increases as l increases.

• If l > 15 lc, fiber is said to be “continuous”• lc is typically around 1 mm for many glass and

carbon fiber-matrix combinations

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Fiber loading when l = lc

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Fiber loading when l > lc

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Fiber loading when l < lc

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Discontinuous fibers

• Discontinuous aligned fibers• Longitudinal tensile strength (parallel to fiber

direction)

• For l < lc, case is beyond our scope• For discontinuous random fibers and for

laminated composites, ditto

fails composite when stressmatrix )TS(

strength fracturefiber )TS(

lllfor )V()TS()l

l(V)TS()TS(

'm

f

ccf'm

cff0

=

=

<<−+−= 1512

1

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Anisotropy vs Isotropy

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Isostress Condition

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Isostress Loading

• Continuous and aligned fibers

• Young’s Modulus (E)

or f

f

m

m

EV

EV

E+=

0

1

mfff

fm

EVE)V(EE

E+−

=10

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Isostrain and Isostress Limits

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Interfacial Bond Strength

• In polymer matrix and metal matrix composites (PMCs and MMCs), it is important that interfacial bond strength be high, because fibers are the stronger phase

• In ceramic matrix composites (CMCs), the matrix is strong but brittle, so desirable for interfacial bond strength to be low (“fiber pullout” is desired to increase toughness or resistance to fracture.)

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Strong and weak interfacial bond

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Fiber Pullout Mechanism of Toughening in CMCs

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Toughening of CMCs

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Large Aggregate Composites

• Particles reinforce the matrix phase

• Most common example is concrete

– rock (coarse) and sand (fine) aggregates

» usually chosen from locally available deposits)

– aluminosilicate (cement) matrix

» Portland cement (Ca-aluminosilicates)

• Weight of concrete used each year exceeds that of all metals combined

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Graded Aggregates for Packing Efficiency

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“Large Aggregate” Composites

ppmm0 VEVEE +=

Upper Bound

mppm

pm

EVEVEE

E+

=0

Lower Bound

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Mixture performance boundaries

Tungsten particles in copper matrix

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Mixture relationship for aggregate composites

nhh

nll

n EVEVE +=0

Where the subscript l refers to the phase with lower modulus and h refers to the phase with higher modulus

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Mixture relationship

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Other particulate composites

• SiC in Aluminum

• Tungsten in Copper• Tungsten Carbide in Copper (cutting

tools)

• Dispersion strengthened materials– very fine particles - usually oxides (small

concentration - 15%) in a metal matrix

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Reinforced concrete(A macrocomposite)

• Concrete is very strong in compression• Reinforced with steel bars• Prestressing is also commonly used

– Added bars are held in tension until the concrete hardens and then the stress is released

– This leaves a residual compressive stress

– Most often used for bridges (large unsupported spans in flexure)

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Honeycomb sandwich composite(A macro composite)

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Composites Summary

• Reinforced composite materials include fiber (both natural and synthetic) and particulate (aggregate) reinforced composites

• Property averaging schemes depend on the quantities, shape and orientation of reinforcement phase

* Optional additional reading» http://www.iastate.edu/mse383/index.htm