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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