Composite Engineering Materials Review

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Composite Engineering Materials Review

A composite is a material consisting of two (or more) different materials bonded together, one forming a ‘matrix' in which are embedded fibers or particles that increase the strength and stiffness of the matrix material.

A natural composite is wood in which cellulose fibers are embedded in a lignin matrix. Concrete is a composite in which particles of stone add strength with a further increase in strength provided by steel reinforcing rods. Vehicle tires consist of rubber reinforced with woven cords.

Plastics are reinforced with glass, carbon and other fibers. The fibers may be unidirectional, woven or random chopped. Metals, carbon and ceramics are also used as matrix materials.

So-called ‘whiskers', which are single crystals of silicon carbide, silicon nitride, sapphire, etc., give extremely high strength.

Typical engineered composite materials include:

Composite building materials such as cements, concrete
Reinforced plastics such as fiber-reinforced polymer
Metal Composites
Ceramic Composites (composite ceramic and metal matrices)

Composite materials are generally used for buildings, bridges and structures such as boat hulls, swimming pool panels, race car bodies, shower stalls, bathtubs, storage tanks, imitation granite and cultured marble sinks and counter tops. The most advanced examples perform routinely on spacecraft in demanding environments.

Elastic modulus of a composite (continuous fibers in direction of load)

Let:
E_f = modulus of fibers
E_m = modulus of matrix
E_c = modulus of composite
r = (cross-sectional area of fibers) / (total cross sectional area)
E_c = r E_r + ( l - r ) E_m

Acronyms for composites

FRP fiber-reinforced plastic
FRT fiber-reinforced thermoplastic
GRP Glass-reinforced plastic
GRC Glass-reinforced composite
CFC Carbon fiber composite
CFRP Carbon-fiber-reinforced plastic
CFRT Carbon-fiber-reinforced thermoplastic

Forms of fibers for composites

Fiber: length over 10 times the diameter; diameter less than 0.25 mm.

Filament: a continuous fiber.
Wire: a metallic fiber
Whisker: a fiber consisting of a single crystal.

Arrangement of fibers in composites

Type	Arrangement	Comments
Unidirectional		Load taken in direction of fibers. Weak at right angles to fibers
Bidirectional		Takes equal load in both directions. Weaker since only half the fibers used in each direction
Multidirectional		Load capacity much reduced but can take load in any direction in plane of fibers
WovenMat		Similar to bidirectional however easy to handle
Random, Chopped		Low in strength but multidirectional. Has handling advantages

Properties of some fibers, wires and whiskers used in composites

Material	Type	Density (kgm^-3)	Young's Modulus (GNm^-2)	Tensile Strength (Nmm^-2)	Filament diameter (µm)
E Glass	Fiber	2,500	62	3,500	205
Carbon	Fiber	2,500	415	1,750	705
Silica	Fiber	2,500	72	6,000	5.0
18/8 Stainless Steel	Wire	7,900	205	2,100	150
Tungston	Wire	19,300	350	2,900	150
Tungston	Wire	19,300	350	3,800	25
Graphite	Whisker	2,200	675	21,000	-
Sapphire	Whisker	4,000	525	6,000	-
Silicon Carbide	Whisker	3,200	690	21,000	-
Silicon Nitride	Whisker	3,100	380	14,000	-