Callister - An introduction - 8th edition

Summary • 665
When l l c , Equation 16.18 is valid.
When l l c , it is appropriate to use Equation 16.19.
• The elastic modulus for discontinuous and randomly oriented fibrous composites
may be determined using Equation 16.20.
The Fiber Phase
• On the basis of diameter and material type, fiber reinforcements are classified as
follows:
Whiskers—extremely strong single crystals that have very small diameters
Fibers—normally polymers or ceramics that may be either amorphous or
polycrystalline
Wires—metals/alloys that have relatively large diameters
The Matrix Phase
• Although all three basic material types are used for matrices, the most common
are polymers and metals.
• The matrix phase normally performs three functions:
It binds the fibers together and transmits an externally applied load to the
fibers.
It protects the individual fibers from surface damage.
It prevents the propagation of cracks from fiber to fiber.
• Fibrous reinforced composites are sometimes classified according to matrix type;
within this scheme are three classifications: polymer-, metal-, and ceramic-matrix.
Polymer-Matrix Composites
• Polymer-matrix composites are the most common; they may be reinforced with
glass, carbon, and aramid fibers.
Metal-Matrix Composites
• Service temperatures are higher for metal-matrix composites than for polymermatrix
composites. MMCs also use a variety of fiber and whisker types.
Ceramic-Matrix Composites
• With ceramic-matrix composites, the design goal is increased fracture toughness.
This is achieved by interactions between advancing cracks and dispersed phase
particles.
• Transformation toughening is one such technique for improving K Ic .
Carbon–Carbon Composites
• Carbon–carbon composites are composed of carbon fibers embedded in a pyrolyzed
carbon matrix.
• These materials are expensive and used in applications requiring high strengths
and stiffnesses (that are retained at elevated temperatures), resistance to creep,
and good fracture toughnesses.
Hybrid Composites
• The hybrid composites contain at least two different fiber types. Using hybrids it
is possible to design composites having better all-around sets of properties.