Characterization and control of the fiber-matrix interface in ceramic ...

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Equation (6) shows that the strength of the composite is determined by
the ultimate strength of the fiber reinforcement and not simply matrix
failure stress.
The previous equations describing the elastic behavior of a
composite assume that the fibers are continuous and unidirectionally
aligned in the direction o f the applied stress.
An allowance can be made
for fibers that are not aligned in the stress field (50,51). The
contribution of a fiber is obviously reduced as it is rotated out of the
direction of the applied stress or strain. A correction factor X can he
added to the equations for stress and modulus. The value sf X is
approximately 0.38 for a composite containing randomly oriented fibers
lying in one plane (two-dimensional reinforcement).
Accounting for the
misalignment of the reznforcement, the expressions describing stress in a
composite becomes
Many ceramic matrix composites contain residual porosity that OCCU~S
during processing. Porosity reduces both strength and modulus in
ceramics (54-56).
The presence of pores in a structure reduces cross-
sectional area exposed to a load, and the voids act: as stress
concentrators.
The strength of a ceramic body decreases exponentially
with increased porosity. The equation,