A simple equation which can describe the compressive behavior of unidirectional composites has been derived. With this equation, the effects of resin stiffness, fiber anisotropy, fiber misalignment and waviness, fiber/matrix interface, and void content on composite compressive strength can be assessed quantitatively and qualitatively. Furthermore, the equation can help determine the failure modes of composites under compression.

Damage accumulation and cyclic degradation in a random short-fiber SMC com posite subjected to tensile fatigue loading are studied. Fatigue damage in various forms of microcracking is examined. The transient nature of the nonlinear, monotonic stress- strain curve is investigated first, and subsequent property degradation and hysteresis loop changes are examined. Contrary to the behavior of certain metals and polymers, a cyclic stable state is never reached in general; cyclic softening is always observed in this class of materials. Owing to the random microstructure of the SMC material, the fatigue damage is viewed as being macroscopically homogeneous and uniform, and the damage growth is treated in a continuous sense. A parameter is then introduced to define the degree of the homogeneous damage. A power-law relationship among the rate of damage evolution, loading variables, and cyclic history is established. The homogeneous fatigue damage decreases rapidly with the loading cycle due to combined effects of rapid depletion of microcrack initiation sites and presence of various crack arrest mechanisms.

A study on stiffness degradation of glass fiber-reinforced composite laminates subjected to uniaxial tensile, pure torsional, and biaxial cyclic fatigue loading at cryogenic temperature is presented. The tensorial nature of stiffness change in the composite material is discussed first to establish a basis for fatigue experiments. Uniaxial tensile and pure torsional cryogenic fatigue tests were conducted to provide references for the study of more complicated multiaxial fatigue. Biaxial cryogenic fatigue under cyclic tension-torsion loading was then performed to examine the effect of complex states of cyclic stress. Stiffness deterioration in terms of changes of elastic constants during fatigue is studied for the cases of various loading conditions. The fundamental nature of cryogenic fatigue degradation in the composite material is investigated. Basic damage mechanisms and mechanics associated with the stiffness change during cyclic fatigue are discussed. Implications of the cyclic stiffness degradation on cryogenic fatigue failure and on the design and analysis of cryogenic composite structures and components are also discussed.

The fracture behavior of random chopped-strand, glass-fiber reinforced SMC composites was studied. Experiments were conducted on notched SMC-R50 composite plates subjected to inplane loading. Fracture mechanics analyses based on recently developed conservation laws of solid mechanics were performed to evaluate notch-tip stress intensities in the specimens. Fracture toughnesses, K IQ and K IIQ , of this material under individual mode-I and mode-II loading were determined. Fracture tests and subsequent analyses were also conducted on the notched SMC-R50 subjected to combined-mode loading. Mixed-mode fracture criteria were investigated for the random short-fiber composite. The suitability of the Whitney-Nuismer two-parameter model was also examined for determination of notched SMC composite strength.