The success of any progressive failure analysis of composite structures is influenced by the failure criteria and the associated material property degradation models. The failure criteria are the conditions for the prediction of the occurrence of material damage. The degradation models are mathematical representations of the residual properties for each material damage state predicted by the failure criteria. A brief summary of the major classes of failure criteria pertaining to the degradation models is followed by a review of degradation models that have been developed for unidirectional polymer matrix composite laminates. The review is organized around the relationships of the various models to associated failure criteria as well as the various constitutive frameworks for finite element implementation. Models that invoke residual properties as a one-time sudden degradation of the original properties are described followed by models where the mathematical representation of at least one property invokes gradual property degradation as a function of some other evolving field variable.

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In this paper, a micromechanical model of a composite lamina material with fiber waviness is described. Results are presented and discussed with regard to stiffness and strength predictions for composite lamina. A micromechanical model of a unit cell from periodically distributed unidirectional waved cylindrical fibers embedded within matrix is proposed to withdraw the different material stiffness parameters. Finite element analysis of the periodic unit cell characterizing the structural stiffness of the composite material is carried out to determine the average stress and strain components. The composite stress-strain relations are then employed to determine the stiffness parameters. Numerical results for a typical composite constituted of polymer matrix and carbon fibers in the form of periodically hexagonal packing and initially sinusoidal waviness are presented for different amplitude to wavelength ratios and a range of fiber volume fractions. The results reveal the presence of local periodic-antisymmetric stresses that are usually unaccounted for in conventional structural analysis. The potential influence of these stresses on failure prediction is discussed.