Residual stresses in composites are induced during fabrication and by environmental exposure. The theory formulated can describe the shrinkage commonly observed after a thermal expansion test. Comparison between the analysis and experimental data for [02 /± 45] s laminates of various material systems indicates that the residual stress-free temperature can be lower than the curing temperature, depending on the curing process. Ef fects of residual stresses on ply failure including the acoustic emission characteristics are discussed.

SUMMARYThe present paper examines compression failure mechanisms in unidirectional composites. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear material properties and initial fiber curvature is proposed.The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant macroscopic-scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, for stiff resins failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a lesser extent, were found to irlcrease with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.

The paper discusses a characterization of fatigue behavior of [0/±45/90] s glass/epoxy laminate in terms of the following parameters: static properties; S-N relationship; reliability; effect of thickness variation; damage initiation and growth; temperature increase; secant modulus change; effect of preloading on residual modulus and strength; and effect of ply failure on compression buckling strength. Some of the findings are the following. The primary failure process responsible for up to about 106 cycles of fatigue life is the wear-out followed by the chance failure. Change of secant modulus can be used as a measure of damage extent. Preloading to a high level has negligible effect on the residual tensile strength when the fatigue stress is low. However, ply failure and partial delamination result in a moderate loss of compression buckling strength.

The paper presents a concept of proof testing for composite materials. A unique relationship between the static strength and time to rupture is demonstrated for a unidirectional glass/epoxy composite subjected to static fatigue. The maximum significance level at which a Weibull distribution is applicable to represent scatter is 35% for the static strength and 61% for the fatigue life. Limitations and variations of the strength degradation model for the life prediction are discussed.

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