Fatigue Behavior of Composite Laminates

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. SCOPE NIDIRECTIONAL CONTINUOUS fiber composites are known to be highly in-Usensitive to fatigue in the fiber direction. The excellent fatigue resistance can be easily understood if one notes that the load in unidirectional composites are mainly carried by fibers and also that fibers such as boron and graphite exhibit excellent resistance to fatigue. In actual applications composites are mostly used in the form of laminates. Because of difference in orientation. in each ply, some plies are weaker in the loading direction than the others and fail early before the final fracture [1, 2, 3, 4]. Here ply failure is physically defined to be the appearance of micro-cracks within a ply. These cracks originate at locations of high stress concentration and low strength. In homogeneous materials a crack, once it is formed, rapidly grows leading to final fracture. In composites, however, even visible cracks do not lead to immediate fracture; rather, they may even grow in such a way as to relieve stress concentration on fibers. Further, since they occur throughout the volume of weak plies in a laminate, decrease of modulus may also result. J. COMPOSITEMATERIALS, Vol. 10, (April 1976), p. at PENNSYLVANIA STATE UNIV on May 17, 2016jcm.sagepub.comDownloaded fro