Internal Load Distribution Effects During Fatigue Loading of Composite Laminates

Two fundamental processes govern the reduction of strength and stiffness of composite laminates caused by cyclic loading, commonly called the fatigue effect. One process is the reduction of the strength and stiffness ofthe individual constituent materials in the composite, caused by events such as localized nonconservative deformation, microcrack formation, and substructure variations. This process is essentially the source of the fatigue effect homogeneous materials, except that macrocrack formation and fracture dominated by a single crack generally does not occur as a consequence of this process in composite materials. In laminated composites there is another fundamental process which plays a major role in determining residual properties. That process is the damage-dependent redistribution of load sharing among the plies of the laminate caused by the continuous variation of the elastic stiffness properties of each ply induced by microdamage development in those plies. Experience suggests that this process is critical and, in some cases, dominant in the determination of the residual strength and life of a laminate under cyclic loading. This paper addresses the experimental and analytical aspects of this fundamental process. The results of the present work indicate that strain (and stress) redistributions in regions of highly localized damage are large and significant. The general nature of these redistributions has been established, which provides the first firm foundation for the formulation of the philosophy needed to interpret these physical damage states in terms of residual strength and life.