Long-Term Fatigue Behavior of Composite Materials Available to Purchase

There is now a significant body of literature that provides a fairly detailed account of the types of damage that commonly develop during the tensile and compressive fatigue loading of high-modulus fibrous composite laminates. Although this body of information is by no means complete, many detailed descriptions of microevents that accompany such loadings are available, and some models have been developed, including successful attempts to represent and predict the stiffness changes which accompany certain types of damage. However, at least two basic and essential questions concerning such fatigue response remain unanswered. First, exactly how does damage reduce laminate strength during long-term fatigue loading? And, second, how does damage (during a long-term fatigue test) combine to cause the fracture event which defines the life of a laminate? The present paper addresses these two questions. Laminates of several types and materials were loaded in cyclic tension or compression at load levels which corresponded to long times to failure. At these “high cycle” levels, the reduction in residual strength was maximized. The corresponding damage states were monitored using an array of nondestructive and destructive techniques. Special attention was given to the damage states that developed just prior to failure. Various concepts and models were applied to the interpretation of the data. Damage development in tension and compression was contrasted.