Effect of Ply Constraint on Fatigue Damage Development in Composite Material Laminates Available to Purchase

Laminae of composite materials are bonded together to form a laminate for the purpose of achieving strength and stiffness in certain desired directions. Although a direct use of laminae strength and stiffness properties can be used to determine the properties of an undamaged laminate, it is well known that the manner in which local damage initiates, grows, and combines is not the same in an unconstrained lamina as it is when the same lamina is constrained by other laminae in a laminate. This paper addresses the question of constraint effects directly and discusses the consequences of such effects on the development of fatigue damage in graphite epoxy laminates. Results of monotonic tension tests, tension-tension fatigue tests, and nondestructive investigations are reported for several constraint situations and are compared with the results of several analyses.