Deformational Behavior of a Unidirectional Graphite/Epoxy Composite Under Compressive Fatigue

Abstract

Deformations of a unidirectional lamina, loaded in compressive-compressive fatigue in the fiber direction, perpendicular to the fibers, and in shear, were recorded continuously during fatigue tests. The deformations were separated into cyclic (AC) and creep (DC) components, and these values were used to define AC and DC moduli for each of the three modes of loading. The phase between stress and strain was monitored as well. The response to loading in the fiber direction was elastic, with negligible nonlinearity. Perpendicular to the fiber direction, the specimens crept slightly during the initial phase of the loading, and this was accompanied by a minor increase in dynamic stiffness. The shear loading caused large creep deformation, similar to creep observed in metals, and large change in phase, but the change in dynamic stiffness was minor. The results of creep are needed for modelling load transfer between layers in laminated composites, and the measurements of dynamic modulus have a potential for being a nondestructive test method for detecting fatigue life limit.