A hysteresis-based damage parameter for notched composite laminates subjected to cyclic loading

With the increasing use of polymer matrix composites in aircraft primary structure, it is critical that damage propagation during operation is accurately detected and monitored. Unlike metals, composite materials can sustain damage that is not readily detectable by visual or nondestructive inspection. Fiber optic strain sensors have performed satisfactorily in laboratory specimens but require the installation of a large number of sensors in real structures to monitor localized damage zones. In addition to complexity and installation issues, strain sensors need to be located near potential areas of damage that are not easy to determine a priori. This paper introduces a more sensitive damage parameter that can be readily measured in practice and takes into account the varied damage mechanisms present in composites. These include matrix crazing, microcracking, fibers breakage, delamination, interfacial failure and fiber pullout.