Fatigue Studies on Impacted and Unimpacted CFRP Laminates

Abstract

Carbon fiber reinforced polymer (CFRP) composites are widely used in safety-critical aerospace structures and experience low velocity impacts, which deteriorate their residual strength and residual life. This paper presents the results of damage accumulation during fatigue loading, estimated through stiffness degradation under constant amplitude and the programmed version of FALSTAFF spectrum loading (Prog FALSTAFF) of [0/90]8 and quasi isotropic CFRP laminates. The laminates are prepared by a hand lay up technique; specimens of size 250 mm by 45 mm by 4.5 mm thickness are extracted from the laminate for testing. Some of the specimens are subjected to drop impact at different input impact energies (23J, 35J, and 51J), and the fatigue response after impact loading is studied. The rate of degradation of stiffness for impacted CFRP specimens under a Prog FALSTAFF spectrum loading is less compared to constant amplitude loading. The sequence of intermediate block cycles of constant ampltiude in Prog FALSTAFF is organized either in an ascending order of magnitude, low to high (Lo Hi), or in a descending order of magnitude (Hi Lo) to study the effect of load sequencing on damage accumulation. Lo Hi sequencing results in greater damage compared to Hi Lo sequencing for all the orientations of CFRP laminates studied. In the case of unimpacted specimens, there is a marginal increase in stiffness in the early stages, followed by a drop in stiffness; the stiffness degradation rate in unimpacted specimens is much less compared to impacted specimens. Computed X ray tomography has been carried out on specimens under the following conditions: pristine; after fatigue cycling; after impact damage; and postimpact, postfatigue cycling to understand the damage progression. As an exploratory study, investigation of damage through active infrared thermography is carried out. The presence of delamination could be identified through the variable specimen cooling curves of CFRP specimens using the active thermography technique.