A Study of Fatigue Damage in Composites by Nondestructive Testing Techniques

Abstract

The methodology and experimental results of damage mechanism studies in graphite/epoxy composites using a modified X-ray nondestructive testing technique and continuous acoustic emission monitoring (AEM) method are presented. Fracture specimens were fabricated from Narmco 5208/T300 and Narmco 5208/ Modmor II graphite/epoxy composites with different ply orientations chosen to exhibit fiber failure and linear and nonlinear matrix failures. The specimens with center slits were subjected to tensile ramp loading, constant-amplitude cyclic loading, and an air combat fighter randomized flight-by-flight spectrum loading. During tensile ramp loading, the specimens were monitored by an X-ray “movie” to observe the damage initiation and growth in the areas around the slit tips. An opaque additive was injected in the slit opening to enhance the damage indications. For specimens undergoing cyclic loading and fatigue spectrum loading, periodic X-ray records were made for damage growth monitoring. Continuous AEM was conducted on specimens undergoing tensile ramp loading. The actual fatigue damage initiation and growth within the composite laminates can be observed by the periodic X-ray records of the specimens. These observations were correlated with the AEM results. Three parameters measurable by the AEM system— accumulated counts, amplitude distribution, and frequency spectrum—characterized fatigue damage initiation and growth in different fiber directions. Damage redistribution mechanisms in the fatigue specimens can be observed from the damage growth study.