CAI fatigue testing in CFRP: is the test representing what happens in real structures?

Abstract

Impacts on carbon fiber reinforced composites (CFRP) can produce internal damage comprising of matrix cracks, multiple delaminations and fiber fracture, which are hard to detect by visual surface inspection. This situation is known as barely visible impact damage (BVID). There is evidence that BVID can propagate in compression fatigue in the form of delamination growth causing failure [1,2]. In the past decades, great attention has been paid to this scenario by the aerospace research community, since low velocity impacts are frequent in aviation and BVID can potentially grow unnoticed during an aircraft’s operational life, exposing structures to the risk of unexpected failure. The study of fatigue after impact propagation has been conducted usually by tracking the growth of projected delaminated area using periodic ultrasound inspections. To compare results of different experimental campaigns, the ASTM D7137 static compressive test fixture has often been used also in case of fatigue tests. Results of the previously described class of experiments [1,2], showed first a phase of almost no growth in the projected delamination area, followed by a fast propagation of delamination in the perpendicular to loading direction. However, the reason why this no-growth phase is observed is still unknown. It is also not clear how dependent the observed growth pattern is on the adopted test fixture. In the present work, we use the results of a series of CAI fatigue tests to critically discuss and redefine these two knowledge gaps