A study of microcracks and delaminations in composite laminates

Abstract

With the rapid increase of composite usage on aircraft primary structures, the damage of composite laminate and its nondestructive testing are of high interest to the aerospace industry. Composite laminates, typically without reinforcement in the out-of-plane direction, are prone to impact damage. While the characteristics and the NDT of impact-induced delaminations are well known, the precursor to failure has not attracted equal attention. In this paper, we report an investigation of microcracks and delaminations in carbon composite and glass composite laminates fabricated from unidirectional and woven prepregs. Microcracks were induced by low energy impact and by thermal cycling to cryogenic temperature. The density, distribution and orientation of the microcracks caused by impact were mapped out in detail by photomicrographs. Specimens containing microcracks were then fatigued to observe the initiation and development of delaminations. With before-and-after micrographs, the changes of individual microcracks were followed and documented. The delaminations grown from microcracks in a woven glass laminate during fatigue were found to be localized, short, and tight. These "micro-delaminations" are characteristically different from the "macro-delaminations" caused by impact energy above the delamination threshold. Unlike macro-delaminations, the ultrasonic detection of micro-delamination zone has proven more challenging. © RILEM 2013.