Impact fatigue in adhesive joints

One of the forms of a vibro-impact effect in engineering components is impact fatigue (IF) caused by a cyclic repetition of low energy, low-velocity impacts, for instance, in aerospace structures. It can have a highly detrimental impact on performance and reliability of such components, exacerbated by the fact that in many cases it is disguised in loading histories by non-impact loading cycles with higher amplitudes. Since the latter are traditionally considered as most dangerous in standard fatigue, IF has not yet received deserved attention; it is less studied and practically unknown to specialists in structural integrity. Though there is a broad understanding of the danger of high-energy single impacts, repetitive impacting of components has been predominantly studied for very short series. This paper aims at the analysis of IF of adhesively bonded joints, which are becoming more broadly used in aerospace applications. The study is implemented for two types of typical adherends - an aluminium alloy and a carbon-fibre reinforced composite - and an industry-relevant epoxy adhesive. Various stages of fatigue crack development in adhesively bonded joints are studied for the conditions of standard and IF. The results obtained - in terms of crack growth rates, fatigue lives, and microstructures of fracture surfaces - are compared for the two regimes in order to find similarities and specific features.