Influence of core joints in sandwich composites under in-plane static and fatigue loads

Highlights: • This study quantified the reduction in axial fatigue life due to core junctions; • Under cyclic axial load (R =-1), lateral expansion and contraction between foam and core joints were more significant than anticipated; • Despite facesheets' primary in-plane axial load carrying mechanism, traditional butt-core joint reduced the axial fatigue life of the structure; • Cracks were initiated in the facesheets for the butt-joint and in the core for the scarf-joint samples; • This study will hopefully alert composite engineers and designers to better apprehend and test the influence of core junctions; Most designers are unaware of the influence of core junction in lightweight sandwich structures under axial load. Quantifying this effect and its criticality on the life of the structure is still a challenging task. In this study, a novel testing methodology was used to characterize this effect. Scarf and butt core joints in foam core sandwich composites were subjected to axial static and fatigue loads (R = 0.1 and R = − 1). Under cyclic axial load, differential movement (through-the-thickness) between the foam and core joint was more significant than anticipated. Non-destructive evaluation techniques were used to locate the damage and assess the failure mechanisms. The root-cause-failure analysis showed that cracks were initiated in the facesheets for the butt-joint, and in the core for the scarf-joint samples, respectively. Consequently, at 80% residual strength, the butt-joint reduced the predicted fatigue life by 42% and 32% at low and high cycle fatigue, respectively. Residual tensile tests revealed the sizeable damage induced by the traditional butt-joint design. This research confirmed that despite the facesheets' primary in-plane load carrying mechanisms, core junction will substantially influence the axial fatigue life of the structure.