Fatigue behavior of foam core sandwich beams with sub-interface impact damage

The fatigue life of impact-damaged foam core sandwich beams with GFRP faces is investigated for constant load-amplitude cyclic loading. The damage produced by the low velocity/energy impact is characterised as a sub-interface cavity with surrounding crushed core whereas the face sheet is undamaged. The effect of bridging owing to the crushed core in the peripheral regions of impact damage, revealed in the preceding static study Shipsha et al. [1,2] is assessed under cyclic loading. The fatigue test results are analysed in a stress-life S-N diagram and compared with published data from undamaged beams. The effect of two load ratios R = 0.1 and -1 is investigated. The fatigue threshold load levels are experimentally quantified from the S-N curves. Furthermore, a modified point-stress criterion based on the threshold stress intensity factor, K-th, is proposed to estimate the "no-crack growth" limit loads. The results show good agreement with performed experiments.