Failure analysis of adhesively-bonded metal-skin-to-composite-stiffener: Effect of temperature and cyclic loading

The aim of this research is to analyse the failure of a Fiber Metal Laminate (FML) skin adhesively bonded to a Carbon Fiber Reinforced Polymer (CFRP) stiffener, under quasi-static loading at different environmental temperatures (−55 °C, Room Temperature RT and +100 °C) and under fatigue loading at RT. This bonded joint was tested using stiffener pull-off tests, which is a typical setup used to simulate full-scale components subject to out-of-plane loading. The failure sequence for all test conditions consist of: (1) damage initiation at the noodle of the CFRP stiffener; (2) damage propagation by delamination from the noodle to the stiffener foot; (3) detachment of the stiffener from the skin. Increasing the temperature, decreases the joint stiffness (40% when compared to RT) and decreasing the temperature decreases the maximum load (50% when compared to RT). The fatigue life initiation of the joint presents a very large scatter but the fatigue life propagation presents more stable results. The fatigue threshold (no damage) is reached at approximately 30% of the maximum load level. The fracture surfaces indicate a predominant inter and intra-laminar failure of the composite under mixed mode I/II. The CFRP stiffener is the weakest link of the bonded FML-skin-to-CFRP-stiffener both for static and fatigue loading.