Combined DIC and FEA method for analysing debonding crack propagation in fatigue experiments on wrapped composite joints

Wrapped composite joint is a novel joining technology which connects steel hollow sections through bonding, completely avoiding the welding in the load transferring mechanism. Fatigue performance of wrapped joints has been experimentally shown to be superior over their welded counterparts. Aiming to enable development of prediction methods for fatigue life of wrapped composite joints, this paper proposes a combination of 3D Digital Image Correlation (DIC) technique and FE analysis as a method for monitoring debonding crack propagation at a complex composite-to-steel interface covered by a non-uniform thickness laminate. Fatigue tests on wrapped composite X-joints under tensile load are used for the method application and to analyse crack propagation in the brace and chord, including their interaction. Variation of strain distribution on surface of composite wrap obtained in DIC is corelated to length of the debonding crack at the composite-to-steel interface by the means of 3D finite element model of such joint. Crack development obtained from the combined DIC and FEA method is correlated to strain energy release rates calculated from FEA. With the help of FEA, the failure mode is characterised by debonding on the chord at the early stage of cyclic loading, followed by debonding on the brace.