Slow-growth damage of bonded composite-metal joints subjected to fatigue loading

This study investigates the slow-growth damage of bonded composite-metal (CFRP-Aluminium) hybrid double lap joints. First, static tension tests were conducted to measure the residual strength of the partially disbonded joints. Finite element models were developed to predict the residual strength as a function of disbond crack length. The model was calibrated using the static test results and the characteristic distance method. Then, constant amplitude fatigue tests were conducted at a practical load range, determined on the basis of joint static strength and safety factor requirements, to measure disbond growth rates and joint fatigue life. Further numerical analyses were conducted, where the extended finite element method (XFEM) and virtual crack closure technique (VCCT) were applied to predict energy release rates at the disbond crack tip of the joints. The crack growth rates measured from the fatigue tests and the energy release rates from the parametric numerical analyses consistently indicated that slow-growth behaviour was present. By combining the crack growth rates and energy release rates, modified Paris laws were established to predict the disbonded crack growth and fatigue life of the joints with satisfactory results obtained. In addition, an important observation was made, that is, in the fatigue tests the disbond from the taper end would not migrate to generate delamination, the mechanism of which was convincingly revealed by a detailed FEM analysis. This study successfully implemented the proposed framework for the slow growth damage prediction of adhesively bonded joints and demonstrated its effectiveness.