Theoretical and experimental research into optimal edge taper of bonded repair patches subject to fatigue loadings

Finite element (FE) analyses were conducted for double lap, metal-to-metal bonded repair joints with different linear edge taper angles, or optimal taper profiles using the shape optimisation approach. Various adhesive failure criteria were applied and compared. A fracture mechanics approach was also applied in the FE analyses, in which an initial crack was pre-set at the edge of the adhesive bond line. The energy release rates were calculated and the results were used to evaluate the fatigue resistance. FE results predicted that the taper angle should strongly affect the fatigue performance of the repair patch. Compared with the 90° taper case, the peak stresses in the 6° taper case reduced by around 60%, and the stresses in the 3° taper case reduced by around 80%. The optimum design was able to reduce the peak stresses by about 50% compared with the widely used 6° linear taper (i.e. 1:10) with the same taper length. Thus, it appeared to be the best in terms of the fatigue resistance vs. taper region length. The calculated energy release rates also indicated a similar trend for the fatigue resistance as a function of the edge taper configuration. Experimental results agreed well with the numerical predictions. In particular, the predicted crack initiation loads, based on estimated threshold value of mode 1 energy release rate, correlate well with experimental results and the high fatigue resistance of the optimal design was confirmed.