Initiation and progression of composite patch debonding in adhesively repaired cracked metallic sheets

Failure analysis of composite bonded repairs in cracked metallic structures is often given superficial treatment. Apart from crack growth, which is prevented during the design of the repair by ensuring that the stress intensity factor will remain in safe levels, patch debonding is a very critical failure mode since its presence reduces the effective area of the patch. In this paper, a finite element-based progressive failure model was used to investigate the geometrical effects on patch debonding initiation and progression induced by mechanical loading. A metallic sheet containing a central through-thickness crack loaded in tension and repaired using a double-sided rectangular tapered composite patch was used as the basic configuration. In all cases examined, the debonding is due to cohesion or adhesion failure caused by high shear stresses. It was found that, depending to the patch thickness, the debonding initiates either at the upper patch edge (being catastrophic for the repair) or at the crack faces (being not catastrophic). Given the adhesive shear strength, the debonding initiation load increases significantly with increasing the patch thickness, adhesive thickness and tapered length. Based on these findings, specific suggestions for the enhancement of composite bonded repairs against debonding are made.