Analysis of the effectiveness of repairing cracked aircraft structures: patch-performance criteria interaction

If hot bonding of the composite patch to the cracked plate leads to an improvement in the adhesion energy between these two protagonists, it is a source of residual stresses in the plate, the adhesive and the patch near their interfaces. Using FEM, this study aims to improve the performance of hot plate-patch joining and patch-crack interactions by optimizing the patch shape. Thus, all the patch shapes developed in this study arise from an initially rectangular shape. This results in six patch shapes: elliptical, orthogonal, star, H, double arrow and butterfly. This optimization is analyzed in terms of improvement in both the fracture energy gain of the plate (stabilization of the SIF with the evolution of the crack size K), the mechanical energy gain of the adhesive (reduction of the risk of rupture of the adhesive by a drop in the level of shear stresses in the adhesive t) and of the mass gain of the patch m (reduction of the risk of peeling and delamination and of debonding by an improvement of the aerodynamic resistance of the patch). These three gains, K, t, m, developed for the first time in this study, constitute characteristic criteria for the performance of composite patch repair. This is where the originality of this work lies. This study highlights that improving the fracture energy gain alone is not a sufficient condition for the effectiveness of hot repair. Thus, the simultaneous satisfaction of these three criteria is a necessary condition for the durability and performance of the repair. This therefore constitutes the originality of this study which lies in the development of these criteria. It appears from this study that the repair using an optimized double arrow-shaped patch leads to the satisfaction of these criteria. This shape simultaneously ensures stabilization of the three components of the repair: the plate by stabilizing the crack, the adhesive by reducing shear stresses, the patch by reducing its mass. The risks of damage (debonding, peeling, delamination) due to these stresses and the size and thickness of the patch are clearly minimized. It also emerges from this study that, contrary to previous works, hot bondings of the patch to the cracked plate doesn't in any way affect the performance of the hot repair using an optimized patch.