Progressive damage analyses of patch repaired composites using acoustic emission and digital image correlation

This paper investigates the damage behaviour in patch-repaired composite structures. Quasi-isotropic carbon/epoxy composites with a [45/90/–45/0]_S stacking sequence was used as the parent plate, drilled with a central hole. High toughness Araldite 2015 adhesive was used to bond circular patches on both sides of the plate. Unnotched and notched specimens were prepared for comparison. Quasi-static tensile tests were performed on all specimens, with acoustic emission (AE) and digital image correlation (DIC) used for damage monitoring. Results showed a 75 % recovery in ultimate tensile strength in the repaired composite. DIC results indicated that longitudinal strain concentrated at the patch centre, showing successful load redistribution. Strain concentration occurred at the four corners of the parent plate around the hole, indicating stress concentration relaxation due to the patch repair. AE data suggested damage initiation with matrix cracking in the 90° plies, followed by fibre/matrix debonding, delamination, and fibre breakage in the parent plate. The patches and adhesive layer remained intact throughout the test.