Characterization of fatigue behavior of bonded composite repairs

Composite patches are bonded to a cracked metallic surface either symmetric ally (double sided) or unsymmetrically (single: sided) to extend service li fe, The stresses in the metallic panel are greatly affected by the repair s ymmetry, Unsymmetric repairs present the greatest challenge because of the presence of out of plane bending. Thermal residual stresses are: present be cause of the thermal coefficient mismatch, of the patch and the aluminum pl ate. Debonding along an adhesive-adherend interface can reduce the patch ef fectiveness, a simple analysis with Mindlin plate theory is investigated to model the host and the repair plate. The two plates are connected by an ad hesive layer modeled by effective springs. Large deflection theory is used in the ease of unsymmetric repairs. The springs are ineffective in the debo nd zone and are removed, Both the aluminum and the debond cracks are charac terized by fracture mechanics by use of the stress intensity factor and str ain-energy release rate, respectively. Experiments on aluminum 2024-T3 plat e, AS4/3501-6 carbon/epoxy composite patch and FM73 adhesive include determ ining the thermal residual stresses in the aluminum plate and observation o f debond development by use of an ultrasonic C-scan. Tests are conducted to examine the metallic and debond crack growth interaction on unsymmetric re pairs.