Crack growth performance of aluminum plates repaired with composite and metallic patches under fatigue loading

Composite patches, bonded on cracked or corroded metallic aircraft structures, have shown to be a highly cost effective method for extending the service life and maintaining their structural efficiency. However, there are some detrimental effects of patching dissimilar materials using a room temperature cured adhesive. To evaluate the impact of the two different patch materials on the performance of fatigue crack growth rate, we have conducted a comparative study on the patch repair performance, using composite versus metallic patches, of cracked aluminum plates. Experiments involved fatigue tests of 2 mm thick aluminum specimens with edge crack repaired with 50 mm2 patch. The notched specimens were pre-cracked to 3mm crack length before being repaired with a composite (carbon fiber) or a metallic (aluminum) patch. However, there are differences in the fatigue life using dissimilar patching materials using a room temperature cured adhesive. Fatigue test are conducted on the single sided adhesively patched 2024 T3 aluminum specimens. The crack growth behavior was monitored for different test configuration and the obtained results were compared and analyzed. In fact, although the composite material is much stronger than the metallic patches, the fatigue life of specimens repaired with metallic patches are found to be lower than that offered by composite patches. It was found that there is 15% increase in fatigue life of specimen repaired with composite patch compared to aluminum patches. Consequently, the effect of stress ratio should also be considered while bonding the cracked specimen because it has also a significant effect on the fatigue crack growth rate.