Fatigue and failure testing of a hybrid CFRP-aluminum wing box at elevated temperature

A hybrid wing box, consisting of CFRP skins bolted to an aluminum inner structure, is tested in cyclic and quasi-static bending/twisting loading at temperatures up to 90 degrees C. Besides bolted joints, the test object contains ply drop-off regions with embedded artificial defects and impact damages in the CFRP skins. Thermally and mechanically induced strains are measured and a good correspondence is achieved in comparison to the previously computed strains. The magnitude of the thermally induced strains is significant, 10-20% of the mechanically induced strains at static design load. Fatigue failure of several titanium bolts, and fatigue cracks in the bolt holes in the aluminum plate, in the main joint are detected. No delamination growth of the artificial defects in the ply drop-off regions nor from the impact damages is observed by C-scan inspections. The final failure took place at 128% of the design load and was initiated by a combination of bearing and bolt bending failure in the main CFRP-aluminum joint. This work demonstrates and emphasizes the importance of large-scale tests for the purpose of inducing multiple interactive damage and failure mechanisms, as well as thermal effects in hybrid CFRP-aluminum structures.

Automated summary

This study tests a hybrid wing box under cyclic and quasi-static bending/twisting loading at high temperatures, validating the accuracy of computed strains. The study finds a significant difference between thermally induced strains and mechanically induced strains, with thermal strains accounting for 10-20% of the mechanical strains. Additionally, fatigue failure of titanium bolts and fatigue cracks in bolt holes are detected, but no delamination growth is observed in artificial defects or impact damages. The final failure is initiated by a combination of bearing and bolt bending failure in the main CFRP-aluminum joint. This work emphasizes the importance of large-scale tests in inducing various damage and failure mechanisms as well as thermal effects in hybrid CFRP-aluminum structures.