Experimental Verification of Rose's Constant K Solution in Bonded Crack Patching

Abstract

Bonded composite patches have been used for two decades to extend the lives of fatigue-damaged F-16, F-111, B-1B, C-141B, and many other aircraft. One of the key features of the technology is extremely slow crack growth under the bonded repair. Researchers have performed hundreds of experiments on repaired cracked panels, and have reported near-constant crack growth rates for a variety of relatively thin sheet (t < 3 mm or 0.125 inch) configurations and constant amplitude load cases. Constant crack growth rates rely on the existence of a constant crack tip cyclic stress intensity factor, Δ K, underneath the patch.

The paper describes the results of experimental stress analyses carried out on cracked aluminum panels with bonded composite patch repairs. Experimental strain gage and photoelastic measurements of K underneath a bonded repair validated Westergaard's analytical stress field description. These measurements, combined with fatigue crack growth studies, have verified that a constant K condition (predicted by L. R. F. Rose) indeed exists for cracks under bonded repairs. For the configuration tested, this held true while the crack size was less than roughly 80% of the repair width. These results are key to providing accurate predictions of crack growth rates and subsequent nondestructive inspection intervals in service.