Modified DCB and CLS specimens for mixed-mode fatigue testing of adhesively bonded thin sheets

The fatigue behavior of adhesive joints made with commercially coil-coated thin aluminum sheets has been measured using modified double cantilever beam (DCB) and cracked lap shear (CLS) specimens that avoided yielding. The thin sheets were bonded to a stiffener using a reinforcing adhesive layer. Experimental and finite element analyses demonstrated that the reinforcing adhesive layer had an insignificant effect on the stress state at the crack tip and the resulting fatigue behavior. The modified DCB test geometry was then used to measure the effect of the loading phase angle and test environment on the fatigue performance of the pretreated aluminum sheet. It was observed that the phase angle affected the threshold strain energy release rate, Gth, and the crack growth rates. The presence of moisture in the test environment greatly reduced the fatigue threshold. It was also observed that the Gth was sensitive to the rolling line orientation on the surface of the sheet.