The effects of test temperature and humidity on the mixed-mode fatigue behavior of a toughened adhesive aluminum joint

The effects of test environment humidity and temperature on the fatigue threshold and crack growth behavior of P2-etched and commercial coil-coated (CC) aluminum adhesive joints were studied under mixed-mode loading using aluminum asymmetric double cantilever beam (ADCB) specimens. Under dry conditions, increasing the temperature to 80 °C had an insignificant effect on the fatigue threshold, but caused an increase in the crack growth rates. At 40 °C, the fatigue behavior was insensitive to moisture at higher crack growth rates, but became sensitive to moisture level in the test environment as crack growth rates slowed to the threshold. The effect of moisture and temperature were explained by the observed changes in the crack path, which in general moved progressively closer to the more highly-strained adherend as the applied strain energy release rate, and consequently the crack growth rate, decreased. Furthermore, the residual adhesive thickness on the more highly-strained adherend, tr, increased with increasing temperature, and the crack path shifted to the hydrated aluminum oxide interface when the test environment was saturated with moisture. The degrading effect of a hot-wet environment was similar for both P2-etch and CC pretreatments. At higher crack growth rates, the joint fatigue performance was degraded solely due to the effect of the increased temperature, whereas at low crack growth rates, the fatigue performance was degraded predominantly because of elevated moisture.