Load-control vs. displacement-control strategy in fatigue threshold analysis of adhesives: Effects of temperature

For the design of adhesive joints for service in fatigue, parameters such as the rate of fatigue crack growth and the fatigue threshold energy release rate are of extreme importance. These parameters can be obtained through fatigue fracture tests using different strategies, namely Constant load control and constant displacement control. Despite the extensive studies conducted on the fatigue performance of bonded joints, the role of testing methodology on the fatigue fracture behaviour of adhesives is less considered, especially in conjunction with the effect of temperature in these tests. Accordingly, in this study, a structural polyurethane adhesive suitable for automotive applications is characterized in terms of its fatigue behaviour in pure mode I using DCB (double cantilever beam) specimens. Fatigue tests were performed at different temperatures under both constant load control and constant displacement control conditions. A compliance-based method was used to measure the crack size throughout tests. The threshold energy release rate was calculated for different test temperatures and the two testing strategies were compared. It was seen that a constant displacement control methodology is superior to a constant load control methodology when testing for the threshold energy release rate, due to its shorter duration and low sensitivity to initial testing conditions. Differences observed between fatigue testing methodologies were exacerbated by the effect of temperature on the adhesive.