Exploring strain rate variation in the adhesive layer during constant speed mode I fracture tests: Loading speed and test temperature effects

Abstract

The existing literature primarily explores fracture energy concerning test rate. However, for numerical simulations and bonded joint design, understanding the fracture energy strain rate sensitivity is key, not test speed. Even at a constant speed in fracture tests, adhesive strain rate varies during crack propagation, complicating strain rate sensitivity analysis. The current study aims to comprehend the strain rate sensitivity of the fracture energy, as opposed to the loading rate sensitivity of two different polyurethane adhesives subjected to mode I loading conditions. The study experimentally examines strain rate variations under constant loading rates and investigates how test speed affects the strain rate in mode I fracture tests. The impact of testing temperature on the strain rate is also explored by performing tests at −30 °C and 23 °C. Digital image correlation (DIC) and tracking methods are employed to improve the understanding of strain rate characteristics in these tests. To analyse the effect of loading rate on strain rate, the tests were conducted at different speeds as well. The influence of adhesive thickness on the strain rate was also explored. It was experimentally found that the strain rate at the crack tip varies even at a constant loading rate. The variation in strain rate is mainly evident in tests performed at higher loading rates, where the strain rate range increases. The tests performed at low temperature revealed less variation of strain rate than the ones tested under the same loading rate condition at 23 °C. For the same adhesive, thicker bondlines are less likely to cause a substantial error in fracture energy results than thinner bondlines.