Experimental measurement of mode-I fracture toughness of dissimilar material joints with thermal residual stresses

The current study presents a novel test method to experimentally cancel out the thermal stresses in dissimilar material joints. For the commonly used double cantilever beam test the presence of thermal stresses results in a significant mode mixity at the crack tip, which varies with applied load even if the elastic properties of the adherends are similar. This is particularly a challenge for fibre reinforced plastics bonded to metals due to the large difference in thermal expansion coefficients. The presence of mode-II loading is likely to provide a higher fracture energy from experiments than if tested under pure mode-I loading, which can lead to non-conservative results when using standard test methods. To overcome this challenge a novel test method inspired by the mixed mode bending test is developed. It is shown that the thermal stresses can be cancelled by applying initial constant loads during testing, and that the fracture toughness under pure mode-I loading can be obtained under specific conditions. The test method is validated by carrying out virtual compliance calibration experiments using cohesive zone finite element modelling. As the test method relies solely on analytical calculations and can be used with standard test equipment, it is relatively simple to apply in practice.