Quasi-static and intermediate test speed validation of SHPB specimens for the determination of mode I, mode II fracture toughness of structural epoxy adhesives

The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest, such as the automotive industry, in order to improve mechanical behaviour, reduce weight and simplify manufacturing. Furthermore, due to high standards in terms of safety required in the automotive industry, namely the occupant's safety during collisions, the behaviour of adhesives under impact loads is becoming a prominent object of study. This work proposes a novel and simplified method to experimentally obtain a custom set of TSLs (traction-separation laws) and estimate the fracture energy both for mode I and II that can be tested from quasi-static to impact conditions resorting to a SHPB (Split Hop-kinson Pressure Bar) equipment with especially designed specimens. The method is validated by comparing those results to fracture energy values obtained using DCB (Double Cantilever Beam), ENF (End Notch Flexure) and prior validated mixed mode apparatus Finally, the SLJ (Single Lap Joint) behaviour is predicted by resorting to the custom TSLs and comparing model performance to experimental results. With the proposed method the obtained fracture energy was found to be equivalent to that obtained with more traditional fracture mechanical tests used in the field of adhesive bonding. The custom TSLs were able to predict the behaviour of SLJ fairly accurately.