DCB tests at constant strain rate using crash-resistant epoxy adhesives: A numerical and experimental approach

The use of adhesive joints in the automotive industry has increased significantly as it enables the design of multi-material structures with a high strength-to-weight ratio. Considering the high safety standards held by the automotive industry, especially regarding collisions, there is a need to fully understand the behaviour of adhesives when subjected to dynamic loads if bonded connections are to be used in structural applications. The automotive industry is, therefore, constantly searching for numerical tools that are able to simulate the behaviour of bonded joints under impact. Fracture mechanics tests such as the double cantilever beam (DCB) test are valid and proven tools for the determination of adhesive toughness and to provide critical data for the development of damage models, able to predict adhesive behaviour. However, when loaded at high strain rates, these specimens load the adhesives at strain rates that vary significantly as the testing progresses. The present work describes a special testing procedure that precisely controls and varies the loading rate applied to DCB specimens to ensure that a consistent strain rate is generated in the adhesive layer. The variable loading rate is calculated using a finite element model and then programmed into a servo-hydraulic testing machine for experimental testing. The results of this study demonstrate that the R-curves obtained using the suggested procedure, are similar to those obtained by testing the DCB at constant test speed, although the results appear to be more consistent, a conclusion sustained by the decrease in the standard deviation of the results.