Investigating the effects of loading system on the fracture behavior of DCB specimens considering T-stress

Double cantilever beam specimen is a standard specimen for assessment of mode I failure and separation in adhesively bonded joints and also composite materials. Among the several load introduction systems, the piano hinges and end loading blocks are more common. It has been accepted that the fracture toughness results of the two cases are different, but the effect of the loading system on the load-displacement data and fracture mechanisms is not entirely known yet. Therefore, in this study, the two loading concepts are compared both by precise finite element simulations and experimental tests. The adhesive layer is modeled with its own material properties, and the failure of adhesive is investigated by known LEFM procedures. The results reveal that the load block makes the double cantilever beam specimen stiffer and exhibit more non-linear behavior. Moreover, double cantilever beam with the load block system fails in higher loads and lower crack opening displacements compared with the same specimen loaded by the hinges. To study the effect of loading arrangement in more details and including the softening phase, cohesive zone model was utilized. A correction for strain energy release rate based on the parameter T was proposed, and the role of the T-stress on the traction-separation law of the cohesive zone model and the load-displacement behavior were investigated. It was concluded that the T-stress as a crack tip constraint parameter can successfully justify the difference between the two cases. Applying the correction to the traction-separation law of CZM proves the validity of proposed correction in justifying the experimental results.