Extracting interlaminar cohesive laws from displacement field measurements

Cohesive/interface elements are one of the successful numerical tools that have been recently applied to many different problems and shown good results in modelling crack development. However, most applied cohesive laws are hypothetical and are not based on experimental measurements. The aim of this study is to investigate the damage development at a delaminating interface and to propose a method by which the mode-II traction-separation law can be extracted experimentally. To find the traction-separation law, both variables (traction and separation) need to be evaluated separately at the interface. Mode-II separation can be found by subtracting the displacements on each side of the interface, however finding the value of traction (shear stress) at the interface is more difficult. As a first step, the shear strain field is found from the available displacement field measurements and then, the shear stress is worked out using the shear constitutive law. After validating the proposed method against FE simulations, the experimental displacement field is used to find the traction-separation law. The overall form of the traction-separation law can be approximated by a trapezoidal shape.