Enhancement of Fracture Resistance by Multiple Cracks in Layered Structures under Mode I and Mix Mode Loading

Layered structures are susceptible to delamination because they often exhibit low interlaminar fracture resistance. Through-thickness stresses e.g. due to manufacturing defects or geometric discontinuities, can result in growing interlaminar cracks which may lead to loss of structural integrity [1]. As a result, a number of techniques have been developed to improve the through-thickness fracture resistance of layered structures e.g. fibre reinforced composites. In the field of composite materials, two directions to develop damage tolerant composites can be identified: a) material improvements (e.g. tougher matrices and interleaves) and b) modifications of the fibre architecture (e.g. stitching, z-pinning, knitting and braiding). These techniques aim to increase the fracture resistance by making the damage prone areas stronger.
In the present work, a third approach is explored. It is shown, through cohesive zone modelling, that the fracture resistance can be improved by introducing weak layers that result in multiple delaminations next to the damage prone areas. Our model is motivated by the experimental results of Rask and Sørensen [2] who observed that by changing the ply thicknesses of composite beams bonded together with a thermoset adhesive, more delamination cracks could be developed next to the main/primary adhesive/laminate crack. An analytical model, based on the J integral, was developed for multiple delaminations [3]. It is shown that the maximum possible increase (upper limit) of the steady-state fracture resistance, JR,ss, scales linearly with the number of delaminations in agreement with the observations of Rask and Sørensen.