Mode I delamination toughness of laminated composites with through-thickness reinforcement

Through-thickness reinforcement is an effective way to suppress delamination in laminated composites. Micromechanics based models are developed to study the effect of through-thickness reinforcement (stitching) in improving the mode I delamination crack growth resistance of laminated composites. In the development of these models, two types of stitch geometries are considered. In the first case, the stitches are assumed disconnected, as in many cases the top and the bottom surfaces of the stitched laminates are ground off to remove surface in-plane waviness caused by stitching loops. The force in the stitches in this case is estimated from frictional bonding between stitches and the matrix. In the second case, interconnected stitches are considered and the force carried by the stitches is modelled as a Winkler elastic foundation type of stress-separation relation. The effect of stitches is expressed in terms of a single stitching parameter lambda or beta and closed form analytical expressions for the crack-growth resistance (K sub R ( Delta alpha )) are obtained. The effects of the stitching parameter and various geometric and material properties are examined for Kevlar stitching of CFRP.