Analysis of stitched laminated ENF specimens for interlaminar mode II fracture toughness

Abstract

Two simple micromechanics based models are proposed to predict the effect of through-thickness reinforcement (stitching) on the improvement of delamination crack growth resistance in end-notched flexure (ENF) specimens. In the first model, it is assumed that stitches stretch elastically and then rupture when the load carried approaches the failure load. In the second model, it is assumed that stitches are discontinuous and that the stitch thread-matrix interface is completely frictional. Approximate closed form solutions for energy release rates are obtained, and the effects of stitch density, matrix-stitch thread interfacial shear stress, stitch thread diameter, volume fraction of stitches, critical energy release rate and Young's modulus are then examined. A simple design study for sizing the ENF specimen to minimise geometric nonlinear response is presented. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.

Bending effect of through-thickness reinforcement rods on mode I delamination toughness of DCB specimen. I. Linearly elastic and rigid-perfectly plastic models

“…Eq. (2) is the same as to that proposed by Jain and Mai (1994b) for a discontinuous or independent stitch. However, it should be emphasized that there is only one z-rod at each z-reinforcement location in the present model whereas there are two stitch threads at each stitch reinforcement entity.…”

Mode II delamination toughness of z-pinned laminates

“…Given the critical potential energy release rate of the composite laminate for mode II fracture, G IIC , the shear stress ahead of the crack-tip at any location r can be obtained from Eq. (8). Here, the critical energy release rate, G IIC , is considered a material constant.…”

“…Jain and Mai [8,9] have also studied mode II fracture toughness of stitched laminated EndNotched-Flexure (ENF) specimens by using first-order shear deformation laminated plate theory and beam theory. In the case of stitching, the bridging force comes from the elastic stretching of the threads.…”

Experimental study on the stitching reinforcement of composite laminates with a circular hole

“…In 1990s, Mai et al [6][7][8][9][10] Mouritz and Cox [11] proposed an approach to evaluate the properties of the stitched laminates. These studies showed that the load-carrying capacity of the laminates can be improved through a proper stitching reinforcement.…”