Further validation of the Jain and Mai models for interlaminar fracture of stitched composites

Micromechanical models proposed by Jain and Mai to predict the improvement in Modes I and II interlaminar fracture toughness ( G R) of polymer composites when reinforced by through-thickness stitching have been further validated against a large number of measured G R values. Experimental G R values were obtained from the literature for a variety of carbon- and glass-fibre-reinforced polymer composites stitched over a wide range of densities with different yarn materials (polyester, carbon, glass, Kevlar ®) of varying thickness. Good agreement was found between the measured and theoretical interlaminar fracture toughness values for both the Modes I and II conditions, despite some major assumptions having to be made about the failure mechanism and mechanical properties of the stitches. An assessment of the experimental Mode II G R data reveals that the commonly used end-notched flexure method is not suitable for the interlaminar fracture toughness testing of stitched composites. It is recommended that Mode II delamination resistance should be measured by using the end-notched cantilever method.