An energy based mode III fracture criterion for composites

In this work, a new fracture hypothesis referred to as the Z-criterion is developed for mode III cracks in orthotropic composite materials. The new theory predicts critical crack propagation conditions and the crack propagation direction. The Z-criterion (Zhang et al., Engng Fracture Mech. 34, 749–769 (1989) (S-criterion) (Sih, Proc. 10th SES Annual Meeting, 221–234, Boston (1975)) removes certain deficiencies by considering separately the dilatational and distortional strain energy density factors. It suggests that under mode I conditions, crack initiation is controlled by the dilatational strain energy density factor and for mode II and mode III conditions, crack initiation is controlled by the distortional strain energy density factor (Zhang et al. A new Z-criterion… submitted to Engng Fracture Mech.). These controlling parameters allow extension to mixed mode and three dimension crack analysis. Eleven different composite materials are used to test and verify the proposed Z-criterion.