Effective Stress Fracture Models for Unnotched and Notched Multidirectional Laminates

Three models are proposed for the ultimate strength prediction of multidirectional unnotched and notched composite laminates. These models utilize a characteristic length and a proposed failure criterion that requires the longitudinal strength parameters and stress distribution of a lamina within a laminate. The characteristic length for the second model: effective average stress model is independent of the opening shape and that for the third model: minimum strength model can be related in a simple form. Comparisons show that the ultimate strength predictions for unnotched and notched laminates both agree very well with experimental data for graphite/epoxy materials system. A direct relation between the fracture toughness and the unnotched laminate strength (determined analytically) is also obtained.