ABSTRACT
Two lamina-based fiber failure criteria are presented for composite materials with nonuniform stress state. Using these criteria three models are developed for the ultimate strength prediction of multidirectional unnotched and notched composite laminates. These models utilize a characteristic length and a proposed lamina-based fiber failure criterion that requires the longitudinal strength properties and the 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 written 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. If we use the characteristic length that was determined based on a quasi-isotropic laminate, then the notched strengths for a variety of laminate layups can be predicted very well. These models could be very useful for composite designs.
