Determination of Fracture Strength in Orthotropic Graphite-Epoxy Laminates

A fracture test program performed on orthotropic graphite-epoxy laminates is reported. The test data are used to evaluate the effects of specimen size and specimen configuration on the measured value of laminate fracture strength. It is shown that apparent fracture strength is independent of specimen configuration and specimen thickness, but varies significantly with respect to crack length. As crack length increases, however, the apparent fracture strength is found to asymptotically approach a limiting value, taken to be the valid laminate fracture toughness.

The test data are also used to evaluate two analytical models for predicting laminate fracture toughness. The first model links basic ply properties and the fracture toughness of angle-ply laminates, while the second model relates the fracture toughness of an arbitrary orthotropic laminate to the fracture properties of its angle-ply components. Both models show good agreement with the test data.

It is concluded that linear elastic fracture mechanics does provide a meaningful characterization of crack growth in orthotropic composite laminates, if some specified conditions are met. Study of cases which do not meet these conditions is recommended for future work.