This paper discusses the critical conditions for an interface-related fracture in epoxy-based graphite fiber-reinforced structural laminates. The specific fracture mode under consideration is that of a splitting crack along the fiber direction. Fiber-wise splitting often occurs in laminates that have holes, sharp notches, or cutouts due to stress concentration near these locations. The problem has been treated as an interface shear-dominated fracture, and methods of fracture mechanics are frequently employed for the purpose of analysis. Within this context, suitable mixed-mode fracture criteria are required. Past studies have used the strain energy release rates in the formulation of the desired criteria, and, depending on the test model used, the formulated criteria came in a variety of forms. Among several possible forms, the one form involving the total strain energy release rate (GI + GII) = constant has often been recommended. Recently, however, a form employing the stress intensity factors KI and KII has been deduced from a set of fiber-wise splitting test data. An apparent difference existed between criterion deduced based on G and that deduced based on K. Theoretically, of course, the two deduced criteria should be mutually equivalent.
This paper discusses the reasons for the apparent difference and offers a consistent interpretation of the respective set of test data. When this is done, a unified criterion, be it expressed in G or in K, can then be deduced without contradiction.