The use of slender double cantilever beam specimens for measuring the mode I interlaminar fracture toughness was critically evaluated. Experiments were performed with unidirectional composites to judge the validity of using data from multiple crack jumps on a single specimen. Three approaches for calculating the strain energy release rate were compared. These included an energy rate determination of the J I -integral, a compliance calibration procedure and an analytical equation based on linear beam bending. The fracture toughness was overestimated by the latter two approaches. Permanent deflection was seen to accumulate in the arms of the double cantilever beam specimens as the cracked surface area increased. This component of deflection must be accounted for in all forms of analysis with DCB specimens.

A computer controlled test procedure for evaluating mode I and mode II interlaminar fracture behavior was used in experiments with eight different resin matrix/graphite fiber composites. Four analytical methods for calculating fracture toughness were compared. These included an energy rate determination of the J‐integral, a compliance calibration procedure, equations based on linear beam bending, and an Area method calculation. Methods that account for nonlinear material behavior, such as the J‐integral, were needed for characterizing the systems with high fracture toughness. The ratio of mode II to mode I fracture toughness ranged from 1.5 to 8.0, depending on the material system. Finally, preliminary work with a technique for constant strain rate testing of mode I DCB specimens is presented.