Interlaminar fracture toughness testing of composite mode I and mode II 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.