A CONSTANT-G DOUBLE CANTILEVER BEAM FRACTURE SPECIMEN FOR ENVIRONMENTAL TESTING

Straight-sided double cantilever beam (DCB) fracture specimens have be en used previously for long-duration environmentally assisted crack gr owth (EACG) testing. However, the applied strain-energy release rate G decreases with crack extension. In this study the width taper require d to achieve constant-G during crack growth in a double cantilever bea m (DCB) fracture specimen is determined for the case of general loadin g. The width taper b(a) is a function of the initial specimen complian ce, C0, the crack length, a, and the stiffness of the loading frame, k . The compliance of DCB specimens tapered for testing under three load ing conditions were verified experimentally. These were constant-load (k --> 0). constant-strain (k --> infinity), and critical spring loadi ng (kC0 --> 0.5). Specimens designed for critical spring loading are s uggested for use in long-term (EACG) experiments. Limitations on the p roposed test method related to nonlinear material behavior (e.g., crac k bridging) are discussed.