An apparatus and a method for measuring and controlling the crack growth rate within a double cantilever beam type test specimen (50). The arms (51,52) of the test specimen are fitted with a pressure-actuated bellows (62) to induce a predetermined load and with a sensing assembly (70) to provide feedback on the amount of beam displacement resulting from application of that load. In this manner a loaded test specimen may be remotely mounted and adjusted inside the reactor pressure vessel or piping of a nuclear reactor in order to maintain a stress intensity which is constant or which varies in a predetermined manner for inducing stress corrosion cracking or corrosion fatigue in the specimen.