[en] A testing program was initiated to develop a procedure for toughness testing of irradiated materials using J integral methods. An evaluation of presently used procedures and improved techniques was completed to increase the reliability of the unloading compliance method for single specimen J/sub Ic/ determination. Three different heats of pressure vessel steels were used to evaluate the reliability and accuracy of the unloading compliance procedure. A new technique for improved sensitivity of displacement measurements was found to be successful on large 4T specimens. This technique consisted of using a small electrical linear actuator to continuously rezero a 0.635 mm (.025 inch) LVDT. The effects of specimen geometry on both J/sub Ic/ and dJ/da were evaluated using various specimen sizes and varying amounts of side grooving