A series of punch test measurements was performed on a broad set of engineering alloys to establish the correlation between J and the equivalent plastic strain, εf at the point of fracture. The punch test has a strong advantage over conventional methods for measuring material toughness because it requires only small coupons of material that can be removed from in-service structures. Tests were performed at both room and elevated temperatures in both weakly and strongly anisotropic materials. Results of the program indicated that the simple linear correlation between J and εf predicted by the theory is too simple to describe such a complex phenomenon as ductile fracture. Furthermore, difficulties were discovered in the testing of strongly anisotropic materials that allow for toughness measurements only on those planes with relatively weak crack growth resistance. A modified punch test apparatus is proposed to address this problem.