For several years the nonlinear energy method proposed by Liebowitz and Eftis has been examined as a failure criterion for static testing of center-cracked and compact tension specimens. Since the method appears to be valid under conditions of crack-tip plasticity, subcritical crack growth and load relaxation, tests have been conducted to ascertain the merit of this method as a failure criterion under cyclic loading conditions. The nonlinear energy toughness for cyclic loading, 4, is obtained from an envelope of the cyclic load-displacement record, which naturally imposes some restrictions on the loading program.
The cyclic toughness parameter, 4, has been evaluated for thin, center-cracked sheets of 2024-T3 and 7075-T6 aluminum alloys. The specimen dimensions were held constant and the load parameters were varied so that a significant variation of the cyclic life was obtained. Both alloys exhibited a significant reduction of G,< with increasing cyclic life in a manner similar to the classical S-N diagram. For example, the ratio of cyclic to static toughness, G,C/G,, was found to be about 0.8 when failure occurred after approximately 150 cycles. There appeared to be a tendency for the curve to level off at this point, which suggests4hat these curves may represent compressed S-N curves. It is felt that this method may serve the design process by allowing the establishment of a fracture toughness parameter capable of including the effects of the entire loading history of a structure into the fracture toughness requirements.