CYCLIC DEFORMATION AND FRACTURE-BEHAVIOR OF AL ALLOY-6061 UNDER THE ACTION OF POSITIVE MEAN STRESSES

Cyclic deformation and fatigue fracture behavior of Al alloy 6061 were studied over an extensive range of positive mean stress. In the regim e of low cyclic stress amplitude, both secondary stage creep rate and fracture process are retarded by decreasing mean stress, but they are accelerated if the cyclic stress ratio is smaller than 0.75. The super imposition of cyclic compressive stress is found to decrease the creep rate, but it has no effect on the relationship between fatigue life a nd mean stress. Characteristic patterns on the specimen fracture surfa ces, representing the interaction between creep and fatigue and also t he mechanism of the fatigue-creep deformation in different mean stress ranges, are discussed analytically. Finally, the concept of equivalen t creep rate is proposed and a method to predict fatigue life with pos itive mean stresses is established and verified using the present expe rimental results.