Effect of several parameters on low cycle fatigue behavior

Abstract

Syracuse University has recently been engaged in studies pertaining to the low-cycle controlled strain fatigue behavior of steel and aluminum alloys. Axial tension-compression and bend cycling fatigue tests were utilized to investigate the effects of a number of parameters including mean strain, stress biaxiality, superimposed stresses, test temperature and type of loading. Also, studies pertaining to accumulated damage were undertaken in an attempt to gain a better understanding of the nature of strain cycling fatigue and the damage mechanisms that are operative during the life of a strain-cycled specimen. These studies have shown that low cycle fatigue phenomena are best analyzed from strain-controlled rather than stress controlled test data. The following are also indicated as a result of these studies: 1. (1) The relationship proposed independently by both Manson and Coffin predicts the failure life of strain controlled specimens, but only for mean strain equal to zero. A modified equation is proposed which accounts for the effect of mean strain. 2. (2) The effect of both stress biaxiality and superimposed stresses is to reduce specimen life at a given strain cycling level. The effect of test temperature can be regarded as a reflection of the effect of temperature on uniaxial fracture ductility. 3. (3) Damage studies indicate that the accumulation of fatigue damage in a strain cycled specimen is not predictable by the same equations that predict failure life. Rather, it appears that “accumulated damage” is governed by at least two, possibly interdependent processes, namely: a loss in available ductility due to strain hardening, and the formation and growth of cracks which finally determines failure.