Instability under Cycles of Plastic Deformation

Abstract

Instability of metals due to cyclic strain accumulation in the plastic range is relatively unknown and has not been thoroughly analysed. One of the simplest forms of cyclic strain accumulation, referred to as cyclic creep by metallurgists, is indicated in Fig. 1 by results of a load controlled test on a uniaxial mild steel specimen. Fig. 1 shows that irreversible increments Δe of permanent axial strain accumulate cycle by cycle until the gradual elongation of the specimen causes necking and fracture as observed in monotonic tension tests. It is worthwhile noting that even in this case, fracture is caused by necking, i.e., tensile instability, and not by low cycle fatigue cracks. Additional test results indicate that cyclic strain accumulation takes precedence over fatigue in causing failure for the higher stress ranges and that Δe increases with increasing stress range.