Cyclic stress–strain behaviour of circumferentially notched cylindrical bars at high temperature

Abstract

Cyclic stress–strain tests were undertaken at temperatures between 550°C and 820°C on plain and doubly-notched specimens of three alloy steels: 20Cr/25Ni/Nb, 316 and 1CrMoV respectively. Integrated axial ‘strains’ were measured between the minimum sections of the (semi-circular) notches using a longitudinal extensometer, while surface hoop strains were measured on one notch by means of a diametral extensometer. Cyclic hardening occurred in plain specimens of 316 steel, cyclic softening occurred in the 1CrMoV steel and the 20Cr/25Ni/Nb alloy showed stable behaviour. These effects were also demonstrated in notched specimens, although to a lesser degree. From hysteresis loops determined during multiple step tests, the cyclic deformation response of the notched regions was expressed in terms of an ‘equivalent gauge length’. Comparison was also made between the equivalent (von Mises) stress–strain curves deduced at a representative (skeletal) point in the minimum section with stress–strain data obtained from uniaxial (plain) specimens. The equivalent curves calculated from standard relations were found to be lower than the uniaxial curves and possible reasons for this are suggested.

Keywords:

• axial/diametral strain
• cyclic hardening/softening
• effective Poisson’s ratio
• equivalent gauge length
• equivalent stress–strain
• failure energy
• low cycle fatigue
• skeletal point