Extensometer probe indentation during low-cycle fatigue of plain and circumferentially notched cylindrical bars at 550 °C

Cyclic stress–strain tests were undertaken at 550 °C on (i) plain and (ii) notched specimens of different acuities in several low- and highly-alloyed ferritic steels (1CrMoV, NF616, TB12M and HCM12A). Axial strains were measured between the minimum sections of the (semi-circular) notches using a longitudinal extensometer, while surface hoop strains were measured by means of a diametral extensometer with probes located at the notch root. Over a period of 100 cycles, softening occurred in both specimen types. Cycle-by-cycle embedding was observed for the (diametral) probe tips and was entirely due to testpiece working in the cyclic state, the sprung-load extensometer force being far lower than that encountered in conventional indentation experiments. An interpretation of the effect is offered (a) in terms of the internal stress state produced during cycling and (b) by an energy argument to explain the ‘ranking order’ of resistance to penetration. The effects of cyclically-induced oxidation, possible mechanisms for the embedding effect and likely reduction in endurance are considered.