Near-threshold fatigue crack propagation and internal cracks in steel

Abstract

The existence or non-existence of fatigue crack growth thresholds is of practical importance and scientific interest, as is knowledge of the stresses enabling crack formation at the surface and the interior of a specimen. Near-threshold measurements have been performed on 12% chromium steels with an ultrasound testing technique at different mean loads in ambient air and vacuum at very high numbers of cycles. Microstructural features and fracture surfaces were investigated in an SEM. The fracture appearance after cycling in vacuum in the near threshold regime is compared with that of the fine granular and smooth areas of internal “fish-eyes”.