Characteristics of crack tip deformation during high temperature straining of austenitic steels

Abstract

Deformation behavior, local distribution of mechanical properties and the microstructure near crack tips originating from notches were studied in 316 stainless steel and in Incoloy 800 H, which had been deformed in tension at 923 K at a strain rate of 1.3 × 10 −6 s −1. Local strain measurement showed that strain is concentrated in a rather large region ahead of the crack, several 100 μm in size; here, a multiaxial stress condition is established and severe cavitation is observed. By means of micro-hardness measurements, work-hardening in this region was found to resemble that which is obtained in the steady state for a uniaxial stress state at a 10 times higher strain rate. The average stress concentration in the crack tip area may be estimated to amount to 30%. It is proposed that a prerequisite for crack growth is the establishement of a critical state with respect to both the creep damage (cavitation) and the degree of work-hardening ahead of the crack. Differences in the crack growth behavior in notched and unnotched specimens are ascribed to whether the critical state is established locally ahead of the crack or homogeneously within the specimen.