Modelling and estimation of critical fracture energy, Gfr in ductile to brittle transition zone for characterising crack growth phenomenon in 20MnMoNi55 RPV steel

Abstract

One of the methods for describing the phenomenon of stable crack growth in ductile materials is the estimation of critical fracture energy, Gfr as proposed by Marie and Chapuliot. These methods of calculation of Gfr are applicable for large crack growth. However, the method for estimating Gfr was re-investigated for short crack growth. It was observed in the previous works that the Gfr values obtained for short crack growth gradually increases and saturates to the value at large crack growth. This saturated Gfr value can be claimed to be a material property. It was also shown that a correction factor (function of critical CTOD and crack growth) when introduced to this saturated value of Gfr helps in obtaining the unsaturated values of Gfr. In the present work, an attempt is made to estimate Gfr using the above mentioned re-investigated idea at the upper part of ductile to brittle transition temperature regime (0 °C to-60 °C). At sub-zero temperatures stable crack growth is much less when compared to the room temperature. The correction factor which is a function of saturated Gfr value, CTOD and crack growth is found to be suitable for the CT specimen results even at low temperature. The values of unsaturated Gfr values thus obtained using the correction factor are then used to simulate load versus load line displacement curves. The simulated load versus load line displacement curves at different temperatures are in good agreement with the experimental results irrespective of change in a/W ratios of the CT specimen.