Further study on the mechanism of cleavage fracture at low temperatures

Abstract

On the basis of finite element analyses of stress and strain ahead of a precrack tip and metallographic observation, the RKR model for cleavage fracture was reviewed, and the following conclusions were made: the critical event for cleavage fracture of a precracked specimen of CMn steel, tested around —100°C, is the propagation of a second phase particle-sized crack into the neighbouring ferrite grain; there is a minimum distance X min from the precrack tip to the initiation position of cleavage fracture. This X min is not determined by the criterion of the principal stress σ yy exceeding the cleavage stress satisfied. The X min has a definite physical meaning at which triaxiality σ m / σ reaches the critical value T c, to prevent a produced crack nucleus from blunting. From the X min and the measured cleavage stress σ f, the lower boundary value of K IC in the lower shelf of the transition curve could be calculated. A physical model for the statistical description of the scattering of the measured value of K IC from its lower boundary value is suggested.