FURTHER STUDY ON THE MECHANISM OF CLEAVAGE FRACTURE AT LOW-TEMPERATURES

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-degrees-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 sigma(yy) exceeding the cleavage stress sigma(f), or the probability assuring the presence of an ''eligible'' cracked particle for which the criterion is satisfied. The X(min) has a definite physical meaning at which triaxiality sigma(m)sigmaBAR reaches the critical value T(c), to prevent a produced crack nucleus from blunting. From the X(min) and the measured cleavage stress simga(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.