ADVANCES IN THE MECHANISM OF CLEAVAGE FRACTURE OF LOW-ALLOY STEEL AT LOW-TEMPERATURE .2. FRACTURE MODEL

The authors discuss in detail the existing cleavage fracture model inc luding the physical model and the statistical model. Based on the disc overy that a minimum distance for initiating the cleavage fracture was revealed to have a definite physical meaning, that the stress triaxia lity reaches the critical value Te at this distance instead of that th e presence of an eligible carbide crack within this distance is assure d; a combined criterion for cleavage fracture, i.e. a critical plastic strain (epsilon p greater than or equal to epsilon pc) for initiating a crack nucleus, a critical stress triaxiality (sigma m/<(sigma)over bar>greater than or equal to Tc) for preventing it from blunting and a critical normal stress (sigma yy greater than or equal to sigma f) fo r its propagation has been proposed to substitute for the criterion of sigma yy greater than or equal to sigma f over a 'characteristic dist ance' suggested by the RKR model. With regard to the statistical model the authors suggested that it is not necessary to describe the fractu re stress distribution by Weibull function. The fracture probability a t a given applied load was modified by multiplying a 'remaining probab ility' taking account of the effect of the preceding: loading process. The authors also discuss the mechanism of transition from a fibrous t o a cleavage crack within the toughness transition temperature range.