Predicting the temperature and strain rate dependence of the cleavage fracture toughness of ferritic steels

Abstract

An accepted micromechanistic model of cleavage fracture is combined with an analytical crack tip stress distribution to give a general expression for the cleavage fracture toughness of a ferritic steel. The toughness of a steel is determined by its cleavage fracture stress, its yield stress, its characteristic distance and its work hardening exponent. The temperature and strain rate dependences of these properties are briefly reviewed. The cleavage fracture stress is broadly independent of temperature and strain rate whereas the yield stress decreases with increasing temperature and with decreasing strain rate. The effects of both the temperature and the strain rate on the yield stress can be represented analytically by a single equation. The characteristic distance of a steel is determined by its microstructural geometry and so is independent of the temperature and the strain rate. Finally, the work hardening exponent varies somewhat with temperature and strain rate, decreasing as test conditions increase the yield stress. The cleavage fracture toughness of a ferritic steel is predicted to increase with increasing temperature. At very low temperature an apparent mechanism control effect operates which invalidates the current analysis. The temperature at the onset of this effect can be readily estimated. The toughness is also predicted to decrease with increasing loading rate under isothermal conditions. Predictions of both the temperature and the strain rate dependences of the toughness are successfully compared with published experimental results. The accuracy of the prediction of this analysis is limited by the available descriptions of the material properties and the crack tip stress distribution. The sensitivity of the prediction to possible inadequacies of these input data is discussed. It is noted that the model applies only when cleavage is induced by slip dislocation motion.