Strain based local approach to cleavage fracture

Local Approach to fracture requires a combination of finite element (FE) calculations and experimental results that are needed to construct the applied load-Weibull stress calibration curve, from which the Weibull stress at failure is derived and eventually, through an iterative process, the m exponent is assessed. Wallin (1984) has shown that for a sharp cracks the m exponent is equal to 4. Milella and Bonora (1997) have recently shown that the value of the m exponent actually depends on the shape of the stress field set up ahead of the notch tip which, in turn, depends on the radius of the notch itself. The sharper the notch, the smaller the exponent with a lower bound value of 4. This poses a serious question as to whether the value of the m exponent inferred from a round bar containing a circumferential notch of given radius can be used to asses the integrity of geometries carrying sharp cracks. Also the Weibull stress at failure, required to build up the failure probability curve, poses some concerns in that the construction of the load-Weibull stress calibration curve may not be so simple, particularly at higher temperature, within the brittle regime and with specimens containing large notch radii, where necking and large deformations develop. In these cases the theoretically derived load-Weibull stress curve can be mesh and code dependent with no mean to experimentally verify the accuracy of the stress state within the plastic zone where the Weibull stress is calculated. Moreover, since the load-Weibull stress relationship is not linear at all, the application of a local approach procedure requires an iterative process to derive the actual Weibull exponent. To overcome these latter problems, local approach has been re-formulated in terms of true strain that can be measured and then more closely checked than stress over the reduced section of the specimen itself and can directly provide, as it will be shown, the actual value of the Weibull exponent without any iteration. The new model was applied to a round notch bar in traction carrying a circumferential notch of 2 mm radius and the material parameters for a low alloy steel for nuclear application were determined. The comparison between the failure assessment with the new model proposed and the Weibull stress based model is also presented and discussed