The Use of a Non-conventional Fracture Mechanics Parameter for Estimating SN Curves of Welded Joints

Abstract

The notch stress intensity factor (NSIF) is an extension of the stress intensity factor (SIF) concept for pointed V-notches which allows the stress field intensity in uncracked geometries to be known. A fatigue crack departing from the weld toe, for example, will be under the influence of the NSIF as long as the weld toe is modeled as a pointed V-notch. If the path and shape of this crack are considered constant, then their driving force, the SIF, can be linked to the NSIF and a valid fatigue crack growth (FCG) analysis can be performed in both the short and long crack regimes. In the present paper, the previsions thus obtained were compared with fatigue strength data taken from the literature and highly conservative results were found. These results, however, confirm that the weld toe in the type of joints studied can be modeled as a pointed V-notch with a definite opening angle and the stress field in the small region near the notch tip can be completely described by the NSIF. The difficulties related to the use of the classical linear elastic fracture mechanics (LEFM) in the short crack regime could be overcome. Also, the LEFM-NSIF-based procedure was capable of reducing the width of the scatter band in SN curves when comparing to the nominal stress concept. On the other hand, the approach was tested for various initial crack sizes and also for another set of Paris’ constants and an undesirable high sensitivity to these parameters was found. Its use in real-world engineering problems is limited and it must always be taken into account that the results are strongly dependent on material parameters and initial values.