PAPER 4 - Prediction of overmatching effects on fracture of cracked stainless steel welds

Abstract

This paper discusses the problem of the accuracy of fracture prediction models when considering crack initiation in a weld in an engineering structure, and how that relates to crack extension in a small scale specimen. Weld metal mismatching consequences may greatly affect the crack growth resistance of such a structure, which makes integrity assessment becoming more complicated. Generally, the fracture mechanics assessment procedure assumes that the crack is located in an homogeneous material of uniform tensile and toughness properties, adopting either the weld metal properties, or the lowest tensile properties and the lowest toughness value of the base metal or the weld metal. However, this solution may lead to overconservatism in some cases and overestimates the critical size of allowable flaws. This paper presents the preliminary results of an original study which aims at determining a fracture criterion that could be applied to welds in conditions where classical fracture mechanics concepts fail. Elastic plastic finite element analyses have been performed for various specimens containing strongly overmatched cracked welds. It has been shown that overmatching can provide a substantial benefit in terms of toughness when considering short cracks in the weld metal. It is recommended also to examine cracks located in the fusion line where difference of strain carrying capacities of base and weld metal generates local concentration of shear stresses, which may affect drastically-the weld toughness properties.