Integrated fracture mechanics approach to analyse fatigue behaviour of welded joints

Abstract

Current fracture mechanics methods for fatigue assessment, including those that consider thresholds for crack propagation, are based on long crack behaviour. The present work is concerned with an attempt to predict the fatigue strength of welded joints using a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, which is a function of crack length. The fatigue strength of butt welded specimens stressed transversely was analysed. Experimental results from the literature were used for comparison. Estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks, and the applied stress distribution along the crack path obtained from simple finite element models. The influence of plate thickness, initial crack length, and reinforcement angle on fatigue strength of butt welded joints was analysed. Results show good agreement with experimental trends. STWJ/425a crack length a f final crack length a i initial crack length a np length of non-propagating crack da/dN crack propagation rate A, C, m environmentally sensitive material constants d microstructural dimension (e.g. grain size) h weld reinforcement height k material constant that takes into account development of DK C k tx stress concentration at given distance x from weld toe surface DK applied stress intensity factor range DK C extrinsic component of DK th DK CR extrinsic component of DK thR DK dR microstructural threshold DK th fatigue crack propagation threshold DK thR fatigue crack propagation threshold for long cracks M k correction factor n constant N total fatigue life R stress ratio (minimum stress/maximum stress) t plate thickness t 0 reference plate thickness w weld reinforcement width x distance from weld toe surface along crack path Y crack shape parameter Y u crack shape parameter for welded joint a weld reinforcement angle r weld toe radius Ds nominal stress adjacent to welded joint Ds e fatigue limit (endurance 10 7 cycles) Ds eR smooth fatigue limit Ds n nominal applied stress range Ds t fatigue strength for thickness t Ds t0 fatigue strength for reference thickness t 0 Ds th threshold stress range for crack propagation s u ultimate tensile strength s ys yield strength Ds(x) local applied stress range s yy (x) non-uniform stress field calculated along notch bisector, without considering presence of crack INTRODUCTIONMechanical fatigue of metallic materials

A simple expression to estimate the fatigue endurance of welded joints

“…Previous works have been done by using the proposed method in order to analyze the influence of different weld geometry parameters on the fatigue resistance of welded joints, like plate thickness, reinforcement angle, weld toe geometry, defect sizes and undercuts [7][8][9][10]. Results showed that the method allows understanding the influence of the different mechanical, geometrical and microstructural parameters on the definition of the fatigue resistance of the joint.…”

“…Finally, it is important to say that although fixed global geometry and loading condition were used in the analysis of the influence of undercuts, the proposed method can scientifically explain the effect and relevance of most of the mechanical and geometrical parameters, as was shown in references [7], [8], [9] and [10].…”

“…The analysis carried out in references [7][8][9][10] includes all details related with the configuration and use only a few hypotheses related with the applied models. In the present work the idea is to use the different results obtained in previous analysis and look for a simplification to be use in standards in a simple way.…”

“…Details of the method and some applications can be found on reference [7,8], [9] and [10]. In this work a simple expression to estimate the fatigue endurance of welded joints is proposed on the bases of the results of previous analysis and results [7][8][9][10].…”

“…To avoid some limitations on the analysis, a fracture mechanics methodology based on the resistance curve concept was proposed by Chapetti et al to analyze welded joint, accounting for the fatigue behavior of short cracks and considering the fatigue crack propagation threshold as a function of crack length [7,8]. Details of the method and some applications can be found on reference [7,8], [9] and [10].…”