Abstract
The present work is concerned with an attempt to predict the fatigue strength of welded joints by means of 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, a function of crack length. The fatigue strength of butt-welded specimens stressed transversely was analyzed. Experimental results from the literature were used for comparisons. Good estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks corresponding to the base metal, and the applied stress distribution along the crack path obtained from simple FE models. The influence of parameters like plate thickness, initial crack length and reinforcement angle on fatigue strength of butt-welded joints can be analyzed and results have shown good agreement with experimental results and trends. In order to verify the ability of the methodology to estimate the fatigue behaviour of welded joints, a dedicated experimental methodology had to be implemented for the detection and monitoring of the development of surface small cracks initiated at weld toes. Some results of a multi-strain gauges technique implemented for those purposes are also shown.
Highlights
To ensure that the full effect of the three key features dominating the fatigue life of welded joints is allowed for in design, most fatigue design rules consist of series of Δσ-N curves based on data obtained from constant amplitude fatigue test on actual weldments [1,2,3], in which use is usually made of the commonly used classification method of specifying design curves in terms of the fatigue strength at a given number of cycles (e.g. 2x106 or 107)
Much effort should be done in order to analyse the influence of short crack effect in the definition of fatigue strength
The influence of short crack effect depends on the applied stress ratio and only can be neglected when the transition from short to long crack regime become similar to the initial crack length
Summary
To ensure that the full effect of the three key features dominating the fatigue life of welded joints (geometric stress concentrations, welding flaws and residual stresses) is allowed for in design, most fatigue design rules consist of series of Δσ-N curves based on data obtained from constant amplitude fatigue test on actual weldments [1,2,3], in which use is usually made of the commonly used classification method of specifying design curves in terms of the fatigue strength at a given number of cycles (e.g. 2x106 or 107). Where m, A and C are environmentally sensitive material-constants obtained from long crack fatigue behavior and N is the total cycle number to failure (fatigue life) According to this expression, the Δσ-N curve is linear on a log-log basis with a slope m equal to that of the Paris law. Radaj and Sonsino have recommended initial crack size ai = 0.1-0.25 mm in welded structures for life predictions [10] These defect depths clearly fall on the short crack regime. The present work is concerned with an attempt to predict the fatigue strength of welded joints by means of a fracture mechanics approach that disregards the fatigue crack initiation life and includes the fatigue crack propagation threshold for both short and long cracks. Details of the fracture mechanics approach can be found in reference [11], where a model to estimate the threshold for fatigue crack propagation can be found
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