The development of high-resolution crack monitoring methods to investigate the effect of the local weld toe geometry on fatigue crack initiation life

Abstract

The effect of weld toe geometry on the fatigue performance of welded joints is investigated, with a primary focus on the fatigue crack initiation (FCI) life of the joints. Available standards prescribe global approaches for estimating total life and crack propagation life, as FCI life has historically been considered to be negligible. However, experimental results from the literature show that FCI can take up a significant portion of the total life of welded joints, especially in the case of good quality welds under high-cycle fatigue loading conditions. Weld toe fatigue crack initiation is highly dependent on the local weld toe stress-concentrating geometry, including the inherent flaws such as undercuts, spatter, cold-laps and inclusions. Such flaws, as well as the inhomogeneous geometry, promote premature fatigue crack initiation. Thus, it is essential to investigate methods of resolving such flaws and characterising them in terms of their geometry and stress concentration. The weld toe stress concentration factor (SCF) is an important parameter for characterising the stress-concentrating features that act as fatigue crack initiation sites.
In this work, interrupted fatigue tests were performed on arc-welded joints manufactured by different welders with different electrodes to obtain a variety of weld toe geometries. To obtain fatigue crack initiation lives (corresponding to the growth of a technical crack of depth 250 µm), a novel multi-probe array of alternating current potential drop (ACPD) probes and strain gauges were positioned across the weld toe. These bespoke systems also located fatigue cracks. The effect of stress on the ACPD measurements was observed to influence results, and once cracks approached through-thickness they were difficult to characterise using this method. Further research to investigate this phenomenon has been identified. Both crack growth monitoring techniques (ACPD and strain gauges) were seen to correlate well with crack depth for isolated cracks. However, due to the large number of crack coalescence events in high stress-range fatigue loading conditions, both techniques proved inconsistent at the onset of crack coalescence. FCI lives and early crack propagation rates were correlated with the local weld toe stresses, which were obtained from a high-resolution (5-10 µm) stress analysis. 3D linear-elastic and elastic-plastic finite element models were developed from X-ray micro-Computed Tomography (µ-CT) scans of each fatigue tested weld toe. From these models, over 5000 SCFs were extracted for approximately every 50 mm of weld toe, thus providing comprehensive SCF distribution maps for each of the interrupted-fatigue tested and scanned weld toes. In obtaining FCI lives in welded joints, the local notch stress-strain approach has been used from the available literature. In this study, multiple variations of the approach have been investigated to potentially identify improvements to the approach when using state-of-the-art technology (industrial µ-CT) for resolving the weld toe geometry and bespoke data-rich crack monitoring techniques. The SCF distribution maps, along with the experimental FCI data, were used to identify the robustness of each of the variations in the literature. The use of fatigue notch factor (Kf) is seen to give significantly non-conservative results when combined with high-resolution SCF data. Elasticplastic SCF data was observed to give satisfactory results in terms of predicting the FCI lives. However, more computation time is required to get elastic-plastic SCF distributions compared to linear-elastic stress analysis. The number of tests performed was low and so were not statistically significant to draw firm conclusions but provide an indication of the methods that have the most potential.

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Identifiers

ORCID for Somsubhro Chaudhuri: orcid.org/0000-0001-9849-5766

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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