Effect of Weld Geometry on the Fatigue Behaviour of Umbilical Super Duplex Stainless Steel Tubes

Abstract

The fatigue behavior of welded components is complicated by many factors intrinsic to the nature of welded joints. The mechanical properties of the material, the welding process and position, the type and geometry of the weld and the residual stress distribution across the weld are a few factors affecting fatigue behavior. Published studies [1, 2] have shown that weld geometry is significantly important in determining the fatigue strength of the weld. For a given weld geometry, the fatigue strength is determined by the severity of the stress concentration at the weld toe or at weld defects and by the soundness of the weld metal. The effect of external weld geometry profile on the fatigue behavior of welded small bore super duplex umbilical steel tubes is investigated. Root cause analysis consisting of fractography, metallography and weld profile measurement is carried out on pairs of fatigue failure samples which were tested at the same stress range but failed at significantly different number of cycles. The samples are selected from Technip Umbilicals Ltd (TU) fatigue database. Following the failure analysis, weld geometric profile measurements are performed on fatigue test samples that were prepared for testing. The weld profile was measured in terms of the external weld cap height, weld width and external linear misalignment. Axial fatigue tests are carried out on these samples which are pre-strained before test to simulate the plastic bending cycles typically experienced during the manufacturing and installation processes prior to operational service. The fatigue tests results are interrogated together with the measured geometric data to identify trends and anomalies. Key weld geometric fatigue performance criteria are subsequently identified. For the welded super duplex stainless steel (SDSS) tubes studied, the height of the weld and the weld toe angle provided the best correlation with fatigue life — shorter lives were obtained from specimens with the highest weld aspect ratio (weld height to width) and lowest weld toe angle.