In general, distortion has significant effects on the assembly process of welded structures and remarkable influences on the strength of the welds. Therefore, this work focuses on the effect of angular welding distortion on fatigue strength to improve transferability of specimen results to components. Experimental investigations cover manufacturing and fatigue testing of three single-sided transversal stiffeners series exhibiting different angular distortions. The fatigue test results of as-welded specimen show a distinct link between fatigue performance and initial angular distortion. However, in case of a high frequency mechanical impact (HFMI)-treated weld toe, the fatigue strength increases up to base material level and is independent of the distortion. A comprehensive numerical analysis reveals a complex interaction between the applied nominal load, initial specimen distortion and the local stress field. In this context, an engineering-feasible assessment is derived to estimate the local effective stresses featuring the acting local stress range as well as the stress ratio. The application of this distortion factor enables the setup of a uniform S/N-curve with a significantly reduced scatter band.
The majority of fatigue tests of welded specimens is based on shop-made samples generally exhibiting minor misalignment. Due to the challenge of ensuring misalignment-free joints in industrial manufacturing processes, investigations focussing on the effect of misalignment on the fatigue strength are important. Therefore, this paper deals with the influence of axial misalignment on the fatigue resistance of butt-welded ultra high-strength steel specimen. In addition, the effect of high frequency mechanical impact treatment (HFMI) on the fatigue performance is researched. In the course of the experimental investigations, specimens exhibiting three different levels of axial misalignment are manufactured. Fatigue tests at a stress ratio of R = 0.1 in as-welded condition reveal a significant drop in fatigue strength with increasing axial misalignment. Fatigue assessments of the as-welded test results based on nominal, structural and effective notch stress approach are performed taking into account the sampledependent misalignment factor. The given equations enable an improved consideration of axial misalignment regarding to fatigue strength. The HFMI treatment increases the fatigue strength compared with the as-welded state; the detrimental effect of misalignment is less pronounced. A comparison to the current IIW guideline for HFMI treatment reveals a conservative assessment if IIW-recommended FAT values for as-welded condition are applied.
The division of the total fatigue life into different stages such as crack initiation and propagation is an important issue in regard to an improved fatigue assessment especially for high-strength welded joints. The transition between these stages is fluent, whereas the threshold between the two phases is referred to as technical crack initiation. This work presents a procedure to track crack initiation and propagation during fatigue tests of ultra high-strength steel welded joints. The method utilizes digital image correlation to calculate a distortion field of the specimens’ surface enabling the identification and measurement of cracks along the weld toe arising during the fatigue test. Hence, technical crack initiation of each specimen can be derived. An evaluation for ten ultra high-strength steel butt joints reveals, that for this superior strength steel grade more than 50% of fatigue life is spent up to a crack depth of 0.5 mm, which can be defined as initial crack. Furthermore, a notch-stress based fatigue assessment of these specimens considering the actual weld topography and crack initiation and propagation phase is performed. The results point out that two phase models considering both phases enable an increased accuracy of service life assessment.
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