Quantifying Weld Undercut Severity on the Fatigue of Marine Structures

Fusion welding is the most common metal-to-metal joining method in the manufacturing of metallic structures. The physical process in the fusion welding requires localized melting of the metallic components with the application of high energy, which then solidifies to merge and form the welded joint. The volatile nature of the welding process also introduces several imperfections in the welded structure, such as undercuts, which affect the mechanical performance and service life of the structure. Undercut imperfections have a significant detrimental effect on the fatigue life of welds, and they can lead to early failure of the structure, potentially risking public safety. Modern manufacturing methods can produce higher-quality welds without continuous undercuts; however, local undercuts are unavoidable due to the stochastic nature of the welding process. While the continuous undercuts have been widely investigated, the research on the effect of the local undercut geometry on fatigue has remained limited. A quantified approach is needed to assess the impact of different undercut geometries on fatigue. This dissertation aims to reveal the influence of the local undercut on the fatigue life and then to develop a characterization method for these undercuts. The research work systematically examines the effect of several individual geometric parameters of the undercut on the fatigue performance of the weld. Based on experimental observations, a parametric undercut geometry model is developed to describe the 3D geometry of the local undercut. Additionally, a geometric library of realistic undercuts is generated for a comprehensive numerical investigation. The research employs high-resolution 3D geometry measurements, fatigue experiments together with statistical analyses and comprehensive numerical simulations. A strain-based fatigue life approach is utilized to investigate the relationships between the local undercut geometry and fatigue crack initiation life. The investigation covers varying local undercut geometries, materials, stress ranges, and weld geometries. The results of the thesis show that local undercuts exhibit a distinct 3D presence with complex geometries, which significantly influences the fatigue strength of welded joints. The shape of local undercuts was found to vary significantly, represented by the considerable fluctuation in the geometric parameters over short intervals of a few hundred micrometers. To characterize these complex geometries, a 3D Undercut Indicator has been proposed, demonstrating a correlation with the fatigue crack initiation life. However, this correlation is highly case-dependent, being sensitive to, e.g., load level and material properties. To address the sensitivity of this correlation, a novel, case-independent Undercut Severity Index (ΨUSI) has been developed. This index quantifies the fatigue-deteriorating effect of the local undercuts effectively, offering a unified approach to estimating fatigue crack initiation life by considering the influence of local 3D undercut geometry, applied stress range, material properties, and global weld geometry. The ΨUSI shows significant potential to promote a flaw-tolerant design philosophy and a fatigue-based weld quality control, offering a comprehensive toolkit for predicting the fatigue behavior of welded joints in marine, transportation, and civil industries.