Fatigue Design of Lightweight Welded Structures – Some Aspects of Size Effect

QC260120

Abstract

Lightweight construction is of great value in a number of industrial sectors, asreduced component thickness contributes to improved performance andincreased load capacity. High-strength steels (HSS) enable lightweight, highperformancestructures through their enhanced static and fatigue strength. Theapplication of these steels is significantly affected by manufacturing processes,such as welding. In welded structures, fatigue failures are typically initiated atthe welded joint, which is often the weakest link due to its lower fatigue strengthcompared with the base material.The size effect is defined in relation to the dimensions of the main plate, weld,and attachment. The fatigue strength of welded joints decreases with increasingplate thickness, a phenomenon known as the thickness effect. Conversely,reducing the thickness of welded structures under fatigue loading can improvefatigue strength, which is referred to as the thinness effect.In the current thesis, the size effect on fatigue strength of welded joints has beeninvestigated. The assessment relies on fatigue test data gathered from theliterature and from experiments carried out in this study, and finite elementmodelling. The Effective Notch Stress (ENS) method, a local fatigue assessmentapproach that relies primarily on peak stress and stress concentration values,has been investigated for welded joints. Furthermore, the probabilistic fatiguefailure in welded joints using a weakest-link modelling approach is also analysedfrom the stress distribution within the joint.Weld quality has a critical impact on the fatigue performance of weldedstructures, as higher-quality welds reduce stress concentrations andconsequently extend fatigue life. A detailed analysis of weld profile data provides valuable insights into the sources of uncertainty and variability in fatiguebehaviour, as well as the relationship between weld geometry and fatiguestrength. In the present study, variations in key weld geometry parameters,obtained from measurements, are thoroughly examined to better understandhow these geometric differences influence fatigue performance and reliability.Such analyses are essential for developing strategies to improve weld quality,enhance fatigue resistance, and ensure more predictable structural behaviourunder cyclic loading.The findings from the work performed during this thesis will contribute to thedevelopment of structural reliability assessment methods for the fatigue life ofwelded joints in relation to size effects. These methods will support the industryin designing lighter, high-performance products with a lower carbon footprint,ultimately contributing to more sustainable welded structures. The findingsindicate a pronounced thinness effect, where thinner plates exhibit higherfatigue strength. In addition, improved weld quality, reflected in larger weld toeradii and angles, contributes to extending fatigue life.

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