Einfluss von Imperfektionen auf die Schwingfestigkeit hochfester Stahlschweißverbindungen

Translated title of the contribution: Effect of defects on the fatigue performance of welded high-strength steel joints

Markus Ottersböck

Research output: ThesisDoctoral Thesis

Abstract

Welding is a common joining technology for manufacturing complex support structures as well as drive train units. Thus, it is also often applied for dynamically loaded assemblies. The welding process might cause design non-conformances such as misalignment or undercuts as surface near imperfections, both influencing the joint's fatigue strength. Therefore, the present work contributes to the fatigue assessment of welds exhibiting such manufacturing process based shortcomings. To begin with, a method to determine local weld geometry parameters from a 2D or 3D surface scan of the weld is introduced as first contribution. Therewith, regions of the weld exhibiting a very high stress concentration can be identified during manufacturing as part of the overall quality assurance process. Secondly, another procedure enables an optical detection of surface fatigue cracks using digital image correlation. Furthermore, this also allows a tracking of surface crack growth during the fatigue test. Axial misalignment and angular distortion of welded sheets induce a secondary bending moment due to axial loading. Butt joint fatigue tests using axially misaligned specimens show a decrease in fatigue strength of about 20% by an axial misalignment of only 10% of sheet thickness. In case of angular distortion, the inital direction of the misalignment is crucial for its effect. Here, T-joint fatigue tests exhibit a drop of 12% in fatigue strength for an angular distortion of 0,5°, whereas -1° increases the fatigue strength of 13%. This work presents fatigue assessment models for both misalignment types based on numerous numerical simulations and measurements. In a comprehensive study, the effect of the local weld topography on the fatigue strength of ultra high-strength steel joints is investigated. 3D surface scans and the subsequent numerical analysis of each specimen is performed. As a key result, the stress averaging approach by Neuber using a microstructural length of ρ∗ = 0,13mm is well suitable for an assessment of variations in weld topography. For a fracture mechanical assessment, the consideration of the crack initiation life is of utmost importance as the presented fatigue test results reveal its fraction to be at least 50% of the total fatigue life. The presented scientific findings support an improved design of welded joints with surface layer based weld toe imperfections and comprehensive axial and angular misalignment methodology, which is especially benefical for high-quality high-strength steel joints.
Translated title of the contributionEffect of defects on the fatigue performance of welded high-strength steel joints
Original languageGerman
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Leitner, Martin, Supervisor (internal)
  • Enzinger, Norbert, Assessor B (external), External person
  • Stoschka, Michael, Assessor A (internal)
Publication statusPublished - 2020

Bibliographical note

embargoed until null

Keywords

  • Fatigue strength
  • Welding
  • Weld defects
  • High-strength steels

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