Schwingfestigkeit geschweißter Strukturen unter Berücksichtigung lokaler Gefügeeigenschaften der Wärmeeinflusszone

Research output: ThesisDoctoral Thesis

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Abstract

The dimensioning of welded pipeline components made of thermomechanically rolled highstrength fine-grained structural steels is a great challenge for application engineers. Several codes, reports and guidelines are nowadays applicable for dimensioning, but due to the difficult estimation of the process influences they are quite conservative. Therefore, the lightweight design potential of such high strength steels cannot be used properly. In this work, a finite element based method for fracture mechanical fatigue assessment of welded high strength steel joints with consideration of local microstructures, residual stresses and a weld post treatment is presented. At first, the microstructures of the heat-affected zone (coarse-grained, fine-grained and intercritical zone) of a representative double submerged arc welded longitudinal seam were reproduced with a GLEEBLE(R) thermal simulator. In addition to tensile tests and hardness measurements, both these microstructural zones and the base metal were characterized by cyclic crack propagation curves and fatigue tests. The extensive experimental investigations show that the long crack threshold depends essentially on the local different microstructures. The weld process changes the microstructure and the residual stress conditions within the heat-affected zone strongly. The investigations proofed that, in case of thermomechanical rolled pipeline steels, the microstructural dependent threshold values within the heat-affected zone can even be higher than the base metal. But the crack propagation rates of the microstructures of the heat-affected zone are above the base metal. The crack length dependency of the threshold value was evaluated by means of microstructure-dependent fatigue crack resistance curves (R-curves). Beside the extensive fracture mechanical investigations, the fatigue strength examinations on small specimens made of the characteristic microstructures as occuring in the heat-affected zone did not reveal any differences in fatigue strength. All characteristic cyclic strength values are below the fatigue limit of the base metal. Thus, the observed fatigue strength behavior on small samples from different structures does not correlate with the determined fracture mechanical parameters. Finally, the accompanying residual stresses of the representative longitudinal weld seam were numerically estimated by an accompanying structural simulation with the software package SYSWELD(R). For the numerical assessment of the fatigue limits at two different stress ratios, a crack propagation tool originally developed at the Materials Center Leoben (MCL) and further enhanced in this work was used in conjunction with the FE package ABAQUS(R). The results of microstructural dependent crack propagation simulations are compared with the fatigue tests of large butt joints joined by submerged arc welding. It can be shown that a conservative numerical load cycle assessment for these large butt joint specimens is feasible with a very slight deviation from the experiment. As exploited by several numerical studies, it is necessary to combine the test results of small specimens, characteristic crack propagation values out of codes and a NASGRO(R) equation extended by the short crack range to obtain a properly matching fracture mechanical lifetime assessment. Furthermore, it can be shown that the positive effect of a weld post treatment process on the fatigue strength of welded samples can be represented numerically.

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Translated title of the contributionFatigue strength of welded structures considering local microstructures in the heat-affected zone
Original languageGerman
QualificationDr.mont.
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Publication statusPublished - 2017