Nonlinear material modelling for fatigue life prediction of welded joints in high strength marine structures
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Insinööritieteiden korkeakoulu |
Master's thesis
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Authors
Date
2022-03-21
Department
Major/Subject
Computational Mechanics
Mcode
Degree programme
Master's Programme in Mechanical Engineering (MEC)
Language
en
Pages
117+12
Series
Abstract
The key challenge that is hindering the development of lightweight ship structures is the lack of a fatigue-life assessment tool for welded thin plates and high-strength steels. The Voce-Chaboche (V-C) cyclic plasticity model is able to model cyclic loading-induced changes in the mechanical properties of a welded joint. This thesis evaluates the suitability of the V-C model to improve the fatigue-life estimation of welded joints in high-strength steel ship structures. To this end, the wave-induced strains in an upper deck longitudinal deck girder of a large cruise ship were analysed using a finite element analysis. Subsequently, a fatigue test was designed that is representative of the fatigue-relevant stresses acting on a cruciform welded joint with a micro notch located at the weld toe. The base material for this welded joint was S355 steel, respectively S690 steel. These specimens were then subjected to a fatigue-relevant service load history. The strain was recorded using three strain gauges placed in each material zone of the weld. The experiment was replicated in commercial finite element analysis software. The material behaviour was described by the V-C model. A linear elastic model was used as a baseline. The V-C model parameters of the different material zones of the welded joints were determined using the accumulated true strain approach. Finally, the recorded strain histories from the experiments and the simulations were compared based on fatigue-relevant criteria related to the strain-life approach. The results showed that the V-C model performs better than the linear elastic model when the material experiences plasticity, like the weld metal of the undermatched welded S690 joint. The definition of the elastic modulus had the largest effect on the fatigue-relevant evaluation criteria. V-C model parameter calibration methods tend to underestimate this parameter, which can lead to conservative fatigue-life results. For future research, it is recommended to perform an extended sensitivity study of the V-C model parameters and their influence on the transition region between elastic and plastic material behaviour. Finally, additional validation load cases should be developed based on available service load histories, that include strain overloads as expected in welded joints in a ship structure during a storm.Description
Supervisor
Remes, HeikkiThesis advisor
Gallo, PasqualeNiemelä, Ari
Keywords
cyclic plasticity, high-strength steel, Voce-Chaboche, fatigue