Influence of weld induced distortions in fatigue strength of thin laser-hybrid welded ship deck panel
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Journal Title
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Volume Title
Insinööritieteiden korkeakoulu |
Master's thesis
Authors
Date
2018-10-29
Department
Major/Subject
Mcode
ENG25
Degree programme
Master's Programme in Mechanical Engineering (MEC)
Language
en
Pages
59+15
Series
Abstract
Shipbuilding industries, as well as other large-scale structural design industries, are following a trend of utilising higher strength materials and thinner structural panels to improve the operational efficiency and performance of their products. The limited knowledge about fatigue behaviour and lack of analytical solutions for distorted thin plates are predominantly limiting their application today. Thinner plates have lower bending stiffness than thicker ones, causing them to distort more during the welding operation. Such distortions and misalignments have an impact on the fatigue life of the structure as observed in the previous research. However, there are no rules devised to describe the methods of estimating such distortions and evaluate the impact of each distortion parameter on the structural behaviour of a distorted plate. This thesis studies the influence of weld-induced distortions and misalignment on fatigue behaviour at the butt weld of a laser-hybrid welded 4 mm thin ship deck panel. The thesis mainly focuses on finding the most influential distortion and/or misalignment parameter that dictates the fatigue behaviour of such thin deck panels. The study is based on Finite Element Analysis of several distortion shapes to evaluate the structural hot-spot stress on the butt weld of the deck panel. A research method tailored to this study is developed and validated with the experimental data from previous research showing a good agreement with the analysis done utilising an accurate geometry. The sensitivity analysis method OAT (one-at-a-time) is employed to identify the influence of the distortion parameters. The results showed that distorted geometry could be simplified with 4-point splines on either side of the butt weld along the longitudinal direction, and 3-point splines along the transversal direction of the deck panel. The 4-point spline is sufficient to describe the local and global distortions of the deck panel. Apart from the axial misalignment, the most important parameters to describe the geometrical shape are local angular misalignment, the distortion magnitude and location of local maximum distortion close to the butt weld. The distortion parameters further away from the weld has a minor influence on the structural stress at the butt weld. It is also observed that the angular misalignment can independently provide a better estimate of stress magnification factor compared to any other distortion parameters. The results demonstrate that the current analytical rules to estimate the stress magnification factor are not suitable for thin distorted panels as they do not consider the non-linear behaviour of thin distorted panels. Current rules underestimate stress magnification factor at low angular misalignments and highly overestimates it at larger misalignments.Description
Supervisor
Remes, HeikkiThesis advisor
Lillemäe, IngritRautiainen, Matti
Keywords
distortion, misalignment, thin plate, laser-hybrid weld, sensitivity analysis, fatigue