Deformations in welded panels

Abstract

Welded panels are commonly used for ships hull manufacturing, and in order to lighten the hull the stiffened panel thickness tends to be reduced, thus increasing the distortion effects due to welding. This project will be part of Chalmers contribution to the benchmark exercise in a specialist committee, Material and Fabrication Technology, for the International Ship and Offshore Structures Congress (ISSC) 2021 conference. The project’s purpose is to compute the deformation for the ISSC2021v.3 benchmark [1] geometry of a stiffened welded panel which comprises of a stiffener 1000 x 100 x 5 mm DH36 plate which is initially tack welded on one side to a 1000 x 400 x 5 mm DH36. The welding procedure consists of a first weld fillet being deposited on one side of the stiffener, followed by a cooling time, and then another weld fillet being deposited on the other side and followed by a second cooling time before the welded panel is finally unclamped. To achieve that, both thermal and mechanical analyses are carried out by relying on the nonlinear Finite Element Analysis (FEA). The commercial FEA program ABAQUS is used for analyses in this project. Those simulations are however performed for a shorter plate and stiffener of length 200mm. The material properties were implemented in ABAQUS input file form, for both the weld and the plate and stiffener. Nonlinear thermal and mechanical properties were taken into account. The specific heat and thermal conductivity were implemented for the DH36 steel, however other properties such as thermal expansion coefficient, Poisson’s ratio etc., were implemented for a similar steel, S355. The weld material and the plate and stiffener are altogether modelled as being the same material except regarding the hardening modulus and the yield stress. When compared to the plate and stiffener material, the weld material itself has been modelled as having a higher yield stress and higher hardening modulus at room temperature. Two thermal models are considered with each having 1000 C as the heat source temperature of the weld. First, the fillet weld is activated simultaneously and in the second, the fillet weld material is activated sequentially by dividing the weld into segments. After each weld pass the model is allowed to cool down for 200 seconds so that the welded panel has cooled to around 50 C at the end of the simulation. The thermal simulations are coupled to mechanical simulations. In mechanical simulations, different clamping sequences are carried out. One with fully fixed and two with sliding clamps. In total six configurations are considered, since there are two thermal models and three clamping conditions for the mechanical simulations. Clamping is taken care of by taking the top and bottom nodes of the plate at designated locations. The same mesh model was used for both the thermal and mechanical simulations, and it consists of linear 8-node hexahedral and linear 4-node tetrahedral solid elements. The linear hexahedral elements are used to model the plate and stiffener and the welding joint is modelled by linear tetrahedral elements. Prior to choosing vi a final mesh a mesh convergence study was conducted by monitoring the temperature variation at a location close to the weld. The resulting mesh had 31200 nodes and elements with an aspect ratio lower than 10 to avoid complications with the mechanical simulations. The results of this project showed that the type of heating source and the clamping conditions have definite effects on the behaviour of the model. The displacements results for the mechanical simulations indicated that the sequential heating results were closer to the intended ones than for the simultaneous heating case. Also, the extent of clamping conditions, i.e., either fully clamped or sliding clamps, along with the methods used to prevent rigid body motions for a sliding clamp are all factors which have an effect on the behaviour of the model. And these differences in the thermal and mechanical loading configurations will be magnified when they are applied to the full length of 1000 mm for the plate and stiffener as per the ISSC2021v.3 benchmark.