Ultimate strength of aluminium panels, considering HAZ effects
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- Institutt for marin teknikk 
Aluminium stiffened panels have been increasingly used in marine, offshore and transportation industries, especially where high strength/weight ratio structures are highly required. However, the ultimate strength of aluminium structures can be seriously deteriorated by heat-affected zones (HAZ) caused by welding in assembly and construction. Existing design codes may not be able to appropriately reflect the structural behaviour and ultimate strength of stiffened aluminium panels with the types of welds and residual stresses considered and discussed in this thesis work. The scope of this thesis work is to systematically investigate the ultimate strength and collapse behaviour of three types of aluminium stiffened panels considering the effect of various types of welds, the residual stresses caused by relevant welds and the combination of welds and the relevant residual stresses, under axial compression and transverse compression respectively. The systematic investigations are carried out by using nonlinear finite element (FE) analysis. Comparisons between the FE results and relevant codes are also conducted to reveal the difference between the FE results and existing design codes. The extensive numerical FE simulations and the comparisons between the FE results and the relevant codes eventually generate and provide information and data that can be used for development of design code formulations for continuous stiffened panels built up of aluminium extrusions or/and welded stiffened aluminium panels, and to help better understand the structural behaviour of aluminium stiffened panels as well. The work in this thesis consists of the following main themes: (1) Overview of key influential factors on the ultimate strength of stiffened aluminium panels; (2) Overview of buckling and collapse behaviour of stiffened steel and aluminium panels, and the arc-length method in nonlinear finite analysis for unstable problems; (3) Meshing approach and convergence study for the models of ultimate and buckling analysis of stiffened panels; (4) Investigation of the effect of various types of welds and residual stresses on the ultimate strength of stiffened aluminium panels subjected to axial compression; (5) Investigation of the effect of various types of welds and residual stresses on the ultimate strength of stiffened aluminium panels subjected to transverse compression; (6) Parametric sensitivity study with respect to the effect of the material strength of the base materials, the material strength in the HAZ and the extent of the HAZ on the structural capacity of stiffened aluminium panels; (7) Proposed a simplified formula for estimating the transverse ultimate strength of aluminium panels with closed stiffeners, where both the stiffener restraint and the HAZ effect is accounted for; and (8) Comparisons of the ultimate strength obtained by FE analysis and relevant existing design codes. The main contributions of this thesis are:The effect of material softening due to various types of welds, residual stresses caused by relevant welds, and the combined effect of material softening due to various types of welds and the relevant residual stresses on the axial and transverse ultimate strength of stiffened aluminium panels has been systematically demonstrated by extensive finite element analyses.Parametric sensitivity study further demonstrates the important effect of material strength in the HAZ and the extent of HAZ on the structural capacity.Material sensitivity study shows that the stress-strain relationship between the elastic limit and the 0.2% proof stress σ0.2 significantly affects the capacity of aluminium panels, but the part above the 0.2% proof stress σ0.2 has a very small effect on the strength, and that the difference of the normalized ultimate strength (divided by the 0.2% proof stress σ0.2) for a same panel made of the different heattreated aluminium alloys is very small.Comparisons of the ultimate strength obtained by FE analysis and the relevant code formulations reveal that existing codes overestimate strength capacity in some cases, and underestimate strength capacity in other cases.Proposed a simplified formula for estimating the transverse capacity of panels with closed stiffeners for which the transverse capacity is underestimated by existing codes since the strong stiffener restraint effect is not accounted for. In the proposed formula, both the stiffener restraint effect and the HAZ effect on the strength are explicitly included.For large multi-stiffened aluminium panels subjected to transverse compression, the panels may collapse in terms of local or global buckling modes (as described in Appendix K). It is necessary to check the buckling modes before carrying out the capacity check for the panels. If the entire deck is governed by global modes, then the capacity should be estimated and checked based on the global mode of the entire panel. If the entire deck is governed by local plate buckling, then conventional design codes and the relevant simplified formulas may be applicable to check the capacity.Information and data generated by the extensive finite element analyses and comparisons between the FE results and the relevant codes are useful for better understanding on the structural behaviour of aluminium panels, and can be used for improving the existing design codes.