New weldable aerospace alloys have up to 80% higher strength than 2024 T3 and at the same time much improved damage tolerance. They allow substantial savings in structure weight, partly by reduced density, but primarily by enabling substantial increases in working stresses. To assess the post-buckling performance of a welded panel subjected to these high stresses, three issues need study: reduction in material strength due to welding, residual stress and geometrical imperfections induced by welding and their influence on (post-)buckling, and failure of welded joints in the post-buckled state. Proper post-weld ageing results in joints with ultimate tensile strengths of around 60% of the base metal strength for fusion welding, and generally more than 80% for Friction Stir Welding (FSW). Weld defects can be difficult to detect, and they can have a significant influence on joint strength, both for fusion and FS welds. However, most welded aerospace structures today are quite thin-walled and slender, and they tend to buckle and collapse at average stresses that are low compared to the strength of the material, even after welding. The more global influence of material strength reduction on (post-)buckling performance may therefore not be the most critical. A high level of welding induced stress can result in geometrical imperfections that can have a significant influence on (post-)buckling behaviour. Methods exist to predict this distortion, and work has been done on full non-linear Finite Element Analysis (FEA) of the impact on post-buckling performance, but methods to more rapidly assess the impact on post-buckling performance for conceptual and preliminary design studies of various types of structure still need development. The level of residual stress from welding can vary from less than 20% of base metal Tensile Yield Strength (TYS) to almost 80% depending on the welding technique. The concentrated energy input fusion welding techniques used for aerospace structures result in small welds, and in those cases the impact on buckling is estimated less than 10%. The low level of residual stress associated with FSW may still cause a reduction in buckling stress of up to 21% in thin (fuselage type) structure since the welds will be relatively large. The influence of residual stress on the maximum compressive edge stress of a thin, simply supported flat sheet in post-buckling was evaluated and estimated to be less than 12% for Laser Beam Welding (LBW) and 23% for FSW. For thicker plates, no significant effect of residual stress on maximum edge stress was found. In post-buckling, stiffened panels and shells may fail because of failure of the welded joint leading to skin-stringer separation. A global-local FE method was developed to resolve the stresses and strains in the welds of postbuckled stiffened panels subject to compression and shear. In future, this method will be used to evaluate weld failure criteria. A similar approach will be used to investigate fracture mechanics-based approaches to the prediction of skin-stiffener separation.


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