As the springback of sheet metal during unloading may cause deviation from a desired shape, accurately predicting springback is essential for the design of sheet stamping operations. Finite-element models have not been successful in predicting springback; hence there is a need for analytical models to make such predictions. In this study, a model based on differential strains after relief from the maximum bending stress is derived for six different deformation patterns in order to predict springback analytically. The springback for each deformation pattern is estimated by the residual differential strains between outer and inner surfaces after elastic recovery. Each of the six deformation patterns has a valid region of applicability, based on elastic modulus, yield strength, applied tension, and bending geometry. Analytical equations for the springback of the sheet deformed under these six deformation patterns are derived. Traditional analytical models for springback prediction have been based on elastic unloading from a bending moment. Traditional models also require the knowledge of the stress distribution through the thickness of the sheet, whereas the residual differential strain model only requires the stress state on the outer and inner surfaces of the sheet. In order to compare the residual differential strain model with the traditional bending moment model, a bending moment model is derived for the same exact deformation patterns. Results from the two models are compared for various materials.

The primary purpose of this investigation was to study welding residual stress relaxation by repeated loading. A coupling finite element analysis model for prediction of the welding residual stress and the mechanical stress relaxation after external repeated loads was presented. The accuracy of this model was verified through experiments. Also, it is found that the residual stress of weldment is relaxed by external loads, and the greatest amount of relaxation was obtained by early repeated loads. As the repetition count increased, the amount of relaxation became smaller than the amount of relaxation in the early stage.

The effects of the microstructural constituents, chemical composition, and retained austenite on high-strength weld metal were studied using preheat-free steels and GMAW solid wires with a low hydrogen content. The cold cracking susceptibility of these GMAW wires was evaluated using the y-groove Tekken test. The results showed that acicular ferrite produced the greatest resistance to cold cracking and that the microstructure of the deposit was more important than the hardness and diffusible hydrogen content in low-hydrogen weld metal. Crack blunting and branching occurred when a crack propagated through fine acicular ferrite because of the fine interlocking nature of the microstructure. Alloying elements for nucleating acicular ferrite, such as Ti, Al, and V, are required for proper austenite grain size, and sequence of inclusion formation was identified in the present paper. Furthermore, the retained austenite was not found to play the role of a hydrogen trapping site and so had no effect on the cold cracking susceptibility at a low preheating temperature (≤100℃) and low heat input (≤1.5 KJ/mm) to the weld metal.

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