Large-strain monotonic and cyclic loading tests of AZ31B magnesium alloy sheets were performed with a newly developed testing system, at different temperatures, ranging from room temperature to 250 °C. Behaviors showing significant twinning during initial in-plane compression and untwinning in subsequent tension at and slightly above room temperature were recorded. Strong yielding asymmetry and nonlinear hardening behavior were also revealed. Considerable Bauschinger effects, transient behavior, and variable permanent softening responses were observed near room temperature, but these were reduced and almost disappeared as the temperature increased. Different stress–strain responses were inherent to the activation of twinning at lower temperatures and non-basal slip systems at elevated temperatures. A critical temperature was identified to account for the transition between the twinning-dominant and slip-dominant deformation mechanisms. Accordingly, below the transition point, stress–strain curves of cyclic loading tests exhibited concave-up shapes for compression or compression following tension, and an unusual S-shape for tension following compression. This unusual shape disappeared when the temperature was above the transition point. Shrinkage of the elastic range and variation in Young’s modulus due to plastic strain deformation during stress reversals were also observed. The texture-induced anisotropy of both the elastic and plastic behaviors was characterized experimentally.

No abstract

No abstract

In this study, we suggest an optimization process for the characteristics of seat foam to improve the seating comfort. The items selected for the optimization of the characteristics of seat foam are the initial comfort, the static comfort and the transient comfort. The evaluation items selected for optimization are the modulus irregularity factor, the amount of compression and the amount of sagging. The properties of the foam material which are the object of the optimization are simulated using fitting equations. The variables which have important influences on the expression for the foam characteristics are selected as the factors for the optimization. Additionally, the normalized relaxation modulus, which describes the time-dependent deterioration in the foam characteristics, is selected as one of the key factors. To design the experiments, a central composite design is selected from among the response surface methods. A second-order regression model is used to create the response surfaces, and the mechanical characteristics of the foam materials that influence the initial comfort, the static comfort and the transient comfort are evaluated by analysis of the response surfaces. It is possible to produce data on the foam characteristics which satisfy all optimization items by optimum designs.

No abstract