The plasma nitriding of tool steel under atmospheric-pressure was performed using a dielectric barrier discharge method, resulting in the formation of a uniform nitrided layer. In this study, the tribology properties of the nitrided layer generated by atmospheric-pressure plasma nitriding were investigated. The results showed that the surface hardness of the tool steel nitrided by the atmospheric-pressure plasma method were increased by more than twofold compared with that of the core material. The surface hardness and the thickness of the nitrided layer were uniform, with values of 1300 HV and 30 m, respectively. In addition, the wear rate of the sample nitrided by the atmospheric-pressure plasma method was decreased by more than 25 times compared with that of the untreated sample. Only the emission of the N 2 second positive system and Ar were detected by the optical emission spectroscopic observation of the generated plasma. For this reason, we consider that the nitriding of this research caused by the dissociation of nonexcited N 2 , NH 3 , NH 2 , and NH etc. on the sample like a gas nitriding.

To satisfy a designer's intention f o r constructing aesthetic shapes such as automotive bodies, we propose a surface generation method. In the surface design process, designers determine shapes according to their great concern f o r the reflected images of vehicle surroundings, shade lines and highlight lines. Since reflection and shading are affected by changes of surface normal, the curvature variation of the surface, which represents the change of the surface normal, should be smooth and distributed as designers want. T h e proposed method controls curvature distribution directly by determining a surface shape f r o m a n evolute, which is a locus of the curvature center of the generatrix and moves along directrices to f o r m the surface. I t first generates evolutes of boundary curves to be generatrices as rational Bezier curves, then interpolates their shapes with the Bezier polygons, and locates the interpolated shape to the corresponding position of the directrices. B y applying this method, we have confirmed that a smooth shape is generated f r o m f o u r boundary curves. © Eurographics Association, 1996 energy of a thin beam for bending and shear force, that is the minimum value of the squares of the curvature and its derivatives 18, 21, 11, 20 . However, since the results do not reflect a designer's intention directly, they do not always satisfy the criterion. The third concerns the clothoidal spline whose curvature varies linearly with arc length 19, 22, 17 . This generates a smooth curvature plot, but it is limited t o a line. So, it is not applicable t o aesthetic shapes. C-188 M . Higashi et al. / Generation of Smooth Surfaces M. Higashi et al. / Generation of Smooth Surfaces

To evaluate the prediction accuracy of the anisotropic yield function, we propose an original cruciform hole expansion test. Displacements on two axes were applied to the cruciform specimens with a hole in the center. The thickness strain in the region near the hole was compared to the simulation results. Because this forming test is free of friction and bending, it is an appropriate method to assess the anisotropic yield function without the influences of friction or the Bauschinger effect, or the need to consider the stress-strain curve in high-strain region. Hill1948, YLD2000-2D, and spline yield function which is an improved version of the Vegter model were selected, and 6000 series aluminum alloy sheets (A6116-T4) were used in this study. The parameter identification method of the spline yield function also proposed in this paper using the pseudo plane strain tensile test and optimization software. As a result, the spline yield function has better predictive accuracy than the conventional anisotropic yield functions Hill1948 and YLD2000-2D.

In making the die of automotive outer panels, the most difficult process is fixing surface deflection. To fabricate high-quality outer panels without modifying dies, it is important to develop an evaluation method and a numerical analysis method for the surface deflection of outer panels. In this study, we developed a new evaluation method that uses the maximum value of curvature, calculated using reflecting curves in the surface. This made the examiner evaluation conform to digital evaluation. The method shows better agreement with the examiner evaluation than the conventional method. We proposed a new analysis method of predicting surface deflection correctly. By the proposed method, plastic deformation is calculated in consideration of stress in the thickness direction, and restriking conditions are examined. We applied our methods to the fabrication of automotive outer panels and verified that they were useful and practical.

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