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3.4. Procedure of MUSMAswe know,material of sheetmetalwill flowin a differentwayfor different stamping process, and the formability of the finalpart will be also different. The sequence of MUSM must meetwith the requirement of actual stamping process in strip lay-out. Therefore, the stamping process must be designed firstlyaccording to the geometry feature and process requirementof the final part. Secondly, the intermediate shape will be cal-culated using MUSM. And the formability will be evaluated toavoid the defects such as crack and wrinkling. The referencesurface and boundary conditions should bemodified until theformability is perfect.Sometimes the surface should be modified again if theshapes calculated at previous steps are not good from thefull view of the strip layout. Finally, the optimum strip lay-out will be obtained after modify the stamping process manytimes.3.5. Formability analysisThe distribution of various physical quantities and the defor-mation of the inner holes can be calculated with the results.And the formability can be predicted to assist the design ofthe stamping process.
3.6. LimitationsThe influence of restriking and springback are ignored duringunfolding process, and there are errors of the simulation resultfor the choice of shell element, which cannot simulate thedeformation of restriking and springback precisely.4. Examples4.1. Example 1: Aided design of strip layoutAs shown in Fig. 4(a), the dominating forming process of a pro-gressive die forming part involves bulging, bending, flangingand restriking. The strip layout is assembledwith six key inter-mediate shapes (Fig. 2) and other auxiliary shapes. As shown Fig. 5 – Intermediate reference surface and trimming line.step in strip layout. Before bending at this local area, the cut-ting line will be calculated and then to bend the sheet metal.So, when unfolding the local bending area, the intermediateshapewill be projected on the reference surface (plane surfaceat current step) at the previous step.
The offsetting of strainneutral layer is calculated.At the local area, the bending radiusis 1.5mm, the offsetting size is 0.03mm with the theoreticalformula (Shuobeng et al., 2002). The error between the sizeof cutting line with offsetting and the experimental result isrelatively little.2nd step. As shown in Fig. 4(c), the flanging area (Fig. 4(a),C and E area) will be unfolded. The counterpart shape is the13th step in strip layout.When unfolding the flanging area, thereference surface is calculated automatically with the featuresurface of previous unfolding shape. Generally, the referencesurface is extruded fromthe boundary curves (feature curves)in the tangent direction of the feature surface. As shown inFig. 5, the flanging contains compression deformation andstretch forming, and it is difficult to determine the cutting lineon the 3D reference surface with experience.Many physical quantities can be obtained with the result,such as the distribution of stress, strain, thickness and form-ing limited diagram (FLD). As shown in Fig. 6, the distributionof thickness strain show that the larger thickness thinning is 10.7% at the stretch deformation area with good formability,and the larger thickness strain is 10.7% at the compressiondeformation area with a little of wrinkle risk.3rd step. As shown in Fig. 4(d), the bending area (Fig. 4(a),F area) will be unfolded. The counterpart shape is the 11thstep in strip layout. The unfolding operation is similar withthe first step. However, due to the larger rate of the bendingradius of curvature and thickness at the bending area, wherethe bending radius is 63.5mm, the offsetting of neutral layercan be ignored.
摘要：多步展开法（MSUM）是为了毛坯的开发设计和复杂的级进模冲压件成形性预测而发展起来的。在该方法中，逆方法（IA）的有限元模型的开发是根据冲压工艺处在一个局部区域，并在局部区域的基准面上展开的中间形状。该模型不仅考虑了连接、压边力和摩擦的影响，而且还考虑了不同曲率半径的应变中性层的偏移对其的影响，从而提高了毛坯形状在每一步的计算精度。主要的中间毛坯形状和初始毛坯形状可以多次使用该方法生成一个逆序列。数值模拟结果表明 ，这种设计方法具有较高的精度和计算效率的同时 ，还可以评价冲压件的成形性。根据成形件的成形性，可以设计出最佳的坯料形状。这说明了MSUM是级进模冲压件的毛坯设计和基准测试与其他实验和数值结果相比较而得到的结果。