Theoretical and Experimental Vertification for Onset and Extrnsion of Surface Deflections of Sheet Metal in Press Forming

在本报告中，我们得到了两个结果。首先，建立了检测金属板材表面变形量及其位置的实验方法。也就是说，将平行直线的光栅投影到冲压成形后的板材产品上，用图像传感相机拍摄光栅的反射图像，并利用逐次逼近法测量表面挠度的大小和位置。表面挠度的位置可以与有限元数值计算结果相联系，这将进一步推动这方面的研究。此外，由于该方法测量的是金属板材表面的倾斜度，因此它比针式轮廓测量仪对表面变形更敏感，表面变形是由屈曲现象引起的，由此发展了检测表面变形的方法。数值分析了板料在压缩状态下的分岔点。对受压缺陷的薄板进行了分析，结果表明，在表面挠度较大的情况下，剪切应力增大。这表明，通过观察剪应力可以发现屈曲的开始。因此，可以很容易地找到板料冲压成形时表面挠度的起始点，而不是使用特征值分析。通过实验观察了压花过程中板材凸缘的表面变形情况。

In this report two results are obtained. First, the experimental method is developed to detect the quantity of surface deflections and its location on the sheet metal. That is, the grating of the parallel straight line are projected on the product of sheet metal after press forming and the reflected image of the grating is taken by an image-sensing camera, from the data of which the quantity and the location of surface deflections are measured by using the successive approximation method. The location of the surface deflection can be connected with the numerical result of the finite element method, which will advance the research more and more. In addition this method is more sensitive to the surface deflections than the needle type contour measuring apparatus because it measures the inclination of the sheet metal surface.Surface deflection is said to be caused by the buckling phenomenon, where the method to detect the surface deflection is developed. The bifurcation point of sheet metal under compression is numerically analyzed. While the sheet metal with imperfection under compression is analyzed, it is shown that the shear stress grows at the instance when the surface deflection becomes large violently. This suggests that the onset of the buckling can be found by the observation of the shear stress. Therefore, the beginning point of the surface deflection of sheet metal under press forming can be easily found instead of using an eigenvalue analysis. Experimentally, the situations of the surface deflection on the flange of sheet metal under embossing are observed.