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

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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.