Measurement of Contact Pressure in Metal Forming by Pressure Sensitive Film

A simple method is presented for measuring the distribution of tool contact pressure in metal forming by using a pressure sensitive film which detects the contact pressure from the change in color density. In the method, a sufficiently hard sheet metal compared to the workpiece is inserted between the workpiece and the pressure sensitive film in order to eliminate the influence of frictional shear stress at the tool-workpiece interface on the measured result. Since the maximum pressure which can be determined by the film is 150MPa, lead is used as a workpiece material. Distributions of tool contact pressure are measured in upsetting of cylindrical billets, in free forging of plates of various shapes, and also in backward extrusion of a can. The measured distributions agree well with those computed by the rigid-plastic finite element method.