Photoelastic Investigation of Metal-forming Processes Using New Sapphire Dies

Sapphire dies (of single-crystal aluminum oxide) are used to obtain interfacial conditions along the contact surface of the die in a strip-drawing operation. Several solutions based on slip-line field analysis and upper-bound techniques are available for metal-forming problems. However, these solutions generally neglect elasticity and work-hardening and assume that friction is constant along the die-work interface. While these analyses are sufficient for estimating forming loads, they cannot predict residual stresses nor do they accurately define the local stresses and deformations. Furthermore, inadequate information on the frictional conditions at the die-work interface precludes quantitative predictions of the actual forming process. In earlier research on the drawing of tin-plated mild steel with transparent dies, interfacial velocities were measured experimentally.'V2 The velocities were then used as the prescribed boundary conditions in the finite-element analysis to provide complete information on the stresses and strains throughout the deformation zone of the strip. There were some experimental difficulties in obtaining accurate velocity profiles along the die-work .interface, especially at the entry and exit regions, due to small contact length. The photoelastic technique was applied to the study of drawing operations about 30 years ago. Since the dies in those studies were made of photoelastic materials such as epoxy resins and fused quartz, only low-strength, nonstrain-hardening materials such as lead could be d r a ~ n . 3 , ~ Thus, the earlier work was not representative of commercial materials processing. By contrast, synthetic sapphire (aluminum oxide, Al203) is hard enough to machine and form commercial materials. The use of transparent sapphire dies in experimental studies of drawing makes the observation of tribological features at the die-work interface possible during processing commercial materials such as steel, aluminum, and copper?