ANALYSIS OF REAL CONTACT AREA AND CHANGE OF SURFACE TEXTURE DEEP-DRAWN STEEL SHEETS

The real shear stress right on the contact spots seems to have more ph ysical meaning than the coefficient of friction. However, to estimate the real shear stress of the interface, the real contact area between tool and sheet needs to be known. Three-dimensional scanning stylus pr ofilometry was used to map the topography of blanks deformed in a deep -drawing process. The real contact area and the contact morphology (nu mber and size of individual contact spots) were evaluated from surface contour maps. Strong dependence between real shear stress aod either the number oi the size of contact spots was observed. The small number of relatively large contact areas accompanied by closed cavities allo wing lubricant to be trapped and pressurized seems to be correlated wi th smaller values of the real shear stress of the interface. This:was explained by assuming different ability to create a good lubricating s ystem. The results could serve in optimizing steel sheet textures by u sing techniques, such as Lasertex and EBT, that permit controlling rea l density and size of asperities. The coefficient beta=R(m)Dq/R is pro posed as a factor characterizing ability of ''as-received'' steel shee ts to develop a larger area of contact, and consequently better lubric ating conditions.