ANALYSIS OF SURFACE-TOPOGRAPHY CHANGES IN STEEL SHEET STRIPS DURING BENDING UNDER TENSION FRICTION TEST

In order to investigate the frictional behaviour and the change in sur face roughness of different sheet topographies, a bending under tensio n friction test, BUT, was used. This test exposes the material to bulk -plastic deformation under conditions similar to those present in the drawing radius of a real tool. Each steel sheet material has been test ed under different lubrication conditions. To measure the sheet surfac e topography a 3D stylus technique was used. For each steel strip the original surface, the area in contact with the tool and the resulting surface topography were measured. When comparing ''standard'' 3D surfa ce roughness data for original and final surfaces, no correlation with friction data was obtained. However, with a special evaluation techni que good correlation with oil retention volume and frictional behaviou r was found for the surface in contact with the tool. By using measure ments from the contact area, filtering the measured surface in two ste ps, firstly with a 5th degree polynom to get rid of the circular form and secondly with a bah filter to get rid of the waviness located in t he topmost layer in the surface and then plotting the bearing area cur ve for the filtered surface on a probability paper, the real area of c ontact for each steel sheet surface can be determined, as well as the number of oil pockets and their mean area. The oil pockets were estima ted by using a software in which areas of peaks were estimated, on a s urface inverted at the level of the real area of contact. By plotting the mean area of the oil pockets versus the coefficient of friction fo r a sheet material with an excess amount of lubricant, a relationship is determined. (C) 1998 Elsevier Science Ltd.