MEASUREMENTS OF DISTRIBUTIONS OF CONTACT PRESSURES BETWEEN GROUND SURFACES USING PET FILMS

A method for measuring a distribution of contact pressures between gro und surfaces using a thin PET, polyethylene terephthalate, film of 0.9 mu m thickness has been developed. Punch specimens were made of carbo n steel, S45C, and their ends were ground. The PET film was inserted b etween ground surfaces of the punch specimens and pressed at a given p ressure for one minute. The PET film was indented by surface asperitie s on the ground surfaces at real contact points. Thus if the relations hip between the areas of the indented parts and the applied pressures were known, the contact pressures could be determined by measuring the indented areas of the PET film. The indented areas of the PET films w ere proportional to the applied pressures in the range of zero to 300 MPa. These areas were measured automatically by image processing throu gh an optical microscope which was modified so that its stage was cont rolled by a microcomputer. The apparent contact area was divided into many small portions with areas of about 0.017 mm(2) and the contact pr essure was determined in each portion. The measured distributions of c ontact pressures for circular and square punches were compared with th e calculated ones. They were in relatively good agreement with each ot her.