EVALUATION OF CONTACT AREA BY REFLECTIVITY

Emanuel DIACONESCU

emdi@fim.usv.ro

 Marilena GLOVNEA

Department of Mechanical Engineering, University of Suceava, ROMANIA

Abstract. Contact mechanics requires experimental investigations to validate the involved hypotheses. Advanced experimental methods offer point to point information upon variable parameters over contact area. For instance, laser profilometry estimates the surface deformations of a metallic body contacting a sapphire window. To use this information, one needs to know the contact area.
This paper answers this question by evaluating the reflectivity of the metallic surface in the above contact. When a laser beam scans this contact, it is reflected by sapphire-metal interface on contact area and by an air-metal interface outside this area. Different values of reflectivity are expected in these situations. Because the literature lacks sufficient information upon reflection on absorbing media, like the metal is, a theoretical investigation of this phenomenon is performed first. This shows that if the light is reflected by an absorbing medium, the intensity of reflected light decreases as the refractive index of primary transparent medium increases. Therefore, on contact area the reflectivity is smaller than outside the contact. Experimental investigations with steel balls confirm this theory and show a reflectivity step at contact boundary. The contact radii measured experimentally agree very well with the predictions of Hertz theory, except at very low loads.
 

Keywords: contact mechanics, contact area, reflectivity, laser profilometry, contact model