  The measurement of real contact area between rough surfaces is one of most
challenge problems in contact mechanics and is of importance to understand some
physical mechanisms in tribology. Based on the frustrated total internal
reflection, a new apparatus is designed to measure the real contact area. For
brass samples with various surface topographies, the relation between normal
load and real contact area is measured. Our experiments verify the linear
relationship between load and real contact area during loading process. More
importantly, the unloading process is firstly considered to distinguish the
contribution of elasticity and plasticity in contact of rough surfaces. It is
found that both elasticity and plasticity are involved throughout the
continuous loading process, different from some present understanding and
assumptions that they play at different loading stages. A quantitative
parameter is proposed to indicate the contribution of plasticity. The present
work not only provides an experimental method to measure real contact area but
figure out how elastic and plastic deformation works in contact of rough
surfaces.
