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Fig. 3.6 Scheme of the location of the contact areas in the contact zone of solids.
The ratio of contact areas is A _r /A _a 0.01-0.1% and A _c /A _a 1-10%
Experimental methods for determining contact parameters
FPC is about 0.01-0.1% of the nominal area. Even at high loads, providing predominantly plastic contact, it does not exceed 40%. FPC monotonically increases with decreasing R _a and increasing the radius of curvature of the tops of the irregularities, as well as with an increase in the normal load and duration t her impact. The effect of t on the FPC is especially typical for the "metal-polymer" and "polymer-polymer" contacts. With an increase in the elastic characteristics and the yield strength of materials of mating bodies А _r decreases. When contacting bodies made of dissimilar materials A _r is determined by the physical and mechanical properties of a softer material and the surface geometry of a harder one.
Experimental determination of FPC And _r presents significant difficulties. In engineering, two methods are most commonly used.
The total internal reflection violation method or the Mehau method is as follows (Fig. 2.2.)



Fig 3.7 Scheme for determining the actual contact area by the Mehau method
The rough surface of the part 1 is in contact with the glass prism 2. A beam 3 of light falls on the prism (luminous flux I ₀ ), experiencing total internal reflection on the face ab. At the points of contact between the protrusions and the prism, the reflection is broken, and the luminous flux is attenuated to I ₁ . The observer sees black spots of actual touch on the mirror background. They are photographed or A _{r is determined} by changing the light flux using a photocell.
The carbon film method makes it possible to study the contact of opaque bodies. A thin (up to 0.5 µm) carbon film is deposited in vacuum on one of the surfaces brought into contact. When the parts are in contact, the film is destroyed in the areas of contact. To determine A _r , the surface of the film is examined with a microscope or photographed.
Rapprochement - a change in the distance between the contacting parts, which occurs during crushing or the introduction of microprotrusions on the contact surfaces. Approach is measured using special mechanical instruments equipped with displacement transducers.
A measuring transducer is a measuring instrument that converts a measured physical quantity (displacement, pressure, temperature, etc.) into a signal (usually electrical) for transmission, processing or recording. The optical approach measurement method uses the phenomenon of interference that occurs in the optical system of the device when contacting samples are loaded.