Ultrasonic measurements of nominal contact area and contact pressure in a wheel-rail system

For more than a century, the contact between wheel and rail has been the su bject of careful investigation by scientists and railway technicians. Many efforts have been made to understand a series of phenomena (wear, pitting, fatigue, failures, etc.) that are closely related to processes occurring in such a small contact patch. A huge amount of numerical and theoretical res earch work (starting from Hertz's theory) has been carried out, but there i s a noteworthy lack of experimental tests, which is probably due mainly to difficulties in accessing the contact area with conventional techniques. In the present paper, an experimental investigation dealing with the evaluati on of the contact area and stresses transmitted across it is proposed: the technique employed is based on a noninvasive ultrasonic method that analyse s the reflection of ultrasonic waves by the contact interface. By using a s et-up commonly employed in many non-destructive testing (NDT) analyses it w as possible to obtain graphic maps of static contact conditions, and severa l tests were carried out to evaluate changes with increases in external loa d. Results show reasonable agreement with Hertz's theoretical predictions a nd suggest a possible future use of the ultrasonic method to investigate co ntact during motion.