Ultrasonic monitoring of thermal stresses in CWR

Ultrasonic systems are since long time used to measure stress on materials, utilizing the acoustoelastic effect. Research has also been carried out in railway engineering, basically to estimate stress on the rails. So far a widely used commercial system, based on ultrasonics or on other methods, does not exist. In this chapter a methodology for monitoring thermally induced loads on continuous welded rails is described, based on the use of ultrasonic waves. The technique allowed an estimation of rail’s neutral temperature and instantaneous longitudinal loads, by means of a new data elaboration method. A complete monitoring system was built and run for about two years on a 3-km track. The method proved to be expensive and time-consuming, if a large amount of railway track is to be monitored, because of the cost of the instrumentation and the necessity of rail adjustment. To find a solution to these problems, a new study was undertaken, aiming to design and test a portable ultrasonic device capable of measuring stress on rails. When using instruments of this kind, several problems must be faced, like those arising from the variation of contact characteristics between ultrasonic probes and rail surface. To solve these problems a double couple probe instrument was designed and built: it proved capable to allow for variation in couplant thickness and type, contact pressure and surface roughness. A calibration with respect to stress and temperature for some of the materials commonly used for rails was carried out. The influence of material’s internal structure and residual stress was also investigated.