Detection method of the neutral temperature in continuous welded rails based on nonlinear ultrasonic guided waves

The continuous-welded rail (CWR) will generate longitudinal stress when the rail temperature deviates from the neutral temperature (NT). Although CWR has been widely used in modern rail transit systems, real-time monitoring of longitudinal stress in CWR, or equivalently NT, is still a technical challenge. In order to explore the possibility of nonlinear guided waves in NT detection, this paper identifies possible pairs of fundamental and second harmonic waves modes in the rail, and investigates the effects of stress and temperature on the nonlinear parameter of each mode pair that could produce cumulative second harmonics. Both of stress and temperature are found to have significant influence on the nonlinear parameter, thus how to decouple the influences of temperature and stress is the key to NT detection. It is also found that the variation pattern of the nonlinear parameter vs. temperature curve could be completely different for different mode pairs. Furthermore, when the temperature of CWR changes monotonically , the nonlinear parameters of almost all mode pairs increase or decrease monotonically, except one mode pair, the nonlinear parameter of which presents a clear local minimum at NT. Thus, this special mode pair can be used to detect NT without considering the coupling influence of temperature and stress.