Detection of thermal cracks in railway wheels using ultrasonic surface waves



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A review is given of the mechanism of initiation and propagation of thermal cracks in railway wheels and the current non-destructive testing (NDT) methods for their detection. Also, the properties of ultrasonic surface waves are reviewed. Two NDT methods using surface waves are proposed for thermal crack detection. In the attenuation method, the increase in attenuation of a surface wave over several circuits of the wheel would indicate cracks in their initial stage of growth. In the pulse-echo method, an echo from a crack would indicate its presence. One-quarter scale model wheels were used and thermal cracks were simulated by inscribing flaws on the wheels' tread. Using a single probe method, surface wave pulses of approximately 200 KHz to 600 KHz frequency were generated on the tread with piezoelectric probes. A reflectoscope was used to send and display the received pulses. A dual probe method was also used to choose optimum combination of probes and to investigate the variation of strength of surface waves across the tread. Experimental results show that both methods are feasible. Using the same probe and at the same frequencies the attenuation method is able to detect shallower flaws than is possible with the pulse-echo method. The former method detected flaws of approximately 0.010 inch depth as compared to approximately 0.040 inch with the latter. Finally, the attenuation and pulseecho methods are compared between themselves and with the current NDT methods.