Cold Climate Ultrasonic Rail Flaw Testing

Abstract

Minor defects or anomalies in rails can cause breakages when stressed by factors such as increased train tonnage or cold weather conditions. To monitor these defects, railroads are primarily reliant on ultrasonic testing (UT). This study was conducted from December 2022 to June 2023 and improved the understanding of the interaction between cold temperatures and ultrasonic testing (UT) of rail materials. The research focused on ultrasonic rail flaw testing using handheld flaw detectors and walking stick flaw detectors with roller search units (RSUs), as well as ultrasonic signal attenuation measurements at varying temperatures. The findings provide insights into how extreme cold affects ultrasonic signal amplitudes and attenuation, with implications for rail inspection practices in low-temperature environments. Results from handheld flaw detector tests indicated a decrease in signal amplitudes from rail samples as temperature decreased. However, due to the short water path present in the contact approach, the cold temperature had a minimal effect on signal amplitude. In contrast, walking stick flaw detector tests using RSUs showed a significant reduction in signal amplitude when detecting a 3.175-mm (0.125-inch) side-drilled hole (SDH) at extreme cold temperatures. The signal dropped from 80% at room temperature to 36% at -40°C. Additionally, signal amplitudes continued to decline as the RSU remained exposed to the cold environment over time, typically for about 20 minutes. A key observation was the UT equipment was calibrated at the same temperature as the test conditions, defect detection improved. For a rail sample with a transverse defect (TD), a 26% signal amplitude difference was observed between results obtained from room temperature calibration and cold temperature calibration at -35°C, with a 22% difference at -40°C. Similarly, a second TD exhibited a 14% signal amplitude difference at -40°C. These findings suggest that calibrating the equipment at the ambient temperature of the inspection site could reduce signal loss and improve defect detection accuracy. Ultrasonic signal attenuation measurements revealed that effective attenuation in rail steel increased as temperature decreased, at a rate of 0.0005 to 0.0007 dB/mm/°C. This suggests that some loss of sensitivity may occur when calibration is conducted at extreme cold temperatures. Additionally, rail material absorbed more acoustic energy at lower temperatures, impacting the clarity of defect signals. Longitudinal wave velocities in two rail specimens increased as temperatures decreased, with an average rate of 0.65 m/sec/°C. Similarly, in an ethanol bath, longitudinal wave velocities increased at a rate of 4.1832 m/sec/°C. These findings highlight the importance of cold-temperature calibration for ultrasonic rail flaw detection.