Non-destructive measurement of near-surface cracks in railheads : with focus on ultrasonic inspections

Abstract: Near-surface cracks in railhead that caused by rolling contact fatigue (RCF) is one of a kind rail defects that degrade rail track quality. Dependent on the trail load, the cracks can reach to severe level quickly. From many studies, the crack growth can be summarized as follows: At the first phase, the crack is initiated at the rail surface due to shear stresses created by the interaction between wheel and rail. The crack then propagates at about 30⁰ angle underneath. After a certain period of transition, the cracks starts propagating horizontally, vertically, or branching. If they propagates horizontally, the cracks potentially cause rail spalling. If they propagates vertically, then the cracks become more severe and dangerous.To overcome such defects, the infrastructure manager remove the top of railhead by performing rail grinding periodically. Prior the grinding, rail tracks need to be inspected to figure out how deep the cracks are by performing non-destructive testing (NDT). Eddy current testing (ECT) is one of the common method to estimate rail surface crack depths. Most of the practices in the industry, ECT estimates only the crack depth, without analyzing any other crack parameters, such as crack angle propagation, crack length, crack area, crack branches, etc. It also has no ability to identify multi-leveled cracks, sub surface cracks, dense cracks etc. Since depth is the only crack parameter that can be provided from ECT, the inspector have no knowledge about how severe the surface crack is. Whereas, information of crack phase, that is known from the crack profile (crack angle, crack depth and crack length) is beneficial to determine whether the crack is in initial or severe level. It also helps to decide the right time for grinding and avoid severe cracks remain long in the rails.Motivated by the benefit of knowing crack parameters, in this study, phased array ultrasonic transducer (PAUT) was used to inspect rail surface cracks. Generally, ultrasonic testing is used to inspect defect of rails at the far-field of the surface, such as at the body or bottom of the rails. Ultrasonic testing is not used to inspect near-surface cracks since the existence of dead zone at the few mm front of the transducer that is caused by piezoelectric crystal ringing inside the transducer. In this study, by utilizing wedge, phased array technique, and setting the optimum gain at the calibration process could decrease the existence of the dead zone. Thus the surface cracks can be observed clearly from the breaking surface to the deepest tip. At the measurement result, crack profile (propagation angle, depth and length), crack branches and multi-level cracks could be observed well. To verify the measurement result, the inspected railheads were sliced into pieces with uniform 0.65-mm thickness. From these pieces, 3D cracks networks were reconstructed. Complete information of crack profiles (angle, depth and length) of all cracks under the inspected surfaces were collected and well presented. From the reconstructed crack images, crack tips, multi-level cracks, and crack branches can be seen. These confirms that the measurement results could be used to observe crack profile well. For a brief description about the results, 3.5 mm crack tip depth and 6 mm crack length were estimated well with 8% and 4% error, respectively. Although the measurement system still have lack of detection for crack path that oriented sharply from the horizontal line parallel to the surface, since the ultrasonic waves could not be reflected back by these paths.At the end of this study, assessment about the potential of measurement speed of the used system when be applied to the rail field inspection was presented. The study is based on the state of the art available at this topic. The discussion is provided in order to motivate the application of the system to be chosen by the rail inspector to use it for the real rail field inspection.