Health monitoring of Indian rail section using AE technique combined with UPVM: an experimental study

Abstract

Railways is a major mode of transportation for passengers and freight in India. Hence, monitoring the health of rail sections is of paramount importance from the viewpoint of passenger safety and economy. Rail sections are prone to damage/defects; thus, their detection and localization are the primary objectives of health monitoring so as to take prompt and necessary remedial measures. In this respect, the Non-Destructive Testing (NDT) technique is effectively used for health monitoring of rail sections. Among the many NDT techniques, the Acoustic Emission (AE) technique is more effective as it helps in damage investigation of rail sections in real-time. In the AE technique, the initiation or growth of damage emits AE waves which are captured using AE sensors mounted on the rail section, and hence, the damage itself acts as an AE source. Major requirements for localization of AE source are finding the AE wave velocity in the media and Time of Arrival (TOA) of AE wave at the sensor(s). In most of the existing methods, the wave velocity is either taken from prescribed sources or is found out through mathematical formulation using the standard properties of the material. But in real-life scenarios, the material of the system may not always be homogeneous and isotropic. Moreover, there is a possibility of existing internal defects or discontinuity etc. inside the material. Therefore, the theoretical wave velocities may often result in erroneous AE source localization. In this context, Ultrasonic Pulse Velocity Method (UPVM) can be utilised to find out the wave velocity in field conditions of the material for rail section. As field measurement of the velocity of rail section using UPVM gives correct information about the rail, the localization using the velocity will be more accurate than the conventional approach followed in the AE technique. Existing literature reviews show that no such investigation in regard to localization of damage using AE along with UPVM has been reported. This seems to be an innovative and accurate approach in heath monitoring of rail section using AE technique. In the present study, a novel approach is proposed for AE source localization whereby a combination of AE technique and UPVM is utilized. In this method, UPVM is used to predict the velocities of the AE wave in the rail section while the obtained wave velocities are utilized through Wavelet Transform (WT) based signal analysis to determine the TOA. The present method is demonstrated through experimental investigation on a rail section (1.9m) of the Indian Railways, in laboratory conditions. A single AE sensor is used to capture the AE waves in different parts of the rail section, e.g. top flange, web and bottom flange, while the AE source is simulated by Pencil Lead Break (PLB). The AGU-Vallen wavelet transform tool has been used to process the AE signals. A comparison of localization results obtained using the proposed technique and the existing technique is done. It is observed that the AE source can be localized very realistically in the Indian rail section using the proposed technique and it is found to be better in such prediction than the existing methodology. It can be concluded that the proposed approach is a very promising and more field-oriented damage localization method for rail sections. It is also worthwhile to mention that error in damage localization is reduced remarkably when obtained through the velocity measurement using UPVM than using the conventional approach, where theoretical velocity is used. The final results for localization are found to be with the lowest error of 0.129%