Air-Coupled Ultrasonic Testing of Rails: Experimental Prototype and Finite Element Simulations

This article describes the main elements of a new inspection system for the detection of transverse and longitudinal discontinuities in railheads. The system is based on ultrasonic guided waves, generated and detected in a noncontact manner through air-coupled transducers. The main drawback of using air-coupled transducers on steel rails is the significant energy loss that occurs at the air-steel interfaces. Finite element analyses were carried out to verify the feasibility for using such air-coupled ultrasonic coupling to detect internal discontinuities in this manner. In order to improve the inherently low signal-to-noise ratio of the air-coupled measurements, the prototype uses a statistical analysis based on multivariate outlier detection that normalizes the measurements for the normal "baseline" of the rail that is being tested. The discontinuity detection results from four runs conducted at low speed during the first field test of the prototype at the Transportation Technology Center in Pueblo, Colorado, were quite encouraging, as they showed good sensitivity of the system to the discontinuities within the tested track. At the current stage of development, the system can be seen as a complement to existing rail inspection technologies to increase their discontinuity detection reliability. KEYWORDS: rail inspection system, noncontact ultrasonic guided waves, surface acoustic waves (SAW), multivariate outlier analysis, finite element simulation of guided waves