Internal defects in rails are a major contributing factor in train derailment related accidents. A non-contact ultrasonic passive rail defect detection system is currently being developed at the Experimental Mechanics, NDE, and SHM Laboratory at the University of California San Diego. This technique has the potential to operate at high speeds and can be mounted on a regular train, thereby facilitating continuous condition monitoring of rails. This paper presents an improved version of the ultrasonic non-contact passive rail defect detection system with the introduction of a non-contact active acoustic source. The previous version of the system utilized air-coupled capacitive ultrasonic transducers in receiving-only mode to passively extract an impulse response function of a rail segment using rail-wheel interaction as the random source of excitation. The use of an active and controlled non-contact acoustic source is proposed for the improved system that aims to enhance the signal-to-noise ratio at the transducer array independent of variabilities of the wheel-induced excitation. Tests were conducted with various types of non-contact sources and source operating parameters to optimize the improved system. Results from field tests conducted by mounting the system on a train with testing speeds of 25 mph, 33 mph and 40 mph are presented.

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