Non-contact interrogation of railroad axles using laser-based ultrasonic inspection

Current nondestructive inspection (NDI) techniques available to the railroad industry require the removal of wheelsets in maintenance shops where inspections are performed on axles and wheels. These techniques also require contact or near contact conditions between the tested wheel or axle and the inspection probe. A non-contact technique is being tested to enable dynamic wayside ultrasonic inspections to be performed on railcar wheels and axles. The laser ultrasonic inspection system is based on the laser air-hybrid ultrasonic technique (LAHUT), a technique that combines laser generation and air-coupled detection of ultrasound. The technique has the unique characteristic of interrogating a specimen with significant distance between the inspection probe and the surface of the specimen. Laser generation apparatus can be several yards away from the interrogated surface while air-coupled detection standoff can be on the order of several inches. The technique also has the capability of interrogating structural materials in their true industrial environment. Dynamic tests can be performed on parts with complex geometry, areas of limited accessibility, and curved surfaces. Also, dark and rough finish surfaces, which significantly reduce the efficiency of optical detection techniques, can be interrogated successfully. These characteristics make LAHUT ideal for the railroad industry. A proof of concept (POC) demonstration was conducted in August 2004 at the Transportation Technology Center (TTC), targeting the critical cracks in railcar axles. The axles were tested for surface breaking cracks in the axle body. The results were successful and repeatable. The technique lends itself for digital collection and automated processing of data, making it a very strong candidate for next-generation railroad wayside inspections.