Rail-flaw detection using a laser-based ultrasonic approach: TTCI evaluates Tecnogamma SPA's laser-based rail-flaw-detection technology in hopes of finding a more-complete rail inspection technique

Railway Track and Structures, June, 2004 by Greg Garcia

A non-destructive testing technique that uses a high-energy laser to introduce ultrasonic signals into rail and air-coupled transducers to monitor various sound wave modes propagating through the rail is being developed by Tecnogamma SPA and evaluated by the Transportation Technology Center, Inc.

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This technology is unique in that it can apply mechanical energy to the rail at any location accessible to the laser beam. Conventional dynamic rail inspection systems are currently limited to applying ultrasonic energy into the rail from the top surface of the railhead between the gauge and field sides of the rail. This limitation is due to special track work (e.g., frogs and switches) inhibiting continuous inspection from the side of the railhead. It is expected that laser-based inspection technology will increase flaw detection reliability by providing a more-complete inspection of the rail, therefore reducing railflaw related accidents.

A feasibility study of the laser based non-contact ultrasonic system was conducted at the Rail Defect Test Facility, TTCI, Pueblo, Colo., in September 2002. TTCI and Johns Hopkins University performed preliminary studies and field tests using the Laser Air Hybrid Ultrasonic Technique patented by JHU.

The LAHUT system, installed on a rail pushcart, was evaluated for its capability to detect transverse defects in the base of the rail and vertical defects located in the head of the rail (vertical split head cracks, VSH). The LAHUT rail pushcart system proved to be very successful with a 100-percent detection rate for VSH cracks and a 90-percent detection rate for rail-base defects.

In an effort to continue the development of this inspection approach, TTCI supplied Tecnogamma SPA, which has obtained a license for the LAHUT system from JHU, with rail containing service and artificially-induced defects to investigate dynamically detecting rail flaws in track. Tecnogamma SPA is an Italian company headquartered in Treviso, Italy.

In March 2004, TTCI conducted a workshop to present the status of research on ultrasonic techniques that apply either phased arrays (currently sponsored by the Federal Railroad Administration) or laser-based (sponsored by the Association of American Railroads Strategic Research Initiatives Program) approaches to rail inspection. As part of the workshop, Tecnogamma demonstrated the laser-based system's capability to detect transverse and vertical defects in the head of the rail and transverse defects located at the base of the rail (Figures 1a, 1b and 1c) in tests conducted at the RDTF.

Description of system

The laser-based ultrasonic detection system consists of a laser, mirror and lens assemblies, air-coupled transducers and a data acquisition system. To ensure system stability and mobility, the complete laser-based system is mounted onto a railroad pushcart (Figure 2). The laser used to generate the ultrasonic signals is a Q-switched Neodymium Yttrium Aluminum Garnet, Nd:YAG, pulse laser. The laser operates at a 1064 nm wavelength and produces four-to seven-ns pulse widths and energy levels of approximately 800 mJ per pulse.

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Water is applied to the rail surface during inspection to provide a constraining layer that optimizes the operating condition by minimizing the amount of energy loss between the source and the surface of the rail. Energy conservation increases the signal-to-noise ratio, making data interpretation more reliable. The laser source is shaped and delivered through a series of beam-steering mirrors and lenses before contacting the surface of the rail. Depending upon the type of defect being studied, the laser beam is focused to either a point or a line.

Capacitive air-coupled transducers are used to detect the incoming signals. The transducers operate with a broadband frequency response varying between 40 KHz and 2.25 MHz. The transducers have the capability to work at long distances from the surface of the rail, allowing for clearance between the transducer and track obstructions.

As described in the January 2002 issue of Materials Evaluation, another advantage of the air-coupled transducer is that the orientation and alignment of the detectors is flexible because of the small angle variations between the surface and the detector. (1)

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Metallurgical examinations of those areas on the rail receiving laser pulses were evaluated November 4, 2002, at TTCI's Metallurgical Laboratory to determine if metallurgical damage was introduced to the rail during inspection. The rail was evaluated at four different locations--one where the rail had not received any pulses from the laser, one with one laser pulse, one with 10 laser pulses and one with 100 pulses from the laser. The examinations showed that there was no metallurgical damage to the rail caused by the laser. (2)

Railhead defect detection

During the March 2004 evaluations, the laser-based ultrasonic detection system was set up in a pitch-catch mode allowing signal monitoring by the air-coupled transducers to be performed on longitudinal, shear and surface waves propagating through the head of the rail.