Laser-air Hybrid Ultrasonic Technique for Dynamic Railroad Inspection Applications

Laser-Air Hybrid Ultrasonic Technique (LAHUT) combines laser generation with air-coupled detection of ultrasound. The technique is non-contact and has the characteristic of operating from remote distances. Acoustic wave laser-generation apparatus can be meters away from the interrogated surface while air-coupled detection standoff can be on the order of several centimeters. The technique has the unique capability of interrogating structural materials in their true industrial environment. Dynamic tests are performed on parts with complex geometry, 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 the LAHUT ideal for many industrial applications including the railroad industry. It was developed for railroad inspections targeting the most critical cracks in rails and wheels. State of the art inspection techniques available to the railroad industry often miss Vertical Split Head (VSH) and Transverse Detail Defect (TDD), which lie in unfavorable positions and orientations in the rail head. No method exists to perform dynamic inspections of the rail base or any part of the railroad wheel. Laboratory experiments were performed for the detection of TDD and Proof Of Concept (POC) field tests were performed for VSH, rail base cracks, thermal fatigue cracks along the wheel flange and tread and subsurface Shattered Rim Cracks (SRC) along the wheel tread. The results were successful and highly repeatable. The technique lends itself for digital collection and automated processing of data making the LAHUT a very strong candidate for next-generation railroad inspection technique. Introduction: In recent years, remote non-contact NDE techniques have been sought in an effort to improve railroad related inspection operations, more specifically, to increase the coverage of rail track and wheel testing and to perform dynamic testing of tracks and wayside inspection of wheels of a moving train. Laser, air-coupled, and water jet ultrasound transduction are the only methods that can comply with the restriction imposed by the nature of railroad operations. Water jet consumes large amounts of water and is more effective when test specimens are in an enclosure that collects and recycles water. Laser detection of ultrasound requires reflective, flat, and smooth surfaces. The dark, curved, and rough surfaces of rail tracks make optical detection of ultrasound very difficult, or impractical for industrial applications (1, . Air-coupled generation of ultrasound is not very effective when inspecting internal flaws in metals. More than 99% of the energy of an acoustic wave propagating from low density air to high density metals reflects back to air. Most of the remaining energy propagates mainly along the surface of the metal . In the present investigation, laser generation is combined with air-coupled detection to create a noncontact and remote hybrid ultrasonic technique that can operate under industrial conditions. The proof of concept field test results, presented in this paper, show that the LAHUT is very flexible and successful in detecting a variety of cracks. Tests are performed on rail tracks detecting VSH, TDD and rail base cracks. The first two are often missed while the latter cannot be detected using conventional dynamic NDE techniques. Proof of concept tests are also performed on railroad wheels detecting thermal fatigue cracks along the wheel flange and tread and subsurface SRC along the wheel tread. No method exists for the railroad industry worldwide to perform wayside inspections of wheels on a moving train. Instead, wheels are removed and inspected in maintenance shops. Based on the proof of concept test results, two prototypes are now underway. In Italy, Tecnogamma Spa is developing a prototype for the dynamic inspection of rail tracks from a moving train while in the United States, the Transportation Technology Center (TTC), is developing a prototype for wayside dynamic inspection stations for wheels of a moving train. Experiment: For the generation of ultrasound, an infrared Nd:YAG pulse laser is used, operating with 4-7 ns pulse width and maximum energy of 800 mJ per pulse. To illuminate the surface of rail tracks or wheels, the laser beam is delivered through a set of beam steering mirrors and shaped through a set of lenses. The laser source is focused to a point or a line depending on the type of flaw inspection to be performed. Water is added to the surface as a constraining