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 metres away from the interrogated surface while air-coupled detection standoff can be on the order of several centimetres. 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 unfavourable 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

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A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.

Laser-based ultrasound and air-coupled ultrasound test methods were used for the inspection of flowed rail, internal, and surface-breaking cracks in railroad tracks. Signals were generated with an infrared pulse laser and detected with a micromachined capacitive air-coupled transducer. A comparison is presented between point source and line source laser-generated signals and their effectiveness in detecting surface flaws. The experiments demonstrate the flexibility and capability of a laser-air hybrid ultrasonic technique to detect cracks using test procedures that are not possible with current contact inspection techniques. The noncontact and remote nature of these methods renders such tests suitable for in-service applications...