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The longitudinal, shear and surface waves have been used to an Ultrasonic wave exploration method to identify internal
defects but it has technical difficulties to detect defects in a limited space with having several millimeters board
thickness. It is applicable to use Lamb wave, a kind of induction ultrasonic wave that has a relatively high inspection
efficiency and defect detection sensitiveness compare to perpendicular or square explorations in terms of the internal
defect detection in these kinds of thin board. The Lamb wave is a special type of induction ultrasonic wave to propagate
to a board and it is effective to inspect such as board structure, cell structure, etc. Recently a laser is being used to
generate Lamb wave but it not being widely utilized because of difficulties in receiving and selective generation in
various modes Lamb wave generating and a low S/N ratio, wide range of generating frequency band when a laser is used.
This paper aims to study that the defect exploration method to specimen's thickness, using an characteristic of an
ultrasonic wave generating in ablation area and thermoelasticity area, has been applied to using a way of receiving it by
Air-Coupled Transducer and transmitting it by an ultrasonic wave and a way of non-contact receiving and transmitting
ultrasonic wave using a laser. The longitudinal, shear waves have been used in 60mm thickness specimen and Lamb
wave in 3mm to run artificial defect detection test. In this way, the usefulness of non-contact laser ultrasonic wave has
been verified. To confirm the possibility of implementation on natural defect existing at real welding part, thickness in
3mm SM45C and STS304 artificial defect specimen has been made and defect detection test using Lamb wave has been
executed. Detected signal by signal treatment has been expressed in visual not RF signal. By doing so the convenience
of defect detection has been attempted and the effectiveness of defect signal visualization using Lamb wave has been
confirmed compare with defect image through SAT.
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