A non-contact method of determining material properties and structural integrity through the analysis of laser generated ultrasound

This paper describes a technique to evaluate the mechanical properties of a structure without involving any physical contact with that structure. The basic principle is to monitor the ultrasonic transfer function of the structure. However by utilising Lamb wave dispersion characteristics whose shape depends on a multitude of parameters, this approach is capable of extracting far more structural data than straightforward compressional mode propagation measurements.
In this paper we shall describe initially how optical techniques are used to both launch and receive Lamb wave signals. Typically a very short harmonically rich pulse of laser light is used to launch a wide spectrum of ultrasonic frequencies. From this impulse response the dispersion curves can be extracted. In turn these dispersion curves can be inverted to produce values for important parameters such as Young's modulus, material thickness and Poisson's ratio.
We demonstrate that this inversion technique is capable of producing values for mechanical parameters with a reproducibility of a few percent. Consequently any deviation in these values becomes immediately obvious. Such deviations can be indicator of structural damage or deterioration. Examples demonstrating this discrimination capability are also included in the paper.