Non-contact measurement of the mechanical properties of materials using an all-optical technique

This paper describes an optically-based measurement mechanism which realizes a totally noncontact assessment of the most important mechanical properties of structural materials-namely effective stiffness and Poisson ratio. These parameters are sensitive indicators of material integrity. The technique uses laser generated broadband ultrasound as a probe and interferometric optical detection as the detector again exploiting the broadband capability of optics in both space and time. Both detection and excitation systems are most conveniently realized in practical systems through optical fiber linkages. Observing the coupled wave forms between source and detector as a function of source: detector separation after a space: time Fourier transform yields a set of dispersion curves for the ultrasonic (typically Lamb wave) transfer function of the sample. This, in turn, can be inverted using curve fitting routines to obtain effective values of modulus and stiffness. An initial assessment of this inversion process is presented and demonstrates that the effective modulus can be extracted with a confidence level of better than a few percent with slightly larger errors in the Poisson ratio. The technique offers future potential for noncontact measurement for in-service monitoring, particularly in applications involving for example high temperatures or highly stressed material structures.