OPTO-ACOUSTICAL INSPECTION OF SURFACE LAYERS

A non-contacting and non-destructive testing method using laser techniques was evaluated by theoretical and experimental research. As the inspection of thin layers was of main interest, the opto-acoustical generation and detection of surface acoustic waves were studied. As surface waves are sensitive to minute changes in the elastic properties of a surface they can be effectively used as a tool to characterize surfaces. Variation of the frequency varies the penetration depth, thus enabling depth profiling. An instrumental set-up was realized to make it possible to study the propagation effects of Rayleigh waves of surfaces of layered media and to be able to study the possibilities of determining layer thickness from dispersion data. Rayleigh-wave theory was implemented in a computer program to be able to compute the dispersion curves for some material combinations by numerical methods. Experimental results show that it is possible to measure phase velocities with an accuracy of about 0.2%. This makes it possible to compute dispersion curves from measured data lo determine the layer thickness (by means of curve fitting) with an accuracy better than about 10%. However, this can only be accomplished by using objects with well-reflecting surfaces; for objects with poorly-reflecting surfaces the signal-to-noisc ratio is poor and the errors will rapidly increase. In that case one has to make surface modifications, thereby more or less loosing the non-contacting property, or one has to use more sensitive, and therefore more complicated, interferometrical detectors