Some applications of caustic foci to non-destructive evaluation

The application of caustic foci to non-destructive evaluation (N.D.E.) yields novel testing techniques with advantages over conventional methods. In situations where the wavelength is relatively small, any wave field can be approximated by a families of rays drawn along the direction of energy flow. Caustics are the envelopes of such rays representing surfaces of high intensity, which implies that they give a general representation of the focusing of the waves. The usefulness of caustics in N.D.E. stems from the fact that they constitute well defined geometric entities inside the test specimen with which to probe its internal structure. The research presented here involves the application of caustics to the ultrasonic testing of rods or pipes and acoustic or optical imaging. The location of small defects inside rods using the caustics formed by the sound scattered from them has been demonstrated. Techniques for locating the ends of extended cracks, both internal and surface-breaking, are also presented. The caustic formed by sound reflected from a cylindrical dish has been used to image both the internal structure and the surfaces of materials. The theory behind a high depth of field optical microscope utilizing caustics is given.