Optical Means of Measurement of Rayleigh Wave Velocity and Attenuation

Rayleigh waves are useful in the study of the properties of metals. For example, flaws, grain structure, and residual machining stress may be examined. Simple apparatus using a laser optical probe of the order of 20 μ in diameter is described, which can yield accurate Rayleigh wave velocities on optically reflecting surfaces. Phase information in the Rayleigh wave is preserved as the probe diameter is more than an order of magnitude smaller than a Rayleigh wavelength up to 15 MHz. Velocities can be measured much higher in frequency. Another apparatus consisting of a Michelson interferometer with a Rayleigh wave traveling on one mirror is described. The mirror with the Rayleigh wave can be, for example, a steel sample with a particular grain structure. The interferometer is most useful for measuring Rayleigh wave attenuations. The use of the optical methods in detecting flaws is discussed. By using a thickness vibrating ceramic transducer as one mirror of the interferometer, a vibration amplitude of 28±3 Å was measured at 5 MHz with a 0.5-mm beam. [Supported by Air Force Materials Laboratory, U. S. Air Force.]