Observation of ultrasonic surface waves on plane and cylindrical solids by optical methods

We present results of observation and velocity measurements of surface waves generated on plane and cylindrical metallic samples submitted to an ultrasonic beam oriented at a slightly greater angle than the second critical angle, which is called the Rayleigh angle. Two different techniques are applied in the experiments. One consists of the visualization of the surfaces of the constant phase of ultrasound in water. The second uses light diffraction by surface-wave corrugations. In this case, the insonified zone is immersed while the observation zone is in contact with the air. This technique allows the determination of the velocity, either directly by measurement of the distance between the diffraction orders or by using the properties of superposition between two echoes. Stainless steel is studied at an ultrasonic frequency near 5 MHz. On the plane surface a unique surface Rayleigh wave is detected. On the cylindrical surface a cylindrical-surface Rayleigh wave is detected in addition to another wave. In the two cases, the velocity of the Rayleigh waves agrees well with the theoretical results: 2875 m/s for a plane and 2900 m/s for a cylinder. The additional cylindrical wave, with a higher velocity (3550 m/s), is assumed to be a circumferential one, and it turns into a higher-velocity wave (4360 m/s) at the transition between a plane and a cylindrical boundary.