A review of the various optical methods to detect ultrasound (bulk and surface waves) at the surface of opaque solids is presented. The most useful techniques are thoroughly analyzed. Their performance when nonideal conditions are encountered, such as vibrations, air turbulence, and rough light scattering surfaces is evaluated. This review includes a description of knife-edge techniques, optical heterodyning, differential interferometry, and velocity (time-delay) interferometry methods, plus a mention of various less-important tech-

Optical Detection of Ultrasound

[45] C. S. Tsai, Le T. Nguyen. M. A. Alhaider, “Wideband Acoustotwo Tilting Surface Acoustic Waves,” Appl. Phys. Lett. 26, optic Guided-Light  Beam  Deflector  Using Tilting  Surface 140-143, February 15,1975. Acoustic  Waves,” 1974 Ultrasonics  Symposium  Proceedings, [47] C. S. Tsai, M. A. Alhaider, Le T. Nguyen and B. Kim, “WideIEEE Cat.. # 74. C H 0 896-1SU. 768-772. band Guided-Wave Acoustooptic Bragg Devices Using Multiple [46] C. S. Tsai, Le T: Nguyen, 8. K.’Yao and M. A. Alhaider, “HighTilting Surface Acoustic Waves,” Talk-36, Conference on Laier performance  Acousto-optic  Guided-light  Beam  Device  Using Engineering and Applications, Washington, D. C., May 1975

III. Optical Probing of Surface Waves and Surface Wave Devices

The potential of Rayleigh wave spectroscopy for the in-depth reconstruction of elastic properties of multilayers for materials with a continuous profile of elastic properties is explored. Two models to calculate the surface acoustic wave (SAW) dispersion spectrum from the profile of the elastic parameters are elaborated and compared. It is found that the relevant elastic parameters for Rayleigh wave dispersion in multilayers are the “effective” Rayleigh velocities, i.e., the Rayleigh velocities calculated for virtually semi-infinite layers. For the solution of the inverse problem, a neural network and a singular value decomposition model are proposed and tested on simulated SAW spectra. The reconstruction techniques are applied to reconstruct the elastic depth profile of shot-peened steel samples from laser-generated and laser-detected SAW data.

Surface acoustic wave depth profiling of elastically inhomogeneous materials

A method for measuring thermal waves in a sample is disclosed. More particularly, thermal waves, which may be used for imaging, are generated in a sample through local periodic heating as, for example, by impinging an intensity modulated beam of energy on the surface of the sample. A laser probe is focused on a portion of the heated area on the surface of the sample in a manner such that the beam is reflected from the surface. The angular displacement of the reflected beam, resulting from the local angular change in surface conditions of the sample due to the thermal waves are monitored. By this arrangement, the displacement of the reflected beam can be used to detect the thermal waves.

Method for detection of thermal waves with a laser probe

A technique for estimating the depth-dependent profiles of a physical property, such as case hardness, multilayers over a substrate, or surface corrosion, is presented. The technique uses surface wave pulses with sufficient bandwidth to allow the observation of the frequency spectrum over the range of frequencies required for the estimation procedure. Typically, this range must include those frequencies corresponding to wavelengths smaller and larger than the characteristic dimensions or depths of the physical property gradients in question. The technique uses pulse data acquisition at two locations of the receiving transducer and, through the use of one of the pulses as a reference, calculates the deconvolved Fourier transform of the other pulse and displays the dispersion diagram of the phase velocity. With the use of an inversion algorithm previously reported, the phase velocity variation with depth is obtained and is correlated to variations of the physical property gradient. The significance of this work is the ease and speed with which the data are obtained and processed to get the desired result. Another important consequence is that this technique provides a dense data set, thereby providing much greater precision than that obtainable by previously reported tone-burst measurements.

Determination of Physical Property Gradients from Measured Surface Wave Dispersion

Visualization of surface elastic waves on structural materials

The predictions of the Gauss-Hermite beam model are compared to those obtained by the finite-element method for a model problem. This is motivated by the desire to examine the trade-offs between computational speed and accuracy in the Gauss-Hermite model. In the model problem, a contact strip transducer radiates through an isotropic layer of ferritic steel into an anisotropic layer of austenitic stainless steel with various directions of the preferred axis of columnar grain alignment. Comparisons are made of time-domain waveforms in a common observation axis in the austenitic material. The predictions of the two models are found to be in good agreement near the center of the beam, with deviations developing as one moves away from the central ray. These are interpreted to be a consequence of the Fresnel approximation made in the Gauss-Hermite model. However, the region that contains most the energy is in the vicinity of the central ray, where there is excellent agreement between the two models. This loss in accuracy is accompanied by a several orders of magnitude increase in computation time. >

Predictions of the Gauss-Hermite beam model and finite element methodf or ultrasonic propagation through anisotropic stainless steel

Comparison between experimental and computational results for elastic wave scattering

Under certain conditions, cast stainless steels develop highly aligned grain structures. When viewed macroscopically, such polycrystalline aggregates exhibit considerable elastic anisotropy and variation of wave speeds with direction. Stainless steels with aligned microstructures are often found in nuclear reactor components and require special attention in the design and interpretation of ultrasonic testing. Such phenomena as beam skewing and excess beam divergence, which are caused by elastic anisotropy, can severely confuse or constrain the detection and evaluation of flaws. Furthermore, when these components are integrated into structures by welding, the direction and degree of alignment in adjacent regions may be different and hence inhomogeneities are presented. Proper inspection of these layered, anisotropic materials is made more effective by the availability of a numerical model to predict the propagation of the ultrasonic fields through them.

R. B. Thompson

Papers

Determination of Physical Property Gradients from Measured Surface Wave Dispersion

Visualization of surface elastic waves on structural materials

Predictions of the Gauss-Hermite beam model and finite element methodf or ultrasonic propagation through anisotropic stainless steel

Comparison between experimental and computational results for elastic wave scattering

Validity of the Gauss-Hermite Beam Model in an Anisotropic Layered Medium: Comparison to the Finite Element Method