R. B. Thompson

Bio: R. B. Thompson is an academic researcher from Iowa State University. The author has contributed to research in topics: Ultrasonic sensor & Ultrasonic testing. The author has an hindex of 21, co-authored 104 publications receiving 1817 citations. Previous affiliations of R. B. Thompson include Rockwell International.

A Model for the Electromagnetic Generation and Detection of Rayleigh and Lamb Waves

Abstract: Calculations and measurements of the efficiencies of electromagnetic transducers for Rayleigh and Lamb waves are presented. Existing solutions for the response of a solid to periodic surface stresses are applied to cases where the stresses arise from Lorentz forces on currents flowing in a static magpetic field. Results are presented for both electri- cally insulating and conducting solids. In the former case, the current must be passed through wires bonded directly to the surface, while in the latter case they may be induced in the metal surface by a nearby coil with no physical contact. Included are expressions for the efficiencies of transmitting and receiving transducers along with a brief discussion of ap- propriate reciprocity relations and equivalent circuits. The theoretical predictions are compared to experimental results for the transduction of 130 kHz flexural waves on 3/8-in aluminum and steel plates. With no adjustable parameters, the theory closely predicts the absolute level of performance on the aluminum plate. Data on steel is qualitatively the same but suggests that magnetostrictive coupling is of comparable magnitude to the Lorentz force coupling mechanism. The dependence of transducer performance upon separation from the plate is also found to be accurately described by the model.

Excitation of horizontally polarized shear elastic waves by electromagnetic transducers with periodic permanent magnets

Abstract: Theoretical and experimental results obtained with a new type of noncontact electromagnetic transducer are presented. The transducer differs from previous configurations in that a periodic permanent magnet and an axially wound coil are combined to produce a transversely directed periodic surface stress. It is demonstrated that this configuration can be used to generate horizontally polarized shear waves traveling along, or at a predetermined angle to, the surface of a thick metal object, horizontally polarized shear modes of a thin plate, or torsional waves in a tube. A model is presented for the radiation resistance of the transducer in semi‐infinite samples and plates and supporting experiments are reported. Some advantages of this transducer in ultrasonic nondestructive evaluation of structural materials are summarized.

A model for the electromagnetic generation of ultrasonic guided waves in ferromagnetic metal polycrystals

Abstract: A model is presented which predicts the efficiency of the electromagnetic generation of ultrasonic Rayleigh or Lamb waves in ferromagnetic metal polycrystals. The practically important case in which the static magnetic field, the dynamic magnetic field, and the propagation direction are parallel is considered. Two distinct forcing mechanisms are explicitly included, magnetostrictive stresses, and Lorentz forces on induced eddy currents. The ability of the resulting formulas, which depend only upon the applied magnetic field and independently known elastic and magnetic properties of the material, to correctly predict such important quantities as the field dependence of generation efficiency (both phase and amplitude), the relative importance of magnetic permeability, magnetostrictive coefficients, and electrical resistivity in determining this efficiency, and the decrease in efficiency with distance of the transducer from the sample, is demonstrated by direct comparison to measurements in iron. Results are included in a form useful in predicting the efficiencieosf transducers in new applications.

Interfacial Spring Model for Ultrasonic Interactions with Imperfect Interfaces: Theory of Oblique Incidence and Application to Diffusion-Bonded Butt Joints

Abstract: The quasi-static distributed spring model is used to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence. The results are then incorporated in a model for the corner reflection from a diffusion-bonded joint between two abutting plates, the corner being defined by the bond plane and the common lower surface plane of the plates. An immersion-inspection geometry is assumed, and seven categories of corner reflections are identified and examined in detail. These fall into two classes: those having parallel incident and exiting rays in water (φ′=φ), and those having nonparallel water rays (φ′ ≠ φ). The φ′ = φ categories are suitable for single probe (pulse-echo) inspections of the joint. Based on the amplitude of the outgoing corner-reflected signal, two φ = φ′ geometries appear promising. These employ, respectively, a corner reflection involving only longitudinal waves with the interface illuminated at near-grazing incidence (LLL), and a corner reflection involving only transverse waves with the interface illuminated at near 45° incidence (TTT). In addition, two practical φ′ ≠ φ geometries are indicated; these both involve mode conversion upon reflection from the interface, with the incident or outgoing longitudinal wave traveling nearly parallel to the interface. Model predictions for LLL and TTT reflections are compared to measurements on diffusion-bonded Inconel specimens, and techniques for applying the model results to more complicated bond geometries are discussed.

Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers

Abstract: Recent molecular dynamics simulations of the growth of $[{\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}/\mathrm{Au}]$ multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

A Baseline and Vision of Ultrasonic Guided Wave Inspection Potential

Abstract: Ultrasonic guided wave inspection is expanding rapidly to many different areas of manufacturing and in-service inspection. The purpose of this paper is to provide a vision of ultrasonic guided wave inspection potential aswe move forward into the new millennium. An increased understanding of the basic physics and wave mechanics associated with guided wave inspection has led to an increase in practical nondestructive evaluation and inspection problems. Some fundamental concepts and a number of different applications that are currently being considered will be presented in the paper along with a brief description of the sensor and software technology that will make ultrasonic guided wave inspection commonplace in the next century.

Ultrasonic scattering from imperfect interfaces: A quasi-static model

Abstract: A quasi-static model for the ultrasonic transmission and reflection at imperfect interfaces is developed. The interface is represented by a distributed spring, determined by the change in static compliance of the medium with respect to one with a perfect interface, and a distributed mass, representing excess mass at the interface. Comparison of the model predictions to exact solutions for two simple cases illustrates its accuracy at low frequencies. The spring stiffnesses can be derived from existing solutions for the elastic displacement of materials containing cracks and inclusions under static load. Results for a variety of cases are reviewed. Applications of the model to study the characteristics of partially contacting surfaces in several problem areas of current interest are discussed.