B. R. Tittmann

Ferromagnetic Resonance in Epitaxial Garnet Thin Films

The chemical vapor deposition process for growing epitaxial yttrium iron garnet (YIG) has matured to the point where films of varying geometry and thickness from ½ to as great as 40 μ can be readily grown on several orientations of gadolinium gallium garnet, yttrium aluminum garnet, and YIG, itself. As a measure of the quality of the films, line widths (at 9.4 GHz and room temperature) as small as 0.6 and 1.4 Oe were obtained in samples with substrates removed and intact, respectively, without a major effort toward improvement. Magnetostatic modes, exchange modes, and ferromagnetic normal modes having comparable exchange and demagnetization energies were observed in these films and in thin bulk disks and were explained in detail by a new theory which applies not only to films, but also to samples of arbitrary shape and size, and includes the effects of exchange and demagnetization energies. The frequencies of the general normal modes were calculated by casting the linearized equation of motion of the magn...

Surface wave scattering from elliptical cracks for failure prediction

The scattered radiation patterns from surface cracks irradiated by acoustic surface waves are interpreted to provide estimates of crack length and aspect ratio, geometric crack parameters needed to enable failure prediction. The technique is demonstrated for circular and elliptical cracks as small as 100 μm in depth with an accuracy of about 10%. The key features are the positions and spacing of peaks and nulls in angular frequency dependence of scattered surface wave intensity. A simple model based on optical diffraction theory is demonstrated on cracks in commercial hot‐pressed silicon nitride studied at 100 MHz and on spark‐eroded slots in commercial aluminum studied at 2–10 MHz. The results are used to calculate the stress intensity factors and to describe the direction of crack propagation for a variety of real and simulated cracks. Implications of the technique with respect to crack closure and effects of stress and time are also discussed.

Fatigue lifetime prediction with the aid of SAW NDE

The present studies concentrated on predicting the remaining fatigue life for single fatigue cracks in the Paris regime of macrocrack propagation. Acoustic surface waves were used to interrogate the crack during cyclic fatigue. The inversion of the obtained scattering data provided crack depth and crack length as a function of the number of cycles applied in tension-tension fatigue. Auxiliary experiments were conducted to study the acoustic response of the crack to tensile and compressive loads, thought to open and close the crack. The technique may allow for new insights into the physics of the “crack closure” effect.

Scattering of longitudinal waves incident on a spherical cavity in a solid

Experimental data are compared with theoretical calculations for the scattering of ultrasonic pulses from a single spherical cavity embedded in a metallic solid. Except at high frequencies, the data are in reasonable agreement with theory, bearing out the notable features predicted for the angular dependence and the mode conversion between longitudinal and transverse waves. At high frequencies the disagreement can be accounted for by the lack of monochromicity in the ultrasonic pulse, and the technique for analyzing the data taking into account the broadband spectrum must be applid. The experimental parameters were chosen to span the regime where the wavelength is of the order of the size of the spherical cavity (0.8 mm) on samples of diffusion bounded Ti–6% Al–4% V.

Estimation of surface‐layer structure from Rayleigh‐wave dispersion. III. Sparse data case—Interpretation of experimental data

Elastic surface waves penetrating into solids to a depth proportional to the wavelength are expected to be dispersive in the presence of gradients in physical properties such as density, chemical composition, internal stress, or metallurgical microstructure. This paper presents the results of studies to use this phenomenon to determine physical property profiles in a nondestructive manner. In a previous paper, a mathematical model was presented giving, for discrete data sets, a probabilistic description of the possible results of measurement, including measurement errors. The model also yields auxiliary measures pertaining to bias, data‐vs‐model dominance, resolution, and a posteriori variance. Here, the model is applied to actual experimental data consisting of the phase velocities of Rayleigh surface waves measured as a function of frequency for three typical profiles: a thin layer on a thick substrate, a thin layer embedded near the surface of a thick substrate, and a smoothly and monotonically varying...

Estimation of surface layer structure from Rayleigh wave dispersion. II. Sparse‐data case—analytical theory

The ill‐posed nature of the estimation of subsurface properties from dispersion data necessitates the application of special approaches in which a priori information supplements the experimental data. Here we consider a particular approach involving the use of estimation theory combined with a nonparametric mathematical model of the measurement process. In a previous paper we considered the somewhat artificial case of continuous dispersion data (or, in practice, data that is sufficiently dense and extensive that interpolation and extrapolation can be carried out with impunity using standard techniques). Here we treat the sparse‐data case using an estimator that is optimized for an input composed of a discrete data set. The treatment starts with a mathematical model giving a probabilistic description of the possible results of measurement including measurement errors. As usual, the estimator (a function providing the estimated profile of subsurface properties in terms of measured data) is optimized in a le...

Technique for the inversion of backscattered elastic wave data to extract the geometrical parameters of defects with varying shape

A technique is described for the inversion of elasticscattering data in order to extract the geometrical properties of a scatterer embedded in a solid. The technique is in part based on the Kirchhoff approximation. Results are presented for the inversion of theoretical and experimental scattering data for a variety of shapes including spherical, ellipsoidal, and disc‐shaped voids as well as a compound void consisting of two overlapping spherical voids of different radii.

A self-calibrating technique for the ultrasonic discrimination of solid-state welds

This paper presents the description and application of a self-calibrating technique for ultrasonic weld discrimination. A broad-band ultrasonic transducer is bonded to one side of the component containing the weld. The transducer transmits and receives short pulses in the pulse-echo mode. The received short pulses are processed to provide data on weld quality. The technique is self-calibrating in the sense that the capture of two waveforms in a single measurement provides enough information to remove the transducer transfer function and transducer bond losses. A key feature of the technique is the elimination of the deleterious effects of ultrasonic bond variability, which was found to be the major limitation for the materials investigated in this study. Another feature is the requirement of access to one side of the weld only. The technique is applied to solid-state welds involving two types of steel, i.e., martensitic and austenitic steel. For the martensitic steels, substantial improvement over previous studies was seen and first- and fourth-class welds could be discriminated. On the other hand, the austenitic steel welds are examples which demonstrate the need for still further improvements, since the discrimination between first- and fourth-class welds was only marginal.

Ultrasonic Characterization of Diffusion Bonds

The characterization of weak (low ultimate tensile stress) diffusion bonds is a generic problem which continues to challenge the QNDE community.1 Here we present preliminary results for an approach based on ultrasound reflection from, and transmission through, a planar diffusion bond. The samples consisted of two bonded stainless steel cylinders, each 12.7 mm in diameter and 63.5 mm in length. The bonds were made by means of thin Ag layers deposited on each of the matching ends of the cylinders. Here the ultrasonic transducers are placed at the ends of the cylinders. For this geometry, some of the ultrasonic energy impinges on the walls of the cylinders and undergoes mode conversion, leading to a complex train of pulses. The appropriate formulae were obtained to identify the various signals in the pulse train and to associate their amplitudes and arrival times with their ray path histories. The technique used is based on a pulse-echo method in which the reflection and transmission coefficients of the bond are combined in such a way that the transducer transfer functions, including coupling variations, are eliminated. Two types of measurements were made on each sample: ultrasonic measurements at 15 MHz and tensile tests to ultimate failure. The samples were bonded under controlled conditions in which bonding time, temperature, and pressure were varied. The ultrasonic measurements suggest that the effective reflection coefficient of the bond may be a candidate for a potential correlation with bond strength.

Elastic wave scattering from irregular voids

Results are presented for elastic wave scattering from irregular voids embedded in Ti alloy by the diffusion bonding process. The defects examined are: two overlapping spherical voids of unequal radii, two overlapping voids consisting of a sphere and a prolate spheroid, and a spherical void with an encircling crack. Representative plots are given for the raw waveforms, magnitude of the deconvolved Fourier transform, and in some cases the time impulse response function. The data are compared to and analyzed in terms of two current theoretical approaches. While good quantitative agreement was observed over certain ranges, the comparisons point to definite (in some cases not unexpected) limitations in either the pertaining theory or experiment or both. The results are discussed with an eye toward applications to nondestructive evaluation.