Reflection and transmission of guided ultrasonic plate waves by vertical stiffeners.

Ultrasonic guided waves are very useful for structural health monitoring. They have the potential to interrogate and detect damage in a structure over a large area with few transducers. Guided plate modes (Lamb waves) are used and proposed for damage detection at a distance in semi-monococque structures such as airplane fuselages and spacecraft structures. The interaction of guided plate waves with stiffening members such as ribs, stringers, or the integral stiffeners used in spacecraft structures limits the distance over which structural health monitoring non-destructive evaluation systems can detect damage. This paper develops a simple explanatory model for the scattering of low-order ultrasonic Lamb waves crossing a stiffening device. The model illuminates the underlying mechanics of waves crossing a stiffener. The model shows that stop-bands for transmission of S(0) (longitudinal pressure) waves across a stiffener line up with flexural resonances of the stiffener. It also demonstrates why transmission of A(0) (flexural) waves is more complicated and harder to predict. The model is shown to agree well with both boundary element method calculations and experimental measurements.

Review of air-coupled ultrasonic materials characterization.

This article presents a review of air-coupled ultrasonics employed in the characterization or nondestructive inspection of industrial materials. Developments in air-coupled transduction and electronics are briefly treated, although the emphasis here is on methods of characterization and inspection, and in overcoming limitations inherent in the use of such a tenuous sound coupling medium as air. The role of Lamb waves in plate characterization is covered, including the use of air-coupled acoustic beams to measure the elastic and/or viscoelastic properties of a material. Air-coupled acoustic detection, when other methods are employed to generate high-amplitude sound beams is also reviewed. Applications to civil engineering, acoustic tomography, and the characterization of both paper and wood are dealt with here. A brief summary of developments in air-coupled acoustic arrays and the application of air-coupled methods in nonlinear ultrasonics complete the review. In particular, the work of Professor Bernard Hosten and his collaborators at Bordeaux is carefully examined.

Guided plate wave scattering at vertical stiffeners and its effect on source location.

This paper examines guided wave transmission characteristics of plate stiffeners and their influence on the performance of acoustic noise source location. The motivation for this work is the detection of air leaks in manned spacecraft. The leaking air is turbulent and generates noise that can be detected by a contact-coupled acoustic array to perform source location and find the air leak. Transmission characteristics of individual integral stiffeners are measured across a frequency range of 50-400kHz for both high and low aspect-ratio rectangular stiffeners, and comparisons are made to model predictions which display generally good agreement. It is demonstrated that operating in frequency ranges of high plate wave stiffener transmission significantly improves the reliability of noise source location in the plate. A protocol is presented to enable the selection of an optimal frequency range for leak location.

Locating air leaks in manned spacecraft using structure-borne noise.

All manned spacecraft are vulnerable to leaks generated by micrometeorite or debris impacts. Methods for locating such leaks using leak-generated, structure-borne ultrasonic noise are discussed and demonstrated. Cross-correlations of ultrasonic noise waveforms from a leak into vacuum are used to find the location of the leak. Four methods for sensing and processing leak noise have been developed and tested and each of these can be used to reveal the leak location. The methods, based on phased-array, distributed sensor, and dual sensor approaches, utilize the propagation patterns of guided ultrasonic Lamb waves in the spacecraft skin structure to find the source or direction of the leak noise. It is shown that each method can be used to successfully locate the leak to within a few millimeters on a 0.6-m2 aluminum plate. The relative merits of the four methods are discussed.

An ultrasonic array sensor for spacecraft leak direction finding.

We have developed an ultrasonic array sensor useable for locating air leaks in manned spacecraft and have found that this sensor locates leaks in a 1-m(2) plate to within 2 cm. The sensor consists of a 63-element multiplexed array plus a reference element, all constructed from a single PZT disc and a printed circuit board. Cross-correlations of signals from the array elements with signals from the single reference element provide a measurement of the leak noise passing through the spacecraft skin under the array. A spatial Fourier transform reveals the dominant direction of propagation. Triangulation from multiple sensor locations can be used to find the source of the leak.

High contrast air-coupled acoustic imaging with zero group velocity lamb modes.

The well known zero in the group velocity of the first-order symmetric (S1) plate wave mode has been exploited in air-coupled ultrasonic imaging to obtain significantly higher sensitivity than can be achieved in conventional air-coupled scanning. At the zero group velocity point at the frequency minimum of the S1 mode, a broad range of wavenumbers couple into the first-order symmetric mode at nearly a constant frequency, greatly enhancing transmission at that frequency. Coupled energy remains localized near the coupling point because the group velocity is zero. We excite the mode with a broadband, focussing, air-coupled transducer at the frequency of the zero group velocity point in the S1 mode. By exploiting the efficient coupling at the zero group velocity frequency, we have easily imaged a single layer of Scotch tape attached to a 6.4-mm thick Plexiglas plate and 3.2-mm Teflon inserts in a composite laminate.

Material property estimation in thin plates using focused, synthetic-aperture acoustic beams.

The method developed here exploits the wide angular range of focused acoustic probes and the large synthetic aperture of scanned transducers to permit a rapid and reliable estimation of material properties in thin plates. It is found in several tests with various materials that estimates of elastic behavior using this method agree with contact measurements to within less than 5%. The method utilizes transmission (or reflection) coefficient reconstruction for an infinite thin plate, across a wide range of frequency and wave number, from which elastic property estimates are made. Data collected over a large synthetic acoustic aperture are processed with temporal and spatial Fourier transforms applied to change the acquired data from the coordinate and time domains to the wave number and frequency domains. Extrinsic real-beam effects on the data are accounted for with a complex transducer point analysis. Transmission measurements yield reconstructed data extending to the mode cutoffs, permitting easy and nearly unambiguous estimation of a subset of the elastic stiffnesses. For anisotropic plates, elastic stiffnesses are estimated with an inversion procedure that uses only limited data carefully selected from different portions of the measured scattering coefficient. Estimates are made by reconstructing in a stepwise fashion, based on sensitivity studies, where only one stiffness is estimated from the data at any one time, restricting the optimization to a robust one-dimensional search.

Microwave excitation of ultrasound in graphite-fiber reinforced composite plates.

In this paper is demonstrated the effect of microwave beam polarization on the thermal generation of acoustic waves in continuous fiber-reinforced composite laminates. It is found that beam polarization strongly influences the dielectric interaction that leads to thermal losses, bulk expansion, and acoustic wave generation. The oriented graphite fibers in the composite laminate effectively short the microwave fields and reduce the generation efficiency nearly to zero. Ultrasonic waves at several hundred kHz generated in the composite are detected by air-coupled acoustic transducers located on the opposite side of the plate specimen from the 9.41 GHz incident microwave beam. With some averaging signal-to-noise ratios of better than 26 dB are obtained. Applying a conventional model of electromagnetic wave scattering in anisotropic media to this experiment yields good agreement between calculations and measured data. Implications for microwave-acoustic testing of graphite-reinforced composites are also discussed.

Equivalence of Gaussian and piston ultrasonic transducer voltages.

A plane-wave decomposition of collimated beams and electromechanical reciprocity relations are used to demonstrate fundamental differences and unusual similarities about transducer fields and transducer voltages at ultrasonic frequencies under various conditions. It is shown that the voltage induced by a transmitting acoustic piston transducer (constant particle velocity over the transducer surface) radiating into an ideal fluid medium on a second identical piston transducer, operating as a receiver, is nearly identical to the voltage observed when the two transducers have instead a Gaussian radial surface velocity distribution. The strong similarity in induced voltage begins when the two devices are separated by only several acoustic wavelengths, still well within the nearfield of both transducers, and the similarity increases with separation. Cases of transducer velocity distributions different from piston, such as Lorentzian or triangular, yield results nearly identical to the piston case. Transducers of differing size are also treated. It is further shown that an "equivalent transducer" can be derived for any combination of radiators, whose field is identical to the voltage measured using the pair. Moreover, with this concept the calculation of voltage in two-probe experiments is as simple as well-known approximations to the Rayleigh integral for a single transducer. These results have substantial consequences for calculations, either analytical or numerical, that predict the voltage measured in two-transducer experiments.

Acoustic waves generated by pulsed microwaves in viscoelastic rods: modeling and experimental verification.

The acoustic wave generation in a specimen irradiated by a pulsed microwave is predicted theoretically. The specimen is a viscoelastic rod inserted into a wave guide. The model is based on Maxwell's equations, heat equation and thermoviscoelasticity theory. Computations show the presence of temperature oscillations due to the electromagnetic interferences in the irradiated rod if its electromagnetic absorption is low. An experimental method to infer indirectly the detailed behavior of microwave-generated acoustic waves in polymer rods, including the influence of electromagnetic wave reflection at the rod ends, is presented. The method consists of measuring the oscillations in the particle acceleration detected at the end of the rod that are induced by variations in the polymer rod length. The oscillations are caused by changing electromagnetic standing-wave conditions within the rod. It is found that these oscillations are in agreement in period, amplitude, and phase, with independent values of the complex dielectric constant and complex acoustic slowness of the polyvinyl chloride samples used in the study.

Modified reentrant Dewar for magneto-optical flux detection in superconductors.

The design and construction details of a Dewar tailpiece for magneto-optical studies on superconducting films is described. The modified design allows the convenient use of high-power hot-stage objectives, increasing considerably the spatial resolution of the microscope.