Review of progress, QNDE

Highlights of the Conference

This review introduces several areas of importance in acoustic emission (AE) technology, starting from signal attenuation. Signal loss is a critical issue in any large-scale AE monitoring, but few systematic studies have appeared. Information on damping and attenuation has been gathered from metal, polymer, and composite fields to provide a useful method for AE monitoring. This is followed by discussion on source location, bridge monitoring, sensing and signal processing, and pressure vessels and tanks, then special applications are briefly covered. Here, useful information and valuable sources are identified with short comments indicating their significance. It is hoped that readers note developments in areas outside of their own specialty for possible cross-fertilization.

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Review on Structural Health Evaluation with Acoustic Emission

Research into the use of nonlinear ultrasonic guided waves for nondestructive evaluation is expanding at a high rate because of the great potential benefit that they possess for early detection of material degradation. However, development of inspection and testing strategies is complicated because (i) the underlying physical principles are complex, (ii) there is a broad spectrum of possible solutions but only a limited number that have been shown to be effective, and (iii) the nonlinearity is weak and thus its measurement is challenging. This Tutorial aims to provide a foundation for researchers and technology-transitioners alike, to advance the application of nonlinear ultrasonic guided waves and ultimately transform how the service lives of structural systems are managed. The Tutorial focuses on the physical principles of nonlinear ultrasonic guided waves leading to the so-called internal resonance conditions that provide a means for selecting primary waves that generate cumulative secondary waves. To detect material degradation, we are primarily interested in nonlinearity stemming from the material itself, which is represented as hyperelastic. For the special case of plates, internal resonance points have been identified and case studies are presented to illustrate some of the applications. The Tutorial has one new result not published in a research paper; finite element simulation of energy transfer from shear-horizontal primary waves to symmetric Lamb waves at the second harmonic.

Nonlinear ultrasonic guided waves—Principles for nondestructive evaluation

Ultrasound is now a clinically accepted modality in the management of osteoporosis. The most common commercial clinical devices assess fracture risk from measurements of attenuation and sound speed in cancellous bone. This review discusses fundamental mechanisms underlying the interaction between ultrasound and cancellous bone. Because of its two-phase structure (mineralized trabecular network embedded in soft tissue—marrow), its anisotropy, and its inhomogeneity, cancellous bone is more difficult to characterize than most soft tissues. Experimental data for the dependencies of attenuation, sound speed, dispersion, and scattering on ultrasound frequency, bone mineral density, composition, microstructure, and mechanical properties are presented. The relative roles of absorption, scattering, and phase cancellation in determining attenuation measurements in vitro and in vivo are delineated. Common speed of sound metrics, which entail measurements of transit times of pulse leading edges (to avoid multipath interference), are greatly influenced by attenuation, dispersion, and system properties, including center frequency and bandwidth. However, a theoretical model has been shown to be effective for correction for these confounding factors in vitro and in vivo . Theoretical and phantom models are presented to elucidate why cancellous bone exhibits negative dispersion, unlike soft tissue, which exhibits positive dispersion. Signal processing methods are presented for separating “fast” and “slow” waves (predicted by poroelasticity theory and supported in cancellous bone) even when the two waves overlap in time and frequency domains. Models to explain dependencies of scattering on frequency and mean trabecular thickness are presented and compared with measurements. Anisotropy, the effect of the fluid filler medium (marrow in vivo or water in vitro ), phantoms, computational modeling of ultrasound propagation, acoustic microscopy, and nonlinear properties in cancellous bone are also discussed.

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Mechanisms of Interaction of Ultrasound With Cancellous Bone: A Review

Ultrasonic attenuation in pearlitic steel

In this article, the effects of uniaxial compressive loading on the ultrasonic scattering from polycrystalline grains are shown for 10 MHz ultrasound in annealed, 1018 steel. The results show a decreasing value of the stress-dependent backscatter coefficient for normal incident ultrasound when the compression loading is perpendicular to the scattering direction. The change due to scattering is about 2 orders of magnitude greater than changes observed by others using ultrasonic wavespeed measurements. It is anticipated that this research can serve as the basis for many methods associated with nondestructive determination of stress in structural materials.

Stress-dependent changes in the diffuse ultrasonic backscatter coefficient in steel: Experimental results

The effects of lamellar duplex microstructure within grains that contain alternating phases of cementite and ferrite on ultrasonic scattering in railroad wheel steel are evaluated using a diffuse ultrasonic backscatter technique. A new singly scattered response (SSR) model that considers the lamellar duplex microstructure within grains is developed based on a previous SSR model. The results show that the amplitude of ultrasonic scattering decreases with decreasing lamellar space. Corresponding experiments are performed with 10 MHz and 15 MHz focused transducers by scanning both unquenched and quenched wheels. The experimental results show that the ultrasonic scattering amplitudes drop dramatically near the quenched tread surface, a result which is attributed to the creation of duplex microstructure (pearlite phase) within grains due to the quenching process. The lamellar spacing within grains increases progressively from the tread surface to the deeper locations due to the non-uniform cooling rate. The distribution of lamellar spacing within grains as a function of depth is quantified with the modified SSR model. Good agreement with optical microscopy is observed. The diffuse ultrasonic backscatter technique exhibits strong sensitivity to microstructure changes, an outcome that may be applicable for quality control during manufacturing.

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

A system consisting of a circular multilayered thin-film elastic plate and a piezoelectric actuator, which is generally used for ultrasound generation in air, is studied in this paper. Effects of the electrode dimension of a circular thin-film piezoelectric actuator lying on a clamped multilayered elastic plate are discussed theoretically, while the first-order theory of asymmetrically laminated piezoelectric plates with consideration of coupled extension and flexure of the reference plane is used. Numerical results show that the deflection of the elastic plate can be optimized by adjusting the radius of the top electrode.

Optimization of a circular thin-film piezoelectric actuator lying on a clamped multilayered elastic plate

In this article, the measurement of quench depth in railroad wheels is demonstrated using a diffuse ultrasonic backscatter technique. A singly-scattered response (SSR) model that accounts for the gradation of lamellar spacing (duplex microstructure) within grains in the direction of ultrasonic propagation is developed based on the previous single-phase model. The effects of a graded microstructure on ultrasonic scattering are observed by comparing the spatial variance curve measured from the tread surface to that measured from the cross section. The experimental results show that the graded SSR model fits the spatial variance curve from the tread surface much better than the uniform SSR model. The spatial variance peaks increase in amplitude and shift to later times when the material path is increased (deeper focus). The experimental variance curve from the tread surface is then fit with the model in a least-squares sense such that the quench depth can be estimated. The diffuse ultrasonic backscatter technique can be used not only to distinguish microstructure changes, but also to quantify the quench depth, an outcome that may be applicable for quality control during manufacturing.

Hualong Du

Papers

Ultrasonic attenuation in pearlitic steel

Stress-dependent changes in the diffuse ultrasonic backscatter coefficient in steel: Experimental results

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

Optimization of a circular thin-film piezoelectric actuator lying on a clamped multilayered elastic plate

Measurement of Quench Depth in Railroad Wheels by Diffuse Ultrasonic Backscatter