The following is a publications that is representative of
previous research efforts. Each publication includes up to three links: (1) If
javascript is enabled, an abstract is available, (2) most papers include a link
to the "accepted" version of the manuscript that is available for free
download, and (3) a link to the final published paper at the publisher's website,
which typically requires a subscription or fee for download.
- J. S. Hall and J. E. Michaels, "Minimum variance ultrasonic
imaging applied to an in situ sparse guided
wave array", IEEE Transactions on Ultrasonics,
Ferroelectrics, and Frequency Control, 57
(10), pp. 2311-2323, 2010.
Abstract
/
Accepted
/
Published
Abstract: Ultrasonic guided wave imaging with a sparse, or spatially distributed, array can detect and localize damage over large areas. Conventional delay-and-sum images from such an array typically have a relatively high noise floor, however, and contain artifacts that often cannot be discriminated from damage. Considered here is minimum variance distortionless response (MVDR) imaging, which is a variation of delay-and-sum imaging whereby weighting coefficients are adaptively computed at each pixel location. Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing. Simulated data from a through-hole scatterer are used to illustrate performance improvements, and a performance metric is proposed that allows for quantitative comparisons of images from a known scatterer. Experimental results from a through-hole scatterer are also provided that illustrate imaging efficacy.
- J. S. Hall, P. McKeon, L. Satyanarayan, J. E. Michaels, N.
F. Declercq, and Y. H. Berthelot, "Minimum variance guided wave
imaging in a quasi-isotropic composite plate", Smart
Materials and Structures, 20 (2), 025013, 2011.
Abstract
/
Accepted
/
Published
Abstract: Ultrasonic guided waves are capable of rapidly interrogating large, plate-like structures for both nondestructive evaluation (NDE) and structural health monitoring (SHM) applications. Distributed sparse arrays of inexpensive piezoelectric transducers offer a cost-effective way to automate the interrogation process. However, the sparse nature of the array limits the amount of information available to perform damage detection and localization. Minimum variance techniques have been incorporated into guided wave imaging to reduce the magnitude of imaging artifacts and improve imaging performance for sparse array SHM applications. The ability of these techniques to improve imaging performance is related to the accuracy of a priori model assumptions, such as scattering characteristics and dispersion. This paper reports the application of minimum variance imaging under slightly inaccurate model assumptions, such as are expected in realistic environments. Specifically, the imaging algorithm assumes an isotropic, non-dispersive, single mode propagating environment with a scattering field independent of incident angle and frequency. In actuality, the composite material considered here is not only slightly anisotropic and dispersive but also supports multiple propagating modes, and additionally, the scattering field is dependent on incident angle, scattered angle, and frequency. An isotropic propagation velocity is estimated via calibration prior to imaging to implement the non-dispersive model assumption. Imaging performance is presented under these inaccurate assumptions to demonstrate the robustness of minimum variance imaging to common sources of imaging artifacts.
- J. S. Hall and J. E. Michaels, "Model-based parameter
estimation for characterizing wave propagation in a homogeneous
medium", Inverse Problems, 27 (3),
035002, 2011.
Abstract
/
Accepted
/
Published
Abstract: A model-based algorithm is presented that uses minimal a priori information and assumptions to adaptively estimate parameters associated with propagating waves in a homogeneous medium. These parameters include transmitter and receiver transfer functions, propagation distances, dispersion, and propagation loss. The algorithm is described in a general framework that accommodates direct arrivals from two or more transmitter–receiver pairs and can be readily adapted to handle application-specific model assumptions. Experimental validation is performed with two sets of ultrasonic guided wave data that conform to two different sets of model assumptions, demonstrating the impact of these assumptions and the ability to successfully estimate model parameters in a dispersive medium.
- J. S. Hall and J. E. Michaels, "Computational efficiency of
ultrasonic guided wave imaging algorithms", IEEE
Transactions on Ultrasonics, Ferroelectrics, and Frequency
Control, 58 (1), pp. 244-248, 2011.
Abstract
/
Accepted
/
Published
Abstract: Guided wave imaging techniques employed for structural health monitoring (SHM) can be computationally demanding, especially for adaptive techniques such as minimum variance distortionless response (MVDR) imaging, which requires a matrix inversion for each pixel calculation. Instantaneous windowing has been shown in previous work to improve guided wave imaging performance. The use of instantaneous windowing has the additional benefit of significantly reducing the computational requirements of image generation. This paper derives a formulation for MVDR imaging using instantaneous windowing and shows that the matrix inversion associated with MVDR imaging can be optimized, reducing the computational complexity to that of conventional delay-and-sum imaging algorithms. Additionally, a vectorized approach is presented for implementing guided wave imaging algorithms, including delay-and-sum imaging, in matrix-based software packages. The improvements in computational efficiency are quantified by measuring computation time for different array sizes, windowing assumptions, and imaging methods.
- J. S. Hall and J. E. Michaels, "A model-based approach to
dispersion and parameter estimation for ultrasonic guided
waves", Journal of the Acoustical Society of America,
127 (2), pp. 920-930, 2010.
Abstract
/
Published
Abstract: A model-based algorithm is presented that adaptively estimates in situ ultrasonic guided wave system parameters. Dispersion curves, propagation loss, transducer distances, transmitted signal, and mode weighting coefficients are estimated using minimal a priori information and assumptions. The five-part algorithm is scalable to accommodate two or more receivers and one or more propagating modes, provided that mode separation can be achieved prior to use of the algorithm. Algorithmic performance is demonstrated on signals obtained both from theoretical dispersion curves and finite element modeling. Quantitative performance curves are presented that are based on algorithmic performance from multiple simulated test cases with varying amounts of additive noise. Results show excellent agreement between estimated and actual parameters, as well as between modeled and received signals.
- J. S. Hall, P. Fromme, and J. E. Michaels, "Ultrasonic
guided wave imaging for damage characterization", Aircraft
Airworthiness and Sustainment Conference, 2011.
Abstract
/
Published (Free)
Abstract: Guided wave imaging with a sparse array of inexpensive transducers offers a fast, reliable, and cost-efficient means for damage detection and localization in plate-like structures such as aircraft and spacecraft skins. As such, this technology is a natural choice for inclusion in condition-based maintenance and integrated structural health management programs. One of the implementation challenges results from the complex interaction of propagating ultrasonic waves with both the interrogation structure and potential defects or damage. For example, a guided wave interacts with a surface or sub-surface defect differently depending on the angle of incidence, defect orientation, and ultrasonic frequency. Fortunately, however, this complex interaction also provides a mechanism for guided wave imaging algorithms to perform damage characterization in addition to damage detection and localization. Damage characterization provides a mechanism to help discriminate between actual damage (e.g. fatigue cracks) and benign changes, and can be used with crack propagation models to characterize the stress state. This work proposes the combined use of two guided wave imaging techniques to perform both damage localization and characterization. The robustness of conventional delay-and-sum imaging to errors in a priori assumptions make it ideal for use in locating potential damage locations. Minimum variance imaging, which is highly sensitive to errors in a priori assumptions, can then be leveraged to characterize the potential damage location by determining which scattering assumptions best match the measured data. Scattering assumptions are obtained through finite element modeling (FEM). Experimental data from an in situ sparse array are used to demonstrate feasibility of this approach using two through-thickness notches of different orientations, one inside and one outside the array bounds, to simulate damage in an aluminum plate.
- J. E. Michaels, S. J. Lee, J. S. Hall, and T. E. Michaels "Multi-mode
and multi-frequency guided wave imaging via chirp excitations",
Proceedings of the SPIE, 7984, pp. 79840I:1-11, 2011.
Abstract
/
Published
Abstract: Guided wave imaging has shown great potential for structural health monitoring applications by providing a way to visualize and characterize structural damage. For successful implementation of delay-and-sum and other elliptical imaging algorithms employing guided ultrasonic waves, some degree of mode purity is required because echoes from undesired modes cause imaging artifacts that obscure damage. But it is also desirable to utilize multiple modes because different modes may exhibit increased sensitivity to different types and orientations of defects. The well-known modetuning effect can be employed to use the same PZT transducers for generating and receiving multiple modes by exciting the transducers with narrowband tone bursts at different frequencies. However, this process is inconvenient and timeconsuming, particularly if extensive signal averaging is required to achieve a satisfactory signal-to-noise ratio. In addition, both acquisition time and data storage requirements may be prohibitive if signals from many narrowband tone burst excitations are measured. In this paper, we utilize a chirp excitation to excite PZT transducers over a broad frequency range to acquire multi-modal data with a single transmission, which can significantly reduce both the measurement time and the quantity of data. Each received signal from a chirp excitation is post-processed to obtain multiple signals corresponding to different narrowband frequency ranges. Narrowband signals with the best mode purity and echo shape are selected and then used to generate multiple images of damage in a target structure. The efficacy of the proposed technique is demonstrated experimentally using an aluminum plate instrumented with a spatially distributed array of piezoelectric sensors and with simulated damage.
- J. S. Hall and J. E. Michaels, "Analysis of distributed
sparse array configurations for guided wave imaging
applications", Review of Progress in QNDE, 30, pp.
859-866, 2011.
Abstract
/
Accepted /
Published
Abstract: Permanently attached, distributed sparse arrays of piezoelectric transducers have been proposed as a cost-effective solution for rapid interrogation of large, plate-like structures for structural health monitoring. Many proposed methods using these arrays rely on guided wave imaging techniques to interpret and graphically display information for damage detection and localization. Guided wave imaging algorithms, however, contain imaging artifacts that are due to reflections from boundaries and other structural features, even in a noise-free environment. The magnitude relationship between pixel values corresponding to actual damage and imaging artifacts is dependent on a number of variables, including imaging algorithm, number and location of transducers in the array, location and scattering behavior of damage, and structural geometry. This study proposes a metric to characterize the imaging performance of distributed sparse array systems and utilizes numerical simulations to analyze the impact of damage location, imaging algorithm, and physical array configuration on performance in the context of a single flaw in a rectangular aluminum plate.
- J. S. Hall, "Perpendicular fluid flow measurement with a
spatial array of ultrasonic transducers", IEEE International
Ultrasonics Symposium Proceedings, pp. 741-744, 2010.
Abstract
/
Accepted
/
Published
Abstract: The challenge of remotely measuring fluid flow through a plane of interest for a wide range of velocities is addressed using an array of ultrasonic transducers. Unlike traditional transit-time ultrasonic flow measurements, the proposed method uses the angle-of-arrival of a reflected ultrasonic wave to measure flow perpendicular to the path of propagation. The use of angle-of-arrival information avoids the inherent requirement of transit-time techniques to have a non-negligible directional component of the propagation path in the flow direction. A successful proof-of-concept was developed to experimentally validate the proposed technique.
- J. S. Hall and J. E. Michaels, "Model-based in situ
parameter estimation of ultrasonic guided waves in an isotropic plate",
Review of Progress in QNDE, 29, pp. 137-144, 2010.
Abstract
/
Published
Abstract: Most ultrasonic systems employing guided waves for flaw detection require information such as dispersion curves, transducer locations, and expected propagation loss. Degraded system performance may result if assumed parameter values do not accurately reflect the actual environment. By characterizing the propagating environment in situ at the time of test, potentially erroneous a priori estimates are avoided and performance of ultrasonic guided wave systems can be improved. A four-part model-based algorithm is described in the context of previous work that estimates model parameters whereby an assumed propagation model is used to describe the received signals. This approach builds upon previous work by demonstrating the ability to estimate parameters for the case of single mode propagation. Performance is demonstrated on signals obtained from theoretical dispersion curves, finite element modeling, and experimental data.
- N. Gandhi, S. J. Lee, B. Xu, J. S. Hall, J. E. Michaels, T. E. Michaels, and
M. Ruzzene, "Beamforming of wavefield data from embedded sources for rapid
follow-up inspection of inaccessible areas",
Review of Progress in QNDE, 29, pp. 184-191, 2010.
Abstract
/
Published
Abstract: Structural health monitoring systems employing embedded guided wave transducers are being considered for aerospace applications. If these systems detect changes that are potentially caused by damage, rapid follow-up inspection is desirable; however, the suspected damage location may be inaccessible. A method is proposed whereby one or more of the embedded transducers are used as a source, and wavefield data are recorded outside of the inaccessible region via a non-contact method such as a laser vibrometer or scanned air-coupled transducer. These data are then used to form images of sources and scatterers inside the inaccessible region via a beamforming algorithm. Dispersion correction and adaptive source removal are applied to the proposed method to improve its performance. The feasibility of this method is demonstrated for several cases using numerical wavefield data generated by a ray tracing algorithm, and experimental results are also shown.
- J. E. Michaels, J. S. Hall, and T. E. Michaels "Adaptive imaging of damage
from changes in guided wave signals recorded from spatially distributed arrays",
Proceedings of the SPIE, 7295, pp. 729515:1-11, 2009.
Abstract
/
Published
Abstract: Several imaging algorithms are being considered for localizing damage in plate-like structures by analyzing changes in signals recorded from permanently mounted guided wave sensor arrays. Delay-and-sum type algorithms have been shown to be effective for damage localization, but exhibit side lobes that significantly reduce the signal-to-noise ratio. Adaptive algorithms such as MVDR (minimum variance distortionless response) can provide significant reduction in the amplitude of side lobes. Additional improvements in image quality are possible if assumptions can be made concerning the scattering characteristics of the damage site. In the work presented here, the efficacy of the adaptive imaging algorithms is evaluated using both simulated and experimental waveform data. The simulated waveform data is generated by ray tracing and incorporates edge reflections, nominal dispersion curves, and a variety of angular scattering patterns for scatterers with cylindrical symmetry. The effect on image quality of mismatch between actual and assumed scattering patterns is evaluated. Images generated from the simulated waveform data are compared to those generated from experimental data for scattering from a 6 mm through-hole in an aluminum plate. The images are in good agreement, and knowledge of scattering characteristics is shown to significantly improve imaging results.
- J. S. Hall, J. E. Michaels, G. Hickman, and J. Krolik, "Sparse array imaging
of change-detected ultrasonic signals by minimum variance processing",
Review of Progress in QNDE, 28, pp. 642-649, 2009.
Abstract
/
Published
Abstract: Spatially distributed arrays of permanently attached ultrasonic sensors are being considered for structural health monitoring systems. Most algorithms for analyzing the received signals are based upon change detection whereby baselines from the undamaged structure are subtracted from current signals of interest, and the residual signals are analyzed. In particular, delay-and-sum algorithms applied to the residual signals have been shown to be effective for imaging damage in plate-like structures that support propagation of guided waves. Here we consider minimum variance processing of the residual signals, which is an adaptive beamforming method in common use for processing of radar signals where the weights are adjusted at each pixel location prior to summation based upon actual and expected signal amplitudes. Experimental results from a sparse sensor array show that this processing method can provide a significantly improved signal-to-noise ratio by suppressing unwanted sidelobes in the image.
- J. S. Hall and J. E. Michaels, "On a model-based calibration approach
to dynamic baseline estimation for structural health monitoring",
Review of Progress in QNDE, 28, pp. 896-903, 2009.
Abstract
/
Published
Abstract: Sparse arrays of permanently attached ultrasonic transducers are capable of quickly interrogating large areas, thereby reducing or eliminating the need for extensive bulk wave testing in plate-like structures. Current imaging methods often ignore dispersion, analyzing received signals by approximating wave propagation with a nominal group velocity. Dispersion compensation can significantly improve the resolution and accuracy of such methods by more accurately modeling the propagation environment and thereby leveraging a larger percentage of the energy and information contained in broadband signals. By adaptively estimating system parameters such as dispersion and relative transducer locations, algorithms will operate at the time of test with the most accurate model possible and will thus be robust to homogeneous environmental changes. This paper reviews features and limitations of current dispersion estimation algorithms and describes a new model-based approach that avoids a priori information and relies only on the physical nature of the dispersion curves. Performance of two contributing algorithms are evaluated using simulated signals obtained from known dispersion curves.
