Ultrasparse Ultrasonic Synthetic Aperture Focus Imaging by Passive Sensing.

Ultrasonic synthetic aperture focus techniques (SAFTs) using less than the total number of available array elements to transmit ("sparse" transmissions) have been recently used in both medical imaging and industrial nondestructive testing (NDT) imaging to increase test speed and simplify multiplexer hardware. The challenge of sparse arrays is to obtain a reasonable image quality given the reduced transmitter-receiver combinations available to the beamforming process. This article proposes a "ultrasparse" SAFT method that employs a minimum number of transmitter elements (from one to four elements only) to obtain an entire full-matrix capture (FMC) set of waveforms. Specifically, a "virtual" FMC is obtained from normalized cross-power spectra between each array element pair in an implementation of "passive" ultrasonic sensing. In order to maintain high image quality without sacrificing imaging speed (e.g., applying a minimal initial time delay and keeping a short time recording window), several key steps have to be taken in this "passive" imaging mode, specifically: 1) the use of carefully designed segment-averaged normalized cross-power spectrum (NCPS) for robust passive reconstruction of the ultrasonic impulse response function (IRF) between two receivers; 2) the use of both the causal and acausal portions of the passively reconstructed IRFs; and 3) the compounding of multiple wave modes in the beamforming process. These steps also ensure the elimination of the near-field blind zone hence potentially enabling near-field imaging. The article first reviews the theory of passive IRF reconstruction between two receivers, comparing time-averaged cross correlation versus segment-averaged NCPS, and then demonstrates the application to ultrasparse SAFT FMC imaging of drilled holes in an aluminum block using a linear transducer array where only one to four elements are used in transmission.

Application of Sparse Synthetic Aperture Focusing Techniques to Ultrasound Imaging in Solids Using a Transducer Wedge.

This article discusses the application of sparse synthetic aperture focusing techniques (SAFTs) for fast and accurate ultrasonic nondestructive testing (NDT) imaging of solids in cases where a wedge is required between the transducer array and the test medium. A wedge is often used to appropriately direct the ultrasonic beams when testing for structural defects at particular orientations or when inspecting parts with particular geometries (e.g., waveguides). Both the virtual element (VE) and the plane-wave (PW) modalities of sparse-firing SAFT are examined for the wedge case that requires particular considerations in the beamforming algorithms for the wave refractions and mode conversions occurring at the wedge-medium interface. The method of wave mode compounding is also examined for this application to increase the array gain without increasing its physical aperture. Numerical simulations and experimental tests demonstrate the potential improvements in speed and accuracy obtainable by sparse SAFT adapted to wedge-transducer cases compared to a traditional full matrix capture (FMC) imaging mode. A practical implementation of the imaging of transverse defects in rail tracks is also presented.

Railroad Sleeper Condition Monitoring Using Non-Contact in Motion Ultrasonic Ranging and Machine Learning-Based Image Processing.

An ultrasonic sonar-based ranging technique is introduced for measuring full-field railroad crosstie (sleeper) deflections. Tie deflection measurements have numerous applications, such as detecting degrading ballast support conditions and evaluating sleeper or track stiffness. The proposed technique utilizes an array of air-coupled ultrasonic transducers oriented parallel to the tie, capable of "in-motion" contactless inspections. The transducers are used in pulse-echo mode, and the distance between the transducer and the tie surface is computed by tracking the time-of-flight of the reflected waveforms from the tie surface. An adaptive, reference-based cross-correlation operation is used to compute the relative tie deflections. Multiple measurements along the width of the tie allow the measurement of twisting deformations and longitudinal deflections (3D deflections). Computer vision-based image classification techniques are also utilized for demarcating tie boundaries and tracking the spatial location of measurements along the direction of train movement. Results from field tests, conducted at walking speed at a BNSF train yard in San Diego, CA, with a loaded train car are presented. The tie deflection accuracy and repeatability analyses indicate the potential of the technique to extract full-field tie deflections in a non-contact manner. Further developments are needed to enable measurements at higher speeds.

Minimum-Variance Imaging in Plates Using Guided-Wave-Mode Beamforming.

This article presents improvements to ultrasonic imaging of solid platelike structures using the minimum-variance distortionless response (MVDR) beamforming processor. The primary application of this work is in the nondestructive testing (NDT) of platelike components that are widely used in aerospace, marine, and civil structures. The study proposes a new set of weights, or MVDR replica vectors, that are based on the physics of the propagating Lamb modes, including the symmetric mode S0, the antisymmetric mode A0, and the shear horizontal mode SH0. Numerical results show that these wave-mode weights, combined with geometrical spreading, improve the focus of the array by increasing the dynamic range and the spatial resolution of the image. In addition, quite dramatic improvements in image quality are achieved by combining, or compounding, the multiple Lamb modes naturally present in the plate in both transmission and reflection. As shown in recent work applied to bulk waves in 3-D solids, the compounding of Lamb modes in plates increases the array gain without increasing its physical aperture.

Robust passive reconstruction of dynamic transfer function in dual-output systems.

The focus of this paper is the estimation of the dynamic transfer function between two outputs of a linear system subjected to an uncontrolled and generally unknown excitation, and accounting for possible uncorrelated noise present at both outputs. Several applications of this case exist in the passive identification of dynamic systems including the health monitoring and/or non-destructive evaluation of structures subjected to natural "ambient" excitations. It is well known that noise-robust transfer function estimation of a single-input-single-output system can be achieved by a normalized cross-power spectrum operation. This paper shows that, for the subject case of a dual-output system, particular caution must be placed in the choice of the normalization factor to apply to the cross-power spectrum of the two outputs. In particular, an "inter-segment" averaging method is proposed for the normalization factor in combination with the classical "intra-segment" averaging of the cross-power spectrum in order to estimate the transfer function between the two outputs without the influence of the excitation spectrum and of the uncorrelated noise at the two receivers. Validating results are presented for synthetic signals and for experimental signals from an application to high-speed ultrasonic rail inspection exploiting the train wheels as the "ambient" excitation.

Nonlinear guided wave propagation in prestressed plates.

The measurement of stress in a structure presents considerable interest in many fields of engineering. In this paper, the diagnostic potential of nonlinear elastic guided waves in a prestressed plate is investigated. To do so, an analytical model is formulated accounting for different aspects involved in the phenomenon. The fact that the initial strains can be finite is considered using the Green Lagrange strain tensor, and initial and final configurations are not merged, as it would be assumed in the infinitesimal strain theory. Moreover, an appropriate third-order expression of the strain energy of the hyperelastic body is adopted to account for the material nonlinearities. The model obtained enables to investigate both the linearized case, which gives the variation of phase and group velocity as a function of the initial stress, and the nonlinear case, involving second-harmonic generation as a function of the initial state of stress. The analysis is limited to Rayleigh-Lamb waves propagating in a plate. Three cases of initial prestress are considered, including prestress in the direction of the wave propagation, prestress orthogonal to the direction of wave propagation, and plane isotropic stress.

Nondestructive measurement of neutral temperature in continuous welded rails by nonlinear ultrasonic guided waves.

Modern rail construction uses continuous-welded rail (CWR). The presence of very few joints leads to an increasing concern due to the large longitudinal loads caused by restrained thermal expansion and contraction, following seasonal temperature variations. The knowledge of the current state of thermal stress in the rail or, equivalently, the rail neutral temperature (corresponding to zero net longitudinal force) is a key need within the railroad transportation community in order to properly schedule slow-order mandates and prevent derailments. This paper presents a nondestructive diagnostic system for measurement of the neutral temperature in CWR based on nonlinear ultrasonic guided waves. The theoretical part of the study involved the development of a constitutive model in order to explain the origin of nonlinear effects arising in complex waveguides under constrained thermal expansion. A numerical framework has been implemented to predict internal resonance conditions of nonlinear waves in complex waveguides. This theoretical/numerical phase has led to the development of an experimental prototype (Rail-NT) that was tested both in the laboratory and in the field. The results of these experimental tests are also summarized.

Higher harmonic generation in nonlinear waveguides of arbitrary cross-section.

This article concerns the generation and properties of double harmonics in nonlinear isotropic waveguides of complex cross-section. Analytical solutions of nonlinear Rayleigh-Lamb waves and rod waves have been known for some time. These solutions explain the phenomenon of cumulative double harmonic generation of guided waves. These solutions, however, are only applicable to simple geometries. This paper combines the general approach of the analytical solutions with semi-analytical finite element models to generalize the method to more complex geometries, specifically waveguides with arbitrary cross-sections. Supporting comparisons with analytical solutions are presented for simple cases. This is followed by the study of the case of a rail track. One reason for studying nonlinear guided waves in rails is the potential measurement of thermal stresses in welded rail.

Temperature effects in ultrasonic Lamb wave structural health monitoring systems.

There is a need to better understand the effect of temperature changes on the response of ultrasonic guided-wave pitch-catch systems used for structural health monitoring. A model is proposed to account for all relevant temperature-dependent parameters of a pitch-catch system on an isotropic plate, including the actuator-plate and plate-sensor interactions through shear-lag behavior, the piezoelectric and dielectric permittivity properties of the transducers, and the Lamb wave dispersion properties of the substrate plate. The model is used to predict the S(0) and A(0) response spectra in aluminum plates for the temperature range of -40-+60 degrees C, which accounts for normal aircraft operations. The transducers examined are monolithic PZT-5A [PZT denotes Pb(Zr-Ti)O3] patches and flexible macrofiber composite type P1 patches. The study shows substantial changes in Lamb wave amplitude response caused solely by temperature excursions. It is also shown that, for the transducers considered, the response amplitude changes follow two opposite trends below and above ambient temperature (20 degrees C), respectively. These results can provide a basis for the compensation of temperature effects in guided-wave damage detection systems.

Structural health monitoring by extraction of coherent guided waves from diffuse fields.

Recent theoretical and experimental studies in a wide range of applications have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. This letter demonstrates the passive-only reconstruction of coherent Lamb waves (dc-500 kHz) in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. It is further shown that the passively reconstructed waves are sensitive to the presence of damage in the plate as it would be expected in a typical "active" guided wave test. This proof-of-principle study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

Propagation of ultrasonic guided waves in lap-shear adhesive joints: case of incident a0 Lamb wave.

This paper deals with the propagation of ultrasonic guided waves in adhesively bonded lap-shear joints. The topic is relevant to bond inspection by ultrasonic testing. Specifically, the propagation of the lowest-order, antisymmetric a0 mode through the joint is examined. An important aspect is the mode conversion at the boundaries between the single-plate adherents and the multilayer overlap. The a0 strength of transmission is studied for three different bond states in aluminum joints, namely a fully cured adhesive bond, a poorly cured adhesive bond, and a slip bond. Theoretical predictions indicate that the dispersive behavior of the guided waves in the multilayer overlap is highly dependent on bond state. Experimental tests are conducted in lap-shear joints by a hybrid, broadband laser/air-coupled ultrasonic setup in a through-transmission configuration. The Gabor wavelet transform is employed to extract energy transmission coefficients in the 100 kHz 1.4 MHz range for the three different bond states examined. The cross-sectional mode shapes of the guided waves are shown to have a substantial role in the energy transfer through the joint.