Flaw Reconstruction Research Articles

An elastodynamic computational time-reversal method for shape reconstruction of traction-free scatterers

This study proposes an elastodynamic time-reversal imaging method for the shape reconstruction of flaws with the traction-free boundary. The proposed method is a generalization of the synthetic aperture focusing technique (SAFT) implemented with the aid of time-reversal back-propagation computation. The reconstruction formula is derived from the boundary condition applied on the flaw boundary without using a diffraction theory yielding an inherently normalized objective function. No restriction is imposed on the wave source or the material properties as long as the wave medium is linearly elastic and lossless. There are two major advantages with the proposed method. First, it is free from ray tracing. The method can thus work with an arbitrary source and the wave modes including inhomogeneous waves. Second, signal conditioning is not required to focus a scattered field to the correct location in the reconstructed image. Numerical examples are presented to demonstrate the performance and benefits of the proposed method. In the numerical examples, the shape of cavities and cracks in an elastic layer is reconstructed from synthetic data. The results show that the method works well with the reverberating incident fields in which inhomogeneous and body waves coexist. Also demonstrated is that the curvature of the cavities is resolved well at the scale of the incident wave length. Through the numerical examples, the method is shown to be a versatile imaging method potentially useful for ultrasonic nondestructive characterization of small flaws.

Noise processing of flaw reconstruction by wavelet transform in ultrasonic guided SH waves

The ultrasonic guided wave technique is a potential and useful tool for nondestructive testing. The scattered or reflected wave from a flaw can give its qualitative or even quantitative information such as location, size and severity. However, in experiments and in situ tests, the noises always exist together with signals. In this paper, we propose a wavelet-transform based noise processing approach for reflected wave signals to the quantitative reconstruction of surface flaws on a plate using guided SH waves. We suggest two different denoising methods based on wavelet transform (WT) in time and wavenumber domains respectively. Numerical results show that wavenumber-domain WT operation gives a better denoising effect than direct time-domain WT denoising. Using the former, one can successfully perform the inverse reconstruction of flaw by reflected signals with signal noise ratio as high as −5 dB. This research can act as a theoretical reference for practical applications of ultrasonic guided SH waves in quantitative nondestructive evaluation.

Reconstruction of Flaws in Heterogeneous Media Using Image-Based FIT and Time Reversal Approach

Most commercial ultrasonic phased array systems implement B– and C–scan methods which use flight time and amplitude of flaw echoes. However, these methods are based on the fundamental ray–tracing theory in homogeneous media, and they are not directly applicable to heterogeneous media because of the complicated phenomenon of wave propagation. Time reversal techniques are adaptive methods that can be used in nondestructive evaluation to improve flaw detection in heterogeneous media. In this study, we propose a simulation–aided flaw imaging method based on the time reversal approach. Scattered waves from a flaw are recorded using an array transducer, and the time–reversed waves are re–emitted in the image–based FIT simulation. The reemitted waves propagate through the heterogeneous media and focus on the flaw. The shape of the flaw can be visually estimated from the focal point of the ultrasonic wave in the simulation.

Ultrasonic Flaw Imaging by the FSAP Technique Using Array Transducer for Angle Beam Testing

There is a phased array technique which makes use of flaw echoes, measured by every two-element combination as a pulser and a receiver, to synthesize high amplitude beams for any arbitrary angle and/or focal depth. Using this full waveform sampling and processing (FSAP) technique, we show a flaw reconstruction method for the angle beam phased array ultrasonic testing. In the case of the angle beam testing, it is necessary to estimate a beam path from a wedge to a specimen appropriately and to consider the mode conversion at the wedge-specimen interface. The main feature of our method is the use of processed wave data, called “scattering amplitude”, which is extracted from each flaw echo. First, the basic principle of the reconstruction method is described and then some flaw reconstructions are demonstrated. The resolution of the flaw images and the accuracy of flaw sizing can be improved by means of the scattering amplitude.

OS02F026 A Flaw Reconstruction Method in Heterogeneous Media with Image-based FIT and Time Reversal Approach

OS02F026 A Flaw Reconstruction Method in Heterogeneous Media with Image-based FIT and Time Reversal Approach

OS02-3-4 A Flaw Reconstruction Method in Heterogeneous Media with Image-based FIT and Time Reversal Approach

OS02-3-4 A Flaw Reconstruction Method in Heterogeneous Media with Image-based FIT and Time Reversal Approach

Application of the inverse diffraction technique to the problem of reconstruction of complexly shaped flaws by ultrasonic methods

The methods used in inverse diffraction problems and applied to the problem of reconstructing flaws of complex shapes using ultrasonic scanning are reviewed. The application of the theoretical methods developed by the authors is described in more detail. The accuracy in reconstruction of flaws on the basis of a new proposed method is tested.

Flaw Reconstruction from Scattering Amplitude using Full-waveform Sampling and Processing (FSAP)

Flaw Reconstruction from Scattering Amplitude using Full-waveform Sampling and Processing (FSAP)

Application of linearised inverse ultrasonic scattering methods for estimating flaw dimension

Linearised inverse ultrasonic scattering methods based on the Born and Kirchhoff approximations are utilised for flaw reconstruction in an elastic material. The flaw shape can be reconstructed through the FFT of the scattering amplitudes in K-space. Theoretical backscattered wave forms are calculated for cubic defects of different sizes, using the software Ansys 9.0. Inverse scattering methods are then used to reconstruct the defect shapes. From the results of this reconstruction, it can be said that the linearised inverse scattering methods are effective means of shape reconstruction and, for our application, they can reconstruct a defect that has a minimal characteristic size of 2 mm.

Efficient binary reconstruction for non-destructive evaluation using gammagraphy

The localization and the sizing of 3D flaws within a homogeneous metallic medium is a major task for non-destructive evaluation (NDE). This paper addresses the problem of the reconstruction of such flaws using an efficient binary algorithm. Basically, the method rests on the fact that a simple binary constraint suffices for an accurate and robust reconstruction in the context of NDE. A heuristic minimization, computationally attractive, is designed in order to provide fast reconstructions. The proposed algorithm is compared with standard binary (the iterated conditional mode algorithm) and non-binary (penalized approach with convex potential Gibbs random fields) reconstruction techniques.

A Fast Shape Inversion Method of Flaws in Materials for Infrastructures

The Born inverse scattering method is applied to the shape reconstruction of flaws in the concrete materials. To accelerate the inversion process, the algorithm of the fast Fourier transform is introduced in the integration process of the scattering amplitude. The concrete specimens with flaw models are prepared and the scattered waveforms from flaws are acquired in the pulse-echo mode. The shape of flaws is reconstructed by using the measured waveforms and the performance of the inversion method is demonstrated experimentally.

Elastodynamic inversion for shape reconstruction and type classification of flaws

Two linearized inverse scattering methods are investigated to reconstruct the shape of flaws in the elastic solid. One is based on the Born approximation and the other is based on the Kirchhoff approximation. The Born inversion is sensitive to the volumetric flaw but not to a crack-like flaw. On the other hand, the Kirchhoff inversion reacts to both boundaries of volumetric and crack-like flaws. The combined use of Born and Kirchhoff inversions leads to the classification method of flaw type. The performance of the proposed classification procedure is demonstrated by the numerical simulations and then by the experimental measurements for the two-dimensional models of flaws. An example for the three-dimensional shape reconstruction is also shown by using the numerically calculated backscattering amplitudes.

High field resolution for nondestructive testing using sensitive magnetometers

Abstract Advanced image processing and flaw reconstruction algorithms of eddy current (EC) testing techniques are based on a low noise level of the measured response fields. Sensitive magnetometers such as SQUID and flux gate sensors provide a high field sensitivity of between 0.1 pT/√Hz and several pT/√Hz, and an outstanding dynamic range of more than 150 dB/√Hz due to their read out electronics based on a feed back loop. The EC measurements based on a usual 12–16 bit data recording often result in an effective dynamic range of about 65–85 dB/√Hz, due to the quantization noise of the ADC. In the case of typical EC signal processing algorithms, such as subtraction of reference signals or phase rotation a higher dynamic range is required. We, therefore, set up an EC system based on sensitive magnetometer which covers an analog dynamic range of up to 160 dB/√Hz. We report on EC measurements with real 21 and 24 bit field resolution, on several aluminium samples and adapted signal processing for the separation of defect signals or the improvement of the spatial resolution.

Defect detection and classification using a SQUID based multiple frequency eddy current NDE system

The probability of detection (POD) of hidden fatigue defects in riveted multilayer joints, e.g. aircraft fuselage, can be improved by using sophisticated eddy-current systems which provide more information than conventional NDE equipment. In order to collect this information, sensor arrays or multi-frequency excitation schemes can be used. We have performed simulations and measurements with an eddy current NDE system based on a SQUID magnetometer. To distinguish between signals caused by material defects and those caused by structures in the sample, such as bolts or rivets, a high signal-to-noise ratio is required. Our system provides a large analog dynamic range of more than 140 dB//spl radic/Hz in unshielded environment, a digital dynamics of the ADC of more than 25 bit (>150 dB) and multiple frequency excitation. A large number of stacked aluminum samples resembling aircraft fuselage were measured, containing titanium rivets and hidden defects in different depths in order to obtain sufficient statistical information for classification of the defect geometry. We report on flaw reconstruction using adapted feature extraction and neural network techniques.

Quantitative flaw reconstruction from ultrasonic surface wavefields measured by electronic speckle pattern interferometry.

A new method for imaging flaws in plate and shell structures is presented. The method employs two-dimensional ultrasonic surface wave data obtained by optical electronic speckle pattern interferometry (ESPI) techniques. In the imaging method, the measured out-of-plane displacement field associated with an externally excited ultrasonic Lamb wave is processed to obtain the spatial frequency domain spectrum of the wavefield. A free space Green's function is then deconvolved from the wavefield to obtain quantitative images of effective scattering sources. Because the strength of these effective sources is directly dependent on local variations in sample thickness and material properties, these images provide a direct map of internal inhomogeneities. Simulation results show that the method accurately images flaws for a wide range of sizes and material contrast ratios. These results also demonstrate that flaw features much smaller than an acoustic wavelength can be imaged, consistent with the theoretical capability of the imaging method to employ scattered evanescent waves. Reconstructions are also obtained from ultrasonic Lamb wave displacement fields recorded by ESPI in a flawed aluminum plate. These reconstructions indicate that the present method has potential for imaging flaws in complex structures for which ESPI wavefield measurements cannot be straightforwardly interpreted.

Forward solution speed-up for 3D eddy current inversion

The forward solver is the bottleneck in eddy current flaw reconstruction because the solution of a dense system matrix is required. Two methods are developed to increase the solution speed. The cell merging technique gives impressive speed-up for cracks but a fully adaptive version for volumetric flaws has not been found yet. The other algorithm based on sparse representation of rank deficient matrix blocks and the Woodbury procedure is slower than the cell merging technique but it is robust and still significantly faster than the LU factorization. Creating a hierarchical partitioning tree ensures the stability of the latter algorithm.

Non-linear algorithms based on SAFT ideas for reconstruction of flaws

The main idea and the assumptions of time domain synthetic aperture focusing technique (SAFT) are discussed. It is underlined that the major SAFT assumption is that the flaw can be represented by a number of single non-interacting scalar point scatterers, which act as point sources after illumination with the incident wavefield. As a result some important classes of flaws are incorrectly reconstructed by means of SAFT. Among them are cracks from which reflected signals are similar to those from mirror-shaped flaws. Different non-linear SAFT algorithms proposed earlier by the author are discussed and compared. Among them are SAFT with non-linear normalization (NSAFT), second order SAFT, and the radial tomography technique. The principles of these algorithms are discussed. Then their advantages and disadvantages are analysed. Unfortunately each algorithm cannot be considered universal as a classical SAFT, but for many important cases these algorithms are useful for solving inverse problems in ultrasonics. The ideas of non-linear SAFT algorithms are illustrated by physical experiments.

Reconstruction of Cracks with Multiple Eddy Current Coils Using a Database Approach

This paper presents some recent studies dealing with flaw reconstruction using eddy current testing. The locations of parallel planar cracks in an Inconel 600 plate specimen are determined and the shapes of them are reconstructed at the same time using the signals from a multi-pancake coil probe. The reconstruction strategy, consisting of a data base used fast forward solver and a first order optimization algorithm, relies on the minimization of the nonlinear least square residual function. Validity of the inverse analysis method is investigated with respect to the influence of the arrangement of scanning paths. The efficiency of the computation is also discussed.

Rapid prediction of eddy current testing signals using A−φ method and database

This paper describes a new scheme for the fast evaluation of eddy current testing (ECT) signals by using databases and formulation of the A−φ method. As the calculation of flaw signals is localized around the flaw region, substantial computational work can be reduced comparing with the conventional FEM–BEM code even for a conductor with complex geometry. Unlike the approach proposed by Badics et al. (Rapid flaw reconstruction scheme for 3-d inverse in eddy current NDE. In Studies in Applied Electromagnetics and Mechanics, Vol. 12, eds T. Takagi et al., IOS Press, 1997, pp. 303–309), the new scheme can predict ECT signals due to a large and complex-shaped crack rapidly because of its higher order interpolation and the newly introduced special element, say, an element with different media. The efficiency of the new method is verified by comparing its numerical results with the measured impedance for several bench-mark problems. The fast and accurate features make this new forward approach especially feasible in the reconstruction of flaw shapes by combining with the conventional optimization method.

Quantitative flaw reconstruction from ultrasonic surface wave fields measured by laser interferometry

A quantitative imaging method for ultrasonic nondestructive testing of structures is presented. The method employs two-dimensional surface-vibration data that is obtained by laser interferometry methods such as holography and digital phase-stepping shearography. In this method, a surface wave field generated by a narrow-band contacting or noncontacting ultrasonic transducer is Fourier transformed to obtain a spatial-frequency domain representation of the wave field. The appropriate Green’s function is then deconvolved from the wave field to obtain quantitative images of effective scattering sources. Since scattering strength is directly proportional to the local contrast of the structure, these images provide a direct map of internal flaws in structures such as plates and shells. The method is valid for strongly scattering inhomogeneities, and can employ scattered evanescent waves to obtain resolution of flaws much smaller than a wavelength. Simulated and experimental results illustrate the efficacy and quality of the imaging method.