Nondestructive Evaluation Problems Research Articles

Approximate reconstruction of attenuation map in SPECT imaging

The influence of linear attenuation presents certain difficulties for quantitative single-photon emission computed tomography (SPECT) in medical imaging. To apply an attenuation correction algorithm, one has first to estimate the attenuation map from some additional measurements or by the use of SPECT data. The determination of the attenuation map from SPECT-type data may also be considered as a special problem of nondestructive evaluation of materials. A new technique for obtaining the attenuation map from SPECT data is proposed. Unlike other methods, the approach suggested demands no a priori knowledge of the problem. The method has been derived on the basis of the approximate linear relation between SPECT data and the attenuation map. The approach permits local changes of the attenuation coefficient to be reconstructed rather than its actual value. In this way, such details of the attenuation map as spots, curves and edges can be localized. Limitations of application of the suggested technique to real data are considered. Results of numerical evaluation and real data processing are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interferometric comparison of diffuse objects using comparative holography

Comparative holography solves a key problem of nondestructive evaluation: the comparison of the mechanical behavior of two nominally identical specimens subjected to the same loads. The method provides contours of equal differences in displacements of two stressed objects. This feature makes the method particularly attractive in flaw detection. The aim is to briefly outline the principles and practice of comparative holography with an emphasis on recent developments.

On the application of moment methods to electromagnetic nondestructive evaluation

The application of moment methods (MMs) to eddy-current testing problems for nondestructive evaluation (NDE) is considered. A general formulation for the MM that can be used to analyze NDE problems is derived, and calculated results and experimental data obtained from eddy-current testing of an artificially made sample are presented. Good agreement between the calculated results and the experimental data confirms the validity of the method and shows that the MM can be used as an alternative to the finite-element method (FEM) and the boundary-element method (BEM) in NDE. >

Temperature recovery and contrast computations in NDE thermographic imaging systems

Thermographic imaging systems find application in many nondestructive evaluation problems. However, because of the inherent nature of the image-formation process, several degradations arise which preclude a more generalized use of thermographic NDE. In this paper, we analyze radiometric and noise degradations and propose a calibration process for the imaging systems which allows recovery of object surface temperature evolution. Moreover, a constrast computation technique which combines both the time and spatial reference techniques is shown to increase the defect visibility and make possible a more quantitative (sizing) defect evaluation.

Neural networks for the classification of nondestructive evaluation signals

The paper proposes the use of massively parallel learning networks for interpreting signals from electromagnetic transducers used in nondestructive evaluation (NDE) problems. Nondestructive testing techniques are used in a variety of industries for evaluating the structural integrity of critical components in a noninvasive manner. A major aspect of research in nondestructive testing is related to the inverse problem and is commonly referred to as defect characterisation. This involves the classification of the eddy current signals in terms of the shape and size of the underlying defects in the test object. A key contribution of the proposed network is the ability to obtain a rotation and translation-invariant internal representation of the signal. Results showing the merits of this approach as well as a comparison with traditional techniques for the classification of signals are presented.

Ultrasonic NDE using a concentric laser/EMAT system

A pulsed laser source, together with a concentric annular electromagnet acoustic transducer (EMAT) as detector, have been investigated for use as a noncontact ultrasonic inspection system. The characteristics of this hybrid transduction method, and its application to various problems in nondestructive evaluation have been studied. Results are presented for applications in imaging, the testing of diffusion and adhesive bonding, and thickness estimation.

Solution of inverse problems in eddy-current nondestructive evaluation (NDE) : Journal of nondestructive evaluation, Vol. 7, Nos 1–2, pp. 111–120 (Jun. 1988)

Solution of inverse problems in eddy-current nondestructive evaluation (NDE) : Journal of nondestructive evaluation, Vol. 7, Nos 1–2, pp. 111–120 (Jun. 1988)

A MODEL OF EDDY-CURRENT PROBES WITH FERRITE CORES

This paper describes an application of modern methods of computational electromagnetics to the problem of eddy-current nondestructive evaluation (NDE). Specifically, a volume integral equation is developed that can be used to model eddy-current probes with ferrite cores, or it can be used to compute flaw-field interactions. Both of these problems are of considerable importance in applying electromagnetic techniques to NDE, for they are intimately involved in detecting flaws and inverting eddy-current data for reconstructing flaws. The model is fully three-dimensional and the discretized integral equation is solved iteratively using conjugate gradients and FFT techniques. Problems with 12,000 unknowns have been routinely solved on the Alliant FX/1 minisupercomputer in reasonable times. The model is used to compute such important probe parameters as impedance and coupling. It is also used to compute the electromagnetic response of flaws as the geometry of the flaw changes, or as a function of frequency.

Inverse problems in nondestructive evaluation

The goal of nondestructive evaluation is to obtain information about the state of a structural or electronic component that will allow a quantitative assessment of its future serviceability. Because of the ability to penetrate opaque media, ultrasound is one of the most widely used forms of interrogating energy. This paper will consider several current problem areas, ranging from the measurement of material properties such as porosity or grain structure through the characterization of the nature of discrete flaws and the determination of their size and shape. In each case, an interpretation of the measurement in terms of elastic wave scattering theory will be followed by a discussion of the inverse theories used to interpret the data, with emphasis on assumptions required to obtain unique solutions. The presentation will conclude with a discussion of future directions of needed research.

Solution of inverse problems in eddy-current nondestructive evaluation (NDE)

Eddy-current methods of nondestructive testing find widespread application in industry. The growth has been fueled, in part at least, by an increasing ability to extract information from the signal generated by the probe. This paper presents a brief overview of some of the techniques proposed for characterizing defects. Emphasis has been placed on two of the techniques developed by the authors, namely, the Fourier descriptor method and the defect reconstruction scheme using the finite-element model.

Thermal-wave imaging for nondestructive evaluation

We review the applications of various thermal-wave techniques to problems in nondestructive evaluation of materials.

Comparative holographic interferometry: A nondestructive inspection system for detection of flaws

Comparative holographic interferometry addresses a key problem of nondestructive evaluation: comparison of the resistance to strength in real time of two nominally identical specimens. After outlining the basis of comparative holography in flaw detection, the present paper reports on some new and complementary developments in comparative holographic interferometry. The proposed systems stand out by their ability to store the master displacement field in the interferometric setup. The integration of the storage capacity in the instrument considerably augments its potential in nondestructive routine inspection tasks. The visual display of the fringes contouring the difference in mechanical response is shown to improve considerably with the addition of auxiliary phase difference satisfying certain conditions. Methods for the generation of corresponding fringes are considered and their localization investigated in brief. Particular attention is devoted to the formation of the holographic moire fringes. The influence of system misalignment on the moire fringe interpretation is examined. A potential application of comparative holography to the quantitative evaluation of fatigue is described. Experimental evidence supporting the operational feasibility of the technique along with the results obtained in application to flaw detection are finally presented.

Nonadaptive, semi-adaptive and adaptive approaches to signal processing problems in nondestructive evaluation

In signal processing problems associated with estimation or pattern recognition in nondestructive evaluation (NDE), the degree of adaption can vary between wide limits, and is related to the availability or usability of a priori information. We will discuss several examples of NDE signal processing problems involving varying degrees of adaption, namely: (1) a completely nonadaptive approach to the extraction of low frequency properties of scatterers, (2) a semi-adaptive approach to the same problem, and (3) a completely adaptive approach to the classification of acoustical emission signals.