Showing papers in "Journal of Nondestructive Evaluation in 1991"

On the boundary conditions for ultrasonic transmission by partially closed cracks

Abstract: The probability of detecting crack-like defects using ultrasonic techniques can be severly reduced if the crack is closed by a static background pressure. In this paper, we model the contacting faces of a partially closed crack by an array of circular spot-welds randomly distributed over an infinite plane. We give an exact derivation of the reflection and transmission coefficients for a plane elastic wave at such a boundary in terms of the mean interfacial stresses. The latter are estimated in the limit when the contact radius is much smaller than the wavelength and the contacts are sparsely distributed. This calculation is then related to a distributed spring model of the interface. The latter replaces the real interface by an effective homogeneous linear boundary condition which relates the crack opening displacement to the boundary stresses by effective stiffnesses. These unknown parameters are chosen to ensure that the model condition predicts the exact values of the mean interfacial stresses and the reflection and transmission coefficients in the limit already described. Our results are consistent with and complement thoses of Baik and Thompson who introduced the distributed spring model in this and a number of other contexts. Our analysis provides a systematic assessment of the range of validity of the model and suggests ways in which the present estimates may be improved.

Transient scattering of Rayleigh-Lamb waves by a surface-breaking crack: Comparison of numerical simulation and experiment

Abstract: The transient scattering of Rayleigh-Lamb waves by a surface-breaking crack in a plate is investigated in both time and frequency domains by using the hybrid numerical method which combines the finite element discretization of the vicinity of the crack with the Green's function integral representation of the exterior scattered field. The frequency domain response is obtained by solving a set of large sparse unsymmetric complex matrix equations, elements of which are stored in a compacted data structure, by the biconjugate gradient method. The time domain solution is then obtained by using FFT. The source function generated by a steel ball impact is extracted by a direct integration technique. It is then used to simulate some available experimental results. Good agreement has been obtained. Numerical and experimental results show the effect of the crack more in the near field than far away.

One-sided ultrasonic inspection technique for the elastic constant determination of advanced anisotropic materials

Abstract: The subject of anisotropic elastic constant determination with a practical one-sided, multi-mode ultrasonic technique is presented along with comparisons with a more traditional cube cutting technique. Results are presented for destructive static compression tests as well. The Young's Moduli calculated from the measured ultrasonic velocity measurements are shown to compare favorably with those obtained from destructive static compression tests. Limitations of the one-sided method are discussed and further research efforts are also suggested.

Far-field radiation of a vibrating point source in anisotropic media

Abstract: In this paper, Lamb's technique,(1) which was used to study acoustic radiation due to an external force acting in an infinite isotropic solid or over the surface of an isotropic solid, has been extended to the case of general anisotropic media. The far-field radiation is numerically calculated for a vibrating point source in an infinite anisotropic solid or on the free surface of a semi-infinite anisotropic solid. The directivity patterns of a point source are obtained for the three different elastic modes, and a brief discussion of the main features of the numerical results is presented.

Elastic wave propagation with kinematic discontinuity along a non-ideal interface between two isotropic elastic half-spaces

Abstract: The problem of wave propagation along the interface between two elastic, isotropic, and homogeneous half-spaces is studied when the half-spaces are coupled through a vanishingly thin layer of Voigt material. It is assumed that the separation, 2H, between the half-spaces, and the complex rigidity-modulus, μ, of the layer are both vanishingly small, but the complex quantity μ/2H remains finite.

A review of factors influencing defect detection in infrared thermography: Applications to coated materials

Abstract: Infrared thermography is a technique that is used to nondestructively inspect parts for the presence of subsurface defects. The technique normally consists of applying heat to one surface of the part and observing the thermal response, using heat-sensing devices such as infrared cameras, as the part cools. Internal defects such as voids modify the thermal response and produce local hot or cold spots on the specimen surface. For the detection of subsurface defects, the sensitivity of the technique to different parameters such as defect depth, material properties, and heating methods has not been established due in part to the complex nature of the heat/flaw interaction. A finite element model is used here to examine the influence of these parameters on defect dectability. The model shows that the defect detectability decreases with increasing defect depth beneath the surface, and that the technique is most sensitive to the inspection of low thermal diffusivity coatings bonded to high thermal diffusivity substrates. The results also show that the heat pulse duration should be made as short as possible to maximize defect detectability.

Holographic inspection of laminated plates containing two fully-overlapping identical debonds

Abstract: Debonds in laminated plates, particularly those made of glassfiber-reinforced plastics, can be readily detected using double-exposure holography when an incremental vacuum pressure is applied between exposures. With the aid of a model developed in this paper, a method is presented for inspecting a laminated plate containing flaws in the form of two identical circular debonds which are fully overlapped. Two holograms are taken, one on each side of the plate; the anomalous fringe patterns enable not only the detection of the presence of fully-overlapping debonds, but also an estimation of their size and depths. The technique can be applied during the shop testing, i.e., for the detection of defects occurring during the manufacturing of composite laminates, because both sides of the laminate must be inspected. This capability contributes further to realizing the potential of holography in nondestructive testing and evaluation of components.

Experimental and analytical characterization of acoustic emission signals

Abstract: The identification of crack growth signals is extremely important in order to effectively use acoustic emission techniques to detect, locate, and determine the significance of an internal flaw. The results of an analytical methodology, incorporating a source model that is an actual crack propagation and arrest event, are presented in this paper to predict a time dependent acoustic emission signal. The integral equation method is used to calculate the dynamic Mode I stress caused by a crack propagating with a prescribed velocity, after which the displacements at any point are calculated. These time dependent displacements are the analytical form of the acoustic emission waveforms. An experimental procedure, which uses a laser interferometer to measure velocities normal to the specimen's surface, is used to investigate acoustic emission waveforms in a compact tension specimen. The experimental results are used to verify the analytical model.

Eddy Current Image Restoration by Constrained Gradient Descent

Abstract: We present in this paper an image restoration method for recovering the geometry of a flaw in a conducting surface from an eddy current image. Image restoration is formulated as a maximum likelihood estimation problem and is solved using constrained iterative gradient descent. A novel frequency-domain constraint relaxation algorithm is used to control the iterative restoration process. The technique is applied to eddy current images generated by sampling the output of an absolute transducer on a dense grid, creating smoothly blurred images of crack-like flaws. The following results were obtained: (1) Images of a range of sizes of electrical discharge machined (EDM) slots in the surface of a non-ferrous block that encompassed the transducer coil diameter were successfully restored. (2) Excellent results were also obtained for synthetic flaw images with very low signal-to-noise ratios. (3) Synthetic images of multiple flaws spaced less than a coil diameter apart were restored. (4) synthetic images of “V” and pit-shaped flaws were successfully restored. The principal conclusions drawn from this work are: (1) Representing a nonlinear system with layers of linear blurs and non-linear point transformations is general, permitting efficient gradient descent on any analytic system function. (2) Smooth blurring functions in the gradient improve the stability of the algorithm. (3) The accuracy of the estimate is improved dramatically by restoring portions of estimate spectrum which correspond to the highest signal-to-noise ratio band in the observation spectrum first.

An eddy current transducer model for image restoration

Abstract: We describe in this short paper a new imaging model for the spatial magnitude response of an absolute eddy current transducer to a flaw. This model is appropriate for image restoration purposes in that it captures the transducer response with sufficient accuracy for image restoration, yet is simple enough to be computationally practical. The model described in this paper is based on a simple resistive loop approximation to the transducer impedance changes induced by a flaw, and is efficiently implemented as layers of linear blurring functions and nonlinear point operations. The model is shown to accurately reflect the magnitude response of an absolute transducer to an EDM slot in a non-ferrous alloy. Furthermore, the model is shown to produce good restoration results for both synthetic and real images of flaws. The model may be adapted to a particular combination of absolute transducer and surface flaw type by optimizing the model parameters, either by forming the partial derivatives with respect to the parameters and minimizing by gradient descent, or by a straightforward implementation of a neural net back-propagation algorithm. Other types of eddy current transducers may be modeled by simply modifying the spatial layers to combine the local terms appropriately for the geometry of the transducer.

Temperature Recovery and Contrast Computations in NDE Thermographic Imaging Systems

Abstract: 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

A calibration approach to acoustic emission energy measurement

Abstract: An approximate method for the quantitative characterization of acoustic emission sources whose signals are recorded with traditional instrumentation has been developed to evaluate the emission from a Fe-Ni alloy undergoing martensitic transformations. The calibration principle is based upon using pseudo sources of known elastic strain energy to produce a calibration curve relating a parameter of the acoustic emission signal measured (envelope strength) to the strain energy of the source. The curve can then be used to determine the strain energy of naturally occurring sources during martensitic transformations.

Techniques for measuring low levels of hydrogen or hydrogen-bearing materials in solids

Abstract: Three new nondestructive techniques for detecting ppm levels of hydrogen in metals and solids are discussed, along with a review of existing techniques. Two of the new techniques use neutron beams to detect hydrogen. One of these is applicable to steel samples on the order of 1–5 cm in dimension, and has a lower level of detection of 2 weight ppm. The second nuclear technique is applicable to any metal or solid with a mass number greater than about 12, is most suitable for samples on the order of a few millimeters thick, and has a lower level of sensitivity of 0.4 weight ppm. The third technique uses a modulated beam mass spectrometry system, can handle samples up to 100 grams of any size (as long as the sample can be heated uniformly) and has a sensitivity of less than 0.02 weight ppm. Suggestions for possible applications of these techniques are discussed.

Eddy current nondestructive testing by a perturbation method

Abstract: This paper presents an analytical solution for the change in impedance due to a flaw of rectangular cross-section in the case when the conductivity of the flaw is sufficiently close to the conductivity of the surrounding medium. The solution, which is found by the small perturbation method, is used to compute the impedance change as a function of the parameters of the flaw. An eddy current nondestructive method based on this consideration can detect the presence of such flaws and, in some cases, determine their size.

Application of the geometrical theory of diffraction to closed cracks

Abstract: The purpose of this paper is to compare the scattering of ultrasound by means of different types of smooth planar cracks in elastic solids. The boundary conditions on the flaws are of a type that incorporates interfacial forces. A crack that is partly closed by a static background pressure can thus be modeled. The Geometrical Theory of Diffraction (GTD) is used to predict the pulse-echo response from the crack. Previously obtained diffraction and reflection coefficients for cracks with interfacial forces are reviewed and some numerical difficulties are discussed. The backscattered echo amplitude is numerically calculated and compared for some different crack types and crack shapes. The presence of interfacial forces, due to background pressure, significantly influences the scattering behavior of the cracks. For a background pressure of 250 MPa, the backscattered amplitude, using our model of the interfacial forces, is typically reduced by 5–20 dB.

The determination of three-dimensional stress tensors on multilayer thin films

Abstract: The three-dimensional residual stress condition of copper sputtered on Kapton™ films was studied by two nondestructive x-ray diffraction techniques. Four samples were evaluated—one “control sample,” and three heat-treated samples. Because of concerns about steep stress gradients normal to the specimen surface, an experimental technique described by Dolle(1) was used to obtain the residual strain tensors in the near surface region. The resulting stress tensors were compared to data obtained through a modification of the technique described by Dolle, referred to as the differential method.(2)