Showing papers in "Journal of Nondestructive Evaluation in 2006"
TL;DR: In this article, the authors developed a model to quantify the acoustic nonlinearity parameter due to elastic-plastic deformation in anisotropic elastic-polysilicon materials with existing microplasticity strains due to either monotonic or cyclic loading.
Abstract: Acoustic nonlinearity (which is quantified in terms of an absolute material parameter, the acoustic nonlinearity parameter, β) can be caused by several sources, one of which is the elastic-plastic deformation of the material. This paper develops a model to quantify the acoustic nonlinearity parameter due to elastic-plastic deformation. This new model is applicable to general anisotropic elastic-plastic materials with existing microplasticity strains due to either monotonic or cyclic loading. As an example, the developed model is applied to calculate the acoustic nonlinearity parameter of a single crystal copper specimen subjected to cyclic fatigue loading. It is found that the acoustic nonlinearity parameter of this specimen increases monotonically with increasing fatigue cycles.
71 citations
TL;DR: In this paper, a dual angle beam through transmission technique is employed whereby the regions of expected crack initiation are interrogated with vertically polarized shear waves, and a model is developed to explain this relationship for known crack shapes by assuming an ultrasonic energy profile in the plane of the crack.
Abstract: Ultrasonic testing is generally recognized as being the preferred nondestructive evaluation (NDE) method for measuring the size of cracks in metals, particularly when access is restricted. Using ultrasonic methods for structural health monitoring imposes the requirement of fixed transducers, which limits the articulation of transducers that is common for NDE. Here we consider the specific problem of in situ sizing of fatigue cracks emanating from fastener holes in aluminum structural components. A dual angle beam through transmission technique is employed whereby the regions of expected crack initiation are interrogated with vertically polarized shear waves. Fatigue cracks partially block the leaky creeping wave spiralling around the hole, and thus reduce the energy of the received signal. The reduction in received energy is directly related to the extent and shape of any open cracks, and this energy is modulated by the applied load as the cracks open and close. The ratio of energy received from the specimen under load to that received under no load is monitored, and this ratio is empirically related to the total crack area. A model is developed to explain this relationship for known crack shapes by assuming an ultrasonic energy profile in the plane of the crack. Data calculated from the model show very good agreement with the experimental results.
45 citations
TL;DR: In this article, the authors investigated the different mechanisms through which cold work could influence the eddy current conductivity in surface-treated nickel-base superalloys and found that neither the magnetic susceptibility nor the piezoresistivity of the material is affected significantly by cold work up to 50% plastic strain level, but the electrical conductivity does substantially increase due to microstructural changes.
Abstract: Shot-peened nickel-base superalloys exhibit 1–2% increase in apparent eddy current conductivity (AECC), which can be exploited for nondestructive residual stress assessment. Experimental evidence indicates that the excess AECC is due in part to elastic strains, i.e., residual stress, and in part to plastic strains, i.e., cold work. The very fact that the conductivity increases rather than decreases was originally thought to indicate that there was no significant cold work contribution to the observed AECC increase. This assumption was also supported by X-ray diffraction (XRD) results on fully relaxed specimens showing that the cold work induced widening of the diffraction beam only partially vanishes when both the residual stress and the AECC completely disappear due to thermal relaxation. However, we show in this paper that assuming that the conductivity change is entirely due to residual stress via the piezoresistivity of the material could result in an unacceptable overestimation of the magnitude of the compressive residual stress. Therefore, we investigated the different mechanisms through which cold work could influence the AECC in surface-treated nickel-base superalloys. It was found that neither the magnetic susceptibility nor the piezoresistivity of the material is affected significantly by cold work up to 50% plastic strain level, but the electrical conductivity does substantially increase due to microstructural changes. Based on these observations, we suggest that in future research the complex variations caused by cold work should be modeled by at least two main types of cold work parameters rather than by a single one in order to properly account for the otherwise contradictory effects of plastic deformation on eddy current conductivity and XRD measurements.
33 citations
TL;DR: In this article, an ultrasonic technique for the detection of matrix cracks in each of the plies of damaged graphite/epoxy laminated composites is developed for an eight-ply quasi-isotropic composite, having [90/45/0/-45]s lay-up.
Abstract: *Extensive use of laminated composites at cryogenic temperatures is envisaged in the next generation Reusable Launch Vehicles (RLVs). The RLVs will include light fuel tanks fabricated using graphite/epoxy composite laminates and will be required to contain liquid hydrogen (LH2) fuel. These tanks undergo extensive thermo-mechanical loading and develop damage in the form of matrix cracks and delaminations. A build up of matrix cracks in the plies of the laminated composite wall of a tank may lead to the formation of a network of interconnected cracks through the thickness of the tank-wall and provide a path for fuel to permeate to the outside. In this context, the detection of matrix cracks and their connectivity takes on an unprecedented significance. In this work, an ultrasonic technique for the detection of matrix cracks in each of the plies of damaged graphite/epoxy laminated composites is developed. Results are reported for an eight-ply quasi-isotropic graphite/epoxy laminate, having [90/45/0/-45]s lay-up. Matrix cracks in each of the plies of the laminated composite were detected even in the presence of a rather high density of cracks in all of the remaining plies. The ultrasonic data were corroborated by comparing them with the corresponding results obtained by using the more traditional methods of optical microscopy and X-radiography. Excellent quantitative correlation was observed between the results obtained with ultrasonics and the traditional methods.
30 citations
TL;DR: In this article, a series of studies concerning the detection of cracks around fastener holes demonstrating the quality of the simulated data to represent experimental measurements are presented, and a novel calibration technique is also presented to practically evaluate the transformation between model-based impedance calculations and experimental voltage data.
Abstract: To improve the detection and characterization of cracks around fastener holes in multilayer structures without removing the fastener, model-based approaches are proposed to support the design of advanced eddy current (EC) NDE systems. This work demonstrates the validation and application of models to simulate EC inspection as part of the design process. The volume integral method (VIM) and finite element method (FEM) are both used to simulate eddy current inspection of fastener sites for fatigue cracks. Convergence studies, validation with existing models, experimental validation studies and validation through inverse method demonstrations are presented, providing a continuum of methods to ensure the quality of measurement models. Consideration concerning convergence and validation is also given with features sensitive to the sample geometry and flaw characteristics. A novel calibration technique is also presented to practically evaluate the transformation between model-based impedance calculations and experimental voltage data. A series of studies are presented concerning the detection of cracks around fastener holes demonstrating the quality of the simulated data to represent experimental measurements.
26 citations
TL;DR: In this article, the authors demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence.
Abstract: Recently, it has been shown that shot-peened nickel-base superalloys exhibit an approximately 1% increase in apparent eddy current conductivity at high inspection frequencies, which can be exploited for nondestructive subsurface residual stress assessment. Unfortunately, microstructural inhomogeneity in certain as-forged and precipitation hardened nickel-base superalloys, like Waspaloy, can lead to significantly larger electrical conductivity variations of as much as 4‐6%. This intrinsic conductivity variation adversely affects the accuracy of residual stress evaluation in shot-peened and subsequently thermalrelaxed specimens, but does not completely prevent it. Experimental results are presented to demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence. This characteristic independence of frequency can be exploited to distinguish these inhomogeneities from nearsurface residual stress and cold work effects caused by surface treatment, which, in contrast, are strongly frequency-dependent.
18 citations
TL;DR: In this paper, a feature with noise invariance to all non-flaw conditions considered in this study was found through investigating changes in the eddy current response along a circumferential direction away from the hole center.
Abstract: There is a need for research in eddy current (EC) nondestructive evaluation (NDE) to improve the reliability to detect, locate and size cracks around fastener sites in multi-layer structures while minimizing the overall cost of inspection. The objective of this work is to develop feature extraction and classification algorithms for crack characterization with invariance to noise features for eddy current inspection of fastener sites. Model-based parametric studies were first performed to explore potential features under a wide array of crack, noise and material conditions. Through these studies, several features were identified to have some invariance to the characteristic asymmetric response due to gaps between the fastener and hole, probe liftoff variation, and probe skew. In particular, a promising feature with noise invariance to all non-flaw conditions considered in this study was found through investigating changes in the eddy current response along a circumferential direction in an annulus region away from the hole center. To obtain a measure of this localized crack feature, an approach was developed using a fit of a characteristic function to the data through nonlinear least squares estimation. A model-based optimization approach was also implemented to evaluate the best signal processing algorithm design to distinguish between several classes of crack size. Using this approach, an optimized measure was found to be well correlated with subsurface crack size and insensitive to noise conditions included in this study.
15 citations
TL;DR: In this paper, the authors present new damage indicators and current capabilities of a dynamics-based boundary effect evaluation method (BEEM) for damage inspection of large one-and two-dimensional structures.
Abstract: This paper presents new damage indicators and current capabilities of a dynamics-based Boundary Effect Evaluation Method (BEEM) for damage inspection of large one-and two-dimensional structures. Damage introduces new boundaries to a structure, and influences of boundaries on steady-state high-frequency dynamic response are spatially localized effects. The BEEM is a signal processing method that takes advantage of these localized effects in performing area-by-area extraction of damage-induced boundary effects from steady-state Operational Deflection Shapes (ODSs) to reveal damage locations. Steady-state ODSs of a structure can be measured using a scanning laser vibrometer or any other full-field measurement tool, and the BEEM decomposes an ODS into central and boundary solutions by using a sliding-window least-squares data-fitting technique. Numerical and experimental results show that boundary solutions are excellent damage indicators because of Gibb’s phenomenon, and the central solutions can be used to clearly identify actual boundary conditions. Except for experimental ODSs of the damaged structure the method requires no model or historical data for comparison. Experimental results of many one-and two-dimensional structures validate the high sensitivity and accuracy of BEEM for detection and estimation of multiple small defects in structures.
12 citations
TL;DR: In this paper, a state-space approach is used to solve a modal state equation leading to a global transfer matrix that directly links the boundary conditions at the outer surface of the interface layer to those at the inner surface.
Abstract: Scattering of compressional waves in multiphase metal matrix composites containing spherical particles with spherically isotropic graded interfacial layers is investigated using a state-space approach. A continuous transition from the particle to the matrix with the change of volume fraction of one of the constituents is assumed to exist across the thickness of the interphase zone. A simplified multilayer model for the interphase complications including both anisotropy and inhomogeneity is considered. Taylor’s expansion theorem is employed to solve a modal state equation leading to a global transfer matrix that directly links the boundary conditions at the outer surface of the interface layer to those at the inner surface. Numerical calculations reveal the important effects of interphase anisotropy and inhomogeneity on the total scattering cross section and dynamic stress concentrations for a moderately wide range of frequencies and interface layer thicknesses.
7 citations
TL;DR: In this article, a modified Born approximation (MBA) is presented that significantly extends the range of validity of the Born approximation to include the pulse-echo responses of strongly scattering inclusions in an elastic solid.
Abstract: A new modified Born approximation (MBA) is presented that significantly extends the range of validity of the Born approximation to include the pulse-echo responses of strongly scattering inclusions in an elastic solid. The MBA also improves on the doubly distorted Born approximation (DDBA), a similar modification of the Born approximation that has been recently developed. These improvements are demonstrated by comparing the MBA, the Born approximation and the DDBA with the exact separation of variables solution for spherical inclusions in isotropic media. Furthermore, it is shown that the form of the MBA remains valid even for the pulse-echo scattering of an anisotropic inclusion in a general anisotropic elastic medium so that it is potentially applicable to a wide class of flaws and materials.
5 citations
TL;DR: Back propagation (BP) type artificial neural networks have been trained and used for thickness estimations from radiographic images and the average estimation errors were 7% and 9% when two and three synthetic radiography images obtained at different x-ray tube settings were used.
Abstract: Back propagation (BP) type artificial neural networks (ANN) have been trained and used for thickness estimations from radiographic images. Test objects have been assembled from different materials and radiographic images of the test objects were obtained for thickness estimations. While some of the study has been based on the synthetic images formed through the radiographic simulation program XRSIM, the rest of the study has used actual radiographic images. The average estimation errors were 7% and 9% when two and three synthetic radiographic images obtained at different x-ray tube settings were used. With the actual images, the thickness of only one of the materials has been estimated and the material was identified. This has been due to the fact that scattering of x-rays by the test object results in a non uniform gray scale variation in the radiographic images even though the object thickness is uniform.
TL;DR: In this paper, a model-based investigation of inspection standards being developed as a recommended practice for eddy current (EC) NDT of aerospace structures for surface cracks is described, and the expected amplitudes and phase-angle ranges are determined for various test conditions, in order to examine the influence of material electrical conductivity, frequency, and probe types.
Abstract: This paper describes a model-based investigation of inspection standards being developed as a recommended practice for eddy current (EC) NDT of aerospace structures for surface cracks. Compared to experimental investigations that can be laborious and expensive, model-based approach provides cost-effective and quantitative verifications of the recommended practice that covers wide ranges of materials, probes, instruments, frequencies, and test conditions. Probe signals for a number of specified standard notches are predicted and the expected amplitudes and phase-angle ranges are determined for various test conditions, in order to examine the influence of material electrical conductivity, frequency, and probe types. For a few selected situations, the model predictions are validated against experimental data. This study verifies the adequacies of the recommended specifications, especially of the reference notch selections and their response amplitudes for various test conditions, as well as confirming the suitability of using vertical amplitude alone for all the test situations. The influence of probe cable and the issue of three-point calibration are also discussed specifically in this investigation.
TL;DR: In this article, the problem of wave scattering from a periodic interface separating two anisotropic layers in a thick elastic plate is studied in the two-dimensional case and the problem is solved by replacing the exact boundary conditions, i.e., continuous displacement and traction on the wavy interface, by approximate first order conditions on a flat reference surface.
Abstract: Wave scattering from a periodic interface separating two anisotropic layers in a thick elastic plate is studied in the two-dimensional case. The problem is solved by replacing the exact boundary conditions, i.e. continuous displacement and traction on the wavy interface, by approximate first order conditions on a flat reference surface. Numerical results are presented for a number of cases and compared to the exact solution obtained by the null field approach. The conclusion is that the approximate method gives reasonably accurate results as long as the slope of the surface is small and the amplitude of the wavy surface is not too large compared to the wavelength of the incident wave.