Showing papers in "Research in Nondestructive Evaluation in 2000"

Nonlinear Elastic Wave Spectroscopy (NEWS) Techniques to Discern Material Damage, Part I: Nonlinear Wave Modulation Spectroscopy (NWMS)

Abstract: The level of nonlinearity in the elastic response of materials containing structural damage is far greater than in materials with no structural damage. This is the basis for nonlinear wave diagnostics of damage, methods which are remarkably sensitive to the detection and progression of damage in materials. Nonlinear wave modulation spectroscopy (NWMS) is one exemplary method in this class of dynamic nondestructive evaluation techniques. The method focuses on the application of harmonics and sum and difference frequency to discern damage in materials. It consists of exciting a sample with continuous waves of two separate frequencies simultaneously, and inspecting the harmonics of the two waves, and their sum and difference frequencies (sidebands). Undamaged materials are essentially linear in their response to the two waves, while the same material, when damaged, becomes highly nonlinear, manifested by harmonics and sideband generation. We illustrate the method by experiments on uncracked and crac...

Nonlinear Elastic Wave Spectroscopy (NEWS) Techniques to Discern Material Damage, Part II: Single-Mode Nonlinear Resonance Acoustic Spectroscopy

Abstract: The presence of mesoscopic features and damage in quasi-brittle materials causes significant second-order and nonlinear effects on the acoustic wave propagation characteristics. In order to quantify the influence of such micro-inhomogeneities, a new and promising tool for nondestructive material testing has been developed and applied in the field of damage detection. The technique focuses on the acoustic nonlinear (i.e., amplitude-dependent) response of one of the material's resonance modes when driven at relatively small wave amplitudes. The method is termed single-mode nonlinear resonance acoustic spectroscopy (SIMONRAS). The behavior of damaged materials is manifested by amplitude dependent resonance frequency shifts, harmonic generation, and nonlinear attenuation. We illustrate the method by experiments on artificial slate tiles used in roofing construction. The sensitivity of this method to discern material damage is far greater than that of linear acoustic methods.

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Abstract: Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.

Analysis of Defective Carbon-Epoxy by Means of Lock-in Thermography

Abstract: The attention of the present work is focused on the analysis of defects in carbon-epoxy laminates. Different specimens are manufactured by varying the laminate orientation code, the weave type (preimpregnated or nonimpregnated), and the kind of defect. In particular, defects such as inclusions of spurious materials, delamination, and localized lack or excess of resin are artificially created to simulate the most probable kinds of damage occurring in carbon-epoxy products during manufacturing and/or in service. Nondestructive tests are performed by means of lock-in thermography.

Eddy Current Testing of Thin Layers Using Co-planar Coils

Abstract: A new eddy current technique using a co-planar coil configuration (ECCPC technique) has been developed and tested quantitatively in the laboratory and under real conditions in a nuclear reactor tank to detect thin resistive and/or magnetic layers on fuel rods. First a short description of the theoretical model is given, followed by a number of model studies comparing conventional co-axial loops with co-planar loops with respect to sensitivity to layer parameters. Laboratory measurements indicate that the simple model describes the physics of the induction phenomena adequately. First measurements in Switzerland prove that the new method successfully predicts the presence of thin magnetic crud layers that were erroneously interpreted using standard techniques to correspond to thick insulating oxide layers.

Electronic Shearography with Thermal Loading for Detecting Debonds in Thick Polyurethane/Steel Panels for Marine Applications

Abstract: We demonstrate that electronic shearography with thermal loading can be effectively applied to the nondestructive detection of debonds in thick polyurethane/steel structures usually employed in the hulls of military ships. A compact fiber-optics shearographic interferometer has been developed that allows remote full-field measurements in real time. Three known debonds in a polyurethane/steel test panel are clearly detected by observing their increased deformation as they cool down after the application of a thermal load. It is also shown that the debond edges can be identified. The results obtained indicate that the shearographic interferometer is well-suited for field detection and sizing of debonds in such panels. Future refinements to the system will include the use of high-power, pulsed illumination for the imaging of large areas and consequent real-time mapping of possible debonds.

Ultrasonic Measurement of Stress Using a Rotating EMAT

Abstract: The acoustic birefringence method is primarily used to measure differences of principal stresses in regions where principal stresses coincide with material symmetry axes. To determine the differences of principal stresses also requires knowledge of the unstressed birefringence at the measurement locations. Consequently, variability in texture can introduce errors in stress determination. In contrast, the ultrasonic measurement of shear stress is independent of texture. The shear stress causes a rotation φ of the pure-mode polarization directions of the SH waves used in our experiments. We constructed a motorized electromagnetic-acoustic transducer (EMAT) and used it to measure phase as the EMAT is rotated. We developed an algorithm to determine φ and the birefringence B. We measured the shear stress along parallel scanlines in a residual-stress specimen with known stress state. We calculated the shear stress gradient and used it in the stress-equilibrium equation to determine the normal stress acting along the scanline direction. The other plane stress component was determined from an acoustoelastic relation between B and the difference of normal stresses. Good agreement was obtained with theoretical stress values.

A quantitative approach to active thermographic inspection for material loss evaluation in metallic structures

Abstract: This paper presents an approach to the inverse geometry problem pertaining to the characterization of material loss due to corrosion damage in metallic structures. It is based on a simplified geometric representation which admits, in the context of a restricted time frame, a simple analytical form for the contrast response evolution. The inverse problem is then solved by means of a deterministic optimisation scheme applied to a suitably defined objective function. Experimental trials on specimens containing simulated corrosion damage have shown that estimates for residual thickness obtained on the basis of this approach are both precise and robust.

Doppler-Based Airborne Ultrasound for Detecting Surface Discontinuities on a Moving Target

Abstract: This paper presents a novel concept based on the acoustic Doppler effect for detecting defects on the surface of a rapidly moving object. The proof-of-concept tests show the feasibility of this approach. By impinging and detecting airborne ultrasound on a rotating target with surface notches, the Doppler effect was clearly observed in the spectral domain at the time when the transducer passed over the flawed zone. For continuous monitoring, the gated spectral magnitude is used to discriminate the signals returned from a flawed region against those from sound regions. In addition, two real-time signal processing techniques are proposed.

MESSINE, a Parametric Three-Dimensional Eddy Current Model

Abstract: A parametric three-dimensional eddy current model is proposed. It first discretizes the eddy current distribution into current loops. A parametric description of the shape of these loops is given according to the observations made with a finite-element code. The inductances and the resistances of the current loops are then calculated. By considering the system constituted of the coil and the current loops as a ``multitransformer,'' their current intensity is determined. The impedance change can then be deduced. The model is validated in the case of an axisymmetric configuration. The proposed model is applied to a three-dimensional configuration. Comparisons with experimental results are sufficiently conclusive to validate the model approach.

Acoustoelastic Effects in Elastic Media with Nonuniform Initial Stress

Abstract: A perturbative formalism for the treatment of the acoustoelastic effect in material plates with a stress profile in the thickness direction is proposed and compared with a numerical method. The latter consists of an application of the local interaction simulation approach (LISA). The reliability of both the analytical treatment and the numerical approach is tested by comparing the respective results for several cases of propagation of Rayleigh waves in specimens with various kinds of stress fields, including welded steel plates. Finally, the linearization obtained from the perturbation treatment is exploited for the solution of the ``inverse problem,'' i.e., the determination of the stress field from a set of RW times of flight with different wavelengths. A numerical example is provided to illustrate the applicability of the method, starting from a set of synthetic data obtained with LISA.

Calculation and Measurement of Scattering Coefficients for Surface-Breaking Cracks Sonified by a Focused Array

Abstract: A simulation of a focused beam of surface wave motion generated by a line array of surface wave transducers is used to calculate the field scattered back from a surface-breaking crack. The backscattered field is expressed in terms of a scattering coefficient, which is derived from the reciprocal identity for elastodynamic states, and which is calculated numerically by using the Kirchhoff approximation. In an earlier work the validity of the focused beam simulation has been verified. In this paper, the theoretical results for the backscattered field are compared with measurements for two crack sizes, one of which is used for calibration.

Pulsed-laser ultrasound generation in graphite/epoxy plate

Abstract: A laser-based ultrasonic technique using a pulsed laser to stimulate ultrasonic waves in fiber-reinforced graphite/epoxy composite plate is the subject of investigation. For convenience, the material is chosen to be homogeneous and transversely isotropic. The study is strictly limited to the laser power regimes that are suitable for nondestructive evaluation. An elastodynamic methodology is presented based on an integral formulation in order to develop a representation for the dynamic responses in terms of the characteristics of the source that originated the motion. This requires the computation of the elastodynamic Green's function which represents the displacement field from the idealized synthetic sources localized precisely in both space and time. A two-dimensional numerical analysis utilizing a finite difference method for computation of the Green function in a finite plate is developed which provides the basis for quantitative nondestructive evaluation of fiber-reinforced composite materia...

Effects of Compressive Stress on Magnetization Components of a Line Pipe Steel Cube

Abstract: Tensor hysteresis loops are measured for a pipeline steel cube while compressive stresses are applied orthogonally to the applied field direction. Total magnetization vectors are calculated with the applied magnetic field in each of the three cube directions. The magnetization components are compared and analyzed for different applied stresses.

Elastodynamic Response of a Periodic Layer of Spherical Particles Containing Vacancies

Abstract: This paper is concerned with the influence of vacancies on the elastodynamic response of a periodic (square) array of identical spherical elastic inclusions embedded in an unbounded elastic matrix. A response function of the array is defined as H∗(ω) ≡ R∗(ω)/T∗(ω), where R∗(ω) and T∗(ω) are, respectively, the reflection and transmission spectra of the lattice. H∗(ω) was measured for a “perfect lattice,” i.e., one without any vacancies, and was found to be characterized by lattice resonances. H∗(ω) was also measured for lattices containing one and three vacancies within the 72 lattice site area insonified by the ultrasonic beam. A counter-intuitive observation is that the presence of even one vacancy significantly reduces the amplitude of the fundamental lattice resonance. Furthermore, in the case of the specimen containing three closely spaced vacancies, the reduction is not three times the reduction due to one vacancy; it is significantly less than that.

Radioscopic Images Segmentation by ``Edge and Area'' Combined Approach for Weld Geometric Characterization

Abstract: The present paper deals with the geometric characterization of a circumferential weld bead by radioscopy. Weld control can only be performed by tangential irradiation. The series of images obtained by projection, for one rotation of the welded component, is then strongly degraded, as each image contains the projected section of the weld and the superposition of several other sections. The inverse problem is solved using a new image segmentation method, based on an “edge and area” combined approach. The edge approach defines the limits of the weld molten zone by minimum gradient points. The area approach uses a thresholding method based on histogram analysis to position the bottom of the bead exactly. This segmentation leads to fast and automatic control of the weld quality. A validation stage on industrial components shows the method reliability.