Showing papers in "Journal of Nondestructive Evaluation in 1995"

Nondestructive evaluation of corrosion damage using electrostatic measurements

Abstract: We investigate the problem of detecting and assessing, by means of static electrical measurements, damage due to corrosion in a structure. Corrosion damage, which is assumed to occur in an inaccessible part of a specimen, is modelled as material loss. The detection device consists of electrodes which inject DC current and measure voltage potentials in the accessible part of the specimen. The topography of the damaged surface is estimated from the measured data. This research is meant to evaluate if a method based on static electrical measurements has the potential to be developed into a useful nondestructive evaluation tool. We propose computational methods that take the measured data and estimate the unknown damaged surface. The methods are studied in order to understand their properties. Several example calculations from synthetic data are presented. Our findings indicate that such a device has limited resolution. However, it offers several advantages that make it worthwhile to pursue further research.

Rapid inversion of eddy current data for conductivity and thickness of metal coatings

Abstract: A feature-based method that determines the thickness and electrical conductivity of a coating on a metal plate from the change in the frequency-dependent impedance of an eddy-current probe coil is presented. Recently a least-squares solution of this problem was presented, which, however, requires approximately 20 CPU minutes on a DEC 5000 work station for the analysis of each set of measurements. We show that a feature-based approach can reduce the time to a few seconds on the same processor. We start by showing that a three-parameter scaling of the resistive component of the impedance change vs. frequency leads to a simple and nearly universal curve. Consequently these parameters provide a simple and compact way of expressing the data. Next, we show that the three scaling parameters can be used to construct a look-up table that determines the conductivity and thickness of the coating. Finally, we test the method using experimental data.

A simple scheme for self-focusing of an array

Abstract: A self-focusing technique and its application to a linear array system are presented in this paper. By application of the technique the system is capable of both sonification and reception focusing. The array is first excited as an unfocused array. Next a cross-correlation technique is used to determine time delays of reception of the largest amplitude backscattered signals at the elements of the array. The original transducer signal is then reemitted with the appropriate time delays to achieve sonification focusing on the scatterer producing the largest signal. This process is repeated in an iterative mode to focus energy on the strongest scatterer. Once insonification focusing has been achieved the last time-delay calculations are used once more for reception focusing, i.e., to correct the signals received by the individual elements for differences in arrival times. A low cost linear array has been constructed to implement the self-focusing technique. Examples demonstrate the capability of the technique to focus on the largest hole of sets of three holes in an aluminum specimen.

Measurement of attenuation using a normalized amplitude spectrum

Abstract: In conventional material evaluation and attenuation measurement, methods employing multiple ultrasonic echoes in the thickness direction of the samples are often used to calculate the reflection and attenuation coefficients. The successive echoes must be well resolved, and there is consequently a minimum thickness of material which can be tested for a given frequency transducer. For thin specimens, a high frequency probe may be used, but this may be problematic for composite laminates since the reflections from the individual plies can cause interference. In this paper, an alternative method based on the amplitude spectrum of the reflections from the specimen is presented. The technique deduces the attenuation coefficients by using the spectral amplitudes at the anti-resonance (or resonance) frequencies of the normalized amplitude spectrum, and has the advantage that it does not require that the successive echoes be separated in time. Measurements made on glass, epoxy, and composite laminate specimens are used to compare our attenuation measurement technique with the standard method.

Accurate probe-response calculation in eddy current NDE by finite element method

Abstract: A method is presented to evaluate flaw signals in eddy current NDE using the finite element technique. The analysis of the electromagnetic field is based on a three-dimensional finite element scheme that computes directly the electromagnetic field distortions due to defects. This direct field-distortion calculation together with an accurate unflawed field calculation provides accurate total field values in general three-dimensional geometries. The paper shows that the application of the reaction concept and the reciprocity theory allows computations of the probe responses by performing integrals over the flaw region only, even if the analysis is performed by a finite element scheme. Two benchmark problems—a plate with rectangular slot scanned by a differential probe and a tube with axial and circumferential slots scanned by an absolute probe—have been solved to demonstrate the validity and the efficiency of the method. The calculated probe responses show good agreement with the measured trajectories. In order to reach better quantitative agreement, a calibration algorithm that adjusts the parameters of the cylindrical coil model and the lift-off within the range of the geometrical tolerances has been developed.

Ultrasonic Determination of Single Crystal Sapphire Fiber Modulus

Abstract: Single crystal α-Al2O3, (sapphire) fibers of ∼ 100 μm diameter have recently emerged as candidates for stiffening and strengthening high temperature composites. The Young's modulus of these fibers depends upon their crystallographic orientation, ranging from a high of 461 GPa for thec-orientation to a low of ∼373 GPa for orientations 45° off thec-axis. A deviation of the fibers' axial orientation from the c-axis and thus a reduction in the fibers' axial modulus can sometimes occur during the fiber growth process, and so a simple reliable method is needed to characterize the modulus and/or orientation of the fibers. A laser generated-piezoelectric transducer detected ultrasonic method has been used for this purpose. It has been found that a clear correlation exists between the velocity of the first arriving ultrasonic signal and the deviation of the fiber's axis from thec-axis. The measured velocity is found to be in reasonably good agreement with the calculated bar velocity,ν b =√E/ρ, for the fiber, providing an estimate of the fiber's orientation dependent Young's modulus. The small differences between the measured and the calculated velocities are believed to be caused by a combination of measurement errors, uncertainties in the reference elastic compliance constants of α-Al2O3 and the presence of small volume fractions of pores and other (low modulus) aluminum oxide phases in the fibers.

A new ultrasonic technique for detection and sizing of small cracks in studs and bolts

Abstract: If small cracks in stud bolts are not detected early enough, they can grow rapidly and cause catastrophic disasters in industrial facilities such as nuclear power plants Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts This study shows a method of detecting the existence and determining the size of a small crack in a root between two crests in the bolt threads using ultrasound The Rayleigh wave propagating from the tip of a crack to the opening of the same crack is utilized A delayed pulse, due to the Rayleigh wave, is detected between regularly spaced pulses from the threads, with the delay time being proportional to the size of the crack Theoretical explanation is presented and experimental results demonstrating detection of cracks as small as 05 mm are shown

Techniques for depth-selective, low-frequency eddy current analysis for SQUID-based nondestructive testing

Abstract: Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.

Fatigue load monitoring in steel bridges with Rayleigh waves

Abstract: Fatigue load monitoring is a useful tool for safety assessment of highway bridges. Monitoring has been conventionally done using strain gages. Installation of these gages is labor-intensive and requires safety precautions. Noncontact electromagnetic-acoustic transducers (EMATs) offer an attractive alternative. EMATs were used to transmit and receive Rayleigh Waves (RW). Changes in time of flight of RW due to the acoustoelastic effect can in principle be used to monitor stresses resulting from vehicular traffic. We have performed proof-of-concept experiments to demonstrate the feasibility of this approach. Specimens were subjected to bending to simulate the load environment in bridges. RW EMATs were used to measure the relatively low stresses (less than 14 MPa) typically experienced by bridge girders. The signal-to-noise ratio achievable with our system should allow adequate stress resolution for fatigue load monitoring. Factors which could impede technology transfer were considered. The primary obstacle appears to be variability in time of flight (TOF) due to magnetostriction. If the magnetic state is changed (e.g., by scanning of the EMATs) the TOF can change, even at constant stress. We have characterized this effect. If a proper installation procedure is followed, fatigue load monitoring with RW EMATs is feasible.

Limited-angle computed tomography for sandwich structures using data fusion

Abstract: Both conventional and limited-angle computed-tomography (CT) are ill-posed problems, but where conventional CT has a small null space with few dimensions, limited-angle CT has a much larger null space and is severely ill-posed. The nature of the ill-posedness of limited-angle CT dictates that any limited-angle reconstruction method must interpolate from partial data in the Fourier domain. Large aluminum-core sandwich structures, such as those of aircraft control surfaces, simultaneously prevent full range data acquisition for CT and have face sheets that lie primarily in the limited-angle Radon transform null space. Consequently, interpolation of missing data is not possible and limited-angle CT for large sandwich structures is not feasible. This paper presents a novel method for limited-angle CT that addresses this problem. The method uses constraint-based data fusion to reduce the null space of the problem in a manner consistent with the specimen. Measurements of spatial support and face sheet thickness provide data for the additional constraints. By using the method, accurate reconstruction of sandwich structures becomes possible, where it would otherwise be impossible. The method of convex projections solves the inverse problem posed by the fusion system well because it is fast (for this problem) and easily incorporates the fusion constraints. Experimental results based on synthetic data show the improvement in reconstruction accuracy obtained by the fusion method. Real data results show the successful application of the method to a plexiglass phantom.

Characterization of laser hardened steels by laser induced ultrasonic surface waves

Abstract: Ultrasonic surface waves are suitable for the characterization of surface hardened materials. This is shown on laser hardened turbine blades. The martensitic microstructure within the surface layer of surface hardened steels has a lower surface wave propagation velocity than the annealed or normalized substrate material. Because the propagation velocity depends on the ratio of layer thickness to wavelengthd/λ, its measurement allows the determination of the hardening depth. If the surface wave frequency is high enough, the surface wave propagates mainly within the hardened layer. A correlation of the surface wave velocity to the surface hardness has been found. Because the variation of the surface velocity in hardened steels is small, a high measurement accuracy is necessary to obtain the interesting hardening parameters with sufficient certainty. Therefore, a measuring arrangement has been developed where laser pulses, guided by optical fibers to the surface hardened structure, generate simultaneously surface wave pulses at two different positions. The two ultrasonic pulses are received by a piezoelectric transducer. The surface wave velocity is obtained from the time delay between these pulses which is determined by the cross-correlation method. To evaluate simultaneously surface waves with different penetration depths from the same signal acquisition, digital filtering has been used in connection with the cross-correlation.

Ultrasonic leaky interface waves for composite-metal adhesive bond characterization

Abstract: Highly stressed or damaged regions in aircraft structures are increasingly being reinforced by adhesively bonding an overlay of a unidirectional boron fiber-epoxy composite to the aircraft metallic substrate While conventional C-scan ultrasonics can be used to detect disbonding of the overlay, nondestructive methods are also required to detect weak adhesive bonding This paper describes a preliminary experimental investigation of the existence of ultrasonic leaky interface waves between overlays and 2024-T351 aluminum alloy and D6ac steel substrates, and their possible use for detection of weak bonds The approach used was to excite these leaky waves using Rayleigh waves generated by a laser line-source on the substrate The existence of the leaky waves, traveling normal to the composite fibers, between boron-epoxy overlays and either an aluminum or steel substrate, was confirmed by observations of the quasishear pulses leaked to the top of the overlay, and of the Rayleigh wave transmitted to the free surface beyond the overlay When quantitative measurements were possible, measured values of the complex interface wave speed agreed satisfactorily with predicted values This paper also demonstrates one case for which leaky interface waves are sensitive to weak bonding caused by the presence of a contaminant on the substrate surface prior to bonding: application of a low-concentration aqueous solution of hydraulic oil to the aluminum substrate prior to bonding of the overlay caused a marked increase, over that for the well-bonded case, in the observed amplitude of the quasi-shear pulses leaked to the top of the overlay by decay of the interface wave More experiments are needed to establish whether leaky interface waves are sensitive to bond condition for various other combinations of substrate, overlay and contaminant

Wave propagation in a composite with a wavy sublamina

Abstract: The through-thickness waviness in a sublamina in a composite laminate is the subject of study for the purposes of nondestructive evaluation by ultrasound. The model consists of a flat composite laminate with one sublamina which has sinusoidal waviness. The theoretical analysis of a longitudinal wave propagating through such a laminate is presented and the reflection coefficients at various locations on the plate are calculated. The results are validated by experimental ultrasonic C-scan.

Ultrasonic wave scattering from rough, imperfect interfaces. Part II. Incoherent and coherent scattered fields

Abstract: A theoretical model for ultrasonic wave scattering for geometrically irregular and imperfectly bonded interfaces is presented. Part I presents the stochastic interface characterization and a model for its mechanical response based on a micromechanics model of asperity contact. Part II uses this interface representation to write the well used “quasi-static” boundary conditions for scattering from a.flat imperfect interface1 directly on the irregular interface profile. The boundary conditions are then expanded in an asymptotic series in the roughness parameter (standard deviation of the surface height) which is small compared to wavelength. The slope of the profile must also be everywhere small. These equations are solved exactly for the zero-th and second order terms, which are the flat coherent solution and its' first coherent correction, and the first order term, which is the first term in the expansion for the incoherently scattered solution. Results for obliquely incident longitudinal and shear waves show a strong dependence on the roughness in both the coherent and incoherent reflected fields, but little if any dependence on the roughness in the transmitted fields. In particular, the reflected coherent fields show markedly increasing attenuation compared to the flat compliant interface with increasing roughness and increasing ultrasonic frequency, the latter result being in qualitative agreement with results for scattering from an inhomogeneous array of individual scatterers.2 There is evidence in the incoherent reflected fields for the existence of an incoherent leaky interface disturbance which manifests itself as a bulk incoherent shear wave at a scattering angle equal to the critical longitudinal angle. A coherent true interface wave is also supported by the rough interface which is shown to further attenuate the coherent reflected fields compared to the flat compliant interface solution.

Ultrasonic wave scattering from rough, imperfect interfaces. Part I. Stochastic interface models

Abstract: A theoretical model for ultrasonic wave scattering by geometrically irregular and imperfectly bonded interfaces is presented. In Part I, the normal stiffness of interfaces formed by the partial contact of solids with rough surfaces is estimated for two models of contacting surfaces with random roughness in one dimension only. The first model considers nonconforming surfaces with a single-scale of roughness, while double-scale roughness characterizes the surfaces of the second model, which are conforming at the large scale and nonconforming at the smaller scale. The surfaces' profiles are described by Gaussian probability and spectral densities. The surfaces at each contact are modeled by two cylinders under a compressive load and the normal stiffness per unit area of the interface is evaluated by averaging the stiffness of all the contacts, assuming they do not interact with each other. It is shown that the smaller the roughness, the softer the interface; the larger the autocorrelation length, the softer the interface; and the smaller the initial aperture, the stiffer the interface. Furthermore, interfaces described by the second model appear much stiffer than those described by the first model. The interface characterizations and normal stiffness models developed in Part I will be used in Part II to study the scattering of ultrasonic plane waves by such an interface.

Numerical analysis of the radiated fields of ultrasonic transducers

Abstract: A finite element model is implemented for simulating the radiated fields of both planar and curved transducers in acoustic media. The approach is based on general finite element analysis developed for solving the governing equations of elastic wave propagation. The distributions of the wave fields are presented for both the nearfield and farfield regions of the transducer. Three excitation pulses with the same center frequency but different bandwidths are examined and the accuracy is indicated by a comparison of the simulation results for the axial and transverse fields with the analytical results for continuous wave excitation.

Interaction of phase-shifted fields of two single-turn coils situated above a conducting medium

Abstract: An analytical solution is obtained for the problem of the interaction of two phase-shifted electromagnetic fields generated by two coaxial single-turn coils carrying alternating currents of the same frequency, but of different amplitudes and nonzero phase difference, ψ. Two cases are considered: coils situated above a conducting half-space and above a two-layer medium. Numerical results show that ψ is the most important parameter. If the values of ψ and of the other parameters are chosen properly, then the curve representing the change in impedance can lie in any quadrant of the complex plane. These results can be used for developing more sensitive and more selective eddy current testing methods.

Improved DSPI configuration for the inspection of components in the production line

Abstract: In this paper, a digital speckle pattern interferometer configuration for the enhanced nondestructive inspection of components in the production line is presented. The setup used is adapted from the original design employed for digital speckle shearing interferometry. The theoretical principle behind the technique is outlined and its application in the detection of poor bolting in plates is demonstrated.

Modeling and evaluating the scattering of Rayleigh waves by a linear distribution of surface cracks

Abstract: A numerical evaluation model of the reflection coefficient of Rayleigh waves for a linear distribution of surface cracks is developed using the weight function estimation method. The numerical evaluation of the reflection coefficients varying with respect to the crack depth ratios, frequencies, and the number of the cracks for several commonly used engineering materials is performed. The results show that the model can effectively be used to evaluate the reflection coefficient for a linear distribution of the cracks. The crack depth which can be evaluated has been extended substantially and the evaluation results have been improved.