Showing papers in "Quantitative Nondestructive Evaluation in 2004"

Laser‐Ultrasonics: From the Laboratory to Industry

Abstract: A broad overview of the field of laser‐ultrasonics is presented. This overview draws from developments at the Industrial Materials Institute of the National Research Council of Canada as well as elsewhere. The principles of generation and detection are presented, stressing a few key characteristics of laser‐ultrasonics: the material is actually the emitting transducer and transduction is made by light, thus eliminating any contact. These features carry both advantages and limitations that are explained. Another feature, which has been an impediment, is actually the complexity of the “laser‐ultrasonic transducer”, but in spite of this complexity, it can be made very reliable for use in severe industrial environments. It also can be very cost effective for a number of applications. Three applications that are now used in industry are presented: the inspection of polymer matrix composites used in aerospace, the measurement of thin layers in microelectronics and the thickness gauging of hot steel tubing in production. Technological aspects, such as interferometer design, detection lasers and others are also discussed. As an overall conclusion, laser‐ultrasonics that was for a long time a laboratory curiosity has definitely now made its transition to industry. Nevertheless, developments should continue to improve performance, to make it well adapted to specific inspection or characterization tasks and more affordable.

Modeling the Excitation of Lamb and SH Waves by Point and Line Sources

Abstract: The guided wave field excited in a plate‐like structure from any weakly coupled transducer can be calculated from the superposition of the guided wave fields due to a number of suitable point or line excitation sources. In this paper, the fields from various point and line excitation sources are reviewed and relationships between them are demonstrated. The performance of pancake coil EMATs is modeled using the superposition of the fields from point sources and the results compared with experiment.

Deep Crack Detection around Fastener Holes in Airplane Multi‐Layered Structures Using GMR‐Based Eddy Current Probes

Abstract: Rotational and linearly scanned eddy current probes based on giant magnetoresistive (GMR) sensors have been developed for detecting buried cracks and flaws emanating from fastener holes. The use of shaped excitation coils, the original positioning and orientation of the magnetic sensors enhance the sensitivity of these probes to buried defects while reducing the influence of fastener holes edge. Corner cracks of 2.5 mm in length were detected in the second layer of a 13 mm thick two‐layer structure.

Sparse Ultrasonic Transducer Array for Structural Health Monitoring

Abstract: A spatially sparse array of conventional piezoelectric transducers is attached to a part surface to monitor its structural health. Artificial flaws are incrementally added to simulate damage progression. The structure is flooded with ultrasonic energy by transmitting on a single transducer, and waveforms are recorded from other transducers in the array. Simple waveform differencing techniques between pre‐flaw baseline waveforms and post‐flaw waveforms show promise for determining the state of damage progression in both concrete and aluminum samples.

Eddy‐Current Testing with GMR Magnetic Sensor Arrays

Abstract: The advent of GMR magnetic sensors and GMR sensor arrays with frequency‐independent sensitivity offers improvements in speed, depth, and resolution in eddy‐current testing. Arrays of GMR magnetic sensors allow rapid scanning of an area for defects in a single pass. The small size and low power consumption of these solid‐state magnetic sensors enable the fabrication of compact arrays of sensors on circuit boards and even on‐chip sensor arrays. Arrays have been fabricated with sensor spacing as small as 5 micrometers when fine resolution is required. GMR sensor elements can be deposited on active silicon substrates facilitating on‐chip signal processing and multiplexing. This integration simplifies the sensor/signal‐processing interface, minimizes the number of leads, and can reduce the effect of noise. This paper will discuss the technology of fabricating arrays on GMR sensors, especially on‐chip arrays. Integrated sensor arrays with on‐chip signal processing and multiplexing will be described. Finally, some applications of arrays to crack and corrosion detection will be discussed.

Estimation of Single‐Crystal Elastic Constants from Ultrasonic Measurements on Polycrystalline Specimens

Abstract: In past work we reported on measurements of ultrasonic velocity, attenuation and backscattering in nickel‐alloy materials used in the fabrication of rotating jet‐engine components. Attenuation and backscattering were shown to be correlated to the average grain diameter, which varied with position in the billet specimens studied. The ultrasonic measurements and associated metallographic studies found the local microstructures to be approximately equiaxed and free of texture in these cubic‐phase metals. In this paper we explore a method for deducing the single‐crystal elastic constants of a metal using the combined ultrasonic and metallographic data for a polycrystalline specimen. We specifically consider the case seen in the jet‐engine alloys: polycrystalline cubic microstructures having equiaxed, randomly oriented grains. We demonstrate how the three independent elastic constants {C11, C12, C44} can be deduced from the density, the mean grain diameter, the ultrasonic attenuation at one or more frequencies, and the longitudinal and shear wave speeds. The method makes use of the attenuation theory of Stanke and Kino, and the Hill averaging procedure for estimating the sonic velocity through a polycrystalline material. Elastic constant inputs to the velocity and attenuation models are adjusted to optimize the agreement with experiment. The method is demonstrated using several specimens of Inconel 718 and Waspaloy, and further tested using four specimens of pure Nickel.

Fatigue crack closure analysis using nonlinear ultrasound

Abstract: Nonlinear ultrasound is a promising tool for evaluating closed cracks which is difficult with the linear ultrasound Recently, subharmonics was observed and explained either by a parametric or a forced vibration However, because the crack was not completely closed, the behavior of crack was complicated and quantitative comparison between theory and experiment has not been achieved We prepared a closed fatigue crack in an aluminum alloy by controlling the crack opening with in‐situ monitoring of the crack tip echoes When a tone burst of 64 MHz longitudinal waves was obliquely incident on the closed crack, a strong subharmonics at 32 MHz was observed The intensity first increased as the crack closure was reduced by applying a tensile stress, took a maximum larger than the fundamental wave and then decreased moderately as the crack was further opened The subharmonic intensity was theoretically explained by a low‐pass filtered displacement of output crack plane due to the inertia when it is driven by vibrating asperities on the input crack plane at fundamental frequency

Properties of Guided Waves in Composite Plates, and Implications for NDE

Abstract: Guided waves are potentially very useful for the rapid NDE of plate structures. In the aerospace industry in particular, they have been proposed for the structural health monitoring of carbon fibre skin panels, in either an active or a passive configuration. In an active configuration a guided wave is deliberately generated and then later received after it has travelled through the structure; in a passive configuration an acoustic emission sensor is used to listen to sound created by the structure itself. The successful development of these ideas depends on a good knowledge of the properties of the guided waves, yet it has been found that the properties of guided waves in such materials are very much more complicated than those in a simple metal skin. The complications include steering of the direction of the group velocity by the anisotropy of the plate, and attenuation because of damping of the matrix material and scattering by the fibres. The authors have studied these phenomena both analytically and experimentally. This paper presents an overview of the findings and identifies the key implications for practical developments of structural health monitoring techniques.

Evaluation of Amorphous Diffusion Bonding by Nonlinear Ultrasonic Method

Abstract: The characteristic of bond interface in amorphous diffusion bonding, of which evaluation is impossible by conventional method, was quantitatively evaluated by the second harmonic amplitude. Steel bars were bonded with Ni‐based amorphous film. Conventional ultrasonic method, e.g. the echo height reflected from bond interface, could not identify samples manufactured at different bonding temperatures 1050, 1150 and 1250°C. Therefore, nonlinear ultrasonic method was applied for distinguishing the difference of bond strength. The nonlinear ultrasonic method is to measure the higher harmonics generated by nonlinear stress‐strain relationship at weak bonds. Measurements were conducted in contact using piezoelectric transducers in through‐transmission. The fundamental and second harmonic wave frequencies are 5 and 10 MHz. To measure second harmonic wave amplitude, a commercial superheterodyne receiver and pulse inversion method were used. The pulse inversion method is the digital signal processing to extract only second and even orders harmonic wave by superposing two burst waves with a 180° phase difference after corrected time‐lag by cross‐correlation function. These results were compared to destructive tests for examining the relationship between tensile strength and the second harmonics. Besides, elemental analysis by EPMA was performed for manifesting the source of second harmonics generation.

Flash-Quenching for High Resolution Thermal Depth Imaging

Abstract: The Flash lamp “thermal forcing function” is typically an exponentially decaying source. Ideally, one would like to achieve a short rectangular heating pulse simulating a Dirac Delta function. Then heat input is precisely limited so that image analysis can begin, without distortion from incoming heat, immediately following the flash. This allows the earliest temporal resolution of events and thus permits thickness measurements of very thin metal components or thermally thin materials. We will describe a high power handling, compact, electrical approach for lamp quenching that cuts off the lamp exponential tail precisely and present results of its effects on measurement. It can handle up to a 1.2 MW‐average pulse (2400J / 2ms), thus cutting off undesirable tail after 2 ms. These units can be placed in series with every lamp thus optimizing power usage.

A Semi‐Analytic‐FEM Hybrid Model for Simulating UT Configurations Involving Complicated Interactions of Waves with Defects

Abstract: A hybrid model is developed combining advantages of both semi‐analytical and numerical methods. Most of the propagation is computed semi‐analytically (pencil method, code Civa), while wave — defect interaction is computed numerically (FEM, code Athena) in a small region surrounding the defect. Both codes exchange results required to predict responses from defects through an integral formula extending Auld’s principle to the transient case. The theory and its implementation are discussed. Examples illustrate its interest in UT simulations involving complicated interactions.

Modeling of Ultrasonic Attenuation in Uni‐Directional Fiber Reinforced Composites Combining Multiple‐Scattering and Viscoelastic Losses

Abstract: A model coupling viscoelastic and multiple‐scattering losses is developed to predict ultrasonic attenuation in unidirectional fiber reinforced composite of high fiber volume fraction. Complex‐valued stiffness constants accounting for viscoelasticity are inserted in classical multiple‐scattering theory. Waves of various polarities (SH, SV, L) relatively to the fiber direction are considered. SH waves only require a scalar treatment, whereas the others require a vector treatment accounting for mode‐conversions. Comparisons of predicted attenuation coefficients with experimentally measured ones validate the model.

Ultrasonic Signal Processing for Structural Health Monitoring

Abstract: Permanently mounted ultrasonic sensors are a key component of systems under development for structural health monitoring. Signal processing plays a critical role in the viability of such systems due to the difficulty in interpreting signals received from structures of complex geometry. This paper describes a differential feature‐based approach to classifying signal changes as either “environmental” or “structural”. Data are presented from piezoelectric discs bonded to an aluminum specimen subjected to both environmental changes and introduction of artificial defects. The classifier developed as part of this study was able to correctly identify artificial defects that were not part of the initial training and evaluation data sets. Central to the success of the classifier was the use of the Short Time Cross Correlation to measure coherency between the signal and reference as a function of time.

On the Scattering and Mode Conversion of the A0 Lamb Wave Mode at Circular Defects in Plates

Abstract: The sensitivity of a permanently attached guided wave array for the detection of corrosion damage in plates is studied. The thickness reduction due to corrosion is modeled as a circular part‐through hole. The scattering and mode conversion of the A0 Lamb wave mode is calculated numerically employing a three‐dimensional finite element model and verified in laboratory experiments. Good agreement is found and the sensitivity of the guided wave monitoring of large areas can be predicted.

On the Detection of Creep Damage in a Directionally Solidified Nickel Base Superalloy Using Nonlinear Ultrasound

Abstract: A limited experimental study was conducted to investigate the feasibility of using nonlinear ultrasonic technique for assessing the remaining creep life of a directionally solidified (DS) nickel base superalloy. Specimens of this alloy were subjected to creep testing at different stress levels. Creep tests were periodically interrupted at different creep life fractions to conduct transmission ultrasonic tests to explore if a correlation exists between the higher order harmonics and the accumulated creep damage in the samples. A strong and unique correlation was found between the third order harmonic of the transmitted wave and the exhausted creep life fraction. Preliminary data also show an equally strong correlation between plastic deformation accumulated during monotonic loading and the second harmonic of the transmitted ultrasonic wave while no correlation was found between plastic strain and the third order harmonic. Thus, the nonlinear ultrasonic technique can potentially distinguish between damage due to plastic deformation and creep deformation.

Considerations for Using Phased Array Ultrasonics in a Fully Automated Inspection System

Abstract: The University of Dayton Research Institute (UDRI) under contract by the US Air Force has designed and constructed a fully automated ultrasonic inspection system for the detection of embedded defects in rotating gas turbine engine components. The system performs automated inspections using the “scan plan” concept developed for the Air Force sponsored “Retirement For Cause” (RFC) automated eddy current system. Execution of the scan plan results in a fully automated inspection process producing engine component accept/reject decisions based on probability of detection (POD) information. Use of the phased‐array ultrasonic instrument and probes allows for optimization of both the sensitivity and resolution for each inspection through electronic beamforming, scanning, and focusing processes. However, issues such as alignment of the array probe, calibration of individual elements and overall beam response prior to the inspection have not been addressed for an automated system. This paper will discuss current progress in the development of an automated alignment and calibration procedure for various phased array apertures and specimen geometries.

Air‐Coupled Ultrasonic Measurements in Composites

Abstract: Air‐coupled ultrasound is a non‐contact technique and has clear advantages over water‐coupled testing. Research of air‐coupled ultrasonics, especially using capacitance and micromachined transducers, has been extensively reported in the literature. This paper reports our experience of applying piezoceramic air‐coupled transducers for nondestructive evaluation of composites. The beam profiles of air‐coupled piezoceramic transducers, with and without apodization, were mapped out. The transmission of air‐coupled ultrasonic energy through composite plates of different thickness was measured experimentally; model calculation of the transmission coefficient, taking into account the frequency bandwidth of the transducer, agreed with the measurement results. The occurrence of diffraction phenomenon (“Poisson bright spot”) while imaging flaws in composite laminates was investigated. The resolution of scanned images obtained with air‐coupled transducers was investigated for different frequency, focusing, and apodization conditions.

N‐Scan®: New Vibro‐Modulation System for Crack Detection, Monitoring and Characterization

Abstract: In recent years, an innovative vibro‐modulation technique has been introduced for the detection of contact‐type interfaces such as cracks, debondings, and delaminations. The technique utilizes the effect of nonlinear interaction of ultrasound and vibrations at the interface of the defect. Vibration varies the contact area of the interface, modulating a passing ultrasonic wave. The modulation manifests itself as additional side‐band spectral components with the combination frequencies in the spectrum of the received signal. The presence of these components allows for the detection and differentiation of the contact‐type defects from other structural and material inhomogeneities. The vibro‐modulation technique has been implemented in the N‐SCAN® damage detection system providing a cost effective solution for the complex NDT problems. N‐SCAN® proved to be very effective for damage detection and characterization in structures and structural components of simple and complex geometries made of steel, aluminum, composites, and other materials. Examples include 24 foot‐long gun barrels, stainless steel pipes used in nuclear power plants, aluminum automotive parts, steel train couplers, etc. This paper describes the basic principles of the nonlinear vibro‐modulation NDE technique, some theoretical background for nonlinear interaction, and justification of signal processing algorithms. The laboratory experiment is presented for a set of specimens with the calibrated cracks and the quantitative characterization of fatigue damage is given in terms of a modulation index. The paper also discusses examples of practical implementation and application of the technique.

Development of a 3D Electromagnetic Model for Eddy Current Tubing Inspection: Application to Steam Generator Tubing

Abstract: In nuclear plants, the inspection of heat exchanger tubes is usually carried out by using eddy current nondestructive testing A numerical model, based on a volume integral approach using the Green’s dyadic formalism, has been developed, with support from the French Institute for Radiological Protection and Nuclear Safety, to predict the response of an eddy current bobbin coil to 3D flaws located in the tube’s wall With an aim of integrating this model into the NDE multi techniques platform CIVA, it has been validated with experimental data for 2D and 3D flaws

Transient Ultrasonic Guided Waves in Bi‐Layered Anisotropic Plates with Rectangular Cross Section

Abstract: Transient ultrasonic guided waves in anisotropic bi‐layered plates with finite‐width are investigated in this paper Composite bi‐layered plates consisting of GaAs substrate coated with Nb sheath is considered as an example because of its application to electronics and calorimetry The purpose is to investigate the acoustic mode coupling (“pinching”) phenomena for phonon transport A semi‐analytical finite element (SAFE) method is adopted to study the guided wave dispersion behavior in finite‐width elastic plates Nine‐noded quadrilateral elements are used to model the cross section of the finite‐width plate Propagation in the axial direction is modeled by analytical wave functions Elastodynamic Green’s functions are derived using modal summation in the frequency‐wavenumber and time‐space domains Results for dispersion and transient analysis of guided waves in finite‐width plates are presented and compared for different aspect ratios Group velocities are calculated and wave arrival times are computed for different plate cross sections as well as different excitation frequency Numerical results show significant influence of the plate aspect ratio on the dispersion and transient wave response Complex nature of quasi‐mode dispersion and propagation due to pinching phenomena in anisotropic plates require such quantitative analysis to afford easy interpretation These results would be important for nondestructive material evaluation and for characterization of phonon transport in anisotropic bi‐layered plates

Inverse Problem Solution to Find Real-Time Temperature Distribution Inside the Spot Weld Medium Using Ultrasound Time of Flight Methods

Abstract: The scope of this paper is to find temperature distributions in the spot weld at different stages of welding using ultrasonic through transmitted waves. The finite difference modeling provides the temperature distributions in the spot weld. For the given welding schedule the delay of the signal passing through the heated area is unique for every temperature distribution. Based on the experimentally obtained TOF curve it is possible to determine the temperature at any point of the weld.

Defect Detection in Correlated Noise

Abstract: We present methods for detecting NDE defect signals in correlated noise having unknown covariance. The proposed detectors are derived using the statistical theory of generalized likelihood ratio (GLR) tests and multivariate analysis of variance (MANOVA). We consider both real and complex data models. To allow accurate estimation of the noise covariance, we incorporate secondary data containing only noise into detector design. Probability distributions of the GLR test statistics are derived under the null hypothesis, i.e. assuming that the signal is absent, and used for detector design. We apply the proposed methods to simulated and experimental data and demonstrate their superior performance compared with the detectors that neglect noise correlation.

Wireless Eddy Current Probe for Engine Health Monitoring

Abstract: The first prototype wireless eddy current (EC) probe for on‐wing inspection was demonstrated in a F100 PW‐220 engine without external cabling at the Air National Guard overhaul facility in Des Moines Iowa. Wireless NDE probes have potential safety and economic benefits leading to prevention or mitigation of safety significant propulsion system malfunctions. Data from 2 MHz Eddy Current probes was transmitted using a dual‐frequency, phase modulated wireless analog communication system. Notches down to 0.010″ were detected by the system. This is comparable to the wired state‐of‐the‐art EC technology currently used to inspect engines.

Curvature Effect on the Properties of Guided Waves in Plates

Abstract: Inspection techniques using guided waves require an accurate prediction of the propagation properties and the physical behaviour of the waves Properties of these guided waves can be modelled using dispersion curves, which are calculated numerically However, any curvature along the propagation direction can change the behaviour of the modes compared to those in the straight waveguide There are currently few studies of the curvature effect along the propagation direction of any waveguide problem We have used a 2D circumferential waveguide problem to study the curvature effect, from which the phase velocity and the mode shapes have been examined at various curvatures The percentage difference in velocity of the low order plate modes, as a function of frequency and curvature, has been calculated numerically Furthermore the changes in phase velocity have been correlated with the changes in mode shapes that are due to the curvature effect This paper also presents an experimental study to validate the analytical predictions The experiment involved a sensitive measurement of the change of velocity as the curvature of a waveguide was increased Reasonably good agreement was obtained between the experimental measurements and the predictions

Importance of Acoustic Chaos in Sonic IR Imaging NDE

Abstract: In the Sonic IR imaging NDE technique, a short ultrasonic pulse (typically a fraction of a second) is applied to the target to excite the defects to heat up, while an infrared camera images the time evolution of the heating effect to identify the defective areas in the target. Although the ultrasonic sources we use in our systems ordinarily provide only a single frequency, the non‐linear coupling between the ultrasonic transducer tip and the target can cause acoustically chaotic vibrations in the target, namely producing other frequency components that are multiples of rational fractions of the driving frequency. The vibrational behavior is monitored by laser vibrometers. We discovered that this acoustic chaos enhances the IR signal of defects. In this paper, we present examples of the generation of chaotic vibration from the application of an ultrasonic excitation pulse when coupled non‐linearly to a specimen under inspection by sonic IR imaging.

Leak Location in Plates: Compensation for Multi‐Mode dispersion

Abstract: The problem of noise source location in a plate‐like structure using time of flight data recorded at sparsely distributed measurement locations is examined. The cross‐correlation approach to source triangulation is not generally applicable to signals carried by multiple dispersive plate wave modes. This work examines the extensions to the principles of cross‐correlation‐based triangulation needed to robustly determine source location.

On the Feasibility of Eddy Current Characterization of the Near‐Surface Residual Stress Distribution in Nickel‐Base Superalloys

Abstract: In light of its frequency‐dependent penetration depth, the measurement of eddy current conductivity has been suggested as a possible means to allow the nondestructive evaluation of subsurface residual stresses in shot‐peened specimens This technique is based on the so‐called electroelastic effect, ie, the stress‐dependence of the electrical conductivity Unfortunately, the relatively small (∼1%) change in electrical conductivity caused by the presence of compressive residual stresses is often distorted, or even completely overshadowed, by the accompanying conductivity loss caused by cold work and surface roughness effects Recently, it was observed that, in contrast with most other materials, shot‐peened Waspaloy and IN100 specimens exhibit an apparent increase in electrical conductivity at increasing inspection frequencies This observation by itself indicates that in these materials the measured conductivity change is probably dominated by residual stress effects, since both surface roughness and increased dislocation density are known to decrease rather than increase the conductivity and the presence of crystallographic texture does not affect the electrical conductivity of these materials, which crystallize in cubic symmetry Our preliminary experiments indicate that probably there exists a unique “window of opportunity” for eddy current NDE in nickel‐base superalloys We identified five major effects that contribute to this fortunate constellation of material properties, which will be reviewed in this presentation

Numerical and Experimental Study of Eddy Current Crack Detection around Fasteners in Multi‐Layer Structures

Abstract: Previous work on the detection of cracks around fastener holes in multi‐layer structures using eddy current NDE has met with some success; however, challenges remain in detecting smaller corner cracks while minimizing false call rates. To address this issue, a 3D model was developed for this problem and successfully validated with experimental data. Using this model, a series of numerical studies were performed to identify potential features for defect classification.

Nonlinear Ultrasonic Evaluation of Concrete Microcracking

Abstract: Microcracking is usually associated with most types of concrete deterioration. The nondestructive detection of deterioration in in‐situ concrete in its early stages is not practical using current technology. In this project nonlinear ultrasound was used to evaluate the degree of microcracking changes resulting from increasing compression stresses. Second and third harmonic amplitudes were measured and correlated to the degree of microcracking induced in the specimen. These harmonics were found to increase with increasing levels of microcracking.

Structural Health Monitoring of Anisotropic Layered Composite Plates Using Guided Ultrasonic Lamb Wave Data

Abstract: Layered composite plate‐like structures are finding an increasing range of applications in the aerospace industry. Structural Health Monitoring (SHM) of such structures is seen as a paradigm that will embrace efficient non‐destructive testing/evaluation techniques. The present study demonstrates the possibility of SHM using algebraic reconstruction techniques (ART) for tomographic imaging with Lamb wave data measured in realistic materials. Commercially available narrow bandwidth PZT crystals with the center frequency at 1.0 MHz were used as sensors on 5 mm thick 300 × 300 mm layered quasi‐isotropic and cross‐ply composite plates with and without defects. Defects (through holes) were synthetic and have been chosen to simulate high velocity impact damage in composite plates. The problem of Lamb wave mode identification to implement ART in anisotropic composite materials has been circumvented through the identification and use of a fairly general new wave energy‐based reconstruction parameter. To achieve reasonable image quality, conventional cross‐hole configuration is replaced by a new modified cross‐hole configuration that also optimizes the number of sensors.