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Reductions in the beam diameter and pulse duration of focused ultrasound for titanium inspections are believed to result in a signal-to-noise ratio improvement for embedded defect detection. It has been inferred from this result that detection limits could be extended to smaller defects through a larger diameter, higher frequency transducer resulting in a reduced beamwidth and pulse duration. Using Continuum Probe Designer™ (Pat. Pending), a transducer array was developed for full coverage inspection of 8 inch titanium billets. The main challenge in realizing a large aperture phased array transducer for billet inspection is ensuring that the number of elements remains within the budget allotted by the driving electronics. The optimization technique implemented by Continuum Probe Designer™ yields an array with twice the aperture but the same number of elements as existing phased arrays for the same application. The unequal area element design was successfully manufactured and validated both numerically and experimentally. Part I of this two-part series presents the design, simulation and modeling steps, while Part II presents the experimental validation and comparative study to multizone. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Multizone Inspection (MZI) is an ultrasonic inspection system that has been developed in response to FAA recommendations for improved inspection of titanium billet material. The system is capable of achieving [sharp/mesh/hash_synonym]2 FBH sensitivity in billets up to 10 inches in diameter. Honeywell as well as other OEMs are interested in adopting this inspection technique in response to FAA recommendations and to improve the sensitivity of their inspection procedures. To achieve that goal an evaluation of the system capability and variability at the supply base is necessary. This paper summarizes the results of a round robin study conducted at the supply base to evaluate the variability of MZI for 8 inch-diameter Ti-6Al-4V. It was found that on the average there is about 27% amplitude variability and 38% Signal-to-Noise Ratio (SNR) variability at each surveyed facility. It was also found that the variability from one facility to the other was on the average about 28% in amplitude and 30% in SNR. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

The authors have recently developed a new technique for nondestructive Hall coefficient measurement based on inductive sensing of the Hall-Corbino current produced by the injection of high-frequency alternating current into the component under test. In this study, the feasibility of characterizing the combined influence of near-surface residual stress and cold work based on Hall impedance spectroscopy was investigated in shot-peened fully hardened IN718 coupons. First, a simple analytical approximation is proposed that allows the direct prediction of the measured frequency-dependent Hall impedance from the depth-dependent Hall coefficient and conductivity profiles. This approximation is based on the simplistic approximation originally developed for eddy current conductivity depth profiling that has been modified for Hall coefficient measurements. According to this approximation, Hall coefficient measurements exhibit roughly half the penetration depth of eddy current conductivity measurements taken at the same frequency. Typical depth profiles of the Hall coefficient and electric conductivity were estimated from residual stress and cold work depth profiles obtained by destructive X-ray diffraction (XRD) measurements. The corresponding Hall coefficient and electric conductivity depth profiles were determined using gauge factors obtained from experiments previously conducted to study the influence of applied stress and cold work on the Hall coefficient and electric conductivity in fully hardened IN718. These Hall coefficient and conductivity depth profiles were then used to predict the Hall impedance spectra using COMSOL FE simulations and the analytical approximation. The results of finite element simulations were in reasonable agreement with the analytical approximation and validate the predicted lower penetration depth of Hall current compared to the conduction current. Finally, the feasibility of Hall impedance measurements was demonstrated by experiments conducted between 100 kHz and 30 MHz on fully hardened IN718 coupons surface treated to three different shot-peening levels of 4A, 8A, and 12A Almen intensity. [ABSTRACT FROM AUTHOR]

Hall coefficient measurements offer great opportunities as well as major challenges for nondestructive materials characterization. The Hall effect is produced by the magnetic Lorentz force acting on moving charge carriers in the presence of an applied magnetic field. The magnetic perturbation gives rise to a Hall current that is normal to the conduction current but does not directly perturb the electric potential distribution. Therefore, Hall coefficient measurements usually exploit the so-called transverse galvanomagnetic potential drop effect that arises when the Hall current is intercepted by the boundaries of the specimen and thereby produce a measurable potential drop. In contrast, no Hall potential is produced in a large plate in the presence of a uniform normal field at quasi-static low frequencies. In other words, conventional Hall coefficient measurements are inherently destructive since they require cutting the material under tests. This study investigated the feasibility of using alternating current potential drop (ACPD) techniques for nondestructive Hall coefficient measurements in plates. Specifically, the directional four-point square-electrode configuration is investigated with superimposed external magnetic field. Two methods are suggested to make Hall coefficient measurements in large plates without destructive machining. At low frequencies, constraining the bias magnetic field can replace constraining the dimensions of the specimen, which is inherently destructive. For example, when a cylindrical permanent magnet is used to provide the bias magnetic field, the peak Hall voltage is produced when the diameter of the magnet is equal to the diagonal of the square ACPD probe. Although this method is less effective than cutting the specimen to a finite size, the loss of sensitivity is less than one order of magnitude even at very low frequencies. In contrast, at sufficiently high inspection frequencies the magnetic field of the Hall current induces a strong enough Hall electric field that produces measurable potential differences between points lying on the path followed by the Hall current even when it is not intercepted by either the edge of the specimen or the edge of the magnetic field. The induced Hall voltage increases proportionally to the square root of frequency as the current is squeezed into a shallow electromagnetic skin of decreasing depth. This approach could be exploited to measure the Hall coefficient near the surface at high frequencies without cutting the specimen. [ABSTRACT FROM AUTHOR]

We investigated the feasibility of residual stress assessment based on Hall coefficient measurements in precipitation hardened IN718 nickel-base superalloy. As a first step, we studied the influence of microstructural variations on the galvanomagnetic properties of IN718 nickel-base superalloy. We found that the Hall coefficient of IN718 increases from ≈ 8.0 × 10-11 m3/C in its fully annealed state of 15 HRC Rockwell hardness to ≈ 9.4 × 10-11 m3/C in its fully hardened state of 45 HRC. We also studied the influence of cold work, i.e., plastic deformation, at room temperature and found that cold work had negligible effect on the Hall coefficient of fully annealed IN718, but significantly reduced it in hardened states of the material. For example, measurements conducted on fully hardened IN718 specimens showed that the Hall coefficient decreased more or less linearly with cold work from its peak value of ≈ 9.4 × 10-11m3/C in its intact state to ≈ 9.0 × 10-11 m3/C in its most deformed state of 22% plastic strain. We also studied the influence of applied stress and found that elastic strain significantly increases the Hall coefficient of IN718 regardless of the state of hardening. The relative sensitivity of the Hall coefficient to elastic strain was measured as a unitless gauge factor ≈ that is defined as the ratio of the relative change of the Hall coefficient ΔRH/RH divided by the exial stain ε = σ/E, where σ is the applied uniaxial stress and E is the Young's modulus of the material. We ermined that the galvanomagnetic gauge factor of IN718 is κ ≈ 2.6 - 2.9 depending on the hardness level. Besides the fairly high value of the gauge factor, it is important that it is positive, which means that compressive stress in surface-treated components decreases the Hall coefficient in a similar way as plastic deformation does, therefore the unfortunate cancellation that occurs in fully hardened IN718 in the case of electric conductivity measurements will not happen in this case. Additionally, the temperature dependence of the Hall coefficient was measured at three different hardness levels and the influence of thermal exposure was studied in fully hardened IN718 up to 700 °C. [ABSTRACT FROM AUTHOR]

We investigated the feasibility of residual stress assessment based on Hall coefficient measurements in precipitation hardened IN718 nickel-base superalloy. As a first step, we studied the influence of microstructural variations on the galvanomagnetic properties of IN718 nickel-base superalloy. We found that the Hall coefficient of IN718 increases from ≈ 8.0×10-11 m3/C in its fully annealed state of 15 HRC Rockwell hardness to ≈ 9.4×10-11 m3/C in its fully hardened state of 45 HRC. We also studied the influence of cold work, i.e., plastic deformation, at room temperature and found that cold work had negligible effect on the Hall coefficient of fully annealed IN718, but significantly reduced it in hardened states of the material. For example, measurements conducted on fully hardened IN718 specimens showed that the Hall coefficient decreased more or less linearly with cold work from its peak value of ≈ 9.4×10-11 m3/C in its intact state to ≈ 9.0×10-11 m3/C in its most deformed state of 22% plastic strain. We also studied the influence of applied stress and found that elastic strain significantly increases the Hall coefficient of IN718 regardless of the state of hardening. The relative sensitivity of the Hall coefficient to elastic strain was measured as a unitless gauge factor K that is defined as the ratio of the relative change of the Hall coefficient ΔRH/RH divided by the axial strain ε = σ/E, where σ is the applied uniaxial stress and E is the Young's modulus of the material. We determined that the galvanomagnetic gauge factor of IN718 is κ ≈ 2.6 - 2.9 depending on the hardness level. Besides the fairly high value of the gauge factor, it is important that it is positive, which means that compressive stress in surface-treated components decreases the Hall coefficient in a similar way as plastic deformation does, therefore the unfortunate cancellation that occurs in fully hardened IN718 in the case of electric conductivity measurements will not happen in this case. Additionally, the temperature dependence of the Hall coefficient was measured at three different hardness levels and the influence of thermal exposure was studied in fully hardened IN718 up to 700 °C. [ABSTRACT FROM AUTHOR]

Recent studies have shown that various ultrasonic wave mixing techniques can be exploited for assessing the nonlinearity of both intact and damaged materials. It has been reported that one particular type of ultrasonic wave mixing, non-collinear shear wave mixing, is sensitive to the excess nonlinearity caused by plastic deformation and low-cycle fatigue in Al2014-T4 aluminum alloy. In this study we investigated the feasibility of using the same non-collinear shear wave mixing technique to detect plastic deformation and fatigue damage in fully hardened IN718 engine alloy specimens. We implemented numerous technical improvements over the earlier developed non-collinear shear wave mixing system to improve its detection sensitivity and image resolution. In spite of these enhancements, we found that in fully hardened IN718 this technique is sensitive only to plastic deformation and, to a much lesser degree, to highly accelerated low-cycle fatigue, but it is not sensitive to either typical low-cycle fatigue or high-cycle fatigue. These observations highlight a potential deficiency in our current understanding of nonlinear material-wave interaction and suggest that further research is needed to explain why certain classes of dislocation networks remain hidden from this type of inspection. [ABSTRACT FROM AUTHOR]

Non-collinear shear wave mixing at an imperfect interface between two solids can be exploited for nonlinear ultrasonic assessment of bond quality. In this study we developed two analytical models for nonlinear imperfect interfaces. The first model uses a finite nonlinear interfacial stiffness representation of an imperfect interface of vanishing thickness, while the second model relies on a thin nonlinear interphase layer to represent an imperfect interface region. The second model is actually a derivative of the first model obtained by calculating the equivalent interfacial stiffness of a thin isotropic nonlinear interphase layer in the quasi-static approximation. The predictions of both analytical models were numerically verified by comparison to COMSOL finite element simulations. These models can accurately predict the excess nonlinearity caused by interface imperfections based on the strength of the reflected and transmitted mixed longitudinal waves produced by them under non-collinear shear wave interrogation. [ABSTRACT FROM AUTHOR]

Ultrasonic wave mixing has shown promising potential for assessing otherwise hidden subtle imperfections in imperfect diffusion bonds between Ti-6Al-4V components. When interrogating a diffusion bonded specimen using noncollinear shear wave mixing, both bulk and interface nonlinearity will contribute to the transmitted nonlinear signal. Although a recent study has shown that changing the transducer alignment can suppress the intrinsic nonlinearity of the surrounding material to some extent so that the interface nonlinearity could be detected more selectively, it is still difficult to distinguish different levels of bond quality based on the detected transmitted signal only. Analytical and numerical studies showed that an imperfect interface generates the same amount of nonlinear displacement in the reflected and transmitted fields. In this study, we used the reflected nonlinear interface signature to characterize diffusion bonded interfaces. Our results indicate that it is better to use the reflected nonlinear interface signature to assess the bond quality, which is in agreement with our previous analytical and numerical predictions. However, the observed random phase of the reflected signature indicates that existing nonlinear interface models are insufficient for accurately describing the nonlinear interaction of shear incident waves with high-quality diffusion bonded interfaces. [ABSTRACT FROM AUTHOR]

Recent research has indicated that non-collinear ultrasonic wave mixing can be exploited for the measurement of both bulk material nonlinearity and localized interface nonlinearity. In the particular configuration considered in this study, two oblique shear waves are mixed to generate a third longitudinal wave as a result of nonlinear interaction with the material. In contrast with the nonlinearity parameter (β) measured with conventional longitudinal wave harmonic generation, the mixed signal depends only on the second Murnaghan coefficient (m). A simple analytical approximation was developed to determine the amplitude of the mixed signal. It was shown that in the presence of a perfectly reflecting interface the bulk nonlinearity decreases due to the complex linear reflection coefficient of such an interface above the longitudinal critical angle. It was also found that a nonlinear interface produces an additional symmetric nonlinear scattered wave that is the same for both the reflected and transmitted fields. This excess nonlinearity can be exploited to characterize the interface, especially from the reflected part of the mixed signal that is essentially free of bulk nonlinearity contribution from the surrounding host material. [ABSTRACT FROM AUTHOR]

There is an important need to quantify and improve the probability of detection (POD) for NDE inspection. To improve POD, it is important to identify and quantity sources of variability. A standard NDE inspection assessment method uses a specimen containing seeded flaws of known size and character. The specimen is inspected according to an experimental design that will capture the important sources of variability. The commonly used NDE data analysis/modeling method, known as a⁁ versus a, uses a linear regression to relate the NDE signal response to the flaw size. The model behind this method contains only one component of variance for the response. There are, however, many random factors causing variability in NDE inspection. In this paper, we use a Bayesian hierarchical model to identify and quantify the inspection variance components in the presence of data censoring. We use Markov Chain Monte Carlo simulation to estimate the model parameters, including the variance components. We demonstrate the effectiveness of the approach with experimental data from an ultrasonic inspection round-robin experiment. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

As with many aerospace applications, commercial jet engine components are operated in demanding environments, often at extreme temperature and stress conditions. The predominant used surface inspection method used on these components is fluorescent penetrant inspection. Research has been ongoing for a number of years on a new technology using a short burst of low frequency (∼20 KHz) ultrasound to “heat up” cracks and make them visible in the infrared range. The basic premise of the Thermal Acoustic method is to use an energy source with recent efforts using an ultrasonic horn originally intended for use in ultrasonic welding to excite the component. The energy source causes an increase in local heating, which is detectable with infrared cameras typically used in Thermographic inspection. While considerable research is underway, additional information on the sensitivity and applicability of this technique to engine components and alloys is needed prior to widespread use in the aviation industry. The purpose of this program is to provide additional data to determine applicability of this method to engine components. [ABSTRACT FROM AUTHOR]

Cylindrical billets of Titanium alloy are ultrasonically inspected prior to use in fabricating rotating jet-engine components. Although each billet has a cylindrical geometry, its ultrasonic properties are not cylindrically symmetric due to asymmetries in the process used to produce the billet from the original cast ingot. In the inspection process, a calibration standard of the same diameter containing flat-bottomed hole (FBH) reflectors is used to set the initial inspection gain (i.e., the signal amplification level). If the ultrasonic attenuation of the billet to be inspected differs significantly from that of the calibration standard, the inspection gain must be adjusted to maintain the desired defect detection sensitivity. In this paper we investigate several schemes for attenuation compensation. The gain adjustments fall into two broad categories: “global” adjustments (in dB/inch units), which are applied uniformly throughout the billet under inspection; and “local adjustments”, which vary with axial and circumferential position. The schemes make use of the patterns of reflected back-wall amplitude and backscattered grain noise seen in the calibration standard and test billet. The various compensation schemes are tested using specimens of 6″-diameter Ti-6A1-4V billet into which many FBH targets were drilled. Results are summarized and tentative recommendations for improving billet inspection practices are offered. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]