Mark P. Blodgett

Anisotropic grain noise in eddy current inspection of noncubic polycrystalline metals

This letter discusses the role electrical anisotropy plays in the structural integrity assessment of polycrystalline titanium alloys from the standpoint of fatigue crack detection and the related issue of microstructural noise. In eddy current inspection of noncubic crystallographic classes of polycrystalline metals the electric anisotropy of individual grains produces an inherent microstructural variation or noise that is very similar to the well-known acoustic noise produced by the elastic anisotropy of both cubic and noncubic materials in ultrasonic characterization. The presented results demonstrate that although the electrical grain noise is clearly detrimental in eddy current nondestructive testing for small flaws, it can be also exploited for characterization of the microstructure in noncubic polycrystalline materials such as titanium alloys in the same way acoustic grain noise is used for ultrasonic characterization of the microstructure in different materials.

Eddy current residual stress profiling in surface-treated engine alloys

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. According to this approach, the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conductivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using the known external applied stresses. This paper reviews the basic principles, measurement procedures, advantages, and limitations of eddy current residual stress profiling.

Development of eddy current microscopy for high resolution electrical conductivity imaging using atomic force microscopy

We present a high resolution electrical conductivity imaging technique based on the principles of eddy current and atomic force microscopy (AFM). An electromagnetic coil is used to generate eddy currents in an electrically conducting material. The eddy currents generated in the conducting sample are detected and measured with a magnetic tip attached to a flexible cantilever of an AFM. The eddy current generation and its interaction with the magnetic tip cantilever are theoretically modeled using monopole approximation. The model is used to estimate the eddy current force between the magnetic tip and the electrically conducting sample. The theoretical model is also used to choose a magnetic tip-cantilever system with appropriate magnetic field and spring constant to facilitate the design of a high resolution electrical conductivity imaging system. The force between the tip and the sample due to eddy currents is measured as a function of the separation distance and compared to the model in a single crystal copper. Images of electrical conductivity variations in a polycrystalline dual phase titanium alloy (Ti-6Al-4V) sample are obtained by scanning the magnetic tip-cantilever held at a standoff distance from the sample surface. The contrast in the image is explained based on the electrical conductivity and eddy current force between the magnetic tip and the sample. The spatial resolution of the eddy current imaging system is determined by imaging carbon nanofibers in a polymer matrix. The advantages, limitations, and applications of the technique are discussed.

The influence of texture and phase distortion on ultrasonic attenuation in Ti-6Al-4V

A high resolution experimental capability has been developed to map the phase and magnitude of ultrasonic waves transmitted in a solid. The advancement presented in this paper is provided by laser detection of the ultrasonic energy over a microscopic aperture of approximately 50 μm. The system is built around a computer controlled scanner and a confocal Fabry-Perot interferometer, which uses a diode pumped, frequency-doubled Nd:YAG laser as a light source. Wave propagation in the axial and radial directions of a 2.5″ diameter bar of textured Ti-6Al-4V was investigated in this study. Measurements were also taken on samples cut with angles between the surface normals and the axis of the bar of 0, 30, 45, 60, and 90 degrees. The work was motivated by the observation of unusually high apparent attenuation in the axial direction of the as-received bar, thought to be associated with phase distortion rather than actual energy loss. The current phase mapping results, using a focused laser spot, show relatively high wavefront distortion and more nonuniform distribution of the transmitted energy in the axial direction. The contribution to attenuation associated with phase cancellation loss was also investigated. These measurements show the laser detected attenuation to be substantially lower than the piezoelectrically measured attenuation. However, even the relative phase insensitivity of focused laser detection approach clearly indicates the attenuation to be strongest in the axial direction. This paper demonstrates the orientation dependence of attenuation stems from scattering effects associated with texturing and the elongated macroscopic grain structure in the mill annealed Ti-6Al-4V bar generated during processing, which may also affect diffraction and beam divergence.

Residual Stress Relaxation Due to Fretting Fatigue in Shot Peened Surfaces of Ti‐6Al‐4V

Fretting fatigue occurs at locations where the materials are sliding against each other under load. In order to enhance the fatigue life under fretting conditions the surface of the component is shot peened. In general, the shot peening process produces a compressive stress on the surface of the material, thereby increasing the resistance of the material to crack initiation. This paper presents the relaxation of residual stress caused during fretting fatigue. X‐ray diffraction has been utilized as the method to measure residual stress in fretting fatigued samples of Ti‐6Al‐4V.

APPLICATION OF A GIANT MAGNETORESISTIVE (GMR) SENSOR FOR CHARACTERIZATION OF CORROSION IN A LABORATORY SPECIMEN

This paper describes the use of a Giant Magnetoresistive (GMR) sensor to detect thickness variation in a multi‐layered specimen. A series of frequency‐response and modes‐of‐operation tests has been carried out to characterize a GMR sensor. Both the first and second harmonics of the GMR signal for corrosion detection were collected and analyzed. The experimental data showed that phase approach seemed to have better thickness discrimination over that of the magnitude approach.

Scattering of obliquely incident shear waves from a cylindrical cavity

Prior work has proposed the use of ultrasonic angle-beam shear wave techniques to detect cracks of varying angular location around fastener sites by generating and detecting creeping waves. To better understand the nature of the scattering problem and quantify the role of creeping waves in fastener site inspections, a 3D analytical model was developed for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity with arbitrary shear wave polarization. The generation and decay of the spiral creeping waves was found to be dependent on both the angle of incidence and polarization of the plane shear wave. A difference between the angle of displacement in 3D and the direction of propagation for the spiral creeping wave was observed and attributed to differences in the curvature of the cavity surface for the tangential and vertical (z) directions. Using the model, practical insight was presented on measuring the displacement response in the far-field from the hole. Both analytical and experimental results highlighted the value of the diffracted and leaky spiral creeping wave signals for nondestructive evaluation of a crack located on the cavity. Last, array and signal processing methods are discussed to improve the resolution of the weaker creeping wave signals in the presence of noise.

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

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, i.e., 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 inc...

UNCERTAINTY PROPAGATION IN EDDY CURRENT NDE INVERSE PROBLEMS

The probabilistic collocation method (PCM) was introduced to efficiently propagate the distributions of input parameters for eddy current NDE inverse problems. A multilevel approach was also considered to simultaneously address input parameter variability and the uniqueness of the inversion result. A case study is presented for the problem of characterizing material loss in a multi‐layer structure with varying liftoff and material properties. The performance and sensitivity of the uncertainty propagation method using PCM was evaluated under varying conditions.

A Novel Multi‐Frequency Eddy Current Measurement Technique for Materials Characterization

In an effort to meet the needs for high frequency eddy current measurements and be able to distinguish small conductivity variations in different materials, a new eddy current module capable of measuring magnitude, phase, and frequency shift was developed and integrated into a general‐purpose scanning system. Comparisons of three different parameter images are presented. The potential application of the multi‐frequency, multi‐parameter eddy current measurement technique for materials characterization to discriminate small conductivity changes is discussed.