Compensation of the skin effect in low-frequency potential drop measurements

Potential drop measurements are routinely used in the non-destructive evaluation of component integrity. Potential drop measurements use either direct current (DC) or alternating current (AC), the latter will have superior noise performance due to the ability to perform phase sensitive detection and the reduction of flicker noise. AC measurements are however subject to the skin effect where the current is electromagnetically constricted to the surface of the component. Unfortunately, the skin effect is a function of magnetic permeability, which in ferromagnetic materials is sensitive to a number of parameters including stress and temperature, and consequently in-situ impedance measurements are likely to be unstable. It has been proposed that quasi-DC measurements, which benefit from superior noise performance, but also tend to the skin-effect independent DC measurement, be adopted for in-situ creep measurements for power station components. Unfortunately, the quasi-DC measurement will only tend to the DC distribution and therefore some remnant sensitivity to the skin effect will remain. This paper will present a correction for situations where the remnant sensitivity to the skin effect is not adequately suppressed by using sufficiently low frequency; the application of particular interest being the in-situ monitoring of the creep strain of power station components. The correction uses the measured phase angle to approximate the influence of the skin effect and allow recovery of the DC-asymptotic value of the resistance. The basis of the correction, that potential drop measurements are minimum phase is presented and illustrated on two cases; the creep strain sensor of practical interest and a conducting rod as another common case to illustrate generality. The correction is demonstrated experimentally on a component where the skin effect is manipulated by application of a range of elastic stresses

Texture Assessment in SCS-6 Fibers from Ultrasonic Dispersion Measurements

Advanced fibers used to reinforce composite materials exhibit complicated morphology. Typically, the fiber consists of a cylindrical core embedded in a cladding region followed by a distinct interface zone separating the fiber system from the matrix region. In addition, the cladding region itself often consists of subregions which can be identified as more or less distinct layers. According to the simplest micromechanical models these coaxial layers are assumed to be isotropic and homogeneous. At low frequencies when the acoustic wavelength is much larger than the radius of the fiber, such a composite fiber exhibits significant anisotropy of transversely symmetric nature manifested by higher axial stiffness relative to the radial one. This macroscopic anisotropy is caused by the coaxial structure and the possibly imperfect interfaces between the layers. The main goal of this study was to determine whether this structural anisotropy produced by the presence of microscopically isotropic and homogeneous constituents is sufficient to account for all of the macroscopic anisotropy observed in real fibers or, in addition, microscopic anisotropy caused by some texturing in the constituents themselves is needed to properly model the fiber at ultrasonic frequencies. Apparent texturing in the constituents can be caused by either real microscopic anisotropy due to preferred crystallographic orientation of grain growth during manufacturing or by additional structural anisotropy due to strong radial inhomogeneity in the material composition, e.g., increasing carbon content in the silicon carbide caladding

Excess Scattering Induced Loss at a Rough Surface Due to Partially Coherent Double-Reflection

Transmission (and retransmission) through rough surfaces degrades ultrasonic flaw detection and materials characterization. The flaw signal as well as reference signals (e.g. reflections from the front-surface or back-surface of the specimen) become difficult to interpret. In the simplest case, we make two assumptions in order to model the ultrasonic pulse-echo signal. Our first assumption is that the scatterer is large in the sense that it extends laterally for many surface correlation lengths. In this case, the signal has a small variance and is well described by its average value that will be referred to as the “specular” signal. Our second assumption is that the flaw is far from the surface, in a sense to be defined below. Given these two assumptions, the rough surface introduces a loss that is proportional to the square of the rms height, h, and the frequency, f. Further, the loss due to a double-transmission, L d , is just twice the loss due to a single-transmission, L s , i.e., L d ≈ 2 L s .</p

Creep strain measurement using a potential drop technique

AbstractThis paper will demonstrate the use of a potential drop sensor to monitor strain. In particular, the suitability of the technique to high temperature or harsh environment applications presents an opportunity for monitoring strain in components operating under creep conditions. Monitoring creep damage in power station components is a long standing technological challenge to the non-destructive evaluation community. It is well established in the literature that strain rate serves as an excellent indicator of the progress of creep damage and can be used for remnant life calculations. To facilitate the use of such strain rate based evaluation methods, a permanently installed, strain sensitive, potential drop technique has been developed. The technique has very simple and robust hardware lending itself to use at high temperatures or in harsh environments. Strain inversions are presented and demonstrated experimentally; a room temperature, plastic deformation experiment is used for validation and additionally an accelerated creep test demonstrates operation at high temperature (600°C+). Excellent agreement is shown between potential drop inverted strain and control measurements

Measurement of Changes in Surface Roughness using Ultrasonic Reflection Coefficient

The goal of this work is to develop a technique for on-line monitoring of surface topography. The monitoring of aluminum surfaces must provide reliable and fast distinction between “good” and “bad” surfaces. This project has investigated the feasibility of the ultrasonic reflection coefficient to measure differences in surface roughness. The standard deviations for the surfaces are 10–20 μm.</p

Attenuation of Rayleigh waves due to three-dimensional surface roughness: a comprehensive numerical evaluation

The phenomenon of Rayleigh wave attenuation due to surface roughness has been well studied theoretically in the literature. Three scattering regimes describing it have been identified-the Rayleigh (long wavelength), stochastic (medium wavelength), and geometric (short wavelength)-with the attenuation coefficient exhibiting a different behavior in each. Here, in an extension to our previous work, we gain further insight with regard to the existing theory, in three dimensions, using finite element (FE) modeling, under a unified approach, where the same FE modeling techniques are used regardless of the scattering regime. We demonstrate good agreement between our FE results and the theory in all scattering regimes. Additionally, following this demonstration, we extend the results to cases that lie outside the limits of validity of the theory

Coherent and Incoherent Scattering Mechanisms in Air-Filled Permeable Materials

Ultrasonic evaluation of porous materials can take advantage of some very specific acoustic phenomena that occur only in fluid-saturated consolidated solids of continuously connected pore structure. The most interesting feature of acoustic wave propagation in such media is the appearance of a second compressional wave, the so-called slow wave [1,2]. The slow compressional wave represents a relative motion between the fluid and the solid frame. This motion is very sensitive to the kinematic viscosity of the fluid and the dynamic permeability of the porous formation. Certain material properties such as tortuosity, permeability, porosity, and pore size, shape and surface quality are inherently connected to the porous nature of the material and can be evaluated best from the propagation properties of the slow compressional wave.</p