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In this paper, we propose an original method for the physical analysis of electromagnetic interactions between an inductive sensor and any homogeneous conducting plane target. In the first part of this paper, the basic principles of inductive sensors are recalled. Then, we solve Maxwell's equations in order to obtain an analytical model for the relationship between the target properties and the electrical signals for a U-shaped sensor. Next, we emphasize the fact that our analytical relations can be interpreted as an extension of the electrical image method, even in the most difficult case of conducting and magnetic targets. In the last part, we present some measurements illustrating this new electrical image concept.

In this paper, we present an original structure of eddy current sensor meant for nondestructive testing of electrically conducting materials. Initially, the basic physical principles of the sensors are recalled, and afterwards we discuss the fact that it is generally impossible to design a sensor with a good spatial resolution and having at the same time a good range detection. Then, we propose an original idea, which consists of exploiting the good directivity of large cup sensors associated to a particular set of measurements, in order to extract relevant information only about the small part of the eddy currents induced in the target, below the centre of the sensor. The interest of this method is illustrated by scanning graphite composite plates containing small defects with this novel sensor. The comparison of the results obtained with actual sensors and our focused-field sensor demonstrates the efficiency of the structure.

The distributed point source method, or DPSM, developed in the last decade has been used for solving various engineering problems—such as elastic and electromagnetic wave propagation, electrostatic, and fluid flow problems. Based on a semi-analytical formulation, the DPSM solution is generally built by superimposing the point source solutions or Green’s functions. However, the DPSM solution can be also obtained by superimposing elemental solutions of volume sources having some source density called the equivalent source density (ESD). In earlier works mostly point sources were used. In this paper the DPSM formulation is modified to introduce a new kind of ESD, replacing the classical single point source by a family of point sources that are referred to as quantum sources. The proposed formulation with these quantum sources do not change the dimension of the global matrix to be inverted to solve the problem when compared with the classical point source-based DPSM formulation. To assess the performance of this new formulation, the ultrasonic field generated by a circular planer transducer was compared with the classical DPSM formulation and analytical solution. The results show a significant improvement in the near field computation. [ABSTRACT FROM AUTHOR]

Broadband electromagnetic frequency or time domain sensor techniques present high potential for quantitative water content monitoring in porous media. Prior to in situ application, the impact of the relationship between the broadband electromagnetic properties of the porous material (clay-rock) and the water content on the frequency or time domain sensor response is required. For this purpose, dielectric properties of intact clay rock samples experimental determined in the frequency range from 1 MHz to 10 GHz were used as input data in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rods based sensor embedded in the clay-rock. The sensor response in terms of the reflection factor was analyzed in time domain with classical travel time analysis in combination with an empirical model according to Topp equation, as well as the theoretical Lichtenecker and Rother model (LRM) to estimate the volumetric water content. The mixture equation considering the appropriate porosity of the investigated material provide a practical and efficient approach for water content estimation based on classical travel time analysis with the onset-method. The inflection method is not recommended for water content estimation in electrical dispersive and absorptive material. Moreover, the results clearly indicate that effects due to coupling of the sensor to the material cannot be neglected. Coupling problems caused by an air gap lead to dramatic effects on water content estimation, even for submillimeter gaps. Thus, the quantitative determination of the in situ water content requires careful sensor installation in order to reach a perfect probe clay rock coupling. [ABSTRACT FROM AUTHOR]

This paper deals with the problem of imaging defects in metallic structures through eddy current (EC) inspections, and proposes an original process for a possible tomographical crack evaluation. This process is based on a semi analytical modeling, called "distributed point source method" (DPSM) which is used to describe and equate the interactions between the implemented EC probes and the structure under test. Several steps will be successively described, illustrating the feasibility of this new imaging process dedicated to the quantitative evaluation of defects. The basic principles of this imaging process firstly consist in creating a 3D grid by meshing the volume potentially inspected by the sensor. As a result, a given number of elemental volumes (called voxels) are obtained. Secondly, the DPSM modeling is used to compute an image for all occurrences in which only one of the voxels has a different conductivity among all the other ones. The assumption consists to consider that a real defect may be truly represented by a superimposition of elemental voxels: the resulting accuracy will naturally depend on the density of space sampling. On other hand, the excitation device of the EC imager has the capability to be oriented in several directions, and driven by an excitation current at variable frequency. So, the simulation will be performed for several frequencies and directions of the eddy currents induced in the structure, which increases the signal entropy. All these results are merged in a so-called "observation matrix" containing all the probe/structure interaction configurations. This matrix is then used in an inversion scheme in order to perform the evaluation of the defect location and geometry. The modeled EC data provided by the DPSM are compared to the experimental images provided by an eddy current imager (ECI), implemented on aluminum plates containing some buried defects. In order to validate the proposed inversion process, we feed it with computed images of various acquisition configurations. Additive noise was added to the images so that they are more representative of actual EC data. In the case of simple notch type defects, for which the relative conductivity may only take two extreme values (1 or 0), a threshold was introduced on the inverted images, in a post processing step, taking advantage of a priori knowledge of the statistical properties of the restored images. This threshold allowed to enhance the image contrast and has contributed to eliminate both the residual noise and the pixels showing non-realistic values. [ABSTRACT FROM AUTHOR]

This paper presents a newly developed system for bridges post-tensioned devices inspection. These devices are generally composed of cables placed inside a duct, the residual space being filled with a particular cement to avoid corrosion. The nondestructive experimental setup uses a capacitive sensor that gives some relevant information about the electrical properties of the materials located inside the duct. Then, using an original modeling of the interactions between the sensor and the post-tensioned device, a signal processing extracts useful information from the sensors data, like the thickness of an eventual air gap between the cement and the duct. Experimental and theoretical derivations and comparisons are presented in the different parts of this paper. [ABSTRACT FROM PUBLISHER]

Interaction between a cavity or void in a liquid and a converging ultrasonic beam generated by a point-focused acoustic lens is investigated. A semi-analytical technique called the distributed point source method (DPSM) is adopted because no analytical solution is available for this problem involving cavities of different size and the finite element method is not very efficient for modeling high-frequency ultrasonic problems. The solution shows that if the cavity is placed very close to the focal point of the lens then it can be detected by the acoustic lens. The detectability of the cavity at the off-focus position depends on the distance of the cavity from the focal point. The variation of this distance as the cavity moves in horizontal and vertical directions from the focal point is also investigated. [ABSTRACT FROM AUTHOR]

Several investigators have modeled ultrasonic fields in front of transducers by Huygens–Fresnel superposition principle that integrates the contributions of a number of point sources distributed on the transducer face. This integral solution, also known as the Rayleigh integral or Rayleigh–Sommerfeld Integral solution, assumes the strengths of the point sources distributed over the transducer face. A newly developed technique called distributed point source method (DPSM) offers an alternative approach for modeling ultrasonic fields. DPSM is capable of modeling the field for prescribed source strength distribution as well as for prescribed interface conditions with unknown source strengths. It is investigated how the ultrasonic field in front of the transducer varies in different situations: (1) when the point source strengths are known, (2) when the point source strengths are unknown but obtained from the interface condition that only the normal component of the transducer velocity is continuous across the fluid-solid interface, (3) when all three components of velocity are assumed to be continuous across the interface for the no-slip condition, and (4) when the pressure instead of the velocity is prescribed on the transducer face. Results for these different interface conditions are compared with the analytical solutions along the central axis. [ABSTRACT FROM AUTHOR]

Phased array transducers are multi-element transducers, where different elements are activated with different time delays. The advantage of these transducers is that no mechanical movement of the transducer is needed to scan an object. Focusing and beam steering is obtained simply by adjusting the time delay. In this paper the DPSM (distributed point source method) is used to model the ultrasonic field generated by a phased array transducer and to study the interaction effect when two phased array transducers are placed in a homogeneous fluid. Earlier investigations modeled the acoustic field for conventional transducers where all transducer points are excited simultaneously. In this research, combining the concepts of delayed firing and the DPSM, the phased array transducers are modeled semi-analytically. In addition to the single transducer modeling the ultrasonic fields from two phased array transducers placed face to face in a fluid medium is also modeled to study the interaction effect. The importance of considering the interaction effect in multiple transducer modeling is discussed, pointing out that neighboring transducers not only act as ultrasonic wave generators but also as scatterers. © 2005 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including microsystems, microsensors and instrumentation for nanosciences.

Modeling ultrasonic fields in front of a transducer in the presence and absence of a scatterer is a fundamental problem that has been attempted by different techniques: analytical, semi-analytical, and numerical. However, a comprehensive comparison study among these techniques is currently missing in the literature. The objective of this paper is to make this comparison for different ultrasonic field modeling problems with various degrees of difficulty. Four fundamental problems are considered: a flat circular transducer, a flat square transducer, a circular concave transducer, and a point focused transducer (concave lens) in the presence of a cavity. The ultrasonic field in front of a finite-sized transducer can be obtained by Huygens-Fresnel superposition principle that integrates the contributions of several point sources distributed on the transducer face. This integral which is also known as the Rayleigh integral or Rayleigh-Sommerfeld integral (RSI) can be evaluated analytically for obtaining the pressure field variation along the central axis of the transducer for simple geometries, such as a flat circular transducer. The semi-analytical solution is a newly developed mesh-free technique called the distributed point source method (DPSM). The numerical solution is obtained from finite element analysis. Note that the first three problems study the effect of the transducer size and shape, whereas the fourth problem computes the field in presence of a scatterer. [ABSTRACT FROM AUTHOR]

In the field of nondestructive evaluation (NDE), the newly developed distributed point source method (DPSM) is gradually gaining popularity. DPSM is a semi-analytical technique used to calculate the ultrasonic field (pressure and velocity fields) generated by ultrasonic transducers. This technique is extended in this paper to model the ultrasonic field generated in multilayered nonhomogeneous fluid systems when the ultrasonic transducers are placed on both sides of the layered fluid structure.. Two different cases have been analyzed. In the first case, three layers of nonhomogeneous fuids constitute the problem geometry; the higher density fluid is sandwiched between two identical fluid half-spaces. In the second case, four layers of nonhomogeneous fluids have been considered with the fluid density monotonically increasing from the bottom to the top layer. In both cases, analyses have been carried out for two different frequencies of excitation with various orientations of the transducers. As expected, the results show that the ultrasonic field is very sensitive to the fluid properties, the orientation of the fluid layers, and the frequency of excitation. The interaction effect between the transducers is also visible in the computed results. In the pictorial view of the resulting ultrasonic field, the interface between two fluid layers can easily be seen. [ABSTRACT FROM AUTHOR]