Current Skin Depth Research Articles

Eddy current evaluation for thickness loss estimation of aluminum alloys used in aircraft structures

Aluminum alloys are commonly used in the aircraft industry but it tends to corrode and needs to be inspected properly. Eddy current testing is the widely used non-destructive testing (NDT) for aircraft metals. This research studied estimating the thickness loss due to corrosion in an aircraft aluminum sheet metal. The Al1100, Al2024, and Al7075 were 2 mm thick. Aluminum sheet metals were used to create artificial damage in hydrochloric acid. The thickness loss due to corrosion was inspected with eddy current testing (ET) and compared with the measurement from the coordinate measurement machine (CMM). The results showed that the thickness loss due to corrosion could be estimated using the eddy current skin depth frequencies. However, in the practical application, the skin depth could be guessed from the corrosion rate which could reduce the inspection time. In this research, the corrosion rate was measured by using potentiodynamic measurement. It can be used for estimating the thickness loss for an appropriate inspection interval.

Effect of connecting cable combination on performance of electromagnetic forming system

Forming and joining of metallic sheet and tubes through electromagnetic forming (EMF) technique have been gaining relevance in the present industrial scenario that calls for energy conservation, environmental concern and improved quality of products. The technique comes under the domain of high velocity forming that operates at strain rate in the range of 103 to 10 6 s−1. The EMF system broadly consists of capacitor bank, coil/actuator, charging and discharging system and connecting bars and cables. The frequency and magnitude of output current pulse can be influenced by geometrical aspect of coil, length and type of cable connection of discharge wire, internal resistance and inductance of various other components of the system (like capacitor, trigger transformer and generator, bus bars etc.). Generally coil design and other system components cannot be changed or altered because of product and functional requirements. The fundamental requirement of high discharge current frequency for achieving the desired skin depth of current can be achieved through simple modification of discharge cable connection. The present work analyses effect of various cable connection combination to optimally improve the discharge pulse parameters like frequency and amplitude. An aluminium tube of AA6061 was deformed experimentally to analyse this effect and correlate the result for better understanding of discharge current behaviour due to various cable connection. A comparative study has been carried out to validate the deformation of experimental results with simulated deformation for various cable combinations. A significant improvement in the parameters are observed by increasing number of cables in parallel connection as compared to other combinations.

Research on crack monitoring technology of multilayer structure based on TMR-eddy current sensor

The flexible eddy current array sensor has the characteristics of flexibility, bendability and light weight, which has a great application prospect in the field of structural health monitoring. The traditional flexible eddy current array sensor is affected by the skin effect and the crack monitoring depth is relatively low. This paper proposes an eddy current array sensor which uses TMR (tunnel magneto-resistance) sensor as sensing channels. It reduces the excitation frequency and increases the eddy current skin depth. The TMR sensor realizes the quantitative monitoring of cracks by the array arrangement. First, the finite element model of the sensor is established, and the influence of the cracks on the different layers of the multilayer structure on the magnetic induction intensity at the test point is analyzed. The simulation and results show that when the crack passes through the sensor, the magnetic induction intensity at the three test points changes in sequence and gradually increases to the maximum value. The direct relative position of the maximum value of the three test points is approximately the same as the distance among the test points. Then, the simulated crack growth experiment of the corresponding sensor is carried out. The experimental results show that as the crack grows, the signals of the three TMR sensors on the eddy current sensor change in sequence, and the distance between the characteristic points is consistent with the distance between the TMR sensors. The output voltage of the sensor decrease with the crack depth increases. The experimental results are consistent with the simulation results, which further verifies the correctness of the simulation model.

Electromagnetic Modeling of Packaging Structures With Lossy Interconnects Based on Two-Region Surface Integral Equations

Accurate electromagnetic (EM) modeling for packaging structures with lossy interconnects requires to consider the finite conductivity of the conducting interconnects. In the integral equation approach, the conducting loss is usually accounted for through an approximate surface impedance when the skin depth of current is small, but this approximation may not be valid for large skin depth caused by low frequencies or small conductivities. In this paper, we treat the lossy interconnects as dielectric-like media and use the two-region electric field integral equations (EFIEs) to characterize the EM feature of the interconnects. The EFIEs are solved with the method of moments in which the Rao-Wilton-Glisson (RWG) basis function and dual basis function (DBF) are used to represent the electric and magnetic current densities, respectively. The joint use of RWG and DBF can remove the need of wisely formulating integral equations and selecting testing schemes and also alleviate the low-frequency effect in a wideband modeling for multiscale interconnect structures. Numerical examples are presented to demonstrate the approach and its robustness has been verified.

The production and evolution of multiple converging radiative shock waves in gas-filled cylindrical liner z-pinch experiments

A cylindrical liner z-pinch configuration has been used to drive converging radiative shock waves into different gases. On application of a 1.4 MA, 240 ns rise-time current pulse, a series of cylindrical shocks moving at typical velocities of 20 km s−1 are consecutively launched from the inside liner wall into an initially static gas-fill of density ∼10−5 g cm−3. The drive current skin depth calculated prior to resistive heating was slightly less than the liner wall thickness and no bulk liner implosion occurred. Axial laser probing images show the shock fronts to be smooth and azimuthally symmetric, with instabilities developing downstream of each shock. Evidence for a radiative precursor ahead of the first shock was seen in laser interferometry imaging and time-gated, spatially resolved optical spectroscopy. The interferometry diagnostic was able to simultaneously resolve the radiative precursor and the density jumps at the shock fronts. Optical streak photography provided information on shock timing and shock trajectories and was used to gain insight into the shock launching mechanisms.

Increase of energy transfer efficiency in the electromagnetic flux compression technique generating ultra-high magnetic field

Magnetic fields of over 100 T can be generated only through the use of a destructive pulsed magnet. Electro-magnetic flux compression (EMFC) is an efficient method to generate such ultra-high magnetic fields. The EMFC system at Institute for Solid State Physics holds the world record for the highest magnetic field produced in-doors. This system has been used for various sorts of measurements applied to matters under ultrahigh magnetic fields. Recently, we successfully improved the coil system to generate a higher field using less energy injection and more simplified preparation processes. The new system increased the electro-magnetic energy transfer efficiency to at least twice that of the previously employed system. Our new primary coil using a copper current guide has the advantage of shallower high-frequency current skin depth and less contact impedance than previous ones. Therefore the discharge current spark is completely avoided. The improved skin depth resulted in a symmetric implosion of the liner coil with less influence of the feed gap. A high degree of cylindrical liner symmetry was observed during implosion. A fast liner speed of 2.4 km/s was achieved, and 350 T could be generated by 1 MJ and 470 T by 2 MJ.

Comparison of two simple high-frequency earthing electrodes

Lightning strikes on high-voltage transmission lines may create hazardous touch potentials on adjacent substation equipment and damage to control equipment. When designing suitable substation earthing electrodes to overcome this problem and safely dissipate the transient fault currents to ground, it is essential to consider the behaviour of the current flow. The steady-state frequency equivalent to a lightning strike is at least 0.25 MHz, which corresponds to a current skin depth ? of about 10m in homogeneous soil of conductivity 0.01 Sm-1. This surface effect causes the impedance of an electrode system to be considerably larger than the power frequency resistance. Thus, a dedicated electrode is normally placed in parallel with the low-frequency earthing system, usually taking the form of a simple vertical rod of copper-coated steel about 5m long. However, the work presented in this paper suggests that a long rod is not ideal for the purpose. Using a relatively simple numerical finite-difference procedure it has been found that a flat disc electrode parallel to the surface of the ground achieves a significant improvement over the performance of a vertical rod. Both rod and disc have been solved in the frequency domain, but the rod has also been analysed in time-stepping form so that the peak voltage for a given imposed current can be compared with that deduced from the equivalent steady-state complex impedance.

The measurement of surface cracks in metals by using a. c. electric fields

This paper describes some recent studies associated with the development of an instrument, the Crack Microgauge, at University College London, for the detection and measurement of surface cracks in metals by using a. c. electric currents in the metal surface. The design of the instrument has enhanced the accuracy of measurement of surface voltages in the neighbourhood of surface-breaking cracks to the point where it is now advantageous to consider the distribution of surface voltages by mathematical analysis. This enables better use to be made of the instrument since measurements can then be more accurately interpreted in terms of crack size and geometry. In contrast, earlier applications of the method relied on the calibration of the instruments against test blocks. The first generation of users of the Crack Microgauge interpreted their readings according to a simple one-dimensional normal crack model. This, however, is an oversimplification for cracks of finite aspect ratio and it leads to underestimates of centre-line crack depth typically of the order of 30-40% when the aspect ratio is about 3. The first major contribution of the theory was therefore to provide a model for surface current flow around cracks of finite aspect ratio. For this purpose a useful unfolding theorem was deduced in cases in which the current skin depth is small compared with crack dimensions. This allows the surface field to be unfolded into a plane distribution of potential satisfying Laplace’s equation, and it enables us to obtain solutions for many different crack shapes by using standard mathematical methods. An account of these developments is given in § 2 of this paper. The unfolding of the surface field gives the global distribution of surface current around a crack, that is, on the length scale of the crack dimensions. On the length scale of the skin depth, however, the solutions need modification in the neighbourhood of the surface and interior edges of the crack. In developing the unfolding theorem it was argued that these modifications, which we describe as corner solutions, could be neglected. The purpose of this paper is to construct these corner solutions to confirm that this is so. The mathematical theory described in § 2 was developed for the most important case in which cracks develop in planes perpendicular to the surface of the metal. It is clear from the construction of that solution, however, that the unfolding may be applied with the same result to plane cracks inclined to the surface. Thus the global solution is not able to distinguish the inclination of the crack plane to the surface. We shall find, however, that the corner solutions are sensitive to this inclination, and they are given in § 3 for arbitrary angles of inclination. These studies suggest, therefore, that information on crack inclination could be deduced from experimental investigation of these corner solutions. Since the solutions are found to possess a phase shift of 1/4π, it might be possible to extract this information by measuring perturbation signals in quadrature with the main field or perhaps by increasing the skin depth so that the scale of the solutions is enhanced. For a given specimen this could be done by reducing the frequency of the a. c. field, but, if the frequency is reduced significantly, the field distributes itself through the interior of the specimen on the scale of the crack dimensions and may no longer be described as a surface distribution. Further work to describe such field distributions when the skin depth is of the order of the crack depth is in progress.

Effect of Surface Roughness on Eddy Current Losses at Microwave Frequencies

A theoretical investigation has been made of the power dissipation by eddy currents in a metallic surface at microwave frequencies in the presence of regular parallel grooves or scratches whose dimensions are comparable to the eddy current skin depth. The eddy current equation has been integrated numerically for grooves of various shapes and sizes transverse to the direction of induced current flow, and the corresponding losses are calculated and plotted. The power dissipation is increased by about 60 percent over its value for a smooth surface when the root-mean-square deviation of the grooved surface from an average plane is equal to the skin depth; the exact shape of the grooves is not critical. The increase in eddy current losses caused by grooves parallel to the current is shown in a particular case to be only about one-third as great as the increase caused by transverse grooves of similar size. The effect on losses of an isolated narrow crack or fissure transverse to the current is briefly discussed.