Eddy current

Summary

In electromagnetism, eddy currents (also called Foucault's currents) are loops of electric current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnetic field. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid. By Lenz's law, an eddy current creates a magnetic field that opposes the chang

Official source

https://en.wikipedia.org/wiki/Eddy_current

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It is important to reliably characterize cracks in metallic mechanical parts in order to assess the serenity of damage due to fatigue. The most important parameter for this is the crack depth. This is notoriously difficult with standard Eddy current techniques. We show here that this can be achieved by measuring the local magnetic field induced by Eddy currents flowing around the crack. This is done by using a high-density array of micro-Hall sensors integrated in a single CMOS chip with a spatial resolution of 10 mu m. We present the dependence of the signal on varying crack depths, liftoff and yaw angle. Finally, we study the response of the sensor to cracks in a DC magnetic field, i.e. the flux leakage due to the presence of a crack. This is especially relevant to cases where cracks appear predominantly close to edges with complex geometries for which it is difficult to induce clean Eddy currents.

Ios Press2014

The High-Resolution Magnetic Camera: A Novel Sensor for Eddy Current Testing

Hannes Bleuler, David Florian Hippert, Pavel Kejik, Roland Moser, Marco Picasso, Gilles Raphaël Santi

Eddy current techniques are often used to detect and, in some advanced applications, to characterize fatigue surface-breaking cracks appearing in mechanical parts. One of the major difficulties in this approach is to reliably separate the crack contribution to the signal from the others (liftoff, edge effects, material property variations). This is particularly prominent in weld inspection. We present here a novel sensor consisting in an array of 200 micro-Hall sensors on a CMOS chip aligned over a length of 2 mm with a 10 mu m pitch. This sensor, which we call a "high-resolution magnetic camera", gives a direct measurement of the perturbation of the local magnetic field caused by the modification of the Eddy currents flow due to the surface-breaking crack. We show that using this technique, a crack has a very distinctive signature which can be easily distinguished from all other contributions. Finite element simulations are used to understand the signal response of different cracks. In addition, this system can perform "micro-magnetoscopy" on magnetic materials by giving a fine picture of the local variations of the magnetic field caused by the presence of a defect.

Ios Press2014

Image analysis technique applied to lock-exchange gravity currents

Claudia Adduce, Helena Isabel Dos Santos Nogueira

An image analysis technique is used to estimate the two-dimensional instantaneous density field of unsteady gravity currents produced by full-depth lock-release of saline water. An experiment reproducing a gravity current was performed in a 3.0 m long, 0.20 m wide and 0.30 m deep Perspex flume with horizontal smooth bed and recorded with a 25 Hz CCD video camera under controlled light conditions. Using dye concentration as a tracer, a calibration procedure was established for each pixel in the image relating the amount of dye uniformly distributed in the tank and the greyscale values in the corresponding images. The results are evaluated and corrected by applying the mass conservation principle within the experimental tank. The procedure is a simple way to assess the time-varying density distribution within the gravity current, allowing the investigation of gravity current dynamics and mixing processes.

Institute of Physics2013