Eddy current depth penetration photos

Skin effect is the tendency of an alternating electric current AC to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. Skin depth depends on the frequency of the alternating current; as frequency increases, current flow moves to the surface, resulting in less skin depth. Skin effect reduces the effective cross-section of the conductor and thus increases its effective resistance. Skin effect is caused by opposing eddy currents induced by the changing magnetic field resulting from the alternating current.

Immediate download. Please login to continue. There is a drive in industry to reliably detect surface breaking sub-millimetre defects in low electrical conductivity materials, such as stainless steel, titanium, and titanium aluminide using eddy-current testing. The sensitivity of the eddy current method to these materials is hindered in part by their low conductivities, and complex grain structures, which makes the high sensitivity standards required by industry more difficult to achieve. Eddy current measurements for sub-millimetre defects in low conductivity materials are routinely performed using a high excitation frequency, to decrease the skin depth of induced eddy currents into the material, which would otherwise be higher due to the reciprocal relationship between depth of penetration, and material conductivity.

The eddy current skin-effect limits the detection of subsurface defects and the range of thickness measurement. Traditional concepts to estimate the penetration depth basing on plane wave propagation into a conducting halfspace cannot describe the real depth of inspection achievable by state-of-the-art sensors and instruments. The paper presents a more fruitful concept for estimating the noise limited inspection depth. Here, the traditional parameters like frequency, probe dimensions, conductivity and permeability are analysed in combination with all sources of noise and disturbances in eddy current technique.

By Tom Nelligan and Cynthia Calderwood Magnetism, the underlying principle behind electric motors and generators, relays and stereo speakers, is also the force that enables an important category of NDT tools called eddy current instruments. Eddy current testing is widely used in the aerospace industry and in other manufacturing and service environments that require inspection of thin metal for potential safety-related or quality-related problems. In addition to crack detection in metal sheets and tubing, eddy current can be used for certain metal thickness measurements such as identifying corrosion under aircraft skin, to measure conductivity and monitor the effects of heat treatment, and to determine the thickness of nonconductive coatings over conductive substrates.