Eddy-current testing

Eddy-current testing (also commonly seen as eddy current testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.

Eddy-current testing (also commonly seen as eddy current testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials. S = signal voltage response A = flaw area a = far-sided flaw depth B 1 , B 2 = regression coefficients Excel implementation: Columns: ln(S), ln(A), a B 1 , B 2 = LINEST(ln(S), ln(A):a, FALSE) S = A B 1 · e B 2 ·a The functional form aligns with established eddy current physics principles, and with it, flaw depth is estimated at an assumed aspect ratio critical for damage tolerance assessment. The power law term (A B 1 ) reflects the nonlinear relationship between flaw scattering volume and induced eddy current disruption, while the exponential term (e B 2 ·a ) accounts for signal attenuation due to skin depth effects as flaws become more distant from the inspection surface. Flaw area: For semi-elliptical surface flaws, the flaw area A is determined by the flaw depth a and the aspect ratio a/c, where c is half the crack surface length. This allows distinguishing between easier-to-detect longer flaws with an aspect ratio of 0.2 and more challenging semi-circular flaws with an aspect ratio of 1.0, expressed as: A = \frac{\pi a^2}{2(a/c)} This formulation enables systematic evaluation of model performance across different flaw geometries commonly encountered in inspections. However, regression analyses must always be constrained to the range of bounding flaw aspect ratios and depths represented in the experimental dataset. == Applications== The two major applications of eddy current testing are surface inspection and tubing inspections. Surface inspection is used extensively in the aerospace industry, but also in the petrochemical industry. The technique is very sensitive and can detect tight cracks. Surface and volumetric inspection can be performed both on ferromagnetic and non-ferromagnetic materials. Optimized ECT array inspections, with skin depth tuned near material thickness and calibrated on 100-micron wide half-sized component notches, have revealed backside flaw depths at ~60% thickness. Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubing in nuclear plants and heat exchangers tubing in power and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate. A variation of conventional ECT for partially magnetic materials is full saturation ECT. In this technique, permeability variations are suppressed by applying a magnetic field. The saturation probes contain conventional eddy current coils and magnets. This inspection is used on partially ferromagnetic materials such as nickel alloys, duplex alloys, and thin-ferromagnetic materials such as ferritic chromium molybdenum stainless steel. The application of a saturation eddy current technique depends on the permeability of the material, tube thickness, and diameter. A method used for carbon steel tubing is remote field eddy current testing. This method is sensitive to general wall loss and not sensitive to small pits and cracks. ==See also== Eddy current Nondestructive testing Alternating current field measurement Cover Meter Metal detector Skin effect Sweep Frequency Eddy Current testing (SFECT) ==References== ==External links== Eddy Current Array Tutorial An introduction to eddy current testing from the NDE/NDT resource center Intro to Eddy Current Testing by Joseph M. Buckley (pdf, 429 kB) Eddy Current Testing at Level 2, International Atomic Energy Agency, Vienna, 2011 (pdf 5.6 MB). ASTM E3052 Standard Practice for Examination of Carbon Steel Welds Using Eddy Current Array Official web page of Lorentz Force Velocimetry and Lorentz Force Eddy Current Testing Group Video on eddy current testing, Karlsruhe University of Applied Sciences

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