Overview

Eddy current testing is a non-destructive inspection technique proven for flaw identification, as well as for verification of proper heat treat/material structure and component features.  The technology is based upon electromagnetic induction originally discovered by Michael Faraday in 1831.  Later in the 1800s, it was discovered that an electromagnetic field changes its properties when induced in metals of different conductivity and permeability. 

In the 1950s and ‘60s, interest in the technology grew and the technique became widely used for aircraft and nuclear power plant inspections.  Over the last 20 years, the method has been used extensively to test components for flaws and proper heat treat or material structure.

Eddy current crack and flaw detection

In eddy current crack and flaw testing, a metallic component is placed into magnetic fields causing eddy currents to flow through the material.  Just like water flowing around a rock in a stream, if there are cracks or flaws in the metal, the eddy currents must go around them.  Criterion NDT’s eddy current instruments are designed to detect these flow changes.  The instrument can then alert operators and QA personnel to remove defective parts and update their manufacturing processes if necessary.

Crack and flaw testing is performed as a dynamic test.  The eddy current probe must pass over the area being tested.  This can be accomplished by moving the probe, or by moving the part with respect to the probe.  This is done with material handling systems.  Multiple coils are often used in an array pattern to speed up testing.  Testing is usually done at one or two frequencies, and alarm limits are preset into the instrumentation to identify actual flaws versus noise.  Our electronic instruments can then drive an industrial output (I/O) for sorting good parts from bad on the production line.

Crack and flaw testing comparisons

Testing MethodGood for High Speed ProductionRepeatableRequires Operator VisualsRequires Chemicals
Eddy CurrentYesYesNoNo
Dye PenetrationNoNoYesYes
Magnetic ParticleNoNoYesYes
UltrasoundNoYesYesNeed Couplant

 

Eddy current heat treat and material structure testing

In eddy current heat treat and material structure testing, the component under test acts as a transformer core, coupling signals from a primary winding to a secondary winding.  Eddy current instruments drive the primary coil, and detect the output at the secondary coil or set of coils.  Materials with different structures (due to heat treat, case depth, alloy, etc.) will have different magnetic properties or impedance, and the signal transferred will differ. 

Eddy current structure testing is done as a static test.  Eddy current coils are placed over the component under test, or the component is placed inside the coils.  Eddy current structure testing is a comparative test.  Unlike indentation (Rockwell) testing, the method does not yield an actual numeric readout.  Instead, the inspection results signal to the operator that the part under test is different from a known good condition.  Based on the results, the instrumentation will then drive an industrial I/O for sorting good from bad parts on the production line. 

Heat treat and material structure testing comparison

Testing MethodGood for High Speed ProductionLooks at Entire ComponentRequires Operator VisualsDestroys Part
Eddy CurrentYesYesNoNo
Cut, Acid Etch and Visual InspectionNoYesYesYes
Indentation (Rockwell)NoNoNoNo