Surface Eddy Current is applied for detection of cracks on the
surface of conductive materials. The method is very sensitive for detection
of tight cracks. Both non-ferromagnetic (eg stainless steel, inconel, Al
Bronze etc.) and magnetic materials (eg carbon steel) can be
inspected.
Surface ECT vs Liquid Penetrant Testing
Both surface ECT and PT are surface inspection techniques used for detection surface breaking cracking. ECT has basic advantages over Liquid Penetrant Testing in certain applications:
- ECT is significantly more sensitive than PT for detection of tight cracks, eg. SCC in stainless steels
- ECT can inspect through paint coatings
- For inspection of small areas, ECT is much faster. PT is slow because
of the long dwell times.
ECT Theory
Eddy Current method is based on the principle of measuring changes in the impedance of a electromagnetic coil as it is scanned over a surface of conductive material.
The test is performed by a electromagnetic coil that is placed over a conductive material. The coil produces a magnetic field that is induced in the material. To counter the coil's primary magnetic field, eddy currents are produced in the material. Eddy currents produce a secondary magnetic field (H' B) to oppose the coil's primary magnetic field (HB). When the coil is scanned over a defect, the secondary magnetic field is distorted thereby changing the loading on coil. Changes in coil loading directly affect the coil impedance. These changes in coil impedance are related to the defect.
Fig 1. Eddy Current Principle. (a) ect coil in air. The coil produces a
magnetic field - HA. The coil impedance is Zair (b) ect coil over a conductive material.
Eddy currents are produced in the material that oppose the magnetic field of
the coil. The coils magnetic field changes from HA to HB
.The impedance of the coil changes from Zair to ZB
along the Lift-Off Line. (c) ect coil over the crack. Eddy currents are
distorted by the crack, this changes the magnetic field of the eddy currents
from H'B to H'C thereby changing the loading on the
coil. The coil impedance changes to Zcrack.
ECT Depth of Penetration
Flaw detection with eddy currents is limited to the penetration depth.
Penetration depth is inversely proportional to the square root of
conductivity, frequency and permeability. Penetration in non-ferromagnetic
material can be increased by lowering the frequency. For most applications, the penetration
depth in non-ferromagnetic material is limited to approximately 5 mm (0.20
inches). In case of ferromagnetic materials, such as carbon steel, the
penetration depth is extremely shallow because of high permeability.
Inspection of ferromagnetic material is therefore limited to surface flaws
only.
Probes
Surface ECT is performed using
probes that include pencil probes, spot probes, cross-axis probes etc.
Instrument
Surface ECT is normally performed with portable ECT machines, eg. Phasec
2200, Nortec 1000, Nortec 2000, etc.
Tubing ECT is done using advanced computer-controlled equipment eg. TC5700,
MIZ-27, MAD4D etc.
Applications
Applications of surface ECT include
- detection of SCC in stainless steel pipes
- detection of cracks in SS vessels and tanks
- detection of cracks in turbine blades
- detection of cracks in aircraft structures eg. Cessna Caravan Service Bulletin 2003 CAB03-6, CAB02-2
- inspection of aerospace structures
- detection of cracks in bolt holes
Reference:
A. S. Birring and G. A. Marshall, "Eddy
Current Testing in the Petrochemical Industry,"
Materials
Evaluation, November 2003.
NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944 Fax: 281-488-8485
