Eddy current testing (ET) uses a probe carrying an alternating current to induce small electrical currents in a conductive part. Cracks, changes in conductivity and changes in thickness all disturb those currents, and the instrument shows the change. It is fast, needs no couplant, and in most applications works through paint.
- Finds
- Cracks, material and coating thickness, conductivity shifts from heat damage, case depth, heat-treat condition
- Typical use
- Aircraft structure and fastener holes, alloy verification, conductivity survey after a fire or overheat event
What eddy current inspection finds
Surface and near-surface cracks
ET finds fatigue and stress-corrosion cracks at or just below the surface of conductive metals, including aluminum, titanium and nonmagnetic stainless steel, where magnetic particle testing cannot be used. It works through thin paint and coatings, so parts often do not need stripping.
Bolt holes and aircraft structure
Bolt hole and fastener hole inspection, with a rotating or hand-turned probe inside the hole, finds cracks that start in the bore, one of the most common places for fatigue cracks to begin in aircraft structure. Surface probes cover skins, fittings, lugs and the areas around fasteners.
Conductivity, sorting and heat damage
Electrical conductivity changes with alloy and heat-treat condition. Conductivity measurement is used to verify the temper of aluminum parts, to sort mixed material, and to survey structure after a fire or overheat event, when heat may have reduced the strength of the metal without leaving any visible mark.
Coating thickness
ET measures the thickness of nonconductive coatings, such as paint and anodize, on nonmagnetic metals, without damaging the coating.
Heat exchanger, boiler and condenser tubes are a separate application with their own probes and techniques: see eddy current and remote field tubing analysis.
Where it’s used
- Aerospace and defense: aircraft structure, fastener holes, wheels and fittings
- Heat-damage and conductivity surveys after a fire or overheat event
- Alloy and heat-treat verification in manufacturing
- Fine art conservation, where material identification helps the conservator
- Accident investigation
Codes and standards
- Aircraft manufacturers’ NDT manuals and structural repair manuals, which specify most eddy current inspections on aircraft
- ASTM E1004, determining electrical conductivity using the electromagnetic (eddy current) method
- SAE AMS 2658, hardness and conductivity verification of wrought aluminum alloys
- ASTM E376, measuring coating thickness by magnetic-field or eddy current methods
- Customer and prime-contractor specifications
Who does the work
Every Midgard technician who performs eddy current testing holds current certification in that method, issued under a written practice. For most industrial work the written practice follows ASNT’s SNT-TC-1A; for aerospace and defense work it follows NAS410. A Responsible Level III oversees the program: approving procedures and technique sheets, examining and certifying technicians, and answering the technical questions that come up on the job.
If your contract calls for a different scheme, such as ANSI/ASNT CP-189 or NAVSEA requirements, tell us when you ask for a quote.
Training in eddy current testing
Midgard also trains in eddy current testing, along with all the other major methods. Instruction is given at Level I, II and III by certified Level III instructors, to SNT-TC-1A, CP-189, NAS410, NAVSEA or whichever code you certify to, with general, specific and practical examinations at your facility or ours. It suits new hires working toward first certification as well as technicians due for re-examination. See training and certification, or ask about a class.
Common questions
What is eddy current testing used for?
Finding surface and near-surface cracks in conductive metals, especially in aircraft structure and fastener holes, measuring electrical conductivity to verify alloy and heat treatment or find heat damage, sorting mixed material, and measuring coating thickness.
Does eddy current testing work through paint?
In most cases, yes. ET does not need contact with bare metal, so thin paint and coatings can usually stay in place. The procedure accounts for the coating thickness, and thick coatings reduce sensitivity.
Can eddy current testing find heat damage?
Yes. Overheating changes the conductivity of aluminum alloys, often before there is any visible sign. A conductivity survey maps the affected area so the damaged material can be identified and dealt with.
Eddy current or penetrant testing?
Both find surface cracks in nonmagnetic metals. Eddy current is faster, needs little or no surface preparation, works through paint and can find some cracks just below the surface. Penetrant examines a whole area at once and needs no reference standards, but the surface must be clean and bare.
Tell us the part or structure, the material, what you are looking for and the manual or specification you work to. Call 1-844-MIDGARD, email contact@midgardscientific.com, or use the contact page.