Eddy current testing uses electromagnetic induction to detect surface and near-surface discontinuities in conductive materials. ET is a surface method — the symbol is placed below the reference line for arrow-side examination or above for other-side. A2.4 Table A6 lists ET under the designation “Electromagnetic.” ET works on any electrically conductive material, including non-ferromagnetic metals like stainless steel and aluminum.
ET Symbol — Arrow Side (Below Reference Line)
Arrow side — surface method
Other side — opposite arrow
Method
How Eddy Current Testing Works
ET works by electromagnetic induction. A coil carrying alternating current is placed near the conductive test surface, generating an alternating magnetic field. This field induces small circulating electrical currents — eddy currents — in the material. When these currents encounter a discontinuity such as a crack, the current flow is disrupted, changing the impedance of the coil.
The instrument detects impedance changes and displays them for interpretation. ET can detect surface cracks, near-surface discontinuities, changes in material properties, and coating thickness variations. Unlike MT, ET works on any electrically conductive material — not just ferromagnetic metals.
Because ET examines the surface from one side, it is classified as a surface method. Per A2.4 §17.5.2 and §17.5.3, the ET letter designation is placed below the reference line for arrow-side examination or above for other-side examination.
Applications
When ET Is Used for Weld Inspection
Tubing and heat exchangers: ET is widely used for in-service inspection of heat exchanger tubes, boiler tubes, and condenser tubing. The probe is inserted through the tube bore to detect wall thinning, pitting, and cracking.
Aerospace: ET is specified for surface crack detection in aerospace welds and components, where non-ferromagnetic alloys (aluminum, titanium, Inconel) are common.
Coating thickness: ET can measure coating thickness over welds and base metal without removing the coating or requiring surface preparation.
Fatigue cracks: ET excels at detecting fine fatigue cracks in thin-walled components during in-service inspection.
D1.1 and ET: D1.1:2025 does not specifically require ET for structural steel weld inspection. The code primarily references VT, MT, PT, RT, and UT. ET is more commonly specified under ASME codes (such as ASME Section V Article 8) and aerospace standards. When an engineer specifies ET on a structural drawing, it is typically for a specialized application beyond the standard D1.1 NDE requirements.
Material Compatibility
ET vs Other Surface Methods
ET works on any conductive material — carbon steel, stainless steel, aluminum, copper, titanium, nickel alloys. This is its key advantage over MT, which only works on ferromagnetic materials.
MT is preferred for ferromagnetic materials in structural work because it is simpler, cheaper, and well-established in D1.1. PT works on any material (including non-metals) but only detects surface-breaking discontinuities. ET fills the gap for conductive non-ferromagnetic materials where near-surface detection is needed without consumables or surface preparation.
ET requires no couplant, no particles, no developer — the probe does not need to contact the surface. This makes ET ideal for automated scanning and high-speed inspection of repetitive geometries like tube arrays.
CWI Exam Tip: ET is a surface method with side significance, just like MT and PT. The symbol goes below the reference line for arrow side, above for other side. Remember that ET works on any conductive material (including stainless steel and aluminum), while MT is limited to ferromagnetic materials only. A2.4 Table A6 lists ET under the name “Electromagnetic.”
Common Questions
ET Symbol FAQ
What does ET mean on a welding drawing?
ET on a welding drawing stands for eddy current testing, a surface nondestructive examination method listed in AWS A2.4:2020 Table A6 under the designation "Electromagnetic" (ET). When you see ET on the reference line of an NDE symbol, it means the weld must be examined using electromagnetic induction. ET detects surface and near-surface discontinuities in any electrically conductive material, including non-ferromagnetic metals like stainless steel and aluminum. Because ET is a surface method, the symbol placement has arrow-side and other-side significance — ET placed below the reference line indicates arrow-side examination, and ET placed above indicates other-side examination.
How does eddy current testing work?
Eddy current testing works by electromagnetic induction. A coil carrying alternating current is placed near the conductive test surface, generating an alternating magnetic field. This field induces small circulating electrical currents (eddy currents) in the material. When the eddy currents encounter a discontinuity such as a crack, the current flow is disrupted, which changes the impedance of the coil. The instrument detects these impedance changes and displays them for interpretation. ET can detect surface cracks, near-surface discontinuities, changes in material properties, and coating thickness variations. Unlike magnetic particle testing, ET works on any electrically conductive material — not just ferromagnetic metals.
When is ET used for weld inspection?
ET is used for weld inspection in specialized applications rather than general structural steel work. Common applications include in-service inspection of tubing and heat exchanger tubes, aerospace weld inspection, detection of fatigue cracks in thin-walled components, and measuring coating thickness over welds. D1.1:2025 does not specifically require ET for structural steel weld inspection — the code primarily references VT, MT, PT, RT, and UT. ET is more commonly specified under ASME codes (such as ASME Section V Article 8) and aerospace standards. When ET is specified on a drawing, the engineer uses the NDE symbol with the ET letter designation per A2.4 Clause 17.
What is the difference between ET and MT?
ET (eddy current testing) and MT (magnetic particle testing) are both surface examination methods, but they differ in physics and material compatibility. ET uses electromagnetic induction and works on any electrically conductive material, including non-ferromagnetic metals like stainless steel, aluminum, copper, and titanium. MT uses magnetic flux and ferromagnetic particles, so it only works on ferromagnetic materials such as carbon steel and low-alloy steel. Both detect surface and near-surface discontinuities. ET requires no surface preparation or consumables (no particles or ink), while MT requires either dry particles or wet fluorescent suspension. ET provides electronic data that can be recorded and analyzed, while MT relies on visual interpretation of particle indications. On drawings, both symbols have arrow-side and other-side significance because both are surface methods.