10.1 Electromagnetic / Eddy Current Testing

ASNT NDT Level III Basic Examination Exam

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1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Level III Role and Certification Path 1.3 Eligibility, Fees, Scheduling, and Retakes 1.4 Study Strategy and Exam-Day Approach

2 Chapter 2: Materials Fundamentals for NDT

2.1 Ferrous Metals and Iron-Carbon Phases 2.2 Non-Ferrous Metals and Alloys 2.3 Mechanical Properties and Crystal Structure

3 Chapter 3: Manufacturing Processes

3.1 Casting Processes and Cast Discontinuities 3.2 Forging, Rolling, and Forming 3.3 Extrusion, Drawing, and Machining 3.4 Heat Treatment Effects

4 Chapter 4: Welding and Joining Processes

4.1 Fusion Welding Processes 4.2 Weld Metal and HAZ Discontinuities 4.3 Brazing, Soldering, and Mechanical Joining

5 Chapter 5: Discontinuity Classification

5.1 Inherent Discontinuities 5.2 Processing Discontinuities 5.3 Service Discontinuities

6 Chapter 6: Specific Discontinuity Types

6.1 Cracks and Fracture Modes 6.2 Porosity, Inclusions, and Lack of Fusion 6.3 Laminations, Seams, Laps, and Undercut 6.4 Relevant vs Non-Relevant Indications

7 Chapter 7: NDT Method Selection and Inspection Planning

7.1 Method Selection Principles 7.2 Surface vs Volumetric Methods 7.3 Inspection Planning and Flaw Expectations

8 Chapter 8: Ultrasonic and Radiographic Testing

8.1 Ultrasonic Testing Principles and Applications 8.2 UT Limitations, Calibration, and Evaluation 8.3 Radiographic Testing Principles and Applications 8.4 RT Limitations, Image Quality, and Safety Basics

9 Chapter 9: Magnetic Particle, Liquid Penetrant, and Visual Testing

9.1 Magnetic Particle Testing 9.2 Liquid Penetrant Testing 9.3 Visual Testing

10 Chapter 10: Electromagnetic, Leak, Acoustic Emission, and Infrared Testing

10.1 Electromagnetic / Eddy Current Testing 10.2 Leak Testing 10.3 Acoustic Emission Testing 10.4 Thermal / Infrared Testing

11 Chapter 11: Specialty Methods and Method Comparison

11.1 Neutron Radiography 11.2 Magnetic Flux Leakage 11.3 Method Comparison and Supplemental Examinations

12 Chapter 12: SNT-TC-1A Recommended Practice

12.1 SNT-TC-1A Overview and Written Practice 12.2 SNT-TC-1A Training and Experience 12.3 SNT-TC-1A Examinations and Recertification

13 Chapter 13: CP-189 Standard Practice

13.1 CP-189 Overview and Scope 13.2 CP-189 Mandatory Requirements 13.3 SNT-TC-1A vs CP-189 Comparison

14 Chapter 14: CP-105 and NDT Training Programs

14.1 CP-105 Purpose and Topical Outlines 14.2 Developing and Approving NDT Training 14.3 OJT, Mentoring, and Instructor Oversight

15 Chapter 15: Building and Managing Certification Programs

15.1 Building Qualification and Certification Programs 15.2 Administering Examinations and Evaluating Competency 15.3 Program Audits and Managing Certifications

16 Chapter 16: Codes, Standards, Procedures, and Quality

16.1 Interpreting Codes, Standards, and Specifications 16.2 Developing and Approving NDT Procedures 16.3 Acceptance Criteria and Quality Control 16.4 External Audits and Inspection Documentation

10.1 Electromagnetic / Eddy Current Testing

Key Takeaways Explain This Topic AI

Eddy current testing (ET) inspects electrically conductive materials by inducing alternating currents and measuring impedance changes from flaws, geometry, lift-off, and conductivity.

Inspection frequency sets skin depth: higher frequency emphasizes surface cracks; lower frequency increases penetration for near-surface and thin-wall tubing work.

Lift-off (probe-to-surface gap) strongly affects signal amplitude and must be controlled or compensated; coating thickness and probe wobble are classic lift-off sources.

Primary ET roles include surface crack detection, heat-exchanger tubing inspection, and conductivity sorting of nonferromagnetic alloys; ferromagnetic parts need special techniques.

Remote-field testing (RFT) and ACFM are related electromagnetic methods: RFT for ferromagnetic tubing wall loss, ACFM for surface cracks with less cleaning than wet MT/PT in some applications.

Last updated: July 2026

10.1 Electromagnetic / Eddy Current Testing

Quick Answer: Eddy current testing (ET) is an electromagnetic method for electrically conductive materials. An alternating current in a coil induces eddy currents in the part; cracks, thinning, lift-off, and conductivity/permeability changes disturb those currents and shift coil impedance . Frequency sets skin depth ; ET is excellent for surface cracks and tubing but weak for deep volumetric flaws in thick sections.

ET sits on the ASNT NDT Level III Basic method list next to MT, PT, UT, and RT. You are not certifying as an ET Level III here, but you must select ET correctly, explain its physical limits, and know when related techniques such as remote-field testing (RFT) or alternating current field measurement (ACFM) enter the picture.

Conductive Materials Only

Eddy currents require a closed electrical path in the test object. Practical ET materials include:

• Aluminum, copper, brass, bronze, and titanium alloys
• Austenitic stainless steels (nonmagnetic or weakly magnetic grades used in tubing and vessels)
• Carbon and low-alloy steels (with stronger permeability effects and more complex signals)
• Conductive coatings and clad layers when the procedure is designed for them

Nonconductors (most plastics, ceramics, glass, composites without conductive paths) do not support conventional eddy-current flaw detection. Conductive fibers or foils in composites can produce specialized responses, but that is not the Basic default model.

Exam trap: “Metal” is not enough. ET needs electrical conductivity . Magnetic permeability in ferromagnetic steels multiplies complexity: permeability variations can dominate impedance and mask small cracks unless frequency, magnetization, and coils are chosen carefully.

Physics: Eddy Currents and Impedance

A coil driven with alternating current produces a changing magnetic field. In a nearby conductor, Faraday’s law drives circulating eddy currents . Those currents create their own magnetic field that opposes the coil field (Lenz’s law) and loads the coil electrically. Instruments display this as impedance —a combination of resistance and inductive reactance—often plotted on an impedance plane (horizontal and vertical channels after phase rotation).

Anything that changes eddy-current paths changes impedance:

• Surface-breaking or near-surface cracks interrupt or lengthen current paths
• Wall thinning or corrosion reduces the amount of metal carrying current
• Conductivity differences (alloy, heat treat, cold work) change current density for a given field
• Permeability changes (ferritic content, residual magnetism, stress) strongly affect ferromagnetic responses
• Lift-off and fill factor change coupling between coil and metal

Level III selection language: ET is a surface / near-surface electromagnetic method with property-measurement capability—not a deep volumetric replacement for UT or RT in thick forgings.

Frequency and Skin Depth

Eddy current density is highest at the surface and decays with depth. Standard depth of penetration (skin depth δ) is the depth at which current density falls to about 1/e of the surface value. Conceptually:

• Higher frequency → smaller skin depth → surface emphasis (fine cracks, shallow layers)
• Lower frequency → larger skin depth → more subsurface / wall information (within practical limits)

Skin depth also depends on conductivity and permeability . High-conductivity copper shows shallower penetration at a given frequency than lower-conductivity alloys. High-permeability carbon steel has very small skin depth at common ET frequencies, which is one reason surface crack ET on steel often uses specialized setups and why ferromagnetic tubing inspection often moves toward RFT or other techniques for mid-wall and OD defects.

Practical multi-frequency work: Many procedures use more than one frequency to separate lift-off, conductivity, and flaw signals on the impedance plane. Basic exam items rarely demand calculation of δ, but they do expect the qualitative trade-off: raise frequency for surface sensitivity; lower frequency for deeper sensing—until noise, coil design, or material physics stop you .

Lift-Off, Fill Factor, and Geometry

Lift-off is the gap between coil and surface (or the effective gap created by paint, scale, or probe tilt). Lift-off almost always produces a strong impedance shift. Uncontrolled lift-off is a leading cause of false calls and missed cracks. Controls include:

• Rigid fixtures, spring-loaded probes, and wobble-resistant designs
• Phase rotation and mix channels that suppress lift-off loci
• Calibration on the same coating/paint condition as production parts when coatings remain

For internal tubing probes , fill factor (how fully the coil fills the tube ID) plays a similar role: undersize probes reduce sensitivity and alter phase relationships.

Geometry limits ET everywhere edges, holes, thickness changes, and welds create baseline impedance shifts. Scanning strategies and filters must distinguish geometric signals from flaw signals —the ET analog of nonrelevant MT leakage at section changes.

Coils and Inspection Modes (Overview)

• Coil / mode Concept Typical use
• Absolute Single sensing coil vs reference Conductivity, thickness, large property shifts
• Differential Two coils oppose; common-mode rejection Small cracks and local pits along a scan
• Reflection (driver/pickup) Separate drive and receive Optimized SNR and phase control
• ID bobbin (tubing) Coil(s) inside tube Heat-exchanger / condenser tube ID scanning
• Encircling Coil around bar/tube OD High-speed bar, wire, tube mills
• Surface pencil / array Small footprint or multi-coil Aircraft skins, fastener holes, welds

Arrays and multi-coil probes improve coverage and reduce scan time on large surfaces; Basic-level recognition is enough—detailed array processing is method-exam territory.

Signature Applications

1. Surface crack detection — Aircraft skins, fastener holes (with rotating probes), welds in conductive alloys, and critical machined surfaces. ET can find tight surface cracks that might be missed if PT cleaning is imperfect, and it often needs less consumable mess than PT. Orientation still matters: coil current paths and scan direction must interact with expected crack orientation.

2. Tubing inspection — Power-plant and process heat-exchanger tubes are classic ET territory. Bobbin probes detect pits, cracks, and wall loss from the ID. Codes and owner procedures define calibration standards (ASME-style tube standards with artificial flaws), reporting thresholds, and expansion criteria. Ferromagnetic tubing may require RFT , partial saturation ET, or other electromagnetic variants for effective wall assessment.

3. Conductivity sorting — Portable conductivity meters (often %IACS) sort aluminum alloys, verify heat-treat condition trends, and screen mixed stock. Conductivity is not a complete material certification by itself, but it is a fast NDT property check tied to composition and temper.

4. Coating/thickness and other property work — With proper calibration, ET estimates nonconductive coating thickness over metal (lift-off calibration) and can support other specialized measurements.

Related Methods at Overview Level: RFT and ACFM

Remote-field testing (RFT): A low-frequency electromagnetic tubing technique especially useful for ferromagnetic tubes. A separated exciter and detector sense wall loss and some discontinuities via the remote-field zone through the tube wall. Think “ferromagnetic tubing wall assessment” when standard high-frequency ID ET is limited by permeability and skin effect.

ACFM: An electromagnetic surface-crack technique that models magnetic field disturbances above cracks; often marketed for inspection through coatings with less surface preparation than wet MT/PT. Still an electromagnetic surface method—not a volumetric deep-flaw tool.

Strengths and Limitations (Exam Table)

• Strengths Limitations
• Fast, often non-contact or light-contact scanning Only conductive materials
• High sensitivity to surface cracks Limited depth ; not for deep mid-wall flaws in thick sections

No large radiation exclusion zone; no large magnetic yoke fields Lift-off , geometry, and edges create strong signals

Tubing automation and conductivity sorting Ferromagnetic complexity; permeability noise

Immediate electronic readout; recordable channels Couplant not required, but probe access and hole/edge geometry still limit coverage

Can work through thin nonconductive coatings with control Calibration standards and phase/mix setup skill required

Level III Takeaway for Method Selection

Choose ET when the part is conductive , the concern is surface or near-surface (or thin-wall tubing), and you need speed, automation, or conductivity data. Do not choose ET as a substitute for UT/RT on thick volumetric integrity problems. For ferromagnetic heat-exchanger tubes, plan for RFT or specialized ET , not naive high-frequency bobbin practice. Document frequency strategy, calibration standards, acceptance thresholds, and how lift-off and geometry are controlled—those are the procedure hooks Basic exam scenarios revolve around.