Published on 04-Jun-2024

Eddy Current Testing - A Complete Guide

Eddy Current Testing - A Complete Guide

Table of Content

The emergency landing of Aloha Airlines Flight 243 after an unprecedented decompression caused by the breakage of the fuselage is a significant event in the history of aviation. The most probable cause of this incident was multiple site fatigue cracking that negated the safety aspects of the fuselage. The maintenance team failed to detect these failures during the inspection of the flight. How can such cracks be detected? 

Finding a tiny crack inside such a humongous structure like an aircraft is almost impossible. But, with the help of Eddy current inspection, it is possible. Eddy Current Testing is one of the most widely used Non-destructive Testing techniques to detect flaws or defects inside a material. It involves inserting specially designed electronic probes powered by alternating current close to the material.

The alternating current produces a fluctuating magnetic field through electromagnetic induction and generates eddy currents nearby. The variations in this eddy current are analyzed to detect any flaws present. Variations in electrical conductivity, the test part's magnetic permeability, or the existence of any discontinuities will affect the eddy current, which will vary the measured current's phases and amplitude in turn. Defects are found by interpreting the changes as they are displayed on a screen.

Fundamentals of Eddy Current Testing

Fundamentals of Eddy Current Testing

The basic principle behind eddy current inspection is Electromagnetic induction. An alternating magnetic field forms around a conductor, such as a copper coil, when an alternating current passes through it. The magnitude of this field varies according to the changes in the alternating current.

This fluctuating magnetic field around the coil passes through the material. According to Lenz's Law, when the coil is then brought close to another electrical conductor, it produces an eddy current in the conductor. The eddy current, in turn, creates a magnetic field of its own. This "secondary" magnetic field opposes the "primary" magnetic field. Any changes in the conductivity of the material being studied, such as near-surface flaws or variations in thickness affect the intensity of the eddy current.

Detection of this change in the eddy current using either the primary coil or the secondary detector coil forms the foundation of the eddy current testing method. A material's permeability determines how easily it can be magnetized. The depth of penetration decreases with an increase in permeability. At the surface, eddy current density is higher, and defect sensitivity is highest; these characteristics diminish as depth increases.

The metal's "permeability" and "conductivity" affect the rate at which the value decreases. The conductivity of the material influences the depth of penetration. Metals with high conductivity have stronger eddy current flow at the surface. The alternating current's frequency can be changed to control the depth of penetration; the lower the frequency, the deeper the penetration. As a result, deep faults can be detected using low frequencies, whereas near-surface imperfections can be found using high frequencies. 

Eddy Current Testing Procedure