Comprehensive Study Notes on Eddy Current Testing (CND)

Introduction to Eddy Current (EC) Testing

Chapter 5 of the Non-Destructive Testing (C.N.D.) course, specifically within the MI 21 module taught by OUERGHUI A., focuses on the Eddy Current (EC) method. This non-destructive testing technique is strictly applicable to all electrically conductive materials. Its primary purpose is to identify defects that are either surface-breaking (débouchants) or located just beneath the surface (sous-jacents). Beyond flaw detection, the method is extensively utilized for measuring the electrical conductivity of materials and determining the thickness of various coatings.

Fundamental Principles of Eddy Currents

The physical principle involves a solenoid coil (winding) through which an alternating current is passed. This current flow generates an alternating magnetic field around the coil. When this coil is brought into proximity with an electrically conductive material, the alternating magnetic field induces electrical currents within the material, known as Eddy Currents (courants de Foucault). These currents manifest their presence within the material through heating, a phenomenon known as the Joule effect. In the context of CND applications, the core logic relies on the properties that link the intensity of these Eddy Currents to the specific characteristics of the material being inspected, such as variations in shape, thickness, electrical conductivity ($\sigma$), magnetic permeability ($\mu$), and the presence of defects.

The initial winding, carrying an excitation current ($I_1$), produces an alternating magnetic field at a specific point, denoted as point A in the course material. The intensity of this field depends on several factors: the magnitude of the current ($I_1$), the number of turns (spires) in the coil, the diameter of these turns, and the overall length of the winding. When a metallic plate is placed at point A, the variation in the magnetic field induces Eddy Currents in the plate according to Lenz's Law. These induced currents, oscillating at the same frequency as the inductor current ($I_1$), generate their own magnetic field that opposes the primary inductor field. This interaction results in the winding being traversed by a modified current ($I_2$), which corresponds to a measurable change in the coil's electrical impedance.

Defect Detection and Visualization

At the site of a fissure or defect, the intensity and spatial distribution of the Eddy Currents are modified. This disruption leads to a change in the secondary magnetic field ($H$), which is ultimately translated into a change in the impedance of the inspection coil. Observation of these changes is performed by visualizing the variations in electrical impedance on an oscilloscope. By comparing the signal from a healthy zone (zone saine) to a defective zone (zone défectueuse), technicians can identify anomalies. Consequently, any apparatus used for Eddy Current testing must include three essential components: an alternating current power supply of a known frequency, a circuit configured to measure impedance, and a display system to visualize impedance variations.

Excitation and Frequency Considerations

The excitation phase in Eddy Current CND consists of subjecting the part to a time-varying magnetic field to induce the currents. This is typically achieved using a sinusoidal wave with a frequency range that can vary from a few Hertz ($Hz$) to several Megahertz ($MHz$). The selection of the frequency is critical because it conditions the depth of penetration of the induced currents into the material. The sensitivity of the control method is closely tied to the nature of the application and the specific products being tested. The difference in impedance ($Z - Z_0$) is only exploitable for a given application within precise limits of the excitation frequency.

Factors Influencing Impedance Variations

Several factors can influence the resulting impedance variations during an Eddy Current test. These include the relative magnetic permeability ($\mu$) of the material, the electrical conductivity ($\sigma$) of the material, the frequency ($f$) of the current in the coil, and the distance ($D$), often referred to as lift-off, between the coil and the component surface. During a standardized examination, the frequency ($f$) and distance ($D$) are usually kept constant. In this scenario, the evaluation focuses on the fluctuations of ($\mu$) and ($\sigma$), as these two parameters generally depend on the chemical composition and properties of the material.

Perturbation, Revelation, and Specific Applications

The trajectory of induced currents is perturbed by anomalies such as geometric variations or changes in the electromagnetic characteristics of the surface. This local perturbation of current lines leads to a modification of the induced field, causing a change in sensor impedance. This principle results in three distinct types of applications:

1. Detection of surface or slightly sub-surface defects: In this scenario, where coating thickness ($e = 0$) and material properties ($\mu, \sigma$) are constant, the impedance change is a function of defect dimensions: $Z - Z_0 = f(p, L, l)$, where ($p$) is the depth of the defect, ($L$) is its length, and ($l$) is its width.

2. Measurement of coating thicknesses: This applies to paint or electrolytic deposits, as well as detecting the thinning of tube walls. Here, the variation is defined as $Z - Z_0 = f(e)$, where ($e$) is the coating thickness ($e \neq 0$).

3. Material sorting (Tri de nuances): This involves sorting materials based on differing electrical conductivity, which may vary within the same alloy due to heat treatment or the inclusion of different alloy grades. The relationship is expressed as $Z - Z_0 = f(\mu, \sigma)$, where ($\mu_0$) represents the reference permeability.

Skin Effect and Depth of Penetration

The distribution of Eddy Currents under a flat surface follows the skin effect. Approximately $63\%$ of the induced currents pass between the surface and a specific depth denoted as ($\delta$). To achieve penetration depths on the order of millimeters ($mm$), frequencies between $10\,Hz$ and $10^5\,Hz$ are typically used, depending on the material. The transcript provides values for resistivity ($\rho$) in $\Omega \cdot m$ and relative permeability ($\mu$) for various materials:

• Graphite: $\rho = 1000 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1$

• Austenitic Stainless Steel: $\rho = 70 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1$

• Brass (Cu/Zn 30): $\rho = 7.1 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1$

• Aluminum: $\rho = 2.8 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1$

• Copper: $\rho = 1.75 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1$

• Carbon Steel (with saturation field): $\rho = 17 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 100$

• Extra Mild Steel: $\rho = 10 \times 10^{-8}\,\Omega \cdot m$, $\mu \approx 1000$

Implementation Equipment and Hardware

The equipment required for Eddy Current CND includes sophisticated digital multi-frequency generators capable of delivering frequencies (multiplexed or not) adjustable from $1000\,Hz$ to $4\,MHz$. Visualization is managed by multi-channel thermal graphic recorders that track the EC signals. Data storage is handled via digital magneto-optical disks. For probe manipulation, adjustable-speed probe push-pull devices (tireur-pousseur) are employed. Furthermore, a diverse array of sensors is used, including axial probes, rotating probes, and encircling coils, which may or may not include saturation devices. Each sensor is chosen based on the geometry and material nature of the inspected part.

Industrial Applications and Operational Characteristics

Eddy Current testing is widely used for measuring conductivity, searching for surface-breaking cracks, and comparing batch parts against a reference standard (covering material sorting, heat treatment verification, dimensions, and defects). It is used for external probe inspection of bars, tubes, and profiles, as well as internal probe inspection of tubes in industrial components like heat exchangers, condensers, and evaporators, particularly for detecting wear at baffle plates (chicanes).

The advantages of the method include high detection sensitivity, high inspection speed, the availability of probes adaptable to the product shape, and the ability to record results for long-term monitoring. However, the method has significant limitations: it is restricted to conductive materials, offers low penetration (only a few millimeters), and is highly sensitive to disruptive phenomena such as cold working (écrouissage) or surface deposits. Use of a specific calibration standard (étalon) for each control application is mandatory.