Target of Investigation

Eddy current testing (ECT) is used to detect cracks in steel elements. ECT can only detect surface-breaking cracks because the magnetic properties of steel limit the penetration depth of eddy currents into the material. The technology is commonly applied to detect cracks in welds. However, special probes must be used because of the changing weld geometry and complex magnetic and microstructural properties associated with the weld and surrounding heat-affected zone. 

Description

ECT works by scanning a small electromagnetic probe across the surface of a material. The signal produced by the probe is displayed in real time on an ECT screen, and changes in the signals are produced when the probe scans across a discontinuity in the material. Inspectors interpret the changes in the signal to identify cracks and other defects. 

ECT is used to detect surface or near-surface cracks in a wide variety of metal components, such as airplane landing gears, turbine engines, heat-exchanger tubing, pipes, and rods. For tunnels, ECT is applied to detect surface-breaking cracks in steel components. 

Physical Principle 

Figure 1 shows the basic principle of ECT. Eddy currents are generated by a probe consisting of one or more wire coils excited with alternating currents at a regulated frequency. Alternating currents result in a dynamic expanding and collapsing magnetic field surrounding the coil. When an electrically conductive material is placed within the magnetic field, eddy currents are induced in the material through electromagnetic induction. The currents flowing in the material create a secondary magnetic field that opposes the primary magnetic field of the coil, affecting the impedance of the coil. The presence of a discontinuity, defect, or irregularity in the grain structure of the material disrupts the flow of the current in the material, and thus, changes in the impedance of the coil are produced. Impedance changes in the coil are analyzed and interpreted to detect defects in the material. 

Probes must be oriented such that a crack is perpendicular to the flow of currents in the material so the crack interrupts the current flow. Current flow in the material is parallel to the flow of currents in the probe coil, but these currents flow in opposite directions. In other words, the path of currents at the surface of the material is a projection of the coil with opposing current direction. 

The sensitivity of ECT depends on the eddy current density at the flaw location. Eddy current density decreases with increasing depth into the material being tested. The depth of penetration decreases with increasing frequency, conductivity, and magnetic permeability. 

The density of currents in a conductive material can be described by the standard equation for depth of penetration (figure 2), which determines the depth at which the density of currents is reduced to about 37 percent relative to its density at the surface. 

Where: 

δ = standard depth of penetration. 

ƒ = excitation frequency. 

μ = magnetic permeability. 

σ = electrical conductivity. 

When attempting to locate flaws, a frequency is selected that places the expected flaw depth within one standard depth of penetration into the material; this ensures the density of current in the material is sufficient to affect the impedance of the coil and produce a flaw indication. For ferromagnetic materials (e.g., steel), permeability values are high; as a result, the standard skin depth is very small, typically less than 1–3 mm (0.04–0.11 inches) depending on the frequency used. ECT in steel can detect only surface or near-surface cracks. Nonferromagnetic materials (e.g., aluminum) have a relative low permeability; therefore, skin depths are larger and subsurface flaws may be detected. 

Density of a current in a material depends on the intensity of the magnetic field inducing the current. Magnetic fields attenuate exponentially in air, so the distance from the probe coil to the surface of the material under test is an important test parameter. This effect is referred to as the “lift-off factor”—the loss in signal strength resulting from the distance between the probe coil and the surface of the material being tested. In general, uniform and very small lift-off values are preferred for achieving better flaw detection sensitivity. For steel bridges, coating on the steel causes sensor lift-off, decreasing signal intensity relative to uncoated steel. This effect can be mitigated by proper calibration that accounts for the lift-off factor on coated steel.^(2,3) 

Data Acquisition 

Data acquisition for ECT includes an ECT instrument and an appropriate handheld probe. Relatively low-cost, battery-operated, portable instruments are available, making the technology practical for use in the field (figure 3). Test results are displayed on the ECT instrument in real time for interpretation in the field. 

ECT probes are produced in a variety of configurations suitable for different applications. Several typical probes are shown in figure 4. Probes with right-angle heads are designed for inspecting the inside of bolt holes. Very small probes are designed for applications where access is limited. For inspection of welds, a differential probe with two probe coils is typically used; the differential probe reduces the effect of material property variations on the probe output. Figure 5 shows a pencil ECT probe being used to inspect the web of a girder. 

Data Processing 

Minimal data processing is required for ECT because the impedance signal is analyzed directly. Analog frequency and time-based filters are commonly used to improve signal quality. 

Data Interpretation 

An operator completes the interpretation of ECT data in real time during an inspection. The signal output is analyzed as the probe is scanned across the surface of the material. The analysis determines if signal variations represent a relevant indication, such as a crack, or if signal variations represent nonrelevant indications, such as probe lift-off, noncrack surface irregularities, or material property variations. 

Data are plotted using an impedance plane display or a time-series display. An impedance plane display plots the individual components of the impedance signal (reactance and resistance) as x–y coordinates. A time-series display plots the individual components of the impedance separately against time, sample number, or distance. Figure 6 shows an example of a time-series display in which the probe resistance is plotted against distance. The signal shown in figure 6 resulted from scanning the probe over a specimen with three notches of different depths and illustrates the signal displacement observed by the operator as the probe is scanned across each notch. 

Signal output can be dependent on the movement of the probe over the specimen surface. Irregular surfaces or inconsistent movement of a probe (e.g., lifting the probe off the surface or tilting the probe) can produce signals that may be interpreted as defects. Skilled operators can consistently scan specimens and use judgment to determine the nature of the output signal. 

Advantages 

Advantages of ECT include the following: 

• Instrumentation is low cost and portable. 
• Surface preparation is minimal. 
• Coating removal is not required, unlike other crack detection technologies, such as dyepenetrant testing, ultrasonics, or magnetic particle testing. 
• Variations in paint thickness are detectable. 

Limitations 

Limitations of ECT include the following: 

• Only surface-breaking cracks are detectable. 
• Magnetic properties of weld materials can influence results. 
• Testing requires a skilled operator to obtain reliable results. 

References 

1. García-Martín, J., Gómez-Gil, J., and Vázquez-Sánchez, E. (2011). “Non-Destructive Techniques Based on Eddy Current Testing.” Sensors, 11(3), pp. 2,525–2,565, MDPI, Basel, Switzerland. 
2. American Society of Nondestructive Testing. (2005). American Society of Nondestructive Testing (ASNT) Handbook, 5, ASNT, Columbus, OH. 
3. Hagemaier, D.J. (1990). Fundamentals of Eddy Current Testing, American Society for Nondestructive Testing, Columbus, OH.