The ABCs of Eddy Current ...

The ABCs of Eddy Current Weld Inspection
The advantages of eddy current inspection include its effectiveness in detecting surface-breaking flaws and ability to be used on wet surfaces
Currently, several nondestructive examination (NDE) techniques are used to inspect welds for defects: magnetic particle inspection (MP), liquid penetrant inspection (LP), ultrasonic testing (UT), X-ray testing (RT), and eddy current testing (ET). All have their advantages and disadvantages.
  • Magnetic particle is straightforward and relatively easy to use but is not good for welds that have coatings or wet surfaces (surfaces must be dried first).
  • Ultrasonic testing is good for finding subsurface defects but is operator dependent.
  • Dye penetrant is good for surface cracks, but it also requires a dry surface and is operator dependent.
  • X-ray is good for subsurface defects, but the radiation hazard requires additional safety considerations not necessary for other techniques.
  • Eddy current is good for detecting surface-breaking defects, can detect these defects through fairly thick coatings (up to 2 mm), and can be used on wet surfaces (even underwater), but several scans of an individual weld must be performed to ensure a defect is not missed. Eddy current is also an operator-dependent technique.

As eddy current is best used for detecting surface-breaking cracks, its most practical applications relate to the in-service inspection of welded structures that are subject to a cyclical loading that can lead to fatigue crack propagation in critical welded areas.

Fig. 1 - Schematic showing how sinusoidal AC voltage is applied across the eddy current probe or inspection coil.
How Eddy Current Works
In eddy current testing, a sinusoidal AC voltage is applied across the eddy current probe or inspection coil - Fig. 1. This coil creates an electromagnetic field, which in turn causes current flow in the surface of the material being inspected. (The circular nature of these currents has been compared to the eddies in a stream or river, hence the term "eddy current.") When the coil or probe is scanned across the material surface, changes in the material's physical properties, i.e., geometry, temperature, conductivity, material type, flaws, etc., affect the current flow generated by the electromagnetic field induced in the material by the probe. These changes reflect back to the probe. If the voltage response of the eddy current probe is monitored, then changes in voltage amplitude and phase angle shift can be used to show changes in material properties. These changes in magnitude and phase angle are displayed on what is known as an impedance plane display.

Figures 2A and B show typical results from a simple absolute probe (single-winding coil) as it is scanned over surface-breaking flaws in a conductive material. The increasing magnitude of the signal relates to deeper flaws. The lift-off signal is associated with the signal generated by lifting the probe from the material. Simple absolute coils can be limited by the orientation of the flaw.

Flaws are most detectable when the eddy current path is crossed at right angles by the flaw - Fig. 3. Special probes designed for weld inspection help limit this problem.

Figure 4 shows the lift-off response of various thicknesses of nonconductive coatings on a ferrous test block. A clear relationship between the magnitude of the shift of the balance point and the coating thickness can be seen. This allows for assessment of the coating thickness on the actual structure to be inspected, which enables the operator to adjust the instrument gain level to normalize flaw signal response. This, in turn, allows use of alarm gates for go/no go testing and flaw sizing between structures with different coating thicknesses.

Figure 5 shows the typical impedance plane display flaw indications in a Hocking WeldScan test block. The positive y-axis indications are generated with the probe in one orientation. The negative y-axis indications occur when the probe is rotated 90 deg. The indications are greater in magnitude for deeper flaws. (Note: WeldScan probes are differential probes with two orthogonal coils. This design also acts to eliminate signals associated with changes in the material properties in the heat-affected zone as well as minimizing the lift-off effect encountered when inspecting rough welds.) Normally, crack depth can be assessed to approximately 4.5 mm in depth. Beyond that, the signals level out. Crack length can also affect the response if the crack length is smaller than the probe's electromagnetic field area. Crack branching can give an indication the crack is deeper than it actually is.

Fig. 2 - A,B - Typical results from a simple absolute probe as it is scenned over surface breaking flaws in a conductive material.
Practical Applications
There are many practical applications of weld inspection using eddy current techniques. These include the following:

Offshore structures. By far the widest use of eddy current weld inspection occurs in the offshore industry. Offshore structures such as drilling platforms are subjected to cyclical loads twice daily (tides) and, more unpredictably, by severe weather. Fatigue crack propagation can occur topside or underwater and periodic inspection of critical weld areas is required. Frequently, topside inspections are only possible by rope access. Underwater inspections and repairs are often done by divers at acceptable depths and in acceptable water environments. At more extreme depths in cold water areas, remote operated vehicles (ROVs) have been developed to carry out surface preparation, eddy current weld inspection, weld repair, and repair inspection in one unit.

Bridges, cranes, traffic signals. All of these structures are subject to cyclical loading on their welded structures, as well as weather-related loading and weight loading in cranes and bridges. Cantilevered traffic arms are a relatively recent application that has resulted from the failure of such arms because of weather-induced weld fatigue. Such failures have had fatal results.

Ships. Ships are subject to bending and torsional moments in day-to-day use. Extreme weather conditions, shifting loads, and grounding can lead to extreme loading of the superstructure and overstress critical welded joints.

Submarines. Eddy current is often used in the inspection of new welds and for the detection of fatigue cracks in welded joints. Submarines are subject to the same loading as surface ships plus have the added stress of the cyclical loading that accompanies pressure changes associated with submerged operations.

Amusement park rides. Another relatively recent application is the inspection of amusement park rides. Again, these rides are subject to the same cyclical loading as bridges and cranes. Due to public demand, theme parks in warmer climates are often required to be in operation 364 days a year. Every day a ride is unavailable due to maintenance closure a negative public reation is encountered, even if the closure is for safety-critical inspection and maintenance. Eddy current techniques for the inspection of track and cars represent a significant reduction in manpower and downtime, resulting in more cost-effective and higher quality maintenance and inspection along with more paying hours of operational attractions.

Fig. 3 - A flaw is easiest to detect when it crosses the eddy current path at right angles.
Law enforcement. Among the more unconventional uses for eddy current weld inspection are law enforcement applications. The technique can be used to identify welded joints in automobiles where you would not expect to find them. So called "cut and shut" welds across a vehicle's chassis can easily be detected using eddy current technology as well as the welded area surrounding false VIN plates. This can be done without damage to the paintwork on the vehicle being inspected. Also, specially designed probes have been used to detect the removal of material from prison bars in penal institutions. However well the cut is hidden with dirt, shoe polish, or paint, the eddy current technique will ignore the filler material and detect the cut in the bar.
Fig. 4 - Lift-off response of various thicknesses of nonconductive coatings on a ferrous test block.
Rotary eddy current techniques. Special rotary probes were developed to aid in the inspection of welded steel engineering structures after the Kobe earthquake to allow construction engineers to evaluate structures that were under construction during the quake. Rotary techniques can detect flaws in any orientation, allowing the contractor to assess quickly whether cracks in the welds had occurred and to effect repair or demolition as necessary.
Fig. 5 - Typical impedence plane display of flaw indications in a test block.
Summary of Benefits
Eddy current inspection offers several benefits over competing NDE inspection techniques. Traditionally, a suspect weld is stripped, cleaned, and a magnetic particle or liquid penetrant inspection is performed to detect any surface-breaking cracks. Beyond offering effective surface-breaking flaw detection for various welded structures, the eddy current technique offers a higher flaw detection "hit rate," reduced costs and down time, lower inspection consumables cost, minimal or no surface preparation, and the capability to be done underwater. In many applications MP or LP inspection can be done to confirm eddy current results to give more confidence to operators new to the eddy current technique. Additionally, developments in ceramic and stainless steel probe tip design have greatly increased the wear resistance of eddy current probes, significantly enhancing the cost effectiveness of the eddy current technique. Special probe designs for aluminum and nonferritic stainless steel have also extended the usefulness of eddy current inspection to applications where magnetic particle inspection is not an option. Lastly, water-cooled probe designs have been developed for use in the inspection of welds in high-temperature piping and vessels, eliminating the need to take them off-line in order to perform the inspection.
Based on a story from Agfa NDT Inc. Krautkramer Ultrasonic Systems and Hocking NDT, Lewistown, Pa., (717) 242-0347.