Conventional eddy current technology has been used for many years to inspect the surfaces and subsurfaces of various components across a wide range of industries. This electromagnetic technique can reliably detect surface-breaking flaws, such as cracks, and subsurface defects, such as corrosion. Unfortunately, inspecting large surface areas using a single-coil probe is extremely impractical as the process would be too time consuming and the probability for missed flaws too great. Eddy current array (ECA) technology remedies these issues. Furthermore, the capacity of ECA to inspect large surfaces makes it an advantageous replacement for traditional techniques such as magnetic particle or liquid penetrant inspection.

Eddy current array principles

Eddy current array technology features the capacity to drive several eddy current sensors positioned side by side in the same probe assembly. As a result, ECA can be used to inspect large surface areas in a single pass while maintaining high-resolution, thereby improving both the inspection speed and the probability of detection. The results can be displayed using color-coded mapping (C-scan), which facilitates inspection performance and analysis. An ECA system is composed of three fundamental components: an instrument, software, and a probe. ECA probes are either rigid (flat or shaped) or flexible.

Eddy current array probes

There are various characteristics that define eddy current array probes. ECA probe operational modes, such as absolute, differential, and reflection, are determined by the configuration of the individual coils, or sensors. The type of application dictates the appropriate configuration. An ECA probe is also defined by its operating frequency, the size of its coils, the number of coils, the resolution, and the coverage.

Hard-coil ECA probes are either flat or shaped to fit common component geometries. As these hard coil sensors can have varying sensitivities, matching coils must be stringently selected during the manufacturing process.

A more flexible type of ECA probe, made of a PCB-based film, has also recently been developed. During an inspection, these probes can be adapted to components with varying geometries, or they can be mounted on holders that fit certain shapes or curvatures. Because the sensors are printed into the PCB film, their sensitivities are identical.

Eddy current array instrument and software

All eddy current array instruments on the market work in the essentially the same way: they drive the sensors of the array in a multiplexing pattern to avoid mutual inductance. An ECA instrument is characterized by its portability, number of channels, frequency range, and software.

ECA instruments feature varying degrees of portability. Some instruments are considered portable but they require a PC to drive it, while others are completely stand-alone. There are also ECA instruments that are fully integrated into inspection systems in manufacturing plants. Generally, these instruments are not transportable because they necessitate a large number of channels.

Instrument software options vary from manufacturer to manufacturer; however, all ECA instruments offer the possibility to adjust the parameters, perform a calibration, and save the data. They also offer a wide range of analysis tools and images, such as C-scan mapping and strip charts.

Corrosion and crack detection in aircraft

Aerospace companies were among the first users of ECA technology. Today, ECA inspection is referenced in the maintenance manuals of major aircraft companies, mainly for corrosion and crack detection.

A high-frequency eddy current array with small coils can effectively inspect a complete row of fastener holes in only one pass. Because paint removal is unnecessary, using the ECA technique represents substantial time-savings. When an encoder is used to record the position, the inspection results are presented as a C-scan, which enables operators to identify cracks oriented in any direction.

For corrosion detection, an eddy current array with larger coils operating at a lower frequency is used in order to increase the depth of penetration. The C-scan presentation of the results enables operators to clearly determine the position of the fasteners and any corrosion that is located between the skin layers.

Stress corrosion cracks in process piping and pipelines

Stress corrosion cracking (SCC) results from a combination of tensile stress and a corrosive environment. When induced in process piping or in pipelines, SCC can lead to a catastrophic failure. Accurately detecting and sizing these cracks is critical.

Traditionally, liquid penetrant (LPI) or magnetic particle (MPI) inspection has been used for SCC detection and sizing. However, ECA has shown to be a good substitute for LPI and MPI and with significant advantages.

Flexible eddy current array probes mounted on holders of different diameters enable operators to scan different pipe schedules using the same probe. The C-scan results are displayed using a special color palette so that they are identical in appearance to the results provided by LPI or MPI.

Replacing liquid penetrant or magnetic particle inspection with ECA offers the following benefits:

• No need for chemicals
• Minimal surface preparation
• Larger surface-area coverage
• Color-coded mapping (C-scan)
• Data archiving and post-processing

Gear-tooth surface inspection in the mining industry

Gears are a critical component used in a variety of mining equipment. From processing equipment to heavy machinery, each gear must be closely monitored for surface-breaking cracks caused by stress during operation.

Using eddy current array technology for gear-tooth inspection helps keep the surface preparation to a minimum because the gear only needs to be lightly cleaned. Inspecting a gear-tooth with ECA is also very fast. The acquisition speed can reach up to two meters per second, and only one pass is required to scan one full side of a gear tooth.

Manufacturing industry

Eddy current array technology is also integrated into fully-automated in-line testing systems. ECA functions as a complement to ultrasonic phased array technology in verifying the integrity of large-diameter billets (carbon steel, stainless steel, aluminum) on the manufacturing site.

To meet continuous production demands and sustain high production rates, this type of surface inspection requires powerful eddy current array electronics with hundreds of channels capable of parallel firings and numerous probes composed of hundreds of elements capable of covering a large surface area. Both the electronics and probes have to be robust as these systems operate twenty-four hours a day, seven days a week.

An ECA manufacturing inspection system is capable of performing automatic calibration. It adjusts each coil of the array to the same detection level in amplitude and phase. Detection of a surface indication sends a signal to the software, which automatically assesses the degree of severity of the flaw and accepts or declines the billet based on predefined selection criteria.

Conclusion

 ECA as a technology has reached maturity, yet the range of possible applications continues to expand. Because of its capacity to perform rapid and reliable large-surface inspection, ECA has the potential to replace conventional eddy current as well as techniques such as MPI and LPI. Thus, ECA could serve as a means for a wide variety of industries to upgrade and improve their current inspection processes.