Aerospace Manufacturing hears from Olympus’ NDT Applications Team and how eddy current testing of aircraft wheels helps ensure a safe landing.

Aircraft wheels are essential components to aircraft operations, just like the wings and engines. They experience a tremendous amount of stress and brake-generated heat during landing, so they must be strong and lightweight.

Consequently, most wheels are made from an aluminium alloy. To help ensure they maintain their structural integrity, the wheels are regularly inspected using eddy current technology, a non-destructive testing technique that enables trained operators to detect cracks and flaws in the wheel.

Eddy current testing technology works on the principle of electrical magnetic coupling of a probe’s coil when operating close to a test sample (conductive material, ferromagnetic, or non-ferromagnetic). The probe’s induced magnetic field generates eddy currents within the test sample and displays signals on the instrument’s impedance plane.

Defects within the test sample cause a disruption to the induced eddy currents, affecting the magnetic coupling. This will be displayed on the eddy current instrument and interpreted by the inspector. Many inspections are performed with single-element pencil probes. As the name implies, these probes inspect small areas. While they are effective, their small size makes these inspections a time-consuming job.

However, using a custom-shaped probe specifically designed for a wheel’s profile makes this process faster and more efficient. The custom-shaped wheel probe employs dual element shaped coils that match the wheel’s bead-seat profiles. This area is where most of the weight is applied to a wheel during its operations and landing, and where fatigue cracking will occur. The wheel probe’s coils cover the bead-seat profiles and enable faster inspection with a single pass vs. multiple scans with a single-element pencil.

These dual element coils are referred to as a bridge-differential coil configuration and give a differential display that helps minimises lift-off and signal drifting, resulting in fewer false calls during inspections.

Make a safe touchdown

The critical nature of aircraft wheel inspection prompted manufacturers to develop new manufacturing technologies to improve the process. One important technology is 3D printing, which has enabled custom probes for wheel inspection to be created. And because the probes are created with high precision, it helps ensure that the probes provide repeatable data.

The process begins when a customer provides a CAD drawing of a wheel or sends an actual wheel to work with. If they send a wheel, the manufacturer uses a coordinate-measuring machine (CMM) to get exact measurements, which are converted into a CAD drawing. Once the exact drawing is created, the data is sent to a 3D printer.

A 3D printer can print the probe’s coil bobbins instead of having to place the coils by hand, greatly improving the probe’s repeatability. To improve the data quality, the eddy current probes and standards are manufactured to precisely fit a specific wheel’s bead-seat radius. The result of this manufacturing process is probes that are reliable, repeatable, and that can be made quickly.

Using a custom probe, most wheels can be inspected in a single pass, leading to significant time-savings. The shape of the probe covers the entire bead-seat area almost perfectly, increasing the probability of detection.

Advanced manufacturing technology provides the flexibility to design custom eddy current probes for many applications, including just about any type of wheel. Other benefits include: better inspection repeatability; more consistent results and faster inspections, as they cover a large area in a single pass - more than a conventional probe.

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