Inspection of hexagonal material

Eddy-current inspection of hexagonal material with rotating probes

Today the dynamic inspection with rotating probes is standard technology whenever longitudinal defects are to be detected on round semifinished goods such as wires, rods and pipes. Depending on the required minimum defect resolution and test speed, one or more one or more probes scan the material in a helicoidal fashion and thus detect longitudinal and circumferential defects.

Limits of this technique
So far this inspection method cannot be used for large portions of semifinished products. Depending on their diagonal dimension, square, hexagonal and octagonal materials are inspected with round coils or geometrically adapted circumferential probes. During this inspection mainly circumferential defects are detected. Axial defects are only detected, if they have a noticeable transverse component. Thus, the axial longitudinal crack is only detected, if it is recorded track-by-track. If one tries to scan hexagonal material track-by-track, the results are very poor. Even optimized filter settings cannot suppress the interference by the edge enough to yield signals that could be evaluated.

The breakthrough
The breakthrough started with the implementation of an idea: "Identical geometric profiles should yield identical signal images under a probe." Our tests indicated that variations in the distance between the receiver coil and the surface do not influence the test results noticeably and that cracks in the immediate vicinity of the edges are reliably detected.
Following the conclusion of the tests cracks that are deeper than 1mm can be reliably detected at a width across flats of 28mm. During lab tests the detection starts at 0.5mm. The detection for smaller dimension starts at 0.3mm.

The following profiles can be inspected:
• Square,
• Hexagonal,
• Octagonal,
• Polygonal profiles (regular and dual) and natural