ACFM in Detail

Probe Deployment

For standard probes, inspection is carried out by placing the probe on the surface, starting data collection and moving the probe smoothly along the surface.

Mechanical contact between the probe and the surface is not necessary for the technique to work, but sensitivity is optimised if contact is made. In some instances (e.g. with a rough or hot surface) scanning is made easier by keeping the probe off the surface (e.g. by adding a plastic or fibre-glass sheet). In these cases, care should be taken that the distance between the probe and the metal surface is kept constant, otherwise sizing accuracy will be reduced.

Although mechanical contact is recommended, contact pressure should be light to avoid excessive wear of the probe face.

Probe movement is normally linear and parallel to the expected orientation of defects. The probe orientation is such that the current is induced perpendicular to the scan direction. Defects are recognised by the probe passing over one end, along the defect length, then over the other end. The operator is trained to recognise the signals produced by a defect traverse to the scan direction.

One scan inspects a band 10-15mm wide, so many welds, for example, can be inspected with two scans, one along each toe.

The use of a simple linear scan pattern provides several advantages. When inspecting welds, movement of the probe across the weld toe is avoided, along with any effect of the permeability difference present. Also two-man deployment, where the probe operator is remote from the ACFM operator and the data display, is made much easier.

The speed of probe movement will be partially dictated by access constraints, but is typically 20-50mm/second for standard systems. Probe movement should be at a fairly constant speed, but the use of the butterfly plot takes out the effect of any speed variations.

After completing a scan the data remains on the computer screen for analysis. The data can also be replayed if required. Clock markers or pointers can be added to the data to make probe position or the location of the anomalous features. All these mean that there is no need for the probe operator to watch the screen during data collection, leaving them free to concentrate on deploying the probe in the right place.

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