External thread crack detection using eddy current testing

A long-term client asked Plant Integrity's parent company, TWI Ltd, to inspect the external threads on a composite valve block body (CVB) in order to detect any potential cracks in the component.  Non-destructive testing methods, eddy current testing (ET) and alternating current potential drop (ACPD) were used.  The team carried out the inspection on-site in Agotnes, Norway.

The project’s aim thus far was to inspect and detect any cracks in the external threads on the CVB. This was to be achieved by using ET to detect the cracks, followed by application of the ACPD method to measure the approximate depth of any cracks.

An eddy current flaw detector thread probe was used to scan the external thread. To begin with, the sensitivity and phase were adjusted on a calibration specimen to ensure accuracy. Then the phase angle was adjusted to give a horizontal lift-off vector. Finally, the gain was added to achieve the full-screen height. The same section of the thread was used to calibrate the ACPD equipment which was then used to measure the depth of a crack in either the root or flank of the threads.

The inspection was carried out by dividing the circumference of the component into 12 sections, then checking the response from the 0.2mm notch in the calibration specimen, after which the probe was placed inside the threads and component. Subsequently, the probe was placed and balanced in the thread of the component. To ensure full coverage, each thread in each section was scanned separately in turn, and overlapping with the next section.

Figure 1: The component marked up in sections 1-12.

No deposits were visible on the inspected composite valve block body and the threads were in good surface condition. The ET inspection of the thread was successful as it detected several indications, two of which were below the reportable level. However, due to the eddy current noise and unideal contact between the eddy current probe and the thread, it was recommended that the client consider using a magnetic particle inspection for the next stage of the project.

Figure 2: The threads showing good condition.