- ACFM Storage Tank Weld Inspection
Typically, paint coatings need to be removed to ensure accurate defect detection and sizing. However, in this project, we were asked to explore the potential of using ACFM to inspect the floors of storage tanks in order to detect weld defects without removing the paint coating. The first phase of the project involved a study to assess the feasibility of using ACFM without removing coatings. The second phase was to perform ACFM inspection on-site for our client, should the process be proven to be feasible. If successful, the project would avoid the downtimes and extra costs associated with removing coatings before inspection.
The feasibility study was carried out on a manufactured fillet weld that had known embedded defects. This was used as a benchmark for future ACFM inspections and was conducted in accordance with the ASTM Designation E2261-07: Standard Practice for the Examination of Welds Using the ACFM Technique Standard. Due to the size of the weld cap, three scans were carried out; one scan on each toe and one scan on the weld centreline. The three scan lines were sufficient to cover both the entire cap and part of the adjacent weld parent material. The technician inspected both a painted fillet weld (figure 1) and an unpainted fillet weld (figure 2). This was backed up by tests on a second unpainted sample where the defects were easily located. This sample was then coated and retested by our experts who determined that, although the signature was less defined, it was still sufficient to detect and measure the defects accurately (figure 3). The coating caused a decrease in amplitude that delivered different calculated depths between the coated and uncoated samples, making it essential to confirm the thickness and nature of the paint and to include this in the scanning or analysis stage. However, the feasibility study showed that ACFM was effective for detecting defects in the coated materials.
With the first phase completed successfully, the next phase was to inspect the client’s tank floors on-site to detect any surface-breaking defects without removing the coating. Our inspectors travelled to the site and used ACFM to inspect the tank floors. A defect was located (figure 4) which was confirmed with the use of magnetic particle inspection. A post-inspection data analysis step followed before a report was produced detailing defect sizing and defect depth.
Our recommendations determined that defects should be inspected with a volumetric method like ultrasonic testing to understand the extent of the cracking, as ACFM can only detect surface-breaking defects and some sub-surface defects.
This project set and validated the used of ACFM to detect the full integrity of welds from the outer side of the storage tank, although it is recommended that further inspection is carried with local removal of the coating to confirm the indication.
- Track Inspection by Autonomous System (TrakSys)
The Traksys project was created to address the need for faster and more efficient inspections of railway infrastructure using remote control technologies. The aim was to design an autonomous vehicle capable of inspecting rail tracks for defects in line with the appropriate UK / EU standards. By bringing together ACFM with ultrasonic and visual inspection capabilities the project would reduce the number of personnel required on rail tracks during inspections as well as enabling targeted repair along with predicative and preventative maintenance regimes.
This project built on previous developmental work undertaken by TWI, with the solution being to mount the inspection system on an autonomous cart. Our experts used their experience in systems integration to deliver industry standard hardware, embedded systems and radio controllers to manage the control and flow of messages and data between the track side application (Figure 5) and the cart (Figure 6).
TrakSys delivered an agile, mobile inspection system that provides continual track inspection coverage supporting a regime of targeted repair while reducing annual maintenance costs. This successful completion of this project opened a number exploitation routes in autonomous rail inspection for the project partners.
Our knowledge and experience in a range of NDT inspection methods also allows us to undertake comparative studies on behalf of industry, including a project that compared eddy current array (ECA) inspection to dye penetrant testing (PT), conventional eddy current inspection (EC), and ACFM.
- Best Practice Guide for the Use of Eddy Current Arrays
Eddy current inspection uses the electromagnetism of conductive materials to detect surface and near-surface flaws as well as to determine material properties. As a technique, EC offers a number of advantages over other surface inspection methods like magnetic particle or liquid penetrant inspection. However, EC inspection depends upon the skill of the operator and does not offer a permanent record of the inspection conducted.
Eddy current array (ECA) technology created the ability to electronically drive multiple eddy current coils placed side by side in the same probe assembly, so TWI created a project to establish the essential variables that should be controlled when conducting an ECA inspection and determine the performance of ECA systems relative to that of existing, recognised surface inspection methods.
ECA was compared to dye penetrant testing (PT), conventional EC inspection, and ACFM in order to produce a best practice guide for the application of ECA.
To find out more about ACFM at TWI, please email contactus@twi.co.uk.