Thu, 16 April, 2020
TWI conducted a three-phase project aimed at determining the feasibility of using alternating-current field measurement (ACFM) testing, a non-destructive testing (NDT) technique, to inspect train bogies without the need to remove the surface coating.
Conventionally, train bogies are inspected using visual and magnetic particle inspection (MPI) where paint coatings have to be removed prior to inspection. Otherwise, probability of detection (POD) will be low, or the indication of defects might be inaccurate
Under this project, the client asked TWI to firstly explore the feasibility of using ACFM testing on the train bogie without coating removal. Secondly, if proven feasible, TWI was asked to produce the defect specimens and perform the ACFM testing blind trial test on the train bogie.
TWI demonstrated that it was possible to detect defects, through surface coatings, with a high POD of 91% over MPI, without coating removal. The variety of ACFM testing probes with differently angled/lengthened heads also further expedites the inspection on weld joints and areas previously difficult to access using MPI. This reduces the overall downtime and the extra cost for production that is associated with removing coatings.
Another major advantage of ACFM testing demonstrated from the study was that the data is recordable and storable for traceability and future reference. ACFM testing is also applicable for base material or welds, and ferritic or non-ferritic conductive metals, where MPI can only be used on ferromagnetic materials.
ACFM inspection was derived from the methods used in eddy current testing (ECT) and was initially used by divers for the inspection of subsea welds on offshore installations in the North Sea. Soon, ACFM was widely adopted by offshore operators worldwide to inspect not only ferritic steels, but also duplex stainless steel and titanium.
ACFM inspection is fundamentally an advanced form of conventional ECT. It combines the advantages of the alternating current potential drop (ACPD) technique and ECT, where it can technically work without electrical contact, and perform defect sizing without calibration.
ACFM uses unidirectional and constant energising fields. By inducing a current in the surface of the piece to be inspected (ECT principal), any defects in the component will change the direction of these sub-surface currents; these changes are detected by small sensors, and are used to compute the length and depth of the defect. The field component denoted by Bz responds to the poles generated when the current flows around the ends of the crack, introducing current rotation in the component plane. This is indicative of the crack length. The field component denoted by Bx responds to the reduction in current surface density, as the current flows down the crack. This reflects the depth of the defect.
ACFM is typically used to replace MPI as it offers the possibility of allowing quicker inspection than that delivered by MPI; ACFM can work through paint, coatings and dirt, and can differentiate between a surface scratch and an actual surface defect - only detecting defects which have some depth.