The manufacturer provided a selection of samples for TWI to use for trials, some of which were suspected to contain shallow surface cracking. Having reviewed the suppliers’ root cause analysis report, TWI postulated the potential type of flaw that could be present and considered the advantages and limitations of each of the main NDT methods. TWI concluded that eddy current array (ECA) would be the most suitable NDT technique to use, supported by magnetic particle inspection (MPI) for defect confirmation.
Initially, MPI inspection was carried out using an alternating current (AC) electromagnetic yoke, with colour contrast (black) MPI ink, and a wet continuous method of application. After the MPI inspection, ECA was implemented. The ECA equipment manufacturers were consulted, and they advised the use of an I-flex probe with small coils to provide greater sensitivity.
The combination of ECA and MPI worked well together. The two methods were equally sensitive, and both had a good probability of detection, detecting surface cracks as small as 2mm long and 0.6mm deep. However, on occasion the ECA picked up some crack like indications, which MPI did not, and vice versa.
During initial on-site inspections, it became apparent that the I-flex probe was unsuitable due to access constraints. TWI contacted the ECA manufacturer, who created a bespoke T-flex probe that was specifically tailored to have the required sensitivity for this project.