Understanding signal changes when monitoring creep damage in high temperature plant using DCPD

The electrical potential drop (EPD) technique has previously shown promising results using a combination of AC and DC EPD (or DCPD) on large pressure vessel creep tests, detecting final cracking as well as incipient creep cavitation damage in welded P91 steel, with DCPD showing subtle but steady rises of around 5% over ca 10,000 h of testing before rising exponentially at failure. The work presented here has attempted to shed light upon this using a simple numerical model. The model uses an array of spherical cavities to constrain the current path and hence raise the DCPD, however it was only able to show a modest rise in DCPD, and not match experimentally determined rises. Modelled DCPD values were a fifth of those experimentally observed, but both the nature of the model (simplified to aid timely computation) and the assumption that only cavitation is responsible for the changes seen, could be the reason for the discrepancies reported here. The possibility remains that other mechanisms are at play, which could magnify the measured DCPD – particularly those mechanisms that could be associated with embryonic or micro-crack formation, and these are discussed herein.