[6B1] Factors affecting the use of electrical potential drop (EPD) as an on- and offline NDT method for creep life monitoring of pressure vessels
A G Wojcik¹, A S Santos2, M Waitt² and A Shibli³
¹University College London, UK
²Matelect Ltd, UK
³ETD Ltd, UK
To help determine the remaining lifetime of steel pressure vessels under conditions that encourage creep, the authors have previously developed a method and presented promising results using a combination of AC and DC electrical potential drop (EPD) monitoring, detecting both final cracking as well as incipient creep damage. The latter was tentatively ascribed to the development of cavitation damage, but recent modelling has shown that cavitation is unlikely to provide enough of a change in electrical properties to explain all of the variations observed in EPD in field trails where ‘online’ EPD has been employed. Here, we report on an ‘offline’ study using laboratory specimens in a more controlled set of creep experiments, which have permitted a greater insight into the mechanisms at play. Whilst changes in both AC and DC-EPD have been observed that largely accord with the in-field testing, the belief is that the changes seen in the online work cannot be fully explained by cavitation development and that local strain effects must play a large part, as well as microstructural changes such as the phases present and the grain size. Deconvoluting all these effects is not straightforward, and may not even be possible, and the implications of this are discussed in the context of using EPD for remaining lifetime determination.
