Potential Drop Monitoring of Creep Damage

Managing creep is a major issue in the power and other industries, particularly as plant ages, but there is currently no satisfactory method for in-situ monitoring the of progress of creep damage. The proposers have recently conducted a feasibility study that has shown that the progress of creep can be tracked by monitoring the evolution of potential drop anisotropy between directions parallel to and perpendicular to the loading direction. The technique is potentially a very simple method of monitoring creep, but several fundamental issues must be addressed before the method can be applied in industry. To date, only nominally homogeneous, ferritic steels have been tested, and these exhibit significant voiding during creep. Other important materials such as stainless steels can exhibit less voiding so it is necessary to understand better the mechanism of the evolution of the potential drop anisotropy and to investigate its applicability to austenitic steels and nickel base super alloys. In addition, creep often occurs at welds, so it is necessary to determine how the intrinsic conductivity difference between the base metal and the weld affects the apparent anisotropy measured by directional potential drop measurements, and also whether different thermally-induced microstructural evolution in these different microstructures leads to spurious apparent anisotropy changes, and hence limits the detectability of creep damage in welds and their neighbourhood. While monitoring using a permanently attached probe is attractive in some applications, in others such as turbine blades, it is not feasible so it is necessary to investigate whether a deployable probe can be used. This proposal seeks funds to address these scientific and engineering issues, and so to produce a new creep monitoring technique that will particularly benefit the power and related industries.

The project tackles a key current industrial problem whose solution will benefit all users of high temperature plant that is subject to creep deformation. This is recognised by the three collaborating companies, E.ON, RWE npower and Rolls-Royce who are contributing £90k cash and £110k in-kind to the project. The detection of creep damage in high temperature components is very difficult and time consuming; as plant ages, and particularly if the life of critical components working at high temperatures such as boilers is to be extended, it is crucial that the level of any creep damage is known. At present this is done at plant shutdowns by an extremely laborious replica process typically involving surface polishing, etching and pressing a softened plastic foil which moulds itself to the surface, producing an exact copy of the etched surface microstructure that can be analysed under a microscope. E-ON estimate that they do 15000-18000 such examinations per year; this is replicated across the other power companies both in UK and worldwide. Therefore the potential benefits of a technique that will automatically monitor the progress of creep, readings being obtainable at any time during operation, not just at shutdowns, are enormous - the cost of testing at a given location would be reduced, safety would be improved by obtaining data continuously or at regular intervals during operation, and it would be economically feasible to monitor at more locations.

In addition to the use of permanently installed monitoring systems, the project also aims to develop a deployable system for cases where permanently installed monitoring is not feasible, such as aircraft engine blades. Success of this part of the project would therefore benefit the aerospace sector, particularly engine manufacturers, and would help to ensure aircraft safety.

In addition to the direct benefits to the industrial users of the technology, the project will also impact the instrumentation sector where there will be opportunities to manufacture the required test equipment and associated software.