In recent years, there has been an increasing need for component specific testing in nuclear power plant (NPP) materials. This has arisen from changes in fatigue design criteria outlined within the ASME Boiler Pressure and Vessel and Pipe Code (BPVC). These are primarily derived from low-cycle fatigue (LCF) testing on idealized small-scale specimens. For the case of stainless steel (SS), testing is performed under membrane loading on traditional uniaxial round-bar specimens according to standards such as ASTM E606. Here, failure is defined as a 25% drop in load, that typically corresponds to a 3 mm deep crack. Real NPP components however, rarely experience such membrane loading due to their complex geometry and in-service loading (combined primary and secondary stresses). For the case of an internally-pressurized pipe subjected to thermal transients, due to temperature fluctuations in the coolant, an additional through-wall stress field gradient is present. This results in fatigue crack initiation and propagation occurring into a decreasing stress field, which may be expected to result in longer fatigue lives than within small specimen testing and, hence, a large degree of inherent over-conservatism may exist in the ASME design data.
This paper describes the development of a novel test procedure able to incorporate the effect of a stress gradient within fatigue endurance testing. A bespoke eight-point bend (8PB) setup has been developed that is capable of performing load-, displacement- and strain-controlled LCF tests. A series of trials have been conducted in all three control modes with the material response validated against finite element analysis (FEA). It is anticipated that this testing procedure can provide a better representation of NPP components under fatigue loading conditions, providing results that would be used to remove over-conservatism within the current design criteria.