The effects of scoop sampling on the creep behaviour of power plant straight pipes

Abstract

Small specimen testing techniques are potentially highly useful procedures which could be used to determine many fundamental material properties from relatively small amounts of sample material. Small specimens can be manufactured from scoop samples taken from the surface of a component, allowing for continued operation after sampling. In the past, it has been assumed that, provided the wall thickness dimension around the specimen sample site is greater than the minimum design requirement, the action of scoop sampling will not greatly compromise the future operation of the component. Little study has been completed, however, to verify this assumption or to evaluate the likely reduction in remnant life due to scoop sampling. In the present study, a finite element investigation is made into the effects of scoop sampling on the stress states and failure lives of power plant straight pipes under steady-state creep conditions. Loading conditions considered include an internal pressure load only (assuming closed-end conditions), as well as system loading (in the form of additional axial and in-plane bending moment loading). Typically, the application of system loading increases the likelihood of failure being controlled by the stress states in the vicinity of a scoop excavation. This leads to potentially significant reductions (up to approximately 90%) in creep life of straight piping components. Parametric equations (valid for a range of pipe geometries and scoop cut depths) have been proposed, which can estimate the stress riser effect in the vicinity of an excavation sample under closed-end and additional axial loading conditions.