Analysis and Interpretation of Early Stage Creep Crack Growth Behaviour in Type 316H Stainless Steel

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1.1.1 Project SummaryThe current fleet of Advanced Gas Cooled Reactors (AGRs) in the UK is reaching the end of its planned service life. However, plant life extension (PLEX) is highly desirable both from an economic and environmental perspective as nuclear plants have the potential to generate large amounts of emissions free power, and it is unlikely that new build stations will be able to match the current generation capacity in the near future. This life extension is possible on the condition that the continued operation is proved to be safe.Part of the analysis contributing to PLEX is determination of the damage caused by reheat cracking in high temperature AGR plant such as boiler tubes and headers which is made of 316H austenitic stainless steel. This type of cracking is driven by residual stresses as a result of welds which cause the material to creep and eventually crack. The crack growth is predicted through laboratory Creep Crack Growth (CCG) tests, predominantly on Compact Tension specimens. Currently there is a relatively good understanding of the behaviour of CCG in these tests once the crack is fully formed, but much less is known about the initiation stage of the crack. Early stage crack growth data from laboratory tests is either disregarded or likely misinterpreted, meaning that the initiation of a crack is not fully determined. Therefore neither initiation nor early stage growth is accurately characterised. Aims and ObjectivesThe main aim of this PhD is to further understand the early stages of creep crack growth and to attempt to correlate it with a fracture mechanics parameter, either C* or Ct. The objectives set out in order to achieve this are: Accumulate and analyse further experimental CCG data. Tests make use of the Potential Drop (PD) technique for measurement of crack growth. Newly developed hardware which minimises noise will be used in these tests, and the data will be analysed using modified methods which should improve detection of crack initiation. Digital Image Correlation (DIC) will also be used to analyse strain fields. Improve experimental estimate of C* using deflection partitioning of load line response. Experimental determination of C* depends on the load line displacement (LLD) rate due to creep. Partitioning the load line response using uniaxial tensile data in Finite Element Analysis (FEA) as opposed to Ramberg Osgood parameters can improve the accuracy of the creep contribution, leading to better estimates of C*. Characterisation of creep and plasticity interdependency. Although deflection partitioning assumes that deformation due to creep and plasticity are independent of each other, one inevitably affects the behaviour of the other. As such it is necessary to determine to what extent creep deformation affects plasticity and vice versa. Determine whether crack tip parameter such as C* or Ct characterises initiation and early stage growth. Subsequently develop comprehensive methodology to describe initiation and early stage crack growth in 316H at 550degreesC incorporating combined findings from above three tasks.