13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- • Estimates based on relationships describing creep strain with time for applicable stress/ temperature combinations • Strain to Failure, Elongation/Reduction of Area, or other measured value for different conditions extrapolated to the conditions of interest Moreover, although significant information exists for the most widely used boiler and turbine steels; in most cases testing has concentrated on generating parent properties. Thus, less laboratory data exist for weld metals, heat-affected zone (HAZ), or overall weldment performance, even though in many boiler applications, in-service damage in components operating at high temperature frequently occurs associated with welds. EPRI is working with members of the expert group to establish comprehensive data compilations on the most widely used alloys. It is planned that analysis of the data sets compiled will result in published material data sheets suitable for base line type analysis. Alloys for which Data Compilation Books or ‘Creep-fatigue data sheets’ are planned include: – Steels used in Boiler Headers and Piping, for example P22, P91 (X10CrMoV9-1), P92, E911, – Alloys used in turbine rotors and discs (IN718, IN706, X12CrMoWVNbN10-1-1, IN617, etc. An example of high temperature creep rupture, deformation and ductility data for Grade 91 steel is shown in Fig 5. The output analysis of this type of data would be included in the envisaged Creep-fatigue datasheets. The plan to produce master creep, fatigue, and creep-fatigue equations most relevant to plant operational conditions is critically important as the data input often becomes the most important source of uncertainty in life assessment calculations. Before releasing these summary equations and curves, the Creep-Fatigue datasheets will be provided to the EPRI expert working group for evaluation and approval. Adoption of standardized equations will aid current needs for creep-fatigue assessment as well as future developments in more sophisticated analytical approaches and in new materials. 5. Concluding Remarks In general, creep-fatigue design considerations are intended to prevent crack initiation, where crack initiation is defined arbitrarily as the presence of cracks that can be detected visually, for example, 1 mm in length. The difference between crack initiation and failure life in a normal laboratory specimen is often a small proportion of the total life, and it can be argued that the failure endurance of a small specimen corresponds to the endurance at crack initiation in a large component. A recent discussion identified the following as primary classification of component types of concern: • Thick sections, especially welds, • Changes in section and complex geometries, • Welds in low ductility (creep brittle) materials, • Non-stress relieved welds, • Thin section welds with defects. The general consensus regarding methods of assessing crack initiation is the following: • Time-fraction-type stress-based creep damage is insufficient for predicting life reduction due to creep holds. This is especially the case at the small strain ranges of practical interest. This effect cannot be properly described using creep-time fraction on an interaction diagram.
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