13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Interaction. A summary of these Milestone reports is provided in Table 2. It is apparent that the two main methods used to asses creep damage are: • Time fraction • Strain fraction (ductility exhaustion) While these general terms describe the overall methodologies, there are also many variations in the detailed application. These variations and the assumptions that are often required to support the analysis frequently make definitive engineering judgments on accuracy difficult. Table 2. Milestone Reports Year Title Reference 2007 Creep-Fatigue Damage Accumulation and Interaction Diagram Based on Metallographic Interpretation of Mechanisms 1014837 2008 The State-of-Knowledge Report on Creep-Fatigue Interaction 1016489 2008 Review of Creep Deformation and Failure Models for Creep - Fatigue Assessment, 1018233 2008 Creep Fatigue Damage Interaction: Fatigue Deformation and Failure 1018439 2009 Plant Component Assessment for Creep-Fatigue Damage: Component Assessment Methodologies 1017608 2009 Plant Component Assessment for Creep-Fatigue Damage: Case Studies 1020511 2010 Creep-Fatigue Testing and Assessment Guideline: Material Property Data Requirements for Component Assessment 1019778 2012 Review and analyses of Creep-Fatigue data. Metallographic Atlas and examples of damage In press Whether time summation or strain summation is chosen, investigators have usually been at pains to demonstrate an agreement with experimental failure data to within a factor of two. This has generally been managed by increasing use of ‘partitioning’ rules or other degrees of sophistication so that the original models begin to lose their attractiveness – the most robust have proved to be those which are the easiest to use. By tracing some of the history of the damage laws, it appears that the time fraction rule originally took peak stress as the reference and was therefore conservative. Attempts to integrate down a relaxation curve and refine the appropriate time led to non-conservative predictions of life. Thus the interaction diagram was made bi-linear which restored conservatism. There have been similar difficulties with analyzing ductility data for strain fraction. One basic question must be asked - which is the most physically correct ductility to take? The assumption of an average strain rate by taking ductility divided by time to failure is not necessarily an accurate reflection of secondary creep rate if both primary and tertiary stages of creep are large. Nevertheless, all models acknowledge the decreasing ductility (of whichever form) with decrease in strain rate, and there seems now to be a general acceptance that the ductility exhaustion approach is consistent and less prone to difficulties with scatter. In many cases, however, one may be forced to apply the time fraction rule because of data restrictions – it is highly unlikely that in long-term stress rupture
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