13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- The practicality of using complex constitutive equations with many variables also needs to be considered. While design approaches, particularly for turbine components, may be carried out based on detailed knowledge, it is apparent that in-service assessment methods that are slightly conservative and can be applied without the need for extensive materials testing and/or complex stress analysis are required. This is particularly true for boilers and piping. This issue is illustrated by consideration of Fig 4. Here different analytical approaches have been applied to creep fatigue results from Grade 92 steel [7]. The relatively simple strain fraction method results in a consistently conservative prediction of behaviour. In contrast a more detailed modified strain fraction approach shows a much less conservative outcome. Indeed, with this analysis, experimental results and predictions are scattered on both sides of the line showing matching agreement, and all data are well within ±2. The continued validation of simple, sensibly conservative methods for at-risk plant components is necessary because of the implementation of new alloys and operating regimes. Finally, it is important to note that the effect of Oxidation on Creep - Fatigue Damage interaction and Component Performance must be considered in specific circumstances of the component and details of the in-service environment. It is established that surface scales can lead to damage initiation in some component applications. Thus, when developing laboratory test programs it is important to ensure that the experimental damage mechanisms are relevant to in-service behavior. Figure 4. Comparison of Experimental and predicted Creep Fatigue lives for Grade 92 steel tested in the range 600 to 650oC. Analysis was performed using a strain fraction rule in (a) and a more complex Modified Strain Fraction rule in (b). [7] 4. On-going Commitment There are relatively few examples where comprehensive data sets have been established without the need for significant assumptions and/or extrapolations. As an example, challenges with application of the strain fraction/ductility exhaustion methodology often occur because measurements of strain: time behavior are not available. In addition, there are many different suggestions for the appropriate value to use for Strain to Fracture. These variations include: • Data averaged from available information for the alloy • Simplified methods for total strain, for example, made using the product of the minimum creep rate and time to failure
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