13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- • The current standard should be modified to include post-test metallographic analysis to ensure that the dominant crack(s) form within the gage length of the specimen and if bending and/or extensive bulging was present in the test specimen warranting rejection of the data. • A cautionary note should be added in the standard to warn users of the possible effects of heating methods on the C-F lives of test specimens. • A more in-depth test program should be considered using the available material to characterize the C-F properties of P91 steel and to also investigate the effects of heating methods on the C-F lives. In addition to these standards, the experts group has produced a code of practice for short-crack growth under creep fatigue conditions [6]. (a) (b) Figure 3. Comparison of data produced as part of the creep-fatigue RR test program, (a) showing all the data while (b) shows the only data for tests which met the criteria for validity. The 95% confidence interval bands decrease significantly when post-test metallographic evaluation is used to eliminate invalid tests. 3. Discussion It is clear that updated methodologies for estimating damage in service components should distinguish between sequential damage, for example, steady operation leading to creep followed by cyclic performance resulting in fatigue, and interactive damage, i.e. under conditions where the damage processes lead to rapid damage development. This is particularly important for alloys where time and/or cyclic microstructural changes occur. Damage accumulation in creep-fatigue should be described by a formulation that includes a term giving the influence of creep on fatigue and vice versa. In other words, where damage is truly interactive, the capacity for creep must be reduced due to the fatigue, and the capacity for fatigue must be reduced by creep. Moreover, the challenges associated with using curve fitting approaches for parametric fitting of experimental data must be considered because although reasonable fits can be obtained to an existing data set, the accuracy of parametric data extrapolations may not be guaranteed. In all cases, there is a major requirement for the results of experimental and analytical programs to be supported by meaningful post test metallographic examination. Thus, post test examination should be performed to document the type, density and character of damage present recording at the very least whether the primary damage is intergranular or transgranular.
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