13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- tests that specimens were fitted with extensometry to determine relevant strain rates, and further there is no guarantee that associated end-of-test ductility values are available. 2.3. Standardization Many countries have established guidelines and procedures for creep fatigue testing. However, there were differences in detail between many of the recommendations. A key activity facilitated by the EPRI work was thus development of specific ASTM Creep - Fatigue Standards. Currently efforts have resulted in two standards: • ASTM Standard: Test Method for Creep Fatigue Testing, ASTM E2714-09. This method covers the determination of mechanical properties pertaining to creep-fatigue crack formation in nominally homogeneous materials. It is primarily aimed at providing the material properties required for assessment of defect-free engineering structures containing features that are subject to cyclic loading at temperatures that are sufficiently high to cause creep deformation. • ASTM Standard: Test Method for Creep-Fatigue Crack Growth Testing, ASTM E2760-10. This test method is concerned with developing creep-crack growth data under cyclic conditions which is used in some more sophisticated assessments of in-service materials when large flaws may be present. As part of the review and acceptance process by ASTM, the provisional Standards listed above must be evaluated for a precision and bias statement through a round robin test program. A round robin testing program is now complete for ASTM E2714-09[5] (details below) and the precision and bias statement will be added in the next revision of the standard. Planning, preliminary testing, and specimen blank fabrication are now complete for ASTM E2760-10 with round robin testing expected in 2013. The round robin test program for ASTM E2714-09 utilized Grade 91 steel test blanks (modified 9%Cr-1Mo-V) provided by EPRI. Sixteen laboratories around the world agreed to participate in the study with 13 eventually reporting their test results to EPRI and the ASTM Task Group on Creep-fatigue Crack Formation (E08.05.08). Strain controlled creep-fatigue tests were conducted at 625C at three stain amplitude. Each laboratory followed the provisional standard, but variations in specimen geometry, heating methods, and numbers of tests were acceptable. Statistical analysis of the inter- and intra-laboratory variability was conducted. EPRI highly recommends metallographic assessment as part of the post-test evaluation to see if creep or fatigue damage dominated the failure or if a true creep-fatigue interaction was found. Most laboratories did not do this, so additional post-test metallography was conducted. Assessment of the data required development of an improved analytical method, not currently prescribed in the standard, to determine the cycles to crack formation. The analysis of the data found the variability factor for the 95% confidence interval bands increased at longer hold times and lower strains. One significant finding was that post-test inspection of specimens was necessary to determine if a test was valid (or not). Uneven heating due to the use of induction heating methods or failures due to bending were not identified by the testing laboratory, but post-test inspection and metallography resulted in some tests being rejected. Fig 3 shows an example of data before (a) and after post-test assessment for validity (b). The round robin (RR) research produced the following recommendations: • From the results of the RR, the current precision and bias statements in the standard should be modified (this activity is underway)
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