13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 4 Demonstration of temperature-dependent strain-controlled mode 2.3. Strain Control and Measurement A new strain control and measurement technique is introduced for high temperature fatigue testing. For high temperature strain-controlled fatigue tests, implementation of punching dimples or knife-edges within gauge length of specimen are common practices for mounting extensometers. Premature failure on the test section of the specimens were introduced at the punch dimple or knife-edge contact area by implementing these techniques as shown in Fig. 5 A and B, respectively. Figure 5 Photos of premature failure caused by dimple and knife-edge Alternatively, a pair of dimples is punched outside the gauge length of the specimen as shown in Fig. 6. The punched dimples are located on the thicker cross sections which are outside the effective gauge length of the test sections. Therefore, the premature failure will not be introduced by these dimples. However, since the controlled strain is from outside of gage length, a correlation between measured strain and strain within gauge length should be carefully calibrated and verified. The correlation coefficient is defined as (in this case): 0.6" 1" C ε ε = (4) Figure 6 Picture of the specimen with dimples
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