13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Then the specimens were heated up and tested at R ratio of 0.05 where R = Kmin /Kmax. These tests were performed either under continuous pure fatigue (PF) at 2 Hz or with a trapezoidal shape signal 10/X/10 where 10 is the time (in s) to reach the maximum load and to unload the specimen while X designates the hold time at Kmax in seconds. A potential drop technique which was previously calibrated was used to measure the crack growth rate expressed in terms of da/dN or da/dt, where N is the number of cycles and t is the total time (rising time + hold time + unloading time). The stress intensity factor (SIF) K was calculated using an expression which was calibrated previously. The measurement of the fatigue crack growth rate was performed following the standard ASTM E647-08. Scanning electron microscopy (SEM) was used to observe the fracture surfaces of the specimens. A special attention was paid to the fracture surface character, i.e. transgranular with fatigue striations or intergranular. 3. Results The results of FCGR measurements, da/dN expressed as normalized values, are shown in Figure 3 (da/dN)tm / (da/dN)2Hz for a given value of ΔK equal to 25 MPa.m1/2. The results reported by Pédron and Pineau [5] and by Gustafsson et al. [10-12] for a similar ΔK are also included in figure 3. Figure 3. Normalized fatigue crack growth rate (da/dN)tm / (da/dN)2Hz as a function of hold time, tm. Test conditions are given in the insert. Squares and full lines indicate data from this study whereas diamonds/dashed line and triangle/dot line represent data from Gustafsson’s [10] and Pédron’s and Pineau’s [5, 14] work. Results obtained for ΔK = 25 MPa.m1/2.
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