13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 3 illustrates the four-point bend test set-up for the plate specimens. This four-point bend set-up, in contrast to the three-point bend set-up, prevents the plastic deformation caused by the indentation of the loading point to impinge on the plastically deformed crack tip under large deformations, which has frequently occurred in SE(B) specimens made of highly ductile materials. Figure 3. The four-point bend test set-up for the plate specimens. 3.2. Experimental Procedures The experimental procedure separates into two stages: 1) a load-controlled fatigue pre-cracking cyclic load to generate a sharp crack-front along the machined notch; 2) a displacement-controlled monotonic loading coupled with multiple unloading-reloading cycles to monitor the compliance of the specimen, measured by the ratio of the measured CMOD range over the corresponding range of the applied load, throughout the procedure. The increasing compliance measured during the test indicates a growing crack during the test. The experimental procedure monitors the specimen compliance during the fatigue pre-cracking stage, which terminates as the compliance increases by 20% to 30% of the initial compliance of the specimen. Table 1 lists the fatigue pre-cracked sizes for all four specimens. Table 1. Fatigue pre-cracked sizes for the plate specimens Machined Crack Size Fatigue Pre-crack Specimen a (mm) a/c a (mm) a/c SP1 12.0 0.497 SP2 12.0 0.496 SP3 12.4 0.484 SP4 10 0.5 12.3 0.483 Figure 4 shows the load-CMOD measured during the experiment for the specimen SP1 shown in Table 1. The CMOD in Figure 4 refers to the crack mouth opening displacement measured at the center of the surface crack (Z = 0 in Figure 1).
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