13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 6 50.8±0.25 63.5±0.5 30.48±0.25 13.97±0.25 30º 2 60.96 10.16 ±0.05 0 12.7 Figure 3. CT specimen configuration 2.2. Stress corrosion cracking test Fig. 4 shows the schematic illustration of SCC test apparatus. The crack length was monitored by a crack gauge bonded to a crack-expected part of CT specimen. In the SCC tests of AZ61 and AZ61-T5, crack gauges were peeled off frequently since the crack growth rate was low and the testing time was long. So a traveling microscope was also used to monitor the crack length of AZ61 and AZ61-T5 specimens. A CT specimen with a fatigue pre-crack was attached to the creep testing machine equipped with a 3% NaCl solution tank. Prior to the loading, SCC test specimens were held in a NaCl solution at a given cathodic potential for 24 hours under no-loaded condition to charge hydrogen. After the loading, the load was gradually increased every 24 hours until a crack initiated to propagate. Once a crack initiated to propagate, the load was sustained and the crack growth was monitored by a crack gauge or a traveling microscope. In the solution tank, the Ag/AgCl and Pt electrodes were set for the reference and counter electrodes, respectively and the cathodic potential of specimen was controlled by means of a potentiostat (HA-303: HOKUTO DENKO Corp.). The cathodic potentials were controlled to be -1.4 V, -2.5 V, and -3.0 V for AZ31; 0 V, -1.4 V, -2.5 V and -4.0 V for AZ61; and -1.4 V and -4.0 V for AZ61-T5. Figure 4. SCC test apparatus The Pourbaix diagram of Mg [2], which shows the stable phase of an electrochemical system, is shown in Fig. 5. The pH of 3% NaCl solution is 7. The test conditions are also plotted in Fig. 5. It is potentiostat Ag/AgCl reference electrode Pt counter electrode CT specimen tank 3% NaCl solution crack gauge
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