13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- fatigue crack treated with the surface-activated pre-coating method by controlling high-density electric current. 2. Experiments 2.1. Specimens Austenite stainless steel SUS316 was used as the experimental material. The chemical compositions and the mechanical properties of SUS316 are shown in Tables 1 and 2, respectively. The dumbbell-shaped specimens were used, and the schematic is represented in Fig. 1. A notch was introduced at the center of the one-side edge in the specimens. The specimens were treated with stress relief annealing to remove the residual stress caused in machining process. The heat treatment process is as follows. The specimens are heated to 1173 K for 4 hours, and the temperature is kept at 1173 K for 10 minutes. After that, the specimens are cooled slowly to a room temperature in a furnace. The surfaces of the specimens were polished to a mirror plane by using a buffing machine to observe the surface condition. 2.2. Experimental conditions 2.2.1. Fatigue test conditions The tensile fatigue tests were conducted to introduce a fatigue crack with the annealed specimen, and were carried out at the room temperature in the atmosphere under load control conditions with a hydraulic driven testing machine. All of the tests were conducted at a stress ratio of R=0.05 and a frequency of f=10 Hz. The details of the fatigue test conditions are shown in Table 3. The crack length under cyclic loading was measured by in-situ observation using a digital microscope. In this paper, the test conditions and results of two major examples were indicated. Each specimen is named Specimen A and B. For comparison, the specimen which is not applied electric current is named Standard specimen. 2.2.2. Surface-activated pre-coatings The surface-activated pre-coating technique, which eliminates the oxide layer and coats Ni film for preventing reoxidation, was treated on the crack surfaces. The surface-activated pre-coating technique was composed of three stages as follows. The first stage is electrolytic cleaning, the second stage is HCl activating and the third stage is Ni striking. The schematic of surface-activated pre-coating technique is represented in Fig. 2. The specimen is washed by using pure water between every stage. In the stage of the electrolytic cleaning, the specimen is washed in alkali solution. The composition of the alkali solution is 30 g/L NaOH, Na2Co3 and Na4SiO3. The anode was connected to Al plate and the cathode was to the specimen. The electrolytic cleaning current density is 10 A/dm2, a time of current duration is 60 seconds and the alkali solution is maintained at a temperature of 60 C. In the stage of the HCl activating, the oxide layer on the specimen was eliminated in 37 % HCl solutions for 10 seconds. At the stage of Ni striking, the oxide layer on the specimen was eliminated and the Ni film was coated for preventing reoxidation. The composition of the Ni coating solution is 240 g/L NiCl2 and 80 g/L 37%HCl. The anode was connected to Ni plate and the cathode was to the specimen in the coating solution. The current density is 10 A/dm2 and a time of current duration are 60 seconds. 2.2.3 Conditions of electric current application
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