13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- In the test, the load starts from 100 N to 1000 N at a step of 50 N. Result of the plastic zone size during the unloading process is obtained as shown in Figure 4. It is found that the reversed plastic zone size keeps increasing throughout the unloading process and these experiment data shows similar trends to the theory data without plastic closure phenomena. This is very different compared with the results of aluminum alloys and it indicates no crack closure for steel in these tests. Figure 4. Plastic zone size of steel specimens 3. In-situ SEM study for crack closure In order to verify the crack closure indications from optical microscope observation and get further information of the crack tip deformation behavior, in-situ SEM experiment is carried out on the two materials. Further, the behavior of crack closure under variable load is studied for the sufficiency of Keff for crack growth prediction. In mechanics, crack closure behavior under a single overload is studied for the sufficiency of Keff for crack propagation mechanics. In statistics, the crack growth rate under constant loading and overloading load is compared to show the sufficiency of Keff for crack growth prediction. Detailed experimental procedure has been discussed in [13][14] and only a brief description of the experimental methodology is given below. 3.1. In-situ SEM study under constant amplitude loading An in-situ SEM fatigue testing is performed to achieve the high resolution investigation for the hypothesis verification of crack growth kinetics and crack deformation in the vicinity of the crack tip. Compared with experiment under optical microscope, crack tip opening displacement (CTOD) is measured directly to find the crack is closed or not. Some details of the experimental set-up for the in-situ SEM experiment are shown in Figure 5. It consists of the tensile sub-stage and a field emission SEM (JEOL-7400F). The sub-stage is fixed in the SEM and the cyclic loading is applied during the experiment. Multiple resolution images are recorded during the testing for the 0 20 40 60 80 100 120 140 0 5 10 15 20 Diameter (um) K (MPa·m0.5) #1 Reversed Plastic Zone Size Test 1 Test 2 Test 3 Test 4 Test 5 Theory 0 10 20 30 40 50 60 70 0 3 6 9 12 15 Diameter (um) K (MPa·m0.5) #2 Reversed Plastic Zone Size Test 1 Test 2 Test 3 Test 4 Test 5 Theory 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 Diameter (um) K (MPa·m0.5) #3 Reversed Plastic Zone Size Test 1 Test 2 Test 3 Test 4 Test 5 Theory 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 Diameter (um) K (MPa·m0.5) #4 Reversed Plastic Zone Size Test 1 Test 2 Test 3 Test 4 Test 5 Theory
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