13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- which is a requirement for digital image correlation (DIC) analysis. Both surfaces of the specimen should be polished with the sandpaper whose average particle diameter is smaller than 10 um. The final polishing is done using a vibration polishing machine with 1~3um polishing suspension; Step 3: apply load on the specimen under the monitoring of a microscope, while image of the crack tip region is taken during the loading and unloading process. A unit load cycle is divided into several steps in the load profile. At each step, the image around the crack region is taken for strain calculation. The step load is set according to the precision requirement and computation resource limitation. An example profile is shown in Figure 2, which has about forty segments during the loading and unloading path. Figure 2. Load profile Step 4, process the images by DIC software to get the strain field of the crack tip region and calculate the plastic zone using mechanical analysis. The plastic zone size is defined along the crack propagation direction and it can be measured directly from the processed image. The theoretical reversed plastic zone under cyclic loading can be estimated by Eq. (1) without the consideration of crack closure [11] 2 1 2 y K , (1) where y is the yielding strength and ∆ = ∙ ∆ √ = ∙ ( max − min)√ (2) where F is the geometry factor and F = 1.12 − 0.231 ( )+10.55( ) 2 −21.72( ) 2 +30.39( ) 3 (3) where a is the crack length. 2.2. Title and author information With the above described experiment procedure, two sets of experiment are carried out to investigate the crack closure behavior under constant loading in Al 7075-T6 and Steel 4340 separately. The chemical composition aluminum 7075-T6 is listed in Table 1. The aluminum is the balance in the total weight. The basic physical properties are listed in Table 2. Following the above discussed general experiment methodology, the plastic zone size at each 0 100 200 300 400 500 600 700 800 900 1000 0 5 10152025303540Step N Loading Unloading
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