13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- the crack closure behavior under overloading load is investigated in detail. Meanwhile, a statistical crack growth rate under constant loading and overloading load is compared. Finally, a discussion for the existence and insufficiency of crack closure for the fatigue crack growth analysis is given based on the current investigation. 2. In-situ optical microscopy study for crack closure In in-situ optical microscopy test, a unit load cycle is divided into several steps and images are taken at each step. By digital image correlation analysis, then the strain distribution can be calculated. When the load is decreasing, the plastic zone is increasing due to the reversed plastic flow. However, if the crack is closed, the crack surfaces will contact and begin to undertake the reversed stress, thus the reversed plastic zone size will stop increasing. The reversed plastic zone size is a better indicator for crack closure than many other methods due to the advantages of non-contact and a stable precision inherited from digital image correlation method. 2.1. Experiment set-up and procedure The experimental set-up for in-situ optical microscopy experiment is shown in Figure 1. It contains two parts: a palm-sized tensile stage and an optical microscope system. The tensile stage is fixed on the microscope and the cyclic loading is applied to the specimen. A Nikon metallurgical microscope is used to monitor the specimen surface and a high resolution imaging acquisition system is used to record images during the testing. Figure 1. Experiment setup The specimen is a ingle edge notched plate with width W = 8 mm, length L = 52 mm and thickness T = 0.86 mm. Due to the small thickness of specimens, it is assumed to be under the plane stress conditions for a relatively long crack. An edge notch of length 1mm is machined on the specimen. After machining, the specimens are pre-cracked until the initial crack reaches about 1 mm. The pre-cracking procedure follows the ASTM standard E647-08. Following this, both surfaces of the specimen are polished with the sandpaper whose average particle diameter is smaller than 10 um. The experiment procedure contains four major steps. Step 1: specimen manufacturing and pre-cracking. Single edge notched plate specimen is used for testing. After notching, the specimens are pre-cracked under a hydraulic tension machine INSTRON 1331 until the initial crack reaches about 1 mm. The pre-cracking procedure follows the ASTM standard E647-99; Step 2: polish the specimen to form randomly distributed small dark regions on the smooth surface,
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