13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Experiment 2.1. Sample In order to investigate the damage and its evolution of the voids, a group of two-hole stripe sample is selected for the uniaxial tension fatigue test. The sample size is cut to 125mm length, 27mm width and 3mm thickness (Fig.1). The hole pair with the diameter 1mm and the hole center distance 1.5mm are embedded in the center zone around the center point of the plate, and the holes are machined to through the whole thickness, i.e., through hole. The angle of the center line of two holes with the cross section α is selected as 0 and 15 degree. Considering the sample thickness is rather small contrasted to the other sizes, the plain stress condition is applied in this test. The material of specimen is selected as Al-alloy, with the Young’s module 56.36GPa, Poisson’s ratio 0.33, Yield stress 308.6MPa and Ultimate strength 450.8MPa. 2.2. Test The fatigue test is implemented using a testing machine MTS-858, with the loading amplitude 0-16KN and loading frequency 1Hz (Fig.2). As the loading circle increased, the cracks will initiate from the edge of some hole early or late. Simultaneously, a CCD camera is used to record the process of the crack initiation and extension. 2.3. Result It is widely believed that the first initiated crack should start from the inner edge of two holes, such as points B and C; owing to the circle stress in these points is larger than that in the outer points A and D. It is surprised that, for α equals 0, the first crack occurs from one point of the outer edge, the point A, see Fig.3. As the loading circle increases, the crack initiation has a little complicated. For example, in one sample, the second crack starts from the inner point of the other hole edge, the point C (see Fig.4), whereas, in another sample, we also find that the second crack starts form the Fig.1, The sample of the fatigue test
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