13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- outer edge of the other hole, the point D. As the cracks started from the outer side of the hole pair propagate to a distance, such as 0.8mm in our experiment, the first two crack stop, meanwhile, the new crack initiated from the inner edge of the holes. It is the inner edge cracks link each other, the two holes connected (see Fig.5). Thus, as the loading circle increases continually, the outer edge cracks restart propagate till arrive to the edge of the sample. This phenomenon also occurs in the test of the sample with α equals 15 degree. As the loading circle increases, the first crack initiates from the outer edge of two holes, i.e., the point A or D. As the first two cracks propagate to some length, the crack driving force disappears, thus the cracks stop (Fig.6). Next, the inner edge cracks initiate (Fig.7) and propagate to each other till two holes connecte (Fig.8). It is obvious that this phenomenon (the crack initiates from the outer edge firstly and after it propagates to some length, the inner crack initiates and propagates.) cannot be explained by the theories proposed till today, such as the maximum tangential stress theory, the maximum tension stress theory, and so on. Thus, in the next section, the crack initiated feature and the stress field around the holes will be investigated in detail to describe the phenomena, and a new theory based on the equivalent plastic strain field will be proposed. Fig.2, The fatigue test using MTS Fig.3, The first crack initiated from the outer edge of the hole
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