13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- (a) Al 7075-T6 (b) Steel 4340 Figure 8. Crack tip under minimum loading of Al 7075-T6 and steel 4340 This result is consistent to the experiment under optical microscope and they show that the existence of crack closure in Al 7075-T6, but it is not observed in steel 4340 at this stage. 3.2. In-situ SEM study under variable loading with single overload As crack closure is observed both under optical microscope and SEM, its effects on crack growth rate is not negligible for fatigue crack growth prediction for some metallic materials, if it happens. However, it is not clear that the crack closure is the only/dominant mechanisms for aluminum materials, especially under different loading conditions. Thus, the crack closure effect on crack growth rate under variable loading is investigated and is shown below. The procedure of SEM experiment under variable loading is the same with that under constant loading. The only difference is the loading profile. In this experiment, a cycle of over-load is inserted in a constant loading spectrum. The constant loading ranges from 10 to 30 MPa·m0.5 and the overload ranges from 10 to 36 MPa·m0.5. Thus the over load ratio is 1.2 as the ratio of two maximum load. A schematic plot is shown in Figure 9. In order to observe and compare the effects of overload loading, four types of loading cycle are chosen for observations. The first type is the constant loading cycles before the over load (cycle A in Figure 9), which provides the baseline information of the crack propagation kinematics. The second and the third type are the overload cycle (B) and the cycle right after it (C). The last type is the cycle 5~20 cycles after the overload (D). In this study, two experiments data are obtained. In the first experiment, the background stress ratio is 0.33 and the overload ratio is 1.2. In the second experiment, the background stress ratio is 0.025 and the overload ratio is 1.1. In the following discussion, the result of the first experiment will be discussed in detail and the second one will be given as a comparison. Following the experiment procedure and the loading profile, the details of crack growing before, during, and after overload under SEM is obtained and compared. For all testing measurements, the crack opening stress intensity factor at each observing cycle are plotted in Figure 10. The X axis is cycle number and Y axis is the ratio of crack opening SIF over the maximum SIF (Kopen/Kmax). It is shown that during the constant loading in stage 1 (cycle A), Kopen/Kmax is 0.57. In the overload cycle (B) and right after the overload cycle (C), the crack opening slightly decreases and increases to about 75% after more than 10 cycles (D). Another experiment has been performed with stress ratio R=0.025, the maximum SIF of the constant loading is 26.26 MPa·m0.5 and the overload ratio 1.1. Very similar result is observed as shown in Figure 11.
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