13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- factor range (defined as the maximum SIF minus the opening SIF) can be used to uniquely correlate with crack growth rate under different loadings. However, this experiment shows that under single overload loading, crack opening SIF becomes variable and crack starts to kink and bifurcate. These mechanics cannot be explained only by the phenomena of crack closure. Further, a statistical study of crack growth rate under constant load and overload lading is conducted to verify this hypothesis. Several experiments have been completed at stress ratio R=0.5, 0.33, 0.1 and 0.025, and the result of crack growth rate with the effective SIF is plotted in Figure 12. Figure 12 (a) shows the result under constant loading. It can be found that all the data shows a good linear relationship in log-log scale, except one data in red dashed circle. Figure 12(b) shows the data together with the single overload testing. It is observed that the two data points are not consistent with the linear trend from the constant amplitude loading data. This observation further indicates that other mechanism also contributes to the crack growth under single overload loading, such as crack branching and blunting as observed in the experiment. (a) Constant load (b) Constant loading and overloading load Figure 12. Crack growth rate with SIF under constant load and overloading load 3. Conclusions In this paper, in-situ experiment at micrometer scale and nanometer scale has been introduced and the observation of crack closure behavior of steel 4340 and aluminum 7075-T6 under constant loading has been obtained. Besides, preliminary crack closure behavior under single overload on aluminum 7075-T6 is studied. Several conclusions can be made from the experiments as follows. 1. Crack closure is directly observed in Al 7075-T6 under both constant and variable loads, but it is not observed in steel 4340 under constant loading at stress ratio 0.1. 2. The crack closure can significantly change the plasticity distribution in front of the crack tip; 3. Keff defined as the difference of Kmax and Kopen in the classical fracture theory correlates with the unique crack growth rate under constant loading, but it not enough for crack growth prediction under variable loading. Additional parameter for the mechanism is need for the prediction;
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