13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Note the experimental equilibrium size of process zone is measured directly from the fracture surface as shown in Figure 4 (a). The CL model predictions agree very well with observations. It is also interesting to compare eq pz with conventional D-B model prediction. The process zone size DB pz in D-B model is determined by the requirement 0 tot K with tot K defined by tot dr K K K . It is also shown in Figure 4 (b) that the experimentally observed equilibrium size of process zone eq pz is significantly smaller than DB pz predicted by D-B model. In this specific example, the equilibrium size of process zone eq pz is within the range of 1.3~2.8mm, whereas D-B prediction DB pz is between 3.8~8.8mm, three times larger. This could be expected since the D-B model ignores the energy dissipation by cold drawing of original material into the oriented fibers of PZ. There are clear trends in eq pz dependency on load and temperature: 1) For a given temperature, the equilibrium PZ size eq pz linearly increases with increase of load; 2) For the same value of K∞, eq pz increases with increase of temperature; 3) The eq pz in CL model is significantly smaller than the D-B zone size; 4) The difference between the equilibrium PZ in CL and D-B zone sizes increases with increasing load. (a) (b) Figure 4. (a). Fracture surface of a CT specimen test at 80°C (Note: the first striation is resulted from the pre load and thus ignored); (b) Comparison of predicted eq pz values with observations from (a). The PZ and crack thermodynamic forces (Eqs. 1-2) are non-linear functions of crack and process zone lengths. Thus, the system of Eq. 3 despite of its simple appearance is a nonlinear system of ODE, solution of which calls for numerical methods. Below we show two examples of numerical simulation of CL growth in a compact tension (CT) specimen that has the same geometry as SCK except holes. SIF in CT specimen increases with crack length. The first example is shown in Figures 5 (a). It presents a discontinuous, stepwise crack layer growth from one stationary CL configuration to the next one. At the beginning the crack is stationary, whereas PZ grows toward its equilibrium size 9.5 L mm. The first equilibrium size of CL L is reached in about 4 t hours , and is maintained constant until the degradation of PZ material triggers the crack growth into PZ. It is depicted by the lower dash line moving up at 28 t hours . The crack propagates through PZ and gets arrested at the time 33 t hours , when it meets the original material at the tip of PZ. PZ grows accompanying the crack growth, since 0 PZ X during this time; PZ X decreases with an increase of the PZ length and CL
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