13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- 3.3. Influence of water concentration on SCG We investigate the influence of the water concentration (R.H.) on SCG in the zirconia polycrystal. We propose to account for the influence of R.H. by invoking a dependence of the energy barrier U0 with the concentration of water molecules. By considering that an increase of R.H. results in decreasing the activation energy U0(R.H.) that will facilitate the cracking. We consider an activation energy U0 = 160 kJ/mol of the ambient air. We reduce this value by 10 % for an environment with more water and increase the reference value of U0 by 10 % for a drier environment. For each values of n 0 = 900 MPa and 1150 MPa, we report the prediction in terms of V-KI in Fig. 10a and Fig. 10b respectively. We observe that reducing the value of U0 increases the value of the crack velocity for a given load KI, while preserving the slope of the curve V-KI. However, the dependence of threshold load K0 in the water concentration is not affected by the variation of U0( R.H.). This prediction is not in agreement with the experimental observations by Chevalier et al. [2] (see Fig. 1). We now examine the dependence of the threshold stress n 0 of cohesive zone with the concentration of water molecules. When the water concentration increases, we now reduce U0 by 10% (U0 is increased by 10% for a drier environment). Therefore, we consider n 0 = 900 MPa and U0 = 160 kJ/mol as reference values in ambient air. The results in terms of V-KI curves are reported in Fig. 11a. In Fig. 11b, we reported the results of the case with the references values of ambient air n 0 = 1150 MPa and U0 = 160kJ/mol. We observe that reducing the water concentration increases the threshold load K0. These predictions are in agreement with the experimental observations in the influence of R.H on SCG by increasing the crack growth kinetics and reducing of threshold load of SCG K0 with increasing R.H. In order to account to the first effect, we show that the dependence of energy barrier U0 with R.H. takes account the variation of SCG kinetics with R.H. Nevertheless, it is important to take account the dependence of the threshold of activation of reaction-rupture mechanism n 0 with R.H. At least this holds for the zirconia. Figure 10. Influence of water concentration R.H. by its dependence with U0 (R.H.) on the prediction of the V-KI curves and K0 for Polycrystals of Zirconia with G = 0.8 m, (a) n 0 = 900 MPa, (b) n 0 =1150 MPa.
RkJQdWJsaXNoZXIy MjM0NDE=