13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 3. Two-parameter Weibull distribution of experimentally measured Jc-values. 5.3. Calibration of Weibull Stress Modulus The parameter calibration scheme described in Gao et al. [15] and Ruggieri [5] is applied to determine the Weibull stress parameters for the tested pressure vessel steel. The Weibull modulus, m, is calibrated using the SE(B) specimens. Because these specimens were not tested at the same temperature, the present methodology adopts a simple procedure to correct the measured toughness values for temperature in which the cJ -values for the deep crack SE(B) specimens ( = a W 0.5) at = T −80°C are scaled to corresponding cJ -values at = T −60°C using the Master Curve fitting obtained previously. The characteristic toughness values for the deep notch and shallow notch SE(B) specimens are then given as: = 0J 116.5 KJ/m 2 and 215.2 KJ/m2. With the toughness values for the deep and shallow crack SE(B) specimens set at the same temperature ( = T −60°C) and using wσ vs. J curves constructed from the 3-D finite element analyses for both crack configurations, the calibration procedure is then applied to determine the m-value that yields the best correction / 0.2 / 0.5 ( ) 0 ( ) 0 = = → a W a W SE B SE B J J . Table 1 provides the calibrated Weibull stress modulus, m, for the tested material based on different definitions for wσ : 1) 0=γ (no plastic strain correction); 2) 1=γ and 0 =β (linear plastic strain correction); 3) 1 2 =γ and 0 =β (square root plastic strain correction) and 4) Beremin plastic strain correction. Table 1. Calibrated Weibull parameter, m, for different forms of wσ Weibull Stress Model m Standard Beremin ( 0=γ with no plastic strain correction) 10.4 Generalized Weibull Stress with 1=γ and 0 =β (linear correction) 28.5 Generalized Weibull Stress with 1 2 =γ and 0 =β (square root correction) 28.0 Plastic Strain Modified Beremin Model 29.0
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