13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- cracks. The fracture surface has strong stereoscopic impression. Table 2. Fatigue crack growth rate of Q&T HPS 485W steel Regression Parameters Thickness R m C（m/cycle） logC Correlation coefficient S 0.1 2.84 1.05×10-11 -10.98 0.96 0.06 0.5 4.13 2.37×10-13 -12.62 0.97 0.05 7.5mm 0.8 3.26 3.64×10-12 -11.44 0.96 0.05 0.1 2.53 2.19×10-11 -10.66 0.87 0.12 0.5 2.72 1.48×10-11 -10.83 0.84 0.11 12.5mm 0.8 3.16 6.11×10-12 -11.21 0.73 0.1 0.1 3.28 2.29×10-12 -11.64 0.99 0.05 0.5 3.29 2.82×10-12 -11.55 0.98 0.05 19.5mm 0.8 3.29 3.24×10-12 11.49 0.99 0.05 错误！未找到引用源。错误！未找到引用源。 (a) Test result for 7.5mm specimen (b) Test result for 12.5mm specimen 错误！未找到引用源。 (c) Test result for 19.5mm specimen Figure 2. log(da/dN) and log(△K) curves for HPS485W specimens 2.3. Threshold value test Threshold value (ΔKth) is a significant factor in fracture control design of steel structures and load decreasing procedure is used for ΔKth test. ΔKth for 19.5mm HPS 485W specimen under R=0.1 is tested and the test result is shown in Figure 4 [7]. The calculated theoretical threshold value is 7.22MPa•m0.5. Figure 3. Fracture surface of specimen Secondary Crack
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