13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- The effective stress intensity range, ΔKeff, was defined by Elber as ΔKeff = Kmax - Kth, where Kth is a SIF threshold value below which no crack propagation occurs and Kmax is the maximal SIF value in a loading cycle [24]. Assuming the stress ratio, R = 0, as applied in the experiment, the threshold SIF value for a mild steel, as used in the experiment, is taken as Kth = 6.8MPa, [28, 29]. For the two cases considered crack propagation life was obtained as shown in Figures 9 a) and b). Figure 8. Ktot and Kappl values The FE analysis for the stiffened panel specimens showed that high tensile residual stresses in the vicinity of a stiffener significantly increase Kres and Ktot, as shown in Figure 8. Correspondingly, the simulated crack growth rate was higher in this region, which is in good agreement with experimental results, as can be seen in Figure 9b. Compressive weld residual stresses decreased the total SIF value Ktot. The model which does not take account of welding residual stresses could not simulate high crack growth rates in the vicinity of the stiffener, as can be seen in Figure 9a. a) b) Figure 9. Fatigue crack growth life for the applied stress range Δσo = 80MPa : a) without residual stresses, b) including residual stresses Fatigue crack growth simulation which takes into account the welding residual stresses provides thus better agreement with experimental results in terms of crack growth rate and total number of cycles. In conclusion, residual stresses in welded stiffened panels should be taken into account for a proper evaluation of SIFs and fatigue crack growth rates. 0 0.02 0.04 0.06 0.08 10 15 20 25 30 35 40 45 50 55 a (m) K (MPa m1/2) K tot K appl 0 1 2 3 4 5 6 x 105 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 N (cycles) a (m) a-N (K eff=Kappl-Kth) Simulation Experiment 0 1 2 3 4 5 x 105 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 N (cycles) a (m) a-N (K eff=Ktot-Kth) Simulation Experiment
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