13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 7 shows the σy stress component along a selected path in the crack line for the crack length a=4.7 mm, where in the model both, residual stresses and the loading stress range, Δσo = 80MPa, are implemented. A higher stress concentration is observed in the crack tip region. Figure 6. Welding residual stresses in the stiffened panel specimen Figure 7. σy stress component along a selected path in the crack line 4.3. Stress intensity factors and fatigue crack growth rate For evaluating SIFs by FEM, the crack tip displacements extrapolation method was implemented [26]. In the general post processing procedure, the ‘‘KCALC” command was used to calculate SIFs. The Mode I SIF values, KI, are determined for a stiffened panel specimen for a loading stress range Δσo = 80MPa, assuming the presence of residual stresses as described above. SIF values with respect to crack length a are given in Fig. 8. Kapp represents the SIF values due to the applied stress range only, without residual stresses. Ktot represents the SIF values for the case when the residual stresses are taken into account along with the external loading stress range. It can be seen that residual stresses significantly increase Ktot values for shorter crack lengths, where tensile residual stresses prevail. Between the stiffeners residual stresses reduce the Ktot values. Assuming material constants of a power law equation, C=5.05*10-11 and m=2.75 [27], fatigue life was simulated by integrating the power law equation as given by equation (5) (The units for ΔK and Δa/ΔN are [MPa ·m1/2 ] and [m], respectively): ݀ܽ݀ܰ ൌ ܥ ൫∆ ܭ ൯ (5)
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