13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 6. Behavior of crack opening stress 3.3. Effect of residual stress on crack propagation rate We conducted crack growth tests with three types of external load conditions; = 25, 50, 75 MPa. First, The results of = 50, 75 MPa are shown in Figure 8. The stress intensity factor range is calculated by equation 4. ( ) a F a K (4) Here, F(a) is a correction factor. The reason why the position of weld line of FSW is different is because these two tests are different in stress range . The maximum of the acceleration ratio of the crack growth rate for the FSW panel against the base material is 1 to 2 mm far away from the weld line after crack tip passes it. In case of = 50 MPa, the acceleration ratio is 2.1, and = 75 MPa the ratio is 2.3. The residual stress influences the crack growth rate. The FSW panels have the same level of residual stress distribution. So in case of external stress is high, the effect of residual stress is expected relatively low. This means the acceleration of crack growth rate for that situation is also low. However in this experiment, the difference on the acceleration ratio for crack growth rate is almost same at this two stress level; = 50, 75 MPa. The result of = 25 MPa is shown in Figure 9. From the figure, it is observed that the crack propagation rate is decelerated by the compressive residual stress when the crack tip is located before the weld line. After the crack tip had moves on the weld line, the crack propagation rate is accelerated by the tensile residual stress. Finally, when the crack tip is located after the weld line, the crack growth rate is gradually approached to that of the base metal. In case of = 25 MPa, the acceleration ratio of the crack growth rate for the FSW panel against the base material is 3.7. The acceleration ratio of each stress range is shown in Figure 10. In summary, the crack propagation rate is affected by the residual stress and stress range . All FSW panel using in this study is made 0 10 20 30 40 50 60 70 80 0 20 40 60 80 Crack opening stress OP MPa Crack length a mm Base metal ( =25 MPa) FSW panel 20 mm ( =25 MPa) Schijve equation ( =25 MPa) Base metal ( =75 MPa) Schijve equation ( =75 MPa) With unti-buckling plate Weld line of FSW [5] A
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