13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 4.2. Fatigue crack growth behavior of heat treated specimens The crack closure behavior of the heat treated specimens is different. Crack closure was observed at both stress intensity factor ranges and for all crack lengths and the typical load-compliance offset curve is shown in Fig. 6a. Fig. 6b shows the effective stress intensity factor range ratio for the heat treated specimens when the crack is at 9 mm from the fusion line and at 13 mm from the fusion line. As a first observation, crack closure is more important at K = 8 MPa∙m½ for equal crack lengths. This is expected since crack closure is more pronounced for low stress intensity factor ranges [14]. Furthermore, up to 9 mm from the fusion line, the effective stress intensity factor range ratio remains above Newman’s theoretical upper bound for both stress intensity factor ranges. This is an indication that tensile residual stresses may have remained after post-weld heat treatment, partially inhibiting crack closure. These results are consistent with previous studies that showed that the post-weld heat treatment may not completely eliminate the residual stresses in CA6NM welds [2, 3]. On the other hand, the effective stress intensity factor range ratio of the crack at 13 mm from the fusion line is below the upper bound predicted by Newman for plasticity-induced crack closure for both stress intensity factor ranges. This is an indication that tensile residual stresses at the crack tip may have relaxed with crack growth and that crack closure is not inhibited. Tensile residual stresses are known to redistribute and eventually relax following the growth of a fatigue crack [15]. After complete relaxation of the tensile residual stresses, common closure mechanisms such as plasticity-induced, roughness-induced and oxide-induced crack closure become fully active and the global closure level is increased. The evolution of the measured effective stress intensity factor range ratio against the crack length is shown in Fig. 8 (triangles). Crack closure is rather constant up to 9 mm from the fusion line, than it increases as the crack grows and eventually stabilizes. A good correlation is observed between the profiles of the fatigue crack growth rate (circles) and the effective stress intensity factor range ratio (triangles). This shows that the fatigue crack growth behavior of the heat treated specimens is directly affected by the variation of crack closure. Figure 8. Relationship between U and the FCGR against the crack length for heat treated specimens a) ΔK = 8 MPa∙m½ b) ΔK = 20 MPa∙m½ a) b)
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