13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 4. Discussion This section presents an analysis of the constant stress intensity factor range tests results. The higher fatigue crack growth rates found in the as-welded specimens are discussed in terms of tensile residual stresses causing the opening of the crack. The fatigue crack growth rate variations observed in the heat treated specimens are discussed in terms of residual stress relaxation and crack closure. Fig. 5 shows the base metal fatigue crack growth curves obtained at load ratios R = 0.1 and R = 0.7 [1], along with the mean fatigue crack growth rate obtained in the base metal from the constant stress intensity factor range test results (Table 4). The mean fatigue crack growth rate in the base metal of the heat treated specimens was calculated from the stabilized data starting at 13 mm from the fusion line. For the heat treated specimens, a good agreement is found between the R = 0.1 base metal fatigue crack growth curve and the fatigue test results, for both stress intensity factor ranges. These results, along with the intra-specimen fatigue crack growth rate variations noted in the last section (Fig. 4b), are discussed further in terms of residual stress relaxation and crack closure. Furthermore, the fatigue crack growth rates in the as-welded specimens at ΔK = 8 MPa∙m½ (empty circles) and ΔK = 20 MPa∙m½ (filled circles) correlate well with the R = 0.7 base metal baseline. Several indications suggesting the presence of tensile residual stresses responsible for the higher effective load ratio seen by the as-welded specimens are discussed next. 4.1. Fatigue crack growth behavior of as-welded specimens The typical load-compliance offset curve for as-welded specimens is shown in Fig. 6a. For both stress intensity factor ranges and at all crack lengths, the specimen’s response does not deviate from the fully-open crack compliance. This indicates that the crack remains fully-open and that the effective stress intensity factor range is equal to the applied stress intensity factor range. This corresponds to an effective stress intensity factor range ratio (U = ΔKeff/ΔK) equal to unity in Fig. 6b (circles). It is well known that, for ductile materials free of residual stresses, plasticity-induced crack closure plays a significant role in reducing the effective stress intensity factor range [11]. Figure 5. Comparison between the mean FCGR in the BM obtained from the constant ΔK tests and the BM fatigue crack growth curve in the Paris regime at load ratios R = 0.1 and R = 0.7 [1]
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