13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 3.2 Fatigue tests Fig. 4 shows the results of the fatigue tests for as-welded and heat treated specimens at ΔK = 8 MPa∙m½ and ΔK = 20 MPa∙m½. Table 4 shows the mean fatigue crack growth rate in the three zones (FM, HAZ and BM) of as-welded and heat treated specimens. When comparing a given zone at a given stress intensity factor range, the fatigue crack growth rate is always lower in the heat treated specimens than in the as-welded specimens, suggesting that the post-weld heat treatment improves the fatigue crack growth resistance. Pukasiewicz et al. found a similar behavior where the fatigue crack growth rate in the fusion zone of an as-welded CA6NM welded joint was considerably lowered after post-weld heat treatment [10]. In as-welded specimens, the fatigue crack growth rate decreases regularly from the filler metal and into the heat affected zone, before stabilizing in the base metal for both stress intensity factor ranges. These intra-specimen variations are believed to be attributed to microstructural effects and are the subject of an ongoing research. The gradual decrease of the fatigue crack growth rate is not observed in the heat treated specimen. At ΔK = 8 MPa∙m½, the fatigue crack growth rate remains fairly constant up to 9 mm from the fusion line (4 mm into the base metal), then decreases by approximately 50 percent and stabilizes at 13 mm from the fusion line. The same behavior, though less pronounced, is also observed at ΔK = 20 MPa∙m½, where the fatigue crack growth rate decreases by approximately 20 percent. Figure 4. Constant ΔK test results a) As-welded specimens b) Heat treated specimens Table 4. Mean FCGR in the FM, HAZ and BM of as-welded and heat treated specimens ΔK (MPa∙m½) Fatigue crack growth rate (mm/cycle) As-welded specimens Heat treated specimens FM HAZ BM FM HAZ BM Up to 9 mm from FL After 13 mm from FL 8 4.2∙10-6 3.7∙10-6 2.9∙10-6 1.9∙10-6 2.1∙10-6 2.2∙10-6 1.1∙10-6 20 7.5∙10-5 6.3∙10-5 5.8∙10-5 3.6∙10-5 3.7∙10-5 3.5∙10-5 2.8∙10-5 a) b)
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