13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- fatigue crack propagation and fatigue crack propagation lives of HAZ and FZ could be estimated by monitoring the natural frequency of specimen. 3.5 The crack propagation lives of BM, HAZ and FZ specimens Table 3 Fatigue crack propagation life of different specimens Specimen Stress/Mpa Total life/cycle Propagation Life/cycle Ratio Np/Nt BM 350 6.08×108 1.72×106 0.28% BM 365 1.83×108 7.27×105 0.40% BM 370 5.60×107 1.66×105 0.29% HAZ 200 3.12×108 8.8×105 0.28% HAZ 220 7.42×107 5.8×105 0.78% HAZ 230 3.02×107 3.0×105 0.99% FZ 150 6.62×106 6.6×105 9.97% FZ 185 1.26×106 9.6×105 76.2% FZ 195 2.18×106 1.08×106 49.5% The natural frequency of welded joint specimen was analysed in this section to clarify the fatigue crack propagation characteristic in VHCF range. It was believed that the fatigue crack start to grow at the moment that frequency of specimen decreased over 20Hz per 20000 cycles. For BM specimen, the period of temperature rise just before the fracture could be regarded as the crack propagation life, as discussed earlier. When the amplitude of temperature variation was larger than 5℃ per 700cycles, fatigue crack propagation was started for BM specimen. Finally, the fatigue crack propagation lives of BM, HAZ and FZ specimens were obtained in Table.3. Similar changes between BM and HAZ specimens were observed. It showed that with the increase in the loading stress, lower crack propagation life and ratio of propagation life to total fatigue life was obtained. The crack initiation process occupied over 99% of the total fatigue life for BM and HAZ in VHCF range. Therefore study on the mechanism of crack initiation was extremely important to clarify the fatigue properties in VHCF. In addition, the life ratio of FZ specimens were much larger than that of BM and HAZ, due to the fact that fatigue crack in FZ propagated directly from the welding defect with no apparent initiation process. 4. Conclusions In this paper, ultrasonic fatigue test was performed on Q345 BM and welded joint to investigate the properties and failure mechanism in super long life fatigue range, the conclusions can be summarized as follows: 1.The fatigue strength of BM and HAZ continues to decline with the increase in fatigue live, and fatigue failure still occurred up to 109 cycles. Compared with BM specimen, the fatigue strength of FZ and HAZ decreased about 60% and 55% respectively. 2. The fatigue crack propagation process would lead to the temperature increment of BM and the decreasing of natural frequency descent in the test. Thus fatigue propagation life could be evaluated
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