13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Fig.8 Comparison between the Finite element analysis values and the experimental results It is interesting that a metallic noise could be heard just before the final fracture due to the decreasing of resonance frequency of welded joint specimen. The natural frequency of specimen was recorded per 20000 cycles in the course of the fatigue experiment and the changes of frequency in term of natural frequency ratio verse fatigue life ratio were shown in Fig.7, where Nf, f0 were the final fatigue life and initial natural frequency, respectively. From the collected data of HAZ, a stable platform of natural frequency ratio could be observed, which occupied almost the whole fatigue life. Further, the fatigue life ratio was larger and the final stop frequency ratio for fracture was smaller with the higher loading stress. But for FZ specimen, the frequency ratio kept going down from the beginning of the test with no platform due to the fact that the frequency descent life was affected by the max size defect located in fusion zone. As mentioned previously, welded joint specimen fractured without obvious temperature-rise period, therefore, it could be due to the reason that fatigue propagation lead to the decreasing of natural frequency of welded joint. In order to verify, FEM model analysis method was employed to ascertain the relationship between natural frequency and fatigue crack size at the surface of specimen under the resonance condition. A half specimen model was established for computation time saving with a crack located in the middle section of the specimen. In the process of the resonance, the state of compression would result in crack surface contact, but any non-linear setting was not included in the analysis in ANSYS, which was neglected automatically, otherwise. To prevent the effect of crack surface compression on the analysis, an additional element layer with the thickness of 0.1mm was attached to the surface of fatigue crack. The modulus of elasticity for extra element is 1GPa, which was considerably less as compared to Q345.With the help of additional layer, the model of specimen remained linear without any penetrations of elements at the crack surface. After the model analysis of specimen with cracks in different sizes, the relation between fatigue crack size and natural frequency of welded joint specimen was obtained as presented in Fig.8. It could be found that with the increase in size of fatigue crack, the rate of natural frequency decreased. In this experiment, the final stop frequencies and max fatigue crack sizes from SEM observation were obtained on five HAZ specimens as plotted with solid triangle in Fig.9. It is noted that the experimental data just located near to the fitted curve of FEM, which confirmed that the decreasing of natural frequency was caused by the
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