Fatigue is a common phenomenon in welded structures. Prediction of fatigue life of welded joints in-service is still an unsolved puzzle by the conventional linear ultrasonic testing method. However, the nonlinear ultrasonic waves or the acoustic nonlinear signal can provide clear signs of the accumulative fatigue damage in materials, as reported by a number of researchers. Hence, the nonlinear ultrasonic testing method has revealed a tremendous potential for fatigue damage evaluation. This paper presents a study to characterize the fatigue damage using the analysis of the signal characteristics and a new nonlinear parameter. Based on the very high-cycle fatigue testing results for a 6N01 aluminum alloy welded joint, the relationship between the amplitude of the second-order harmonic and fatigue cycling has been established. The nonlinear ultrasonic system test results show that the amplitude of the second-order harmonic increases at the early fatigue stage, with further increase in cyclic loading until reaching a peak. Metallographic examinations show that a fatigue crack will nucleate in the weld joint in the stage as the amplitude reaches the peak value. Finally, theoretical and experimental results confirm that the amplitude of the second-order harmonic is useful for assessing the fatigue life of a 6N01 aluminum alloy welded joint.
Ferrite stainless steel has been widely used in foreign automobile vent pipe. During the production of domestic automobile vent pipe, because the crystal grains of welding Joint heat-affected zone are not refined by heat treatment and become very big, forming properties of cold machining of ferrite stainless steel become bad. In order to optimize welding technology, the temperature field of laser welding joint of stainless steel automobile vent pipe is simulated. The temperature field model of laser welding joint of 409 L stainless steel pipe is built based on the pair heat source model. At the same time, the affecting factors are considered, such as thermal radiation, phase change latent heat, thermophysical properties of materials with temperature changes and the spread of convective heat in the liquid metal in the model. The temperature field in different welding parameters are simulated. The impact of process parameters on the temperature distribution is acquired. The experimental results can help works, such as accurately understanding laser welding thermal process, predictive control of welding stress and deformation, and improving cold forming quality of automobile vent pipe.
In the present study, damage evolution in rolled Al-Zn-Mg alloy and its welds is evaluated using the acoustic emission (AE) method and crack initiation is detected using digital imaging during fatigue tests. The AE characteristics and source mechanisms are analysed based on microstructural
and fractographic observations. The experimental results show that AE energies are effective indicators for detecting fatigue crack initiation in Al-Zn-Mg alloys. The results obtained were verified through digital images of the notch tip region of the Al-Zn-Mg alloy samples. For small percentages
of the applied load range close to the peak load, the AE count rates show a reasonable correlation with the crack propagation rates. These correlations can be applied to predict the remaining service life of fatigue-damaged structures. The analyses performed demonstrate that the AE technique
is sensitive to variations in the fracture mode and could be applied to monitor fatigue damage evolution in welded structures.
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