Study Of Fatigue Performance And Fatigue Life Prediction Method Base On Welded Structures

Fatigue are important performance indicators of welded structures and the fatigue life of welded structures is a reasonable decisive factor to measure the structural welding process and structure design whether reasonable. Fatigue experiment is the best way to study welded structures fatigue performance and fatigue life, but, whose characteristics of time consuming and costly cause the method can not be applied frequently in the process of structural design. A variety of fatigue life prediction method avoid the time-consuming and costly shortcoming of experiment, although all of the fatigue life method of prediction have more or less lost in accuracy comparative to experiment method, but the fatigue life estimation method in engineering field is still important. Based on grain aggregate microscopic model, use the fatigue life prediction method(field and energy) to study the performance and fatigue life of welded joints,achieved the following innovative results:1. By means of electron microscopy pictures(EBSD), the skill of image processing and modeling software(UG) to establish a three-dimensional micro model which contain basic material zone(BM), weld material zone(WM), heat affected zone(HAZ)and grain boundaries, also, establish the constitutive model of grain boundaries.Compare with the actual welding joints, it can be found that the micro three-dimensional model can stand for actual welding joints well.2. Execute transient analysis of elastic-plastic deformation to three-dimensional model. Found that larger stress and strain values appear in the heat affected zone, the weld zone, the junction of base metal and heat affected zone. The reason of stressconcentration appear in grains is that, first, coarse grains, which course grains bear greater loads; second, grain distortion, irregular grain shape will directly lead to stress concentration in grains.The reason of stress concentration appear in grain boundaries is that, first, irregular grain boundaries, same to stress concentration appear in grains,irregular grain boundaries shape will directly lead to stress concentration; second, a plurality of grain boundary intersection, when a plurality of grain boundary intersection with a point, the grain boundary withstand more loads. Conclusive, the Q345 B welded joints cracked in the junction of basic material and heat affect zone, and spread along the direction of external loads.3. This paper argues that the fatigue failure region is a zone of plastic deformation of materials, and use field method to estimate the fatigue life of welded joints based on micro model, compare with the results of energy method and experimental data, the suppose can guarantee both accuracy and computation cost are acceptable, furthermore,proving the fatigue crack initiation and propagation with plastic deformation directly correlated. Application of four-point correlation method to estimate Q345 B material parameters and then applied to the calculation of the energy method, compare with the results of field method and experimental data, it found that energy method own high computational accuracy and low computational costs.4. Under low amplitude load, the results of energy method more closer to experimental data, under high load amplitude, the results of field method is more closer to the experiment data. Taken together, the results of the energy method is more conservative and more convenient.