Study On Low Cycle Fatigue Life Prediction Of Steel And Its Welded Joints

Welding connection is the most important connection method in modern steel structures,welded steel structures are widely used in aerospace,bridges and high-rise buildings and other fields by virtue of their own advantages.However,due to the mismatch between the mechanical properties of the metal in the weld zone and the base metal,welding defects such as porosities,slag inclusions,cracks,undercuts,etc.are often introduced during the welding construction.These defects will cause stress concentration and fatigue failure at the weld.Under the action of earthquake,wind load or other equal or variable amplitude alternating loads,the welds of steel frame beam-to-column welded joints are prone to low-cycle fatigue failure,which may even threaten the safety of the entire structure.Therefore,the low-cycle fatigue life prediction research on steel and its welded joints has important scientific research significance and engineering application value.Based on the improved low-cycle fatigue life prediction methods and fatigue cumulative damage theory,this paper predicts the low-cycle fatigue life of steels and the beam-to-column welded joints of steel frame structures.The main research contents include:(1)The improvement of the Manson-Coffin model considering the yield stress.The yield stress is introduced on the basis of Manson-Coffin’s existing improved model,the fatigue strength coefficient is further modified,and two new and improved Manson-Coffin models are proposed.The validity and accuracy of the model are verified by using low-cycle fatigue test data of standard specimens of commonly used materials and engineering components.(2)Prediction of low-cycle fatigue life of beam-to-column welded joints.The finite element numerical simulation of steel frame beam-to-column welded joints is carried out,and the influence of different load conditions on the stress and strain distribution characteristics and failure laws of welded joints is discussed.Based on the local stress-strain method,combined with the improved model proposed in this paper,the low-cycle fatigue life prediction of beamto-column welded joints under symmetrical and asymmetrical loads is carried out,and the influence of mean stress on the fatigue life of welded joints is compared and analyzed.(3)Prediction of low-cycle fatigue life of steel frames.A multi-scale model of steel frame including solid beam-column welded joints is established and finite element analysis is carried out.The influence of the number of solid welded joints on the fatigue performance characterization of the multi-scale steel frame model under seismic load spectrum is discussed.Based on the local stress-strain method,combined with the improved model proposed in this paper,the low-cycle fatigue life prediction of the steel frame structure under earthquake action is carried out.The research work in this paper can provide reference for the weld fatigue research and low-cycle life prediction of beam-column welded joints of high-rise steel frame structures.