Study On Low-Cycle Fatigue Properties And Failure Mechanism Of Welded Plate Component On Steel Structures

When the earthquake occurs,the welding joints of the high-rise steel structure are subjected to extremely large alternating loads,and obvious fatigue cracks may occur which in turn causes damage to the structural welding joints.At this time,the main failure mode of the material is high strain low-cycle fatigue damage and fracture.Therefore,it is necessary to study the low-cycle fatigue failure mechanism and life prediction method of welds in high-rise steel structures to avoid fatigue damage.In this paper,the commonly used welded plate components are taken as the object of study.The fatigue fracture theory of welded components and the Paris formula of crack propagation are first described.The high-precision,high-pixel microscopic measuring instrument was used to observe the development of fatigue cracks in the welded joint during the low-cycle fatigue loading of welded plate components.Combined with the fatigue fracture theory of metal welded structures,the welded joint area of welded plate components was qualitatively analyzed.Experimental studies have shown that the fatigue crack propagation rate is very small in the near-threshold expansion stage,and the crack propagation rate is increased in the high-speed expansion region,and the crack propagation rate is rapidly increased when the maximum stress intensity in the fatigue cycle is close to the material failure threshold.Finally,the main crack penetrates and causes the component to break.Different-segment Q345 welded plate components were subjected to constant-amplitude high-strain low-cycle fatigue tests until the specimens were destroyed.During the loading process,the three-dimensional digital image correlation method(3D-DIC system)was used to test the welds and its heat affected zone in fatigue.Load the strain value in the whole process and measure it.Using the traditional strain data acquisition TDS method and the strain data collected by the 3D-DIC system to form a comparison,the results show that the difference between the readings of the 13 specimens of the two acquisition methods are less than 15%.It is reasonable and effective for the welding member to collect the strain data through the 3D-DIC system for the study.The experimental study shows that with the increase of the number of cycles of low-cycle fatigue loading,the weld zone exhibits a significant strain concentration phenomenon,and the final failure position of the test specimen conforms to the law that the fatigue crack starts from the strain peak.The comparison between the different sets of specimens shows that the low-cycle fatigue performance of the automatic weld is better than that of the artificial weld.Because the equal-thickness specimen does not destabilize prematurely,its low-cycle fatigue performance is better than that of the local thickness-reduced specimen.The weld material produces an austenite to martensite transformation during the welding process.The hard and brittle martensitic structure causes the weld material to harden.Therefore,from the cyclic stress-strain response curve of the test piece,the weld shows a certain cyclic softening characteristics during the low cycle loading.The force-displacement hysteresis curve of the test specimen is biased towards the tension side,and the weld metal exhibits obvious tensile and compressive plastic strain asymmetry during the cycle.This phenomenon is consistent with the Bauschinger effect of the material.Based on the study of the fatigue properties of welded plate components,a low-cycle fatigue life model based on continuum damage mechanics was deduced according to the basic theory of damage mechanics.The model agrees well with the low-cycle fatigue test results of 13 Q345 welded plate components.Under the same fatigue strain amplitude,the low cycle fatigue life calculated by the Manson-Coffin equation method is very close to that calculated by this model.However,the modified general slope method and the elasto-plastic notch stress-strain method have relatively poor accuracy due to the defect of the derivation model itself.The required parameters of the model established in this paper are the basic parameters of the material fatigue analysis,which can be obtained through the material low-cycle fatigue performance test,with high applicability and feasibility.