The Plastic Strain Fatigue Crack Initiation Analysis Of Welding Steel Beam-column Node Under The Strong Earthquake

The steel frame structure has been widely used in building structures because of its unique advantages.However,the damage survey results of the steel frame structure under strong earthquake shows that the weld area of beam-column welded joint is the weak link of the steel frame structure,the crack initiation in the region and then grows to damage.The crack will seriously endanger the normal use of the engineering component,so it's important to study the initiation life of the crack.This paper study the crack initiation life of the beam-column welded joint of the steel frame structure under strong earthquake from view of fatigue,it provides an important basis to evaluating the remaining life of the beam-column welded joint and monitoring the fatigue damage.In this paper,the theory of multi-axial fatigue is used to evaluating the crack initiation life of the beam-column welded joint.Firstly we finish the low cycle fatigue test under plastic strain controlled of butt welding specimen,and we get the relation curve between the plastic strain and the crack initiation life of welding structure from the experimental,we also analysis the experimental results with the fatigue life of material.Then we build the multi-scale model of the steel frame structure,we get the result that the weak link located in the beam-column joint of the first floor.The result of the stress-strain behavior of dangerous points under strong earthquake show that the dangerous points under the non-proportional load.So we use the critical plane approach to evaluate the fatigue life.According to the theory of critical plane approach,we get the angle of the critical plane in MATLAB programming method.The shear and normal strain cycle of critical plane are extracted with rain-flow counting method and be composed an equivalent strain.Then the crack initiation life and fatigue cumulative damage of the beam-column welded joint are got from the relation curve between the plastic strain and the crack initiation life of the experimental.This paper design the experiment scheme of evaluating the crack initiation life of cross beam-column welded joint and build the finite element model.Based on the result of the stress-strain behavior of dangerous points,we use 5 models to evaluate the crack initiation life and compare the result,it is proved that the critical plane approach which is applicable to the non proportional loading condition can predict the crack initiation life of beam column welded joint,and the result is satisfactory.Finally,the relationship between the macroscopic response of the beam and the fatigue life is established.