Experiment Study And The Influence Of The Stress Triaxiality To Structure Steel Fracture

In many cases the steel structure fractured brittlely without not overloading,the plastic deformation is very small,the consequences of which is catastrophic. So any structures require to avoid brittle fracture.but the research of classical theory of metal strength,damage mechanics and fracture mechanics on structure steel settle brittle fracture powerlessly. Therefore, it is very significant to research the influence factors to the structure steel rupture and the relationship of the factors.In order to study crack mechanism in stress triaxiality and fracture design of structure steel, the fracture experiments and numerical simulations were performed on 10 Q235 steel tube with hole,6 aluminum ally plates with notch and 20 Q345 notched bar. Test results showed that crack initiated at the edge of the hole and the minimum section of the notched bars, quickly propagating perpendicular to loading direction. The broken section which had obviously shear fracture trace was perpendicular to loading direction. Smaller plastic deformation happened at the incision with high stress triaxiality and the rupture plane was perpendicular to the direction of principal stress.Conversely,when the plastic deformation accumulates larger,plastic flow occurred along the direction of shear stress with obvious plastic deformation, the fracture plane was ductile rupture induced apparently by shear stress,which shows the quantitative rules of fracture.The "initial cracks are located in the direction perpendicular to the loading hole around the specimen surface and the minimum section of notched bar,"was verified by the experiment,the most dangerous section of triaxial stress in the center of the thickness,where the center of the location of the notch reached its maximum.In addition, at the locations where there were crackles, holes, notches, nook, defects and abrupt change of cross-section were easy to form stress concentration and brittle fracture.The results revealed that simulated curves were consistent with those of experiments,so the stress field can be used to analyze the fracture mechanism. After the analysis of simulated fracture stress field to the bar with holes indicated that fracture strength of structural steel was function of stress triaxiality. The relation of fracture strengthσf and stress triaxialityσm/σe was deduced by regression study of simulated fracture stress field. Stress state obviously affects the rupture pattern,the fracture analysis results show that there is a close relationship between the equivalent strain picked at the fracture point and the average stress triaxialityσm /σe abtained from the fracture plane, by the analysis,a relation curve can be gotten between the stress triaxiality and equivalent strain. More importantly, the experiment verifies stress triaxiality and equivalent strain which are the two important factors governing the crack formation again,making a good prediction of ductile crack formation, which provided detailed experimental data for the follow-up theoretical study.