Experiment Study And The Influence Of Fracture Behavior To Stress Condition

As an important problem in many industries, crack play a vital role in bearing capacity and operational life span of structural members, and there is no exception in steel structure. In the Northridge earthquake in America, 1994 and Hyogoken-Nanbu earthquake in Japan,1995, there were number of brittle failures of beam-to-column connections, and that indicated that people's cognitions on the stress behavior and damage mechanism of structural steel were rather inadequate. In order to avoid the personnel casualty and the property damage, the United States and Japan set up specialized research institutions and proposed a lot of modified programs. But whether these programs are of enough effectiveness only after another large earthquake can we know the result.In order to study crack mechanism in stress triaxiality and fracture design of structure steel, by comparing the results of fracture experiments and numerical simulations were performed on ten Q235 structure steel plates with notch, ten Q345ones, and 18 aluminum ally plates with notch. The experimental results indicated that the initial fracture appeared on the specimen surface of notched groove where stress was concentrated, Then, crack extended along the direction of thickness and width. Finally, the macroscopic crack was formed in stress concentrated positions. At last, compare numerical simulation dates of the six aluminum alloy plates with notch. The results indicated that the stress on the crack tip was obviously larger than that on other positions. Stress field analysis showed that all the initial cracks located in the specimen's surface of peripheral holes and was vertical to the direction of load, Stress concentration laid in regional where cracks appeared; stress triaxiality space lies in the crack tip fields, Concentrating the peripheral of holes and the stress triaxiality was heightened with the increasing of radius of pipesBy the tensile tests of six structural steel pipes with hole and the numerical simulation analysis, it's concluded that the extension of cracks accompanied the yield of metals. Fracture surface broke along the normal plane of the first main stress, and the brittle failure of the pipe with hole occurred in the case of finite plastic deformation. In 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 carrying capacity and plastic deformation of specimens decreased with the increase of the ratio of pipe diameter to pore diameter, if the ratios of pipe diameter to pore diameter were same, the plastic deformation of the pipe with double holes was larger than that of pipe with single pole. And the carrying capacity is larger than that of pipe with single hole. The ductile coefficient was always small whether the ratio of pipe diameter to pore diameter was small or large. The ductile coefficientμget it's max value when the ratio of pipe diameter to pore diameterηwas located in a certain constant value.