| As an important problem in steel structure, crack play a vital role in bearing capacity and operational life span of structural members.At present, many researches are concentrated on beam-to-column connections, they want to solve the problems through improve the modes of beam-to-column connection. But there are only a few researches about structural steel fracture mechanism on mechanics. So, we will seek the crack criterion of structural steel from this way.Base metal and weld metal of structural steels Q345 and Q235 were investigated experimentally, Fracture tests were performed on 20 notched bars made of high strength steel Q345 and 20 notched bars made of steel Q235. The effect of notch radius r and notch depth (d/D) on fracture ductility of high strength steel was studied. Test results show that notched bars fracture at the notched section. Lots of dimples form at the center of notched section going with shear trace. The fracture ductility is low and the fracture strength is high for the specimens with the small notch radius and the deep notch. The probability of brittle fracture increases for the specimens with deeper and sharper notch.Through the generalized yield Doctor Wang Wan-zhen did fracture tests of structural steel ,we know the fracture side of structure steel is sequence and velvet.Refering to the generalized yield function of structure steel,the ellipsoidal fracture function which is based onσm andσm is suitable. Te ellipsoidal fracturefunction is : (σseq/r)2+(σm/q)2=3ty2.At the same time, in order to examine the fracture criterion Doctor Wang Wan-zhen did fracture tests of structural steel, and then did the numerical simulation about these experiments. Through the simulated results of structural steel specimens we know that the fracture criterion has suitable mode and enough precision, the stress concentrates severely at the centre of notch section. The mass fracture parameter of the notched bars are 1.0-1.2.The notched bars fracture while the stress field at the notch section reaches the critical value of the suggested ellipsoidal fracture function.
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