Fracture Model Of Constructional Steel Under Moderate Stress Trixiality

For a long time, macroscopic brittle fracture of metallic materials because of its complexity and the disastrous consequences has become a major research topic in today's world. 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. Thickness of steel plate (Moderate Stress Trixiality) in the middle gap of the transition from plane to plane strain stress state, the fracture mode is unique, therefore, of medium thickness of Q235 steel in regular, low-temperature fracture mode of fracture experiments, theoretical significance and value of the works.In order to study crack mechanism in stress triaxiality and fracture design of structure steel, by comparing the results of fracture experiments were performed on different thickness of notched plates and plates with hole made of structural steel Q235, including ten plates with hole of 16mm, ten plates with hole of 25mm, ten notched plates of 16mm and ten notched plates of 25mm in the room temperature, but in the low temperature, ten notched plates of 16mm and ten notched plates of 25mm were test. The experimental results reveal that the first macro crack initiated in the thickness of notch edge, the crack spread vertical the direction of loading until complete fracture. The fracture ductility is lower for the specimens with the sharper notch, and macro-fracture load of the plates with hole were higher than the notched plates. Fracture mode was combination of shear and normal fracture and fracture section was cup and cone style. Structural steel at low temperature specimen tensile yield strength and ultimate strength was slightly higher than room temperature, tests show low-temperature brittleness.Fracture experiments and numerical simulations were performed on different thickness of notched plates and plates with hole made of structural steel Q235. The results showed that the macro crack in the notch root formation of the cross section is not the same time, the notch root stress concentration, the initial crack formed in the gap in the heart department, and then extended along the width direction to complete fracture. Fracture stress field analysis shows that the article suggests the twin shearing hydrostatical stress fracture criterion and Doctor Wang Wan-zhen's ellipsoid metal fracture criterion has high accuracy. The ellipsoid metal fracture criterion to predict the fracture of structural steel moderate thickness was conservative, and CI is the quantified value of the whole is greater than the proposed twin shearing hydrostatical stress fracture criterion to quantify the value of the fracture parameters of CI . In comparison, this paper suggests the twin shearing hydrostatical stress fracture criterion under moderate stress trixiality steel fracture forecast precision is higher, but the shortcoming is the criterion of coefficient is not due to its generality in use scope has limitations.