Research For Mechanism Of Brittle Fracture Of Structural Steel And Its Engineering Design Method

The working environment of structural steels becomes more complicated as it is widely used. More steel structures are working at low temperature, especially in altiplano and cold zone. High strength steel and thick plate begin to applied in some structures. Low temperature, high strength steel, usage of thick plate and jointing connection usually used in steel structures make the problem of brittle fracture of steel structures more important. Nowdays, the control of brittle fracture of steel structures in code for design of steel structures in China is incomplete, which could not give exact judgement and quantitative analysis on the brittle fracture. This paper researches the mechanical properties and fracture properties with experiments at low temperature, and analyzes the influences of stress status on fracture mode, and finally proposes the applicable brittle fracture prevention design method. The main conclusions of this paper are as following: (1)This paper measures the mechanical properties of structural steels(Q235,Q345 and Q390) and bridge steels(16Mnq and 14MnNbq) with experiments at low temperature, acquires the strength parameters and plastic parameters at temperature 20℃to -70℃. The variation of the properties with temperature is also studied. (2)This paper researches the fracture properties of structural steels(Q235,Q345 and Q390) with experiments at low temperature, measures the fracture toughness CTOD for plate with 12mm to 48mm thickness at temperature 20℃to -70℃, and studies the variation of CTOD with temperature. This paper analyzes the fracture criteria for prevention design based on the experiments and also analyzes the influences of temperature on CTOD, and proposes the equation for CTOD at different temperatur. (3)This paper analyzes the three dimensional stress at crack tip with the FEA method, and analyzes the influences of stress status on fracture behavior in theory. (4)This paper proposes the analytical method for fracture mode and design method for prevention of brittle fracture at low temperature. This paper firstly studies the R-curves of structural steels from the experimental results, then analyzes the fracture behavior of steel structures based on experimental results, and finally proposes analytical method for judgement of fracture mode. The design method for prevention of brittle fracture at low temperature is devided into three degrees. The zero degree and 1th degree is used in the design and for the steel structures satisfying regular inspection. The 2nd degress is for the structure with macro-cracks. The design method is based on fracture mechanics and provides quantitative judgement and analysis for the brittle fracture of steel structures.