Thermodynamic entropy based low-temperature fatigue life prediction for Q500qENH weathering steel in bridges

Weathering steel resist corrosion in specific environments and can effectively reduce the structural maintenance of steel. The research object of this study is a newly designed 500-550 MPa grade high strength easily weldable weathering bridge steel plate (No. Q500qENH), which is suitable for the complex environment of the plateau. However, its low-temperature and high- frequency fatigue performance is not yet known. Therefore, we conducted low-temperature high-frequency fatigue tests at 293 K, 273 K, 253 K, 233 K, and 213 K to evaluate the low- temperature fatigue performance based on the fracture fatigue entropy (FFE) for this material. The test results showed that low temperatures lead to increased fatigue life of the steel and the temperature of the specimen hardly changed during the high-frequency fatigue process. The FFE of a specimen at the same temperature is a fixed value that does not vary with the stress range. For the Q500qENH material, the FFE increased by approximately 1 MJ/m3/K for every 20 K decrease in temperature. The fatigue life of the material increases by about 1 x 106 cycles for every 20 K decrease in temperature when the stress range Delta sigma = 90 MPa. Molecular dynamics modeling calculations showed that low temperatures caused the metal atoms to be in a low- energy state, making positional changes more difficult to occur and making them more resistant to plastic deformation. Combined with finite element analysis and test data, the fatigue limit of Q500qENH steel was found to be 401.1 MPa.