Fatigue crack propagation for Q345qD bridge steel and its butt welds at low temperatures

Steel bridges fabricated with Q345qD steels face critical challenges when operating in cold regions with a low ambient temperature. This study aims to investigate, via an experimental program, the low-temperature fatigue crack propagation behavior of Q345qD bridge steel base material and its butt welds. The testing program comprises a series of Charpy impact tests and fatigue crack propagation tests at the room temperature, -20 degrees C and -60 degrees C. The experimental results demonstrate a reduced crack propagation rate in the base material, but an increasing crack propagation rate in the butt welds, with a decreasing ambient temperature. The base material also shows enhanced fatigue crack propagation thresholds with the decreasing temperature. The ductile-to-brittle transition temperature for fatigue is lower than that for fracture in the base material while the weld metal exhibits an opposite trend. Generally, the butt welds present higher resistance against fatigue crack propagation and larger Charpy toughness values than do the base material at all tested temperatures. The Paris-law parameters measured at the room temperature for the base material leads to a conservative assessment of the crack propagation life for a welded joint under a low ambient temperature.