We investigated the failure types of 50mm thick structural steel plates (SN490) including a weld bead line by conducting tensile tests. The testing temperature, -40℃, was selected in order to simulate the high strain rate. When the weld line made a right angle with the tensile axis, a usual ductile fracture occurred. On the other hand, when a bead line was parallel to the tension axis, brittle fracture occurred. Having a slit in the backing strip in the latter condition, the fracture stress was reduced to a yield strength level. The brittle fracture stress in the slit specimen was estimated by a simple fracture mechanics approach based on the measured fracture toughness K_<IC> (50MPa m^<1/2>), the initial crack size (6.4mm) as determined from SEM observation, and the estimate of residual stress along the weld line.
