School of Mechanical Engineering, Sharif University of Technology
Mechanical Engineering, Buali Sina University of Hamadan
The study of mechanical behavior of the structural steel S400 under quasi- static and dynamic loading has been the subject of this investigation. In oder to obtain different stress - triaxiality conditions the specimens were notched with 1, 1.5, 2 and 3.5 mm notch radius. The results of fractography show as the velocity of tension increases, ductility reduces and a ductile-brittle transition occurs under certain stress triaxiality or strain rates. The observation of load-time history diagrams and SEM micrographs show that, as far as the fracture is ductile, any increase in velocity leads to the reduction of fracture load which is presumed to be due to reduction of plastic deformation. In brittle fracture, however, the velocity increase results in increase in fracture load which is thought to be due to micro-cracks formed at different level near to the fracture surface of specimens, the so called crack shielding and crack branching at high deformation velocities. Notch radius also proved to be highly effective on fracture mechanism which is due to notch strengthening. The change in grain size of some of the specimens shows that the ductile to brittle transition in fracture mechanism can be postponed by a suitable heat-treatment scheme up to a certain strain rate.