Abstract
The fracture tests were performed on a hot-rolled steel containing 0.01 mass% carbon using a multi-specimen type 4-point bending apparatus and fatigue precracked specimens over the temperature range from 4.2 to 290 K and the rates of stress intensity factor from 7×10−1 to 1×103 MPa \sqrtm/s.
The fracture toughness was almost constant at temperatures below 40 K and the fracture surface observed by the scanning electron microscope represented a cleavage mode. Above this temperature, the fracture toughness increased as the temperature increased. This transition range shifted to higher temperatures with increasing loading rate. In this transition range, the fracture occurred still in the cleavage mode. The increase of the fracture toughness was associated with a small quantity of plastic deformation at the crack tip, not observable on the fracture surface. Such plastic deformation is considered to be caused by the emission of a small number of dislocations from the crack tip and the motion of emitted dislocations ahead of the crack tip.
The fracture toughness was plotted against the rate parameter kTlnA⁄K in order to evaluate the temperature and loading rate dependences of the fracture toughness quantitatively. It is found that the fracture toughness is presented as an increasing function of the rate parameter and the predominant process for the fracture mechanism is discussed to be the thermally activated emission and motion of dislocations.
