The Fracture Behavior of Notched Specimen of Iron

Abstract

Low temperature brittleness of vacuum-melted iron was studied through the evaluation of the fracture toughness, critical COD, and yield stress of smooth specimens as a function of testing temperature ranging from 20° to-269°C at static and dynamic tensile rates. The mode of fracture of notched specimens was also investigated in a scanning electron microscope.
The results obtained were as followed:
(1) The ductile-brittle transition temperatures determined by the cleavage percent of 75% were-122°, -68°, and-48°C for specimens tested at deformation rates of 0.5mm/min, 2m/sec, and 10m/sec, respectively.
(2) The fracture strength of notched specimens was decreased with lowering the testing temperature below a critical temperature, which was equal to or somewhat lower the transition temperature determined from the above.
(3) The fracture toughness obtained showed temperature and tensile rate dependences; it was decreased with lowering temperature and with increasing the tensile rate.
(4) The ratio of critical COD to yield stress against the normalized stress fell on a curve for all specimens tested and the critical COD observed was larger than that calculated after Wells or BCS models.
(5) The incipient cleavage crack formed in notched specimens tested at low temperatures was often observed to start at a point of a few grains inner from the notch root.
(6) The effective surface energy calculated from the fracture toughness was from 1.5×10⁶ to 1×10⁷erg/cm², which was about one hundred times larger than that obtained from the dislocation theory of the ductile-brittle transition temperature.