BRITTLE FRACTURE OF MILD STEEL IN TENSION AT - 196 C.

The tensile fracture behavior of a mild steel at - 196 C was studied in some detail. With the aid of long thin strip specimens loaded at controlled crosshead speeds between 0.00089 and 0.16 in.min, the strain pattern and microscopic changes preceding fracture were observed, and the magnitude local strain was measured. Specimens heat-treated to alter the tendency toward brittle behavior, but maintaining ferrite-pearlite structures, were also examined. Under these conditions, all the Luders bands display microcracks in some ferrite grains. However, the as-received and normalized specimens do not fracture at low yield stresses slow loading speeds during the spread of the Luders bands. By raising the loading speed, a critical stress is reached when fracture occurs after a delay time. The formation of microcracks and fracture is always preceded by gross yielding. On further increasing the loading rate, the fracture stress rises along with the yield stress. Deductions from the dislocation pile-up theory of fracture in polycrystalline metals are not compatible with the experimental data. It is concluded that the microcrack model suggested by Low is more appropriate. Some observations on the creep occurring in Luders bands during their propagation at - 196 C are included. Author