Features of fatigue fracture strength and the failure of iron specimens at 77�K after cyclic preloading at room temperature

Abstract

1. It was shown that the structure of low-temperature fatigue fractures depends significantly on the modes of cyclic preloading at 293°K. A stable dislocation structure in iron, created by cyclic loading at 293°K at fatigue limit σw to 107–2·107 cycles — in contrast to the unstable structure formed by loading at σa=σw or σa=σw * 3–4 kgf/mm2 to 104–3·104 cycles — leads to a reduction in the plastic strain of the iron in the event of cyclic deformation at 77°K and to the appearance of shear-type, crystallographic, ductile fracture sections in the stable crack growth zone (as in annealed material). 2. A scheme was proposed for the propagation of fatigue cracks at the stable growth stage by means of the shear mechanism of failure. It is proposed that the impeding of plastic strain at the crack tip and the initiation of submicrocracks in front of the macrocrack by the shear mechanism lay conditions for the development of shear-type, ductile, crystallographic fracture sections. 3. Reduction of the test temperature from 293 to 77°K is accompanied by a substantial decrease in stable crack length (from ∼2 to ∼0.35 mm). The investigated modes of cyclic strain at 293°K have almost no effect on stable crack length at 77°K, although the low-temperature fracture toughness of iron in which a stable substructure has been created at room temperature is 16% greater than that of iron having a relatively unstable dislocation structure. 4. It was established that in polycrystalline iron at 77°K, fatigue failure in the zone of stable crack growth occurs partially by the mechanism of the formation of fatigue microbands.