STRUCTURAL FEATURES OF GROWTH OF SHORT CRACKS IN A HIGH-STRENGTH STEEL

In the fatigue fracture surfaces of the high-strength steel the zone of failure borders with the structure-sensitive zone (SSZ), whose microrelief depends on the microstructure of the steel (diameter of the primary grain, the size of the martensite packet and its orientation). Outside the limits of the SSZ there is a regular fatigue relief-secondary cracks and striations with the same orientation. The SSZ dimensions determine the boundary between the short and long cracks. The short cracks in the examined steels are 2-2.5 mm long. At high stresses this boundary is displaced to even shorter cracks and may be smaller than a millimeter. When using the Δl/ΔN (l) coordinates, the dependences for describing crack growth in the SSZ and in the zone with the regular fatigue relief differ. In the SSZ the crack growth rate decreases with an increase of the crack length to 200-300 μm; further growth of the crack is described with sufficient accuracy by the logarithmic dependence. In the zone with the regular fatigue relief the growth rate of the short crack is an exponential function of its length and, consequently, the stress intensity factor. © 1992 Plenum Publishing Corporation.