Fractographic Features Associated with General Yielding in Low Strength Steels

摘要

The fractographic potential of the scanning electron microscope was exploited, enabling direct measurements to be made of features associated with the fracture surfaces of low strength structural steels. The work, using three-point bend single-edge notched test specimens of low strength steels, identified a 'stretched zone', the presence of which is associated with the development of plastic deformation ahead of the crack tip. The nature of this zone and its orientation with respect to the plane of the fatigue starter crack was determined. The surface is not featureless but exhibits characteristics resembling ductile flow; and the inclination with respect to the fatigue crack is approximately 45 degrees. Quantitative measurements of the stretched zone size were carried out and correlations made with the fracture toughness as measured in terms of crack opening displacement (COD), based on the principles of general yielding fracture mechanics. The development of the zone was examined at various testing temperatures, and at various stages during loading up to and beyond general yield. The zone is interpreted in terms of micro-crack extension resulting from localised plastic flow along slip directions within the plastic zone developed at the crack tip. (Author)