Impact fracture toughness of porous iron and high-strength steels

The impact fracture toughness of sintered iron and high-strength sintered steels, with densities between 7.0 and 7.25 g/cm3, have been investigated by means of instrumented impact testing on fatigue-precracked as well as 0.17-mm-notched specimens. Experimental results show that the fracture behavior is controlled by the properties of the resisting necks at the crack/notch tip. The materials with impact yield strengths of up to 700 MPa display an increase in fracture toughness as the yield strength is increased. These materials undergo continuous yielding during loading, and ductile fracture takes place once the critical plastic strain is attained within a large process zone. A process-zone model, physically consistent with the fractographic observations, correctly rationalizes their impact fracture toughness. The materials with higher impact yield strengths display an impact curve which is linear up to fracture and are characterized by a fracture toughness which is independent of the yield strength. For these materials, the process zone reduces to the first necks at the crack/notch tip, and fracture takes place once the local applied stress-intensity factor reaches the fracture toughness of the matrix.