Impact fracture toughness of porous iron and high-strength steels
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I. INTRODUCTION
POWDER metallurgy (PM) steels are increasingly employed for heavy-duty applications.[1] From a designing point of view, the knowledge of their mechanical performance in terms of yield strength, fatigue, impact resistance, and mechanical reliability is, therefore, of paramount importance.[2–6] The impact resistance of PM steels, in particular, is strongly impaired by the presence of residual porosity in their microstructure.[7,8] This is very deleterious, because most structural parts contain notches and may experience impact loading in service. Since, in addition, cracks originating from the consolidation process before sintering[9] may also be present in PM parts, it is important to know the impact fracture toughness and understand the micromechanisms of fracture. A number of investigations have been carried out on the static fracture toughness of PM steels.[10–13] Uncertainties in the validity of the experimental Klc values still exist, since the ASTM E 399-90 requirement for the specimen size is not often met. It is argued, however, that, for sintered steels with a high amount of open porosity (densities of up to about 7.2 g/cm3), fracture-toughness values are independent of the dimensions of the adopted specimens.[14] Fleck and Smith,[11] as an example, found that the fracture toughness of sintered iron is independent of the specimen thickness. Sonsino,[12] however, carried out fracture-mechanics tests on a high-strength steel using two different specimen geometries with different widths and ligaments. He found that the specimen with a larger width and ligament produced higher fracture-toughness values. A further experimental peculiarity of these materials is that fatigue precracking is claimed to be unnecessary for fracture-toughness testing, provided that a sufficiently sharp notch is introduced in the specimen, because crack extension is controlled by the morphology of the necks. Barnby et GIOVANNI STRAFFELINI, Senior Researcher, is with the Department of Materials Engineering, University of Trento, 38100 Trento, Italy. Manuscript submitted December 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
al.[15] suggest a notch-root radius of 0.1 mm; Mai et al.[16] relate the notch-root radius to the mean dimension of the pores. Recently, Gegel et al.[17] obtained similar fracturetoughness values by employing either precracked or 0.25 mm–notched specimens, in the case of high-strength steels. The analysis of fracture-mechanics data regarding PM steels reported in the literature shows that these materials display a characteristic behavior, in the sense that they show an increase of static fracture toughness as their yield strength is increased, almost up to yield strengths of about 600 MPa.[18] The PM steels with higher yield strengths do not follow this trend. Indeed, the KIc fracture-toughness values remain rather constant (about 35 MPa!m), altough they show a large variation around this value. Recently, the influence of chemical composition, types of powder, and microstructural state on this
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