Abstract
Porosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of the diameter of the pore, whereas the total volume of material to be damaged between pores is proportional to the cube of the interpore spacing. The present study found that cycles to initiation clearly depended on the volume of highly stressed material adjacent to pores, relative to the volume of void free material between pores. The correlation suggests that porosity effects on fatigue crack initiation are primarily stress concentration effects: pores as crack precursors seem less important. PM/0323
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