Fracture Surface Facets and Fatigue Life Potential of Castings
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FATIGUE failure in metals accounts for 90 pct of all in-service failures resulting from mechanical causes.[1] Consequently, much effort has gone into determining the mechanisms of fatigue failure, namely crack initiation and propagation before the final rupture. In castings, cracks initiate almost always from defects, such as inclusions and pores. However, facets that are flat surfaces on fracture surface have been observed in cast alloys, including Al alloys,[2–5] cast irons,[5–8] steels,[5] magnesium alloys,[9] and Ni-base superalloys.[10–13] These facets have been mostly interpreted to have been generated by ‘‘persistent slip bands’’[2,3] and are assumed to represent the metallurgical or ideal fatigue failure in the absence of defects. In their study on A356 aluminum alloy castings, Nyahumwa et al.[2] investigated the effect of casting techniques and hot isostatic pressing (HIP) on fatigue life at R = +0.1. For each fatigue fracture, the type of fatigue crack initiators was determined through extensive fractography. Nyahumwa et al. found that most specimens failed at fatigue cracks initiated at oxide bifilms and pores, which degraded fatigue life significantly. In the specimens with the highest fatigue lives, however, facets were observed, one of which is presented in Figure 1. These facets, originally interpreted as persistent slip bands, were reported to be involved in the process of fatigue crack initiation and stage I crack growth. The results of Nyahumwa et al. are summarized in Figure 2, which is a cumulative probability plot for MURAT TIRYAKIOG˘LU, Professor and Director, is with the School of Engineering, University of North Florida, Jacksonville, FL 32224. Contact e-mail: [email protected] JOHN CAMPBELL, Emeritus Professor, is with the Department of Metallurgy and Materials, University of Birmingham, Edgbaston, B15 2TT, UK. CHRISTIAN NYAHUMWA, Director of Studies, is with the Department of Mechanical Engineering, Dar es Salaam Institute of Technology, Dar es Salaam, Tanzania. Manuscript submitted May 20, 2011. Article published online September 16, 2011. 1098—VOLUME 42B, DECEMBER 2011
fatigue life for quiescently and turbulently filled castings. Figure 2 shows that the specimens in which facets were observed had higher fatigue life than those without facets. Assuming that facets represented defect-free failure, Nyahumwa et al.[14] stated that the fatigue life potential of cast aluminum alloys is several orders of magnitude higher than the values usually obtained in fatigue tests because of the presence of casting defects, such as pores and oxide bifilms, which are categorized as old and young oxides. The ‘‘old’’ oxides, with a typical minimum thickness of perhaps 10 lm or more, show only coarse wrinkles, and because of the long oxidation time required to form, they are thought to have come with liquid metal from the melting crucible. The ‘‘young’’ oxides with minimum thickness of tens of nanometers or less show fine wrinkles only tenths of microns thick, indicating that the oxides are formed and ent
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