values (dashed lines) are useful. T e researchers compared the
predicted fatigue life based on the long and short crack models using the extreme-value statistics (EVS) estimated maximum pore sizes to the experimental data. As expected, with the upper bound EVS estimate of the maxi- mum pore size, both long crack and short crack models gave a conservative lower bound fatigue life prediction. T ey also performed a com-
microstructures (median +/- 3 sigma SDAS or grain size), the researchers’ model accurately predicted the variation and scat- ter of the fatigue life data. In cast aluminum alloys,
parison of the calculated fatigue life of samples failed by cracked eutectic particles and debonded eutectic with actual fatigue life. T e samples tested at the stress amplitude of 85 MPa and stress ratio R = -1 had coarse unmodifi ed microstructures and large silicon and iron-rich particles. T e fatigue cracks in all three samples were initiated by cracked silicon and iron-rich particles. T e calculated fatigue life was in good agreement
Fig. 4. The fatigue crack was initiated from persistent slip bands in a sand-cast A356-T6 casting. (Image was taken using secondary electron imaging.)
with the actual fatigue life. A comparison of the calculated
fatigue life of samples failed by per- sistent slip bands in both coarse and fi ne microstructures with actual fatigue life showed the predicted fatigue life is in good agreement with the actual fatigue life. By considering the proba- bilistic distribution of characteristic
casting fl aw and microstruc- ture features such as porosity, oxides, eutectic particles and dendrite cell and grain structures are not uniform in the entire stressed volume. T is is due to randomness in the nucleation and growth of casting fl aw and microstructure features during local solidifi cation. It is generally accepted that the characteristic
fl aw and microstructure features in aluminum castings follow extreme value probabilistic distributions. T e upper tail populations in these distri- butions dominate the fatigue behavior. In the presence of second phase
particles, inhomogeneous deforma- tion during cyclic loading results in high internal stress in the particles due to dislocation pileups at particles. T e larger the particle, the higher the internal stress. When internal stress is greater than the particle fracture strength, the particle cracks. Otherwise, decohesion occurs when the internal stress is high- er than interfacial strength between the particle and matrix. Cracking or decohesion of second phase particles can happen in the fi rst few cycles, particularly when the second phase particle is segregated and located in the high stress areas. If the cracked particle size or the
debonded particle area is comparable to characteristic microstructure sizes such as dendrite cells (in coarse microstruc- ture) or grain size (in fi ne microstruc- ture), then the fatigue crack immediately propagates like a long crack. Otherwise, the fatigue crack behaves like a short crack. A fi nal failure mode considered in this work is persistent slip bands, which can form in favorably oriented grains or dendrite cells, particularly close to the grain or boundaries near the free surfaces of the component. In this case, a substantial portion of fatigue life is spent in crack initiation and small crack propagation, which must be included in the total life estimates.
40 | METAL CASTING DESIGN & PURCHASING | May/Jun 2013
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