As-cast specimens and smooth specimens of a AA 319 cast aluminum alloy containing casting porosity were fatigue tested with special attention given to the long-life region ( N < 1.25 x lo8 cycles).Fatigue cracks were observed to initiate from the near-surface casting pores or from discontinuities resulting from the as-cast surface texture. The observed fatigue lives were strongly dependent on the size (6) of these casting defects.The effect of casting defects on the fatigue life was modeled assuming the fatigue life to be the sum of the crack nucleation and the crack propagation life (including both the growth of short and long cracks). The crack growth behavior of (mechanically) short cracks was considered in detail by a developed crack-closure-at-a-notch (CCN) model. The CCN model predicted the fatigue lives for both as-cast and machine-notched specimens. Extension of the CCN model to reliability-based design was attempted using the measured size distribution of the fatigue-initiating casting pores. NOMENCLATURE a = D + x b = fatigue strength exponent for smooth specimen C, m (C', m') = constants for the relationship of da/dN versus AK (AK,,) D = notch depth: for edge notch, D is the full notch length K,, K,, = gross and net-section elastic stress concentration factor AKeR = effective stress intensity factor range AK, = intersection point of low AK region and high AK region AKIho (AKcR,tho) = long crack threshold intensity factor range (effective value) Lo = intrinsic crack length (mm) JASON C. TING and FKEDEKICK V. LAWRENCE P R ( . y ) = a model for effective stress intensity ratio for cracks in a notched specimen W = diameter of a cylindrical bar .Y* = spatial extent of notch stress field .Y, I, =crack length, and initial crack length xi = final crack length at failure p = radius of notch root Y ( x ) = geometry factor for stress intensity factor = fatigue strength coefficient for smooth specimen (T," = mean stress
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