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Despite more than two decades of research on the topic, there are currently few
practical tools for incorporating crack size (i.e., small or short crack) effects into
fatigue life predictions in real engineering applications. This paper examines how
fatigue resistance curves (fatigue R-curves or fatigue threshold R-curves) may be
used to understand and predict such crack size effects. Recently published and
new experimental fatigue R-curve results are presented for human bone and
polycrystalline Al2O3. When fatigue crack growth resistance is plotted as a
function of crack size on R-curves, clear trends of increasing with crack extension
are seen for both materials. Also, when those crack size effects arise from
extrinsic toughening mechanisms, like crack bridging in Al2O3, it is demonstrated
that the fatigue behavior at all crack sizes can be predicted by carefully
characterizing the bridging zone and quantifying the effects of bridging using
only long crack experiments. Experimentally measured short crack fatigue
threshold data agrees well with the predictions based on quantitative bridging
zone characterization.