No abstract

No abstract

No abstract

Near-threshold fatigue crack growth behavior has been analyzed for a broad selection of steels surveyed from the literature. It is clear first of all that apparent values of the threshold stress-intensity factor (ΔKth) can vary widely, roughly an order of magnitude. Though in many instances actual ΔKth values are difficult to define rigorously, a pronounced transition point or “knee” is apparent in the near-threshold region of the conventional logarithmic plot of fatigue crack growth rate (da/dN) as a function of stress-intensity range (ΔK). Though the values of ΔK associated with these transition points (ΔKT) for an individual steel may tend to exhibit a functional dependence on yield strength (σys) or grain size (¯ℓ)—as is the case, for example, with a low-carbon ferritic steel—it is unmistakably clear that for the gamut of steels examined (15 cases), the transition points do not order on the basis of either σys or ¯ℓ alone. Rather, values of ΔKT for the gamut of steels order on the basis of a synergetic interaction of σys and ¯ℓ, according to the equation, ΔKT=5.5σysℓ¯. This relationship was derived in the cyclic plastic zone model of fatigue crack growth established in our prior work with titanium alloys. In further agreement with this model, ΔKT has been identified for these steels as the point at which the cyclic plastic zone attains the mean grain size. The significance and implications of these findings appear far-reaching, as the steels surveyed include those of both high and low strength, a wide range of effective grain sizes (mean free path in the case of high-strength steel), and a host of microstructural types (ferritic, martensitic, pearlitic, bainitic, and austenitic).

No abstract