2.2 Fatigue thresholds
The fatigue threshold ΔK th is determined by the fatigue crack growth rate da /dN versusΔK I, where the growth rate approaches zero. Practically, ΔK th is determined as a fatigue crack growth rate of between 10-7 to 10-8 mm/cycle by tests applying a decreasing stress intensity factor.
The threshold value ΔK th is also expressed by the range K max - K min for 0 ≤R , as shown in Equation (1). When the stress ratio R is negative, the definition of ΔK th is deemed to beK max, in accordance with Equation (1). However, this is problematic, because the effect of cyclic compression stress on the fatigue threshold is not clear, as noted in ASTM E 647. Fatigue crack growth thresholds ΔK th are currently employed in several flaw evaluation documents. These documents, containing thresholds ΔK th for R< 0, are investigated below.