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.