2.1 Fatigue crack growth rate da/dN
It is well known that the fatigue crack growth rate da /dNis expressed by a function of the stress intensity factor rangeΔK I, and that the growth rate da /dNdepends on the applied stress ratio R . The stress ratio is defined by R = σminmax =K min /K max, where σmin is the minimum cyclic applied stress and σmax is the maximum cyclic applied stress. When the stress ratio R is negative, minimum stress σminis a compression stress. K min andK max are the minimum and maximum stress intensity factors.
In accordance with ASTM E 647 [8], the stress intensity factor rangeΔK I is expressed by
ΔK I = K max -K min for 0 ≤ R , and
ΔK I = K max for R< 0.
When the stress ratio is 0 ≤ R , the fatigue crack growth rateda /dN depends on the range of the stress intensity factorK max - K min. When the stress ratio is negative (R < 0), the stress intensity factor range ΔK I = K max - 0. Compression stress does not contribute to the crack growth rate. ASTM E 647 notes that it is conventional to use only the positive portion of the stress range to calculate the crack driving force. ASTM E 647 also notes that increases in fatigue crack growth rates can be observed as the R becomes more negative.