Fatigue thresholds at negative stress ratios for ferritic steels and
aluminium alloys in flaw evaluation procedures

Fatigue crack growth thresholds *ΔK*^{th} given in
the WRC Bulletin, IIW and BS 7910 are constant values at negative stress
ratios. However, the definitions of *ΔK*_{th} at
negative stress ratios are different: *K*_{max}
according to the WRC Bulletin and IIW, and *K*_{max} -
*K*_{min} according to BS 7910, where
K_{max} is the maximum and K_{min} is the
minimum stress intensity factors. From fatigue crack growth tests
conducted on ferritic steels and aluminium alloys in air at ambient
temperatures, the thresholds expressed by *K*_{max} -
*K*_{min} are variable values at negative stress
ratios; that is, the threshold increases with decreasing stress ratio.
The thresholds expressed by *K*_{max} are also
variables. *K*_{max} decreases with decreasing stress
ratio. The thresholds at negative stress ratios are not constant; they
are affected by compression stress. Therefore, the thresholds given by
the WRC Bulletin and IIW are slightly unconservative. The threshold
given by BS 7910 is considerably conservative. A suitable definition of
the threshold at negative stress ratios for application in flaw
evaluation procedures is the variable threshold
*ΔK*_{th} expressed by *K*_{max} -
*K*_{min}.