Fatigue thresholds at negative stress ratios for ferritic steels and
aluminium alloys in flaw evaluation procedures
Abstract
Fatigue crack growth thresholds ΔKth given in
the WRC Bulletin, IIW and BS 7910 are constant values at negative stress
ratios. However, the definitions of ΔKth at
negative stress ratios are different: Kmax
according to the WRC Bulletin and IIW, and Kmax -
Kmin according to BS 7910, where
Kmax is the maximum and Kmin 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 Kmax -
Kmin are variable values at negative stress
ratios; that is, the threshold increases with decreasing stress ratio.
The thresholds expressed by Kmax are also
variables. Kmax 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
ΔKth expressed by Kmax -
Kmin.