6. Summary and conclusions
Fatigue thresholds ΔK th given in the WRC
Bulletin, IIW and the 7910 are constant at under negative stress ratiosR . However, there are two types of definitions ofΔK th: K max in the WRC
Bulletin and IIW, and K max-K min in BS 7910.
From the fatigue crack growth tests conducted on ferritic steels and
aluminium alloys, the thresholds expressed byK max - K min increase with
decreasing stress ratio R , and the thresholds expressed byK max decrease with decreasing stress ratioR . The thresholds ΔK th are not constant at
negative stress ratios; the thresholds at negative stress ratios are
affected by compression stress.
It has been found that the thresholds given in the WRC Bulletin and IIW
are slightly unconservative. The thresholds given in BS 7910 are
strongly affected by compression stress, and the BS7910 gives
considerably conservative thresholds.
Furthermore, the threshold equation at negative stress ratios for each
material can be derived from the effect of compression stress, when the
threshold at R = 0 is known. A suitable way of expressing for the
threshold at negative stress ratios for application in flaw evaluation
procedures is to use variable thresholds ΔK thexpressed by K max- K min.