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.