3.2 Maximum stress intensity factors Kmaxat thresholds
The WRC Bulletin, IIW and BS 7910 give constant thresholdsΔK th at negative stress ratio R . However, the definitions of negative stress ratio are different, as mentioned above: ΔK th = K maxaccording to the WRC Bulletin and IIW, ΔK th =K max - K min according to BS 7910. The thresholds ΔK th given by the WRC Bulletin, IIW and BS 7981 are transformed to K maxusing the stress ratio R , that is K max =ΔK th /(1 - R ), derived fromK min = RK max.
The relationship between K max at the threshold and R for ferritic steels provided by the WRC Bulletin, IIW and BS 7981 is depicted in Figure 3. As shown in Equations (2) and (3), the thresholds K max given in the WRC Bulletin and IIW, expressed by ΔK th =K max are constant at R < 0. However, the thresholds K max given in BS 7910, expressed by Equation (3), decrease with decreasing stress ratioR .
Figure 4 also shows the relationship between K maxat the threshold and R for aluminium alloys provided by Equation (4) according to the IIW and Equation (5) according to BS 7910. The relationship between K max and stress ratioR for aluminium alloys shows the same trend as in ferritic steels, as shown in Figure 3.
The value of K max at R < 0 differs significantly from the values determined via eitherΔK th = K max orΔK th = K max -K min. Furthermore, the expression ofΔK th = K max= constant forR < 0 means that compression stress does not contribute to the value of the threshold, and ΔK th =K max - K min = constant forR < 0 means that the effect of compression stress on the fatigue threshold is significantly large. It is important to define the threshold ΔK th at negative stress ratios.