4.2 Test results for fatigue thresholds
Fatigue crack growth rates da /dN versus stress intensity factor range ΔK I for ferritic steels and aluminium alloys at R = 0 and R < 0 are shown in Figures 5 and 6. The definition of the range isΔK I = K max -K min for R = 0 and for R< 0. When R = 0, the stress intensity factor range becomes ΔK I = K max, because K min = 0. As can be seen in Figures 5 and 6, fatigue crack growth rates da /dN for ferritic steels and aluminium alloys decrease with decreasing stress ratio R .
Tables 3 and 4 show the thresholds ΔK th for ferritic steels and aluminium alloys obtained from Figures 5 and 6. These experimental data are depicted in Figure 7. The fatigue thresholdsΔK th increase with decreasing stress ratioR , as shown in Figure 7.
From the values of ΔK th expressed byK max - K min, eachK max and K min is easily obtained from the stress ratio R . For example, the threshold for A533B is ΔK th = 27.5 MPa√m at R = -5. From the ratio K min /K max = -5, the K max is obtained byΔK I = 27.5 = K max -K min = 6 K max. Then, the threshold expressed by K max at R = -5 isK max = 4.6 MPa√m. Tables 3 and 4 tabulate the maximum stress intensity factors K max at negative stress ratios for ferritic steels and aluminium alloys respectively. The thresholds ΔK th increase and theK max decrease with decreasing stress ratioR. The results for K max suggest thatK max is affected by compression stress.