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Fatigue thresholds at negative stress ratios for ferritic steels and aluminium alloys in flaw evaluation procedures
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  • Kunio Hasegawa,
  • David Dvorak,
  • Vratislav Mares,
  • Martin Kraus,
  • Saburo Usami
Kunio Hasegawa
VSB–Technical University of Ostrava
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David Dvorak
VSB – Technical University of Ostrava Faculty of Materials Science and Technology
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Vratislav Mares
VSB – Technical University of Ostrava Faculty of Materials Science and Technology
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Martin Kraus
VSB – Technical University of Ostrava Faculty of Materials Science and Technology
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Saburo Usami
Hitachi Ltd
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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.