Nomenclature
da/dN : cyclic crack growth rateKmax : maximum stress intensity factorKmin : minimum stress intensity factorR : stress ratio (= σminmax =Kmin/Kmax)ΔKI : stress intensity factor rangeΔKth : fatigue crack growth thresholdΔKth0 : fatigue crack growth threshold atR = 0.0
σmax : maximum applied stress
σmin : minimum applied stress
Introduction
Fatigue of materials in the form of stress-number (S-N) curves has been investigated experimentally, and S-N curves are now common in design codes [1, 2]. It is also well known that fatigue crack growth rateda /dN is expressed by stress intensity factor rangeΔK I, and the relationship betweenda /dN and ΔK I is used for reference fatigue crack growth rate curves applied in flaw evaluation procedures such as fitness-for-service codes [3, 4].
Fatigue thresholds ΔK th are the stress intensity factor range ΔK I values below which fatigue crack growth rates da /dN are negligible. The thresholds of fatigue crack growth rates are important in order to determine whether detected defects propagate or not. The fatigue crack growth thresholds are also employed in several flaw evaluation documents [5-7].
Fatigue crack growth thresholds ΔK th are not easy to obtain by experiments, because it takes a long time to determine whether the crack is growing or not. Almost all thresholds for materials have been obtained under cyclic tensile-tensile loadings, which correspond to positive stress ratios. Compared with the values obtained under cyclic tensile-tensile loadings, there are not sufficient data on fatigue thresholds obtained under cyclic tension-compression loadings; that is negative stress ratios. The definition of the thresholdΔK th at negative stress ratios is not clearly given in flaw evaluation documents, as far as the authors are aware.
WRC (Welding Research Council) Bulletin 194 [5], IIW (International Institute of Welding) [6] and BS 7910 [7] give fatigue thresholds as constant values at negative stress ratios. However, the definitions of the thresholds at negative stress ratios differ between the WRC Bulletin, IIW and BS 7910.
This paper focuses on the thresholds for the ferritic steels and aluminium alloys at negative stress ratios provided by the WRC Bulletin, IIW and BS 7910. Fatigue crack growth tests for ferritic steels and aluminium alloys were conducted in an air environment, and the thresholds were compared with those given by the WRC Bulletin, IIW and BS 7910. It is shown that the thresholds are affected by compression stress. A suitable definition of the threshold at negative stress ratios is proposed for application in flaw evaluation procedures.