Nomenclature
da/dN : cyclic crack growth rateKmax : maximum stress intensity factorKmin : minimum stress intensity factorR : stress ratio (= σmin /σmax =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.