Abstract
Fretting fatigue and normal, or unfretting, fatigue tests of a stainless steel SUS304L and an aluminium alloy A2024-T3 were carried out to investigate the effects of the contact pressure and the stress ratio on the crack propagation behaviour. The crack propagation behaviour was represented by the crack propagation rate da/ dN versus the crack length a or the stress intensity factors ΔK eff and K max In fretting fatigue, crack propagation was divided into two stages, namely S I and S II. The value of da/ dN in the S I stage was very high, even under a stress intensity factor less than the threshold for normal fatigue, and decreased gradually with crack growth because of crack closure and the decreasing fretting effect. The decrease in da/ dN was marked in the case of high contact pressure and low stress ratio such as when R = −0.33, where R denotes the minimum stress divided by the maximum stress. During fretting fatigue crack closure occurred at an oblique short crack in the early stages of crack propagation in both the SUS304L steel and the A2024-T3 alloy; it also occurred at the oblique cracked surface of the shear lips formed in the A2024-T3 alloy during crack growth. However, in the S II stage, which followed the S I stage, da/ dN increased with crack growth as for normal fatigue.
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