Effect of the loading cycle asymmetry on the cyclic cracking resistance characteristics of heat-resisting steels

Abstract

1. In the representation of the experimental data on the relationships of fatigue crack growth in the da/dN-KI max coordinates, the increase of the stress ratio from 0.1 to 0.75 greatly reduces the rate of fatigue crack propagation in medium- and high-amplitude sections for all the steels examined and may exert an ambiguous effect on the relationships of fatigue crack propagation in the near-threshold region. 2. The Paris dependence satisfactorily describes the kinetics of fatigue crack propagation at various values of the stress ratio only in the medium-amplitude section of the da/dN-ΔKI diagram. Use of this dependence at high values of SIF is restricted for 15Kh2MFA (I) steel by the presence of the section of the insensitivity of the rate of fatigue crack propagation at the variation of SIF, whereas for 08Kh18N10T steel it is limited by the development of nonconstricted plastic strains at the tip of the fatigue crack preventing application of the parameters of linear fracture mechanics. 3. An increase in R from 0.1 to 0.75 for the 15Kh2MFA (I) and 15Kh2MFA (II) steels has no effect on the critical stress intensity factor in cyclic loading, but it greatly reduces the critical rate of fatigue crack propagation (da/dN)c. The quantity da/dNc can be accepted as the characteristic of the transition from fatigue to brittle failure and is sensitive to the load cycle asymmetry. 4. The dependence of the rate of fatigue crack propagation on the range of the opening displacement of the crack tip for all the specimens examined is invariant to the load cycle asymmetry both in the elastic and elastoplastic region.