Plastic Fatigue Life of Steel under Periodical Multi-Stage Stress Amplitudes

Abstract

In the calculation of the fatigue life of metallic materials under the varying stress amplitude, it is common to apply the linear damage criterion, Σ(n/N)=1, based on the stress amplitude (i.e. S-N curve). In the present, the author proposes the fatigue fracture criterion derived from the damage function on the basis of the plastic strain in stead of the customary Σ(n/N)=1. The fatigue fracture criterion is represented as1/C0∫N0epaadn=1for the varying plastic strain amplitude.The medium carbon steel S35C (normalized) is tested under the condition of the rotating bending and the repeated tension-compression with constant- and varying-stress amplitude to prove the criterion described above.As a result, the experimental values of the cumulative damage based on this criterionDf=1/C0m∫N0epaadnare scattered closely about unity, although Σ(n/N)=1 is also established in this case of low-cycle fatigue: the scatter of Df-value being almost the same as in the value of Σ(n/N). In all the tests performed here, neither effect of the frequency distribution of stress (or strain) nor of their sequence of application on the fatigue life under the varying stress amplitude has been recognized.