Abstract
As a new method to predict the fatigue lives of metallic materials under the fatigue tests with multiple repeated loads or strains, the fatigue fracture criterion based on varying plastic strain amplitudes instead of the linear damage criterion. Σ(n/N)=1 has already been. verified experimentally by carrying out the low-cycle fatigue tests under tension-compression and rotating bending. However, for the plane bending and torsional fatigue tests it has not been verified so far experimentally whether the fatigue fracture criterion based on varying plastic strains is applicable or not. From such a view point, the authors have carried out the lowcycle torsional fatigue tests by using 0.84%C piano wires under multiple repeated strains, and investigated the applicability of the fatigue fracture criterion based on varying plastic strains experimentally. Furthermore, the applicability of the above fracture criterion has been also discussed on the basis of the experimentally determined correlation between the equivalent stress τeq and the equivalent plastic strain amplitude γp.eq.
The main results obtained are summarized as follows:
(1) The values obtained from the fatigue fracture criterion based on varying plastic strains were slightly closer to unity than those calculated from the linear damage criterion. Therefore, when used for predicting the fatigue lives under the strain control test, the fatigue fracture criterion based on varying plastic strains is better than the linear damage criterion.
(2) From the test results the following exponential relationships were found to hold between the equivalent stress and the equivalent plastic strain amplitude.
γp.eq=c·aτeq up to fracture,
γp.eq'=c'·a'τeq' up to fracture initiation,
Where a, a' and c, c' are constants.
(3) The strain ratio γa2/γa1 or the number of cycle ratio n2/n0 had a little effect on the cumulative damage in this study.
