Although several ways have been proposed to predict low-cycle torsional fatigue life of metals under load controlled cycling conditions with mean stress, none of them is satisfactory for all materials and test methods.
In this study, in order to investigate the influence of mean stress on the fatigue life of piano wires in a constant load cycling test, the low-cycle torsional fatigue tests were carried out on 0.84% C piano wires under load controlled cycling conditions with various stress ratios.
The fatigue characteristic of each specimen was discussed and the fatigue fracture criteria, applicable to the load controlled fatigue test and given by the following equation, were proposed.
D=γ
po/γ
f+κ{1/C
0∫
N0Δγ
padn+∫
N0Δγ
c/γ
fdn}
κ=τ
0maxN
0/τ
maxN
where Δγ
p and Δγ
c are the ranges of plastic strain and permanent strain, respectively,
dn is the infinitesimal increment of the number of strain cycles
n, γ
p0 is the strain for the first cycle,
a is a material constant,
C0 is the average value of the constant
C0 obtained from the equation 1/C
0∫
N0Δγ
padn =1 when the mean stress is zero,
N and τ
max are the number of cycles to fracture and the maximum stress when the mean stress is applied, respectively,
N0 and τ
0max are those at zero mean stress, and γ
f is the fracture strain in the static test.
From the test results, it was confirmed that the above equation is well applicable to the fracture life in low-cycle torsional fatigue tests with mean stress under load controlled conditions.
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