Rotating bending, push-pull and plane bending fatigue tests were carried out in the high cycle fatigue region with unnotched specimens of five kinds of carbon steel at room temperature. The results obtained are as follows.
(1) The fatigue process was divided into three stages based on the variation of plastic strain range ε
pg. In the first stage, ε
pg increased with the number of stress cycles
n and reached to the maximum value ε
pgM at stress cycles
nM. In the second stage, ε
pg decreased with
n and reached to the minimum value ε
pgV at
nV. In the last stage, ε
pg was almost constant or increased slightly.
(2) The ratio of ε
pgV to ε
pgM was nearly constant for various stress amplitudes σ
g as follows,
(ε
pgV/ε
pgM)=
C1where
C1 was an experimental constant dependent upon the kind of test materials.
(3) The increase of ε
pg during the first stage and the decrease of ε
pg during the second stage corresponded with the decrease of fatigue life
N, and the relations between the maximum increase ε
pgM and
nM, and between the maximum decrease (ε
pgM-ε
pgV) and (
nV-nM) were as follows,
ε
pgM·
nMm2=
C2(ε
pgM-ε
pgV)·(
nV-
nM)
m3=
C3where
m2,
m3,
C2 and
C3 were experimental constants and
nM and
nV corresponded with the numbers of stress cycles at micro-crack initiation and macro-crack initiation, respectively.
(4) With regard to fatigue damage, the damage (
D12) for the first and second stages was related to the variations of plastic strain range ε
pg1 in the first stage and ε
pg2 in the second stage as follows,
D12=(ε
pg1/ε
pgM)·(
D1)
max.+{ε
pg2/(ε
pgM-ε
pgV)}·(
D2)
max.where (
D1)
max. and (
D2)
max. were the maximum values of fatigue damage for the first and the second stages, respectively.
(5) With regard to fatigue life, the relation between the life
N and ε
pg was shown as,
{ε
pgM+(ε
pgM-ε
pgV)}·
Nm5=
C5where
m5 and
C5 were experimental constants and
m5≈0.5,
C5≈ε
F(ε
F: tensile ductility) in this study.
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