The fatigue test of 7075 T6 aluminum alloy under constant mean stress σ
m in the range of 0.20 ≤
A≤∞(
A=σ
a/σ
m) was carried out by use of a testing machine of Schenck type. Also, the fatigue test under cyclic varying mean stress σ
m' was performed by a newly constructed cyclic creep testing machine. From these tests, the following results were obtained:
(1) From the results of fatigue test under constant mean stress, the equivalent alternating stress σ
ae was determined. The curve for the relation between σ
ae/σ
a and
A was found to be represented nearly by two linear lines with a junction point at
A=0.5∼0.7. It was also found that the fatigue life curve in the range of
A>0.5∼0.7 was nearly parallel to that under fully reversed stressing (
A =∞) but in the range of
A<0.5∼0.7 it was not parallel.
(2) From the results of fatigue test under cyclic varying mean stress with secondary stress superimposed, the equivalent alternating stress σ
ae was also determined and the curve for the relation between σ
ae/σ
a and the stress ratio of σ
a to σ
m,
A', was represented by two curved lines with a junction point at
A'=0.5∼0.6. In the range of
A'<0.5∼0.6 the curve was nearly a straight line, in the range of 0.5∼0.6<
A'<0.8 it showed a convex toward the upper direction and in the range of
A'>0.8 it was again nearly a straight line.
(3) From the results of fatigue test under cyclic varying mean stress without secondary stress, the equivalent alternating mean stress
Ye and the varying mean stress,
Y=1/√2(σ
m'-σ
m), were determined. It was found that the relation between
Ye/
Y and the stress ratio
A'(=
Y/
X) was represented by a similar curve given in (1) and (2).
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