平均応力が変動する条件下における等価両振り応力に関する一考察

Abstract

The fatigue test of 7075 T6 aluminum alloy under constant mean stress σm in the range of 0.20 ≤A≤∞(A=σa/σm) was carried out by use of a testing machine of Schenck type. Also, the fatigue test under cyclic varying mean stress σm' 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 σae was determined. The curve for the relation between σae/σa 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 σae was also determined and the curve for the relation between σae/σa and the stress ratio of σa to σm, 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(σm'-σm), 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).