Application of an Improved Low-Cycle Torsional Fatigue Testing Machine to Test under Manifold Repeated Load on Two Stress Levels

Abstract

There has been but little study so far made on the low-cycle torsional fatigue on cold drawn wires of small diameters under actual loads. In order to pursue studies on this problem, therefore, we designed and manufactured a low-cycle torsional fatigue testing machine suitable for the test of these wires. There are many types of torsional fatigue testing machines, but the one designed by the authors has a merit in that by using a photo cell the change of the load can be conducted easily and exactly.By using this testing machine we carried out the low-cycle torsional fatigue tests of piano wires under manifold repeated load on two stress levels at room temperature.The main results obtained are as follows:(1) The cumulative cycle ratio Σn/N obtained from the test results were not in agreement with the unit, giving values greater than unity for large stress ratio of τA/τw and those less than the unit for small stress ratio.(2) The time strength at 103 cycles for piano wiresrises whenever it is subjected to multiple repeated loads.(3) The test results show that the plots of total plastic strain versus the number of stress cycles to failure deviate from the linear relation between them which holds under a constant stress amplitude.