Fatigue Tests of CrMo Cast Steel under Varying Stress Amplitude with Varying Temperature and Examinations of Cumulative Damage

Abstract

As a preliminary examination for evaluating the effect of combinations of several varying environmental factors on fafigue cumulative cycle ratio Σ(n/N), the plain bending fatigue tests were carried out with various combinations of stress amplitude and temperature. The combinations used were: (1) a constant stress amplitude and step-up-2 stage or step-down-2 stage temperatures, (2) a constant temperature and step-up-2 stage or step-down-2 stage stress amplitudes, (3) a constant stress amplitude and multiple repetition of 2 stage temperatures, (4) a constant temperature and multiple repetition of 2 stage stress amplitudes, and (5) multiple repetition of 2 stage temperatures and 2 stage stress amplitudes.
The following conclusions have been obtained.
(1) Σ(n/N) became larger than unity under the tests for combinations of the step-up temperature environment and constant stress amplitude, and of the constant temperature and step-up stress amplitude.
(2) In the completely opposite cases to the conditions discribed-above, the values of Σ(n/N) seem to be smaller than unity.
(3) In the cases of the multiple step-down-2 stage for the both factors (500°C_??_300°C, 32kg/mm²_??_28kg/mm²), if the period of stage for the stress amplitude 28kg/mm² at 300°C is short, the values of Σ(n/N) are smaller than unity but, if this period is long, the values of Σ(n/N) are extremely larger than unity.
(4) The scatter band of the cumulative cycle ratio for all the test results was in the range of about 0.2 to 100.
Moreover a new idea for predicting fatigue life of machine parts employed under various combinations of temperature, corrosion environments and stress amplitude is proposed on the basis of the knowledge obtained previously from the tests under various combinations of corrosion environment and stress amplitude and the present informations.