Dynamic Strain Amplitude During Fatigue Process of Mild Steel due to Rotary Bending

Abstract

Annealed mild steel specimens have been subjected to rotation under the bending stress of 21∼31kg/mm2 with the speed of stress repetition at 3∼3000rpm at room temperature to examine its dynamic strain amplitudes and creep strains. A wire strain gage and an ocillogram have been employed by means of a slip ring to record the process.(1) Fatigue process, which is expressed by the dynamic strain amplitude, can be divided into three stages: first stage of softening, second stage of hardening, and third stage of apparent softening.(2) When the speed of stress repetition is slow and the stress amplitude is high, the amplitude of dynamic strain increases to a larger value, and the effect of stress amplitude on the specimens is larger than that of the speed of stress repetition.From the experimental results, it has been made clear that fatigue life is in inverse proportion to the peak of dynamic strain amplitude, i. e. the maximum value of softening process.(3) With the exception of the third stage, the dynamic strain amplitude on fatigue process is proportional to the transient creep strain during fatigue process.