On the Fatigue of Tensile Pre-Strained Mild Steel under Repeated Alternating Combined Stresses

Abstract

It is the aim of this paper to present evidences for the changes of work hardening phenomena caused during the fatigue test under repeated alternating combined stresses in virgin and tensile cold-worked mild steel specimens.
Changing the grade of tensile pre-straining K, the phenomena are measured dynamically with the use of strain gage and statically the dial gage, and then the differences between both measured results are argued.
The aspects of slip band produced on the surface of fatigued specimens are studied by means of electron microscope in connection with the changes of work hardening.
The authors find that it is not possible to draw the S-N curve by the use of the Schenk type fatigue testing machine and this origin is traced.
The main conclusions from this research are as follows:
(1) In the case of uniaxial tensile pre-strained steel the work hardening was saturated at an early stage of the fatigue test, and then the strength falls due to the relieving of an excess of internal stress and development of micro-crack was continued slowly up to failure.
(2) In the case of annealed steel two work hardening peaks were observed and one bottom appeared between those peaks at three kinds of α respectively. The difference of stresses at the 1-st peak and the bottom decreased by changing α from 0° to 90°.
(3) The difference of stresses measured dynamically and statically were remarkable at an early stage of all fatigue tests and it was found that the work hardening phenomenon depended on the cyclic strain rate.
(4) The appearance of slip band was due to the softening of fatigue test.