Effect of Understressing on Fatigue Strength of Low Carbon Steel under Rotating Bending and Torsion

Abstract

In order to investigate the effect of understressing on fatigue strength of low carbon steel, the fatigue tests have been carried out under rotating bending and torsion, using the specimens of Al-killed and quenched rimmed steels having different strain aging abilities. Based on the comparison of the experimental results for both materials under both loading conditions, the fatigue behaviours, particularly the effects of strain aging were discussed. The main conclusions obtained are summarized as follows:
(1) Under both loading conditions, the understressing effect appeared clearly in quenched rimmed steel but not in Al-killed steel.
(2) The understressing effect did not depend on the understress levels.
(3) The specimen of quenched rimmed steel aged at room temperature showed an increase in fatigue strength and hardness compared with the unaged specimen.
(4) The coaxing effects was revealed slightly in Al-killed steel. In quenched rimmed steel, it was remarkable under rotating bending but appeared under torsion only when the coaxing stress increment was small.
(5) The coaxing effect was related to the existence of crack and was most remarkable at the crack length of about 70μ.
(6) It seemed that the main causes of understressing and coaxing effect for quenched rimmed steel were precipitation hardening and strain aging.