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In the present paper, the normalized low carbon steel has been subjected to the fatigue stressing under rotating-beam bending with the constant-and double-amplitude in order to investigate the effect of reversed pre-understressing on the endurance limit and the fatigue life for the overstress, and the same material has been submitted to the fatigue-and-hardness test, in which the fatigue machine was stopped several times during the fatigue stressing, and the MicroVickers hardness was measured at the ferrite matrix of the surface of the specimen to study the effect of reversed pre-understressing on the hardness of the test piece. The followings are the conclusions obtained from the experiments:(1) The endurance limit, the fatigue life for the overstress, and the hardness of the material tested in the present study are all increased comparing with those of the virgin material due to reversed pre-understressing.(2) The increasing rates of the fatigue life and the hardness do not depend upon the level of the primary understress.(3) The higher the level of the primary understress, the less number of stress cycles are necessary to start increasing of the endurance limit and of the fatigue life.(4) The endurance limit and the hardness increase due to reversed understressing, but both are gradually saturated with the increase of the number of stress cycles. The maximum hardness is obtained at the number of cycles of about 107 and the maximum endurance limit will be attained at a greater number of stress cycles.(5) The fatigue life for the overstress also increases due to reversed pre-understressing. In the case of the higher secondary overstress, the increasing rate of the life drops after the number of stress cycles of about 5×106. At the lower secondary overstress, the fatigue life extremely increases with the increase of the number of cycles of the pre-understress.

Fatigue Strength and Hardness of Mild Steel Subjected to the Repeated Pre-Understressing