Low endurance fatigue behaviour of mild steel beams in reversed bending

Abstract

The behaviour of mild steel under conditions of reversed bending in the inelastic range at relatively slow rates of cycling, falls into the category of the low endurance fatigue problem in metals, where failures occur within 10⁵ cycles. This investigation is concerned with an examination of material behaviour in reversed bending (i.e. pure bending) at three frequencies——1, 10 and 100 c/min——and strain ranges in the region from 0.5-12 per cent. Axial extension assumed some significance during the tests at the larger strain ranges, as did cyclic heating at the higher frequency. Some adjustments are made to take account of the effects of axial extension in the tests at the two lower frequencies. A moment-curvature relationship for the material in a cyclic state is established along with a fatigue resistance law for the range of deformations considered; the former being restricted to cyclic operations in the range from 1 to 10 c/min. An indication is given, supported by some experimental evidence, of how the moment-curvature relationship and fatigue resistance law can be used to predict the behaviour of mild steel structural components subjected to the type of loading under consideration.