Energy Stored in a Specimen under Fatigue Limit Loading Conditions

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Abstract
This paper presents results of measuring the energy stored in a specimen made from Ck 35 carbon steel loaded with an alternating one-directional stress whose value is equal to the fatigue limit. The energy stored in the specimen was determined in an indirect way as the difference between the mechanical energy expended in the specimen and the energy released into the surroundings as heat. The proposed technique is a modified version of the dynamic hysteresis loop method. The energy released as heat was measured with an electric modelling method. It was found that the energy stored during one cycle in a specimen loaded with a stress equal to the fatigue limit was larger than zero (i.e., with Nf→ ∝, the stored energy value theoretically also tends to infinity). To make the theory of the experiment compatible with the fundamental assumption stating that the amount of energy needed to cause fracture is constant irrespective of loading conditions, it is necessary to introduce a rheological function accounting for all reversible processes occurring in a material under cyclic loading.