The calculation of equivalent fatigue stresses

The most important problem in diagnosing the strength reliability of parts is predicting their endurance. To calculate the endurance under real operating conditions, the actual loading is approximated by block loads which are reduced to an equivalent stress. This stress is used to determine the number of cycles to fracture on a fatigue curve. The available techniques for calculating equivalent stresses do not consider the accumulation of fatigue damage in parts. Therefore, the total endurance for all blocks has a spread over 100 %, though it must be equal to unity in accordance with the hypothesis of fatigue damage summation. To improve the accuracy of predictive durability assessments, equivalent stresses must be calculated taking into account the accumulation of fatigue damage in the material of parts. This paper first describes the main concepts of the method for calculating equivalent stresses based on the kinetic theory of fracture. The mathematical apparatus developed by analyzing durability tests makes it possible to determine the initial damage of a material and the proposed models provide calculations of the accumulated fatigue damage during cyclic deformation of samples at any stage of the block loading. The possibilities of the method are illustrated by an example of low-cycle fatigue tests on samples made of HS80 steel. The developed models and algorithms make it possible to calculate the accumulated fatigue damage in the material for a finite number of loading blocks, which allows equivalent stresses to be determined, a new fatigue curve corresponding to the accumulated fatigue damage to be set up, and the remaining life of parts under given operating conditions to be estimated.

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