Development of Criteria and Modeling of Cyclic Strength of Elements of Structures

Abstract

Many structural elements are exposed to variable random loads during operation and are destroyed, as a rule, in zones of concentration of local stresses, elastoplastic deformations and energies. Accumulation of fatigue damages in the zones of concentration of structural elements is usually evaluated by the experimental fatigue curves of model elements, represented in nominal stresses, determined without taking into account the effect of concentration or local conditional stresses, calculated as the product of nominal stresses and the theoretical stress concentration coefficient under the assumption of local elastic deformation. The use of these criteria for the generation of fatigue curves introduces additional errors in the assessment of damage and prediction of durability under conditions of random loading of structures. In this connection, a method for estimating damage in concentration zones when predicting durability under conditions of random loading of structures is presented, if we use the experimental fatigue curves of models presented in local stress intensities (force approach), elastoplastic deformations intensities (deformation treatment) or specific energy of shape change (energy concept), which take into account the bulk of the stress-strain state and its kinetics in terms of the number of loads. To obtain these fatigue curves, we propose a solution of the cyclic elastoplastic problem for determining the strength, deformation, and energy local strength criteria specified above. Results of application of local criteria for experimental and analytical modeling of cyclic strength at bending with rotation of shafts with a ring notch are resulted. The use of local strength criteria has made it possible to more reliably estimate the damage to shafts under conditions of overload characteristic during operation.