Fatigue predictions of machine components in multiaxial stress states

Aim of project
The main focus of the project is to develop a method of predicting fatigue strength of machine components with notches subjected to multiaxial loads. Presently, the problems of fatigue durability and strength of materials under combined loads are extensively studied at many universities and scientific centers. The majority of projects aim at problems involving formulation of new or effective implementation of existing fatigue hypotheses.

Fatigue cracks are most often initiated at the notch tip where the highest stress concentration occurs and local plastic fl ow is observed. The problem of prediction of fatigue strength of a material, mainly consists of precise simulation of stress-strain histories occurring at the notch concentration area as a result of application of arbitrary combined loads.

Therefore, it is necessary to determine local-notch-tip strains and stresses. In most situations, measurement is impractical and calculation is required. The finite-element method and the boundary integral method present tools that enable accurate calculations. However, application of these methods for analysis of lengthy cyclic loading histories is time-consuming and demands sophisticated hardware.

In the view of engineering implementations, the above mentioned features lead to simplified approximation formulas that enable to predict material behavior subjected to arbitrary combined loads. For this reason, it will be attempted to generalize energy methods (Neuber, Molski-Glinka) to multiaxial non-proportional stress-strain states within the range of elastic-plastic deformations. Particularly, attention will be paid to formulation of constitutive relations accounting for hardening, softening and stability of material plastic behavior observed during applications of non-proportional loads. The approach will be used to obtain accurate results of numerical simulations of material response near the notch root to combined loads.

Expected results The studies planned in the project are expected to improve prediction accuracy of fatigue durability of notched machine components subjected to multiaxial and random loads. The project results will include numerical procedures for effective and fast calculations of stresses and elastic-plastic strains in the zones adjacent to notch boundary. The simplified models were constructed, in the form of an engineering tool, to be used in practical estimations of fatigue durability of machine components under non-proportional multi-axial loads.