CALCOLO DEL DANNEGGIAMENTO A FATICA SU COMPONENTI MECCANICI IN CONDIZIONI DI CARICO COMPLESSE

This thesis is concerned with the fatigue behaviour of complex, three-dimensional, stress concentrations under multiaxial loadings. The fatigue behaviour of components containing complex three-dimensional (3D) features is a subject of great practical interest to industrial engineers. In fact three-dimensional solid modelling and linear elastic stress analysis, by means of numerical methods, are used to investigate the local stress field at 3D stress raisers in the mechanical components. Moreover, the problem of properly performing the fatigue assessment is further complicated by the fact that such failures are, in general, caused by multiaxial loadings: this makes it evident that engineers engaged in assessing real mechanical assemblies need sound engineering tools capable of accurately and efficiently estimating multiaxial fatigue damage. Starting from the stress field obtained from a linear elastic analysis and taking advantage of the so-called implicit gradient approximation, an effective stress index connected with the material strength is calculated. The effective stress is calculated by solving a second-order differential equation over all the component (the implicit gradient approach) independently of its geometric shape. Besides this thesis summarizes a first investigation into the possibility of applying the implicit gradient approach to real components under both uniaxial and multiaxial loading conditions by introducing an appropriate multiaxial criterion into the implicit gradient framework. Generally speaking many multiaxial criteria could be used to this purpose, namely critical plane approaches, stress-invariant based approaches and integral approaches. Such methods have been proved to be effective for fatigue damage evaluation, at least in cases of smooth components or blunt notches, however, since the aim of the present work is to obtain a numerically efficient other than the effective method, in this first attempt attention has been focused on stress-invariant based approaches that is suitable for high-cycle fatigue evaluation. The damage evaluation is obtained by analyzing the loading path on the 5-dimensinal deviatoric Euclidean space. Explicit analytical solutions of the proposed criterion are available in case of biaxial sinusoidal loads. The method in conjunction with implicit gradient approach has been applied to experimental results generated by testing notched specimens of low-carbon steel containing severe 3D stress raisers subjected to uniaxial and multiaxial in-phase and out-of-phase fatigue loads. The implicit gradient methods for component made of non-linear material is presented in section 8. The non linearity of the material is proposed in order to understand the behaviour of the material upper the endurance limits. The study takes into account different stress raisers: sharp V-notch, plane specimens weakened by lateral U or V –notches and simple welded joints made of steel. The effective stress are obtained by means of the gradient approach in two different ways. For sharp V-notch it has benn used a analytical-numerical procedure based on the asymptotic solution for the near-tip stress in an elastoplastic material (know as the HRR solution) whereas for the plane specimens and for the welded joints a numerical procedure based on the FE methods has been adopted.