ENDOMMAGEMENT EN FATIGUE MULTIAXIALE DES PIECES TRAITEES SUPERFICIELLEMENT

Abstract

After machining of the parts which are subjected to cyclic loadings and in order to improve their fatigue life, in terms of life expectancy, manufacturers look for the best means to reach this aim. For this end, the surface treatments are the major concern. Predict fatigue life of me-chanical parts deemed critical is an asset of the design because the specifications become more demanding. The advent of increasingly early fatigue cracking is a systematic concern in any design process. In the first part of this study we have proposed a predictive methodology of accumulated damage under multiaxial fatigue loadings and therefore the assessment of fatigue life of the structures. The model is based on the damaged stress law. In the case of multiaxial loading with variable amplitude, we linked the cumulative damage law with Crossland fatigue criteri-on, which converts the multiaxial fatigue stress in an equivalent stress. At each point of the structure, stress tensors are calculated using finite element method via the Ansys code. We applied the model on specimen made of steel SM 45C. To do this, we selected two types of loadings: a monotonous traction overlying an alternating bending and monotonous traction overlying an alternating torsion. For each loading, we computed, via a finite element calcula-tion, the maximum hydrostatic pressure during the cycle as well as the square root of the se-cond invariant of the stress tensor amplitudes. Mechanical fatigue characteristics are deter-mined from the Wöhler curves of the material. Applications with cyclic loading up of increas-ing, decreasing and random blocs were performed. The simulations developed are used to determine the damage accumulation curves which feature non-linear portions of pace, as well as estimating the total lifetime. The results achieved by the model were compared with those given by the linear Palmgren-Miner law. Contrary to this law, the proposed model takes into account the load history. The proposed model highlights the influence of the loading sequence on fatigue life. The model was also validated by experimental tests, from the bibliography, in uniaxial fatigue loads on specimens made of C 45 steel. In the second part of this study, we have considered a type of surface treatment in this case the shot peening process. We applied the model taking into account the compressive residual stresses generated by this surface treatment. The benefit effect of compressive residual stress-es has been highlighted by the model. The results obtained by the simulation clearly show that the surface treatment leads to an improvement of fatigue life by extending the life and by de-laying the accumulation of damage in the structure subjected to multiaxial fatigue loads with variable amplitudes.