Papers by Author: H. Michaud

Abstract: ASCOMETAL produces alloy steels used for spring (leaf or coil), where the weak fatigue points are on the surface which is reinforced by shot-peening. So, the fatigue optimization with the steel grade needs a perfect knowledge of the material answer after shot-peening. For that reason, an analytical model has been developed where low cycle fatigue behaviour and all the usual process parameters are integrated (especially the impact position, and the covering-rate). Moreover, through a Monte-Carlos approach, the model permits to analyse the effect of scattering elements like impact speeds, ball sizes, or material fatigue behaviour. With this model several key process parameters have been analysed and validated with residual stress profiles evaluated by X-ray diffraction. So, for spring leaf, the effect of an applied load during shot-peening or shakedown during bending fatigue is described.

Abstract: The fatigue resistance of car components such as crankshafts or diesel injection rails is mainly related to fatigue at geometrical singularities. Its understanding requires the simulations of the different steps of the industrial process resulting in residual stresses generation and their evolutions in service. First concerning crankshaft fatigue damage, a complete analytical model of residual stresses generation and shakedown in fatigue is presented. Then a fatigue criterion is established and validated for this application. Finally, for the high pressure diesel injection rail, this approach can be generalized to the holes intersection singularities.

Abstract: The GFAC (French Association for residual stress analysis) decided in 2007 to work on external reference samples for residual stress analysis by X-ray diffraction as defined in the XPA 09-285 and EN 15305-2009 standards. Seven materials are studied: ferritic steel, martensitic steel, aluminium alloy, titanium alloy, 2 types of Nickel-Chromium alloy and tungsten thin layers deposited on silicon wafers. The purpose of this external round robin campaign is threefold: (i) to give possibilities for each laboratory involved in the campaign test to obtain external reference samples for each material tested, (ii) to validate a common procedure for qualification of external samples and (iii) to commercialise validated external reference samples through the GFAC association. A common approach of X-Ray diffraction parameters, samples geometry and standard procedure has been chosen and adopted by each laboratory involved in these tests. No indication in terms of residual stress calculation method is given; the choice of the method (centroid, middle point, maximum of peak, fitting…) is the choice of the laboratory according to their X-ray diffraction set-ups, softwares and experience. Once all samples are analysed, values given by each laboratory are compared and analysed.

Abstract: In this work a complete analytical model of elastic-plastic bending, tension and / or torsion has been developed to simulate the manufacturing of leaf springs and coil suspensions. A special procedure was also implemented to simulate mechanical surface treatments such as shot peening. These models allow predicting the plastic strains and residual stresses induced by successive loadings of the part. Our approach required first defining the mechanical behaviour of the material. The constitutive laws of the employed steels were therefore identified starting from cyclic loops recorded for tests carried out at various imposed strains. These laws were introduced in the plasticity model to simulate the different steps of manufacturing: forming of the part, presetting operations, shot peening, elastic-plastic shake down produced by the first cycles of fatigue. In order to validate the model, residual stress maps were also characterized by X-Ray diffraction. However, to retrieve the maximum of information from this kind of measurement, the experimental data has been analysed through a global method which processes the whole acquired diffraction peaks at once.

Abstract: In this work, the effect of steel grade on the fatigue resistance of deep-rolled crankshafts is analysed. In the first part of this paper, the mechanisms leading to the increase of the fatigue resistance brought by the deep rolling treatment, is presented. This reinforcement is mainly linked to crack arrest due both to a decrease of the in-depth stress concentration factor and to remaining compressive residual stresses induced by the deep rolling. In a second part, an analytical model of residual stresses generation by deep-rolling and fatigue is presented. In this model the low cyclic fatigue behaviour of the steel is taken into account, and the residual stress stability with bending fatigue cycling can be predicted. After a presentation of the experimental validation on two different microstructures (baintic and ferrito- perlitic), this model is used for analysing the main parameters of the deep-rolling process and fatigue resistance.

Abstract: X-ray diffraction is used to analyse the fatigue behaviour of carbo-nitrided steel layers. Measurements are therefore carried out on the two major phases of the material, i.e. the martensite (a') and the retained austenite (g). On such gear material, X-ray residual stress evaluations are particularly difficult for three reasons. First, the studied material is multiphase. For that reason, in each phase, the stress component in the direction normal to the surface is non negligible. Second, the diffraction peaks obtained on the martensite are broadened, due to the overlap of different reflections of the tetragonal structure. Third, the material contains also carbide and nitride clusters, which lead to incoherent and diffuse scattering of X-rays thus making quantitative phase determination difficult. In our paper the methods used to solve these problems are presented. The development of a new quantitative phase analysis method which accounts for the variation of diffuse scattering of X-rays is shown first. The second part deals with an iterative micromechanical model implemented for the evaluation of the carbon content and the residual stress components of each phase. These methods are used for analysing the evolution with cycling fatigue of all the parameters derived from our enhanced analysis (phase volume fraction, carbon content, stress components, peak width).

Abstract: In the new types of diesel motors, the crankshaft loads increase dramatically. For that reason, to satisfy the requirements and needs of its motorist’s customers, ASCOMETAL has developed a special bending fatigue test applied to “Baldwin” specimens reinforced by deep rolling. Such test is representative of the main in-service loads existing in crankshaft crankpin fillets (one of the weakest point of the component). Thus, using this test, the mechanisms leading to the increase of the fatigue resistance brought by the deep rolling treatment, could be identified. This reinforcement is mainly linked to crack arrest due to both a decrease of the in-depth stress concentration factor and to remaining compressive residual stresses induced by the deep rolling. In this paper, the experimental methods (in particular X-rays diffraction) used to characterize the stress fields generated by the deep rolling treatment are described first. The equations of equilibrium and compatibility of the particular toric symmetry of the crankshaft crankpin fillets have been used then to analyze the surface and in-depth stress profiles of the tested sample. This allowed characterizing the shake down and stress relaxation induced by fatigue. The crack propagation behaviour and the associated fracture limits were also defined.