G. Lormand

Development of a Three-Dimensional Semi-Analytical Elastic-Plastic Contact Code

A three-dimensional elastic-plastic contact code based on semi-analytical method is presented and validated. The contact is solved within a Hertz framework. The reciprocal theorem with initial strains is then introduced, to express the surface geometry as a function of contact pressure and plastic strains. The irreversible nature of plasticity leads to an incremental formulation of the elastic-plastic contact problem, and an algorithm to solve this problem is set up. Closed form expression, which give residual stresses and surface displacements from plastic strains, are obtained by integration of the reciprocal theorem. The resolution of the elastic-plastic contact using the finite element (FE) method is discussed, and the semi-analytical code presented in this paper is validated by comparing results with experimental data from the nano-indentation test. Finally, the resolution of the rolling elastic-plastic contact is presented for smooth and dented surfaces and for a vertical or rolling loading. The main advantage of this code over classical FE codes is that the calculation time makes the transient analysis of three-dimensional contact problems affordable, including when a fine mesh is required. @DOI: 10.1115/1.1467920#

A Physically Based Model for Endurance Limit of Bearing Steels

Available models are not suitable for calculating the load rating of bearings when an infinite life is required. This paper presents a new model that has been developed from a detailed analysis of the damage mechanism responsible for failure of bearings operating under EHD pure rolling conditions. It is based on the comparison between the local shear stress concentration built up around inhomogeneities present in steels and the microyield stress of the martensitic matrix. The predictions of this model are in good agreement with some literature data concerning the fatigue limit of bearing steels. Finally, the influence of material parameters and operating conditions on the endurance limit of bearings is investigated.

New Methodology to Evaluate the Rolling Contact Fatigue Performance of Bearing Steels With Surface Dents: Application to 32CrMoV13 (Nitrided) and M50 Steels

A new methodology is proposed to evaluate the rolling contact fatigue (RCF) performances of bearing steels in presence of surface dents. The experimental procedure consists in denting the raceway of the test specimen with a hardness machine using spherical diamond tips of different radius, i.e. 200, 400 and 600 μm, and normal loads ranging from 5 to 50 daN. Analysis of various dent geometries yields to an analytical law with five parameters useful to fit experimental profiles for contact simulation. Besides local residual stresses and plastic strains around the dent have been obtained by finite element simulations of the indentation process. RCF tests performed on a two-disk machine have shown better performances of nitrided 32CrMoV13 steel compared to M50 reference steel. The dominating role of sliding has been highlighted and two areas where damage initiates were identified, while the effects of the normal load and hoop stresses are less marked.Copyright

A dislocation model for internal damping due to the thermal expansion mismatch between matrix and particles in microheterogeneous materials

Abstract A transient low frequency internal damping, called δ T , has been observed in various aluminium-based microheterogeneous materials (Al-12.8wt.%Si alloy containing large silicon particles, Al/12 vol.%SiC, Al-7075 alloy/15 vol.%SiC). On cooling δ T is characterized by a low temperature broad poorly fined maximum. Moreover, δ T increases with increasing the cooling rate and decreasing the frequency. It is shown that the observed phenomena are linked with the internal stresses owing to the thermal expansion mismatch between particles and matrix. Finally, a model based on the movements of dislocations in the vicinity of the interface particle-matrix is suggested to explain the experimental features of δ T .

On the influence of residual stresses in determining the micro-yield stress profile in a nitrided steel by nano-indentation

Abstract A new method has recently been developed for obtaining the local micro-plasticity properties of materials from nano-indentation measurements. This method is based upon the measurement and analysis of remanent displacement e r versus the maximum load W , produced during successive nano-indentation loading-unloading cycles at an increasing W . The present paper is concerned with the influence of residual stresses in determining the local micro-yield stress using this method. First, the influence of residual stresses on e r is evidenced experimentally on nitrided 32CrMoV13 samples by studying the effect of the redistribution of residual stress that occurs when the initial component is sectioned in different ways. Second, the actual residual stress at any measurement point is calculated for every sample configuration. This is achieved by identifying the volume dilatation strain profile in the nitrided layer. Third, the influence of residual stresses on e r is simulated using a three-dimensional elasto-plastic contact code. In this way, we obtain the remanent displacement that would be measured if the material were free of residual stress. We then propose a new method for obtaining the local micro-yield stress in the presence of residual stresses. This method is then applied to the determination of the intrinsic micro-yield stress profile of nitrided 32CrMoV13 steel. The results show that in some specific configurations the micro-yield stress may be overestimated by some 20%.

Application of a New Physically Based Model to Determine the Influence of Inclusion Population and Loading Conditions on the Distribution of Bearing Lives

The principal cause of failure in bearings under E.H.D. conditions is deep spalling initiated beneath the surface. The corresponding damage mechanisms have been clearly identified (fatigue “butterflies”, crack nucleation and propagation to the surface). In this context, a statistical model, based on a mixed approach combining micromechanics and physical metallurgy, has been developed for predicting the distribution of bearing lives from the inclusion population in the steel. The model has been validated by comparing its predictions with experimental failure probability curves determined from flat washer fatigue tests performed on a steel whose inclusion distribution had been accurately established. The existence of non zero survival rates for very long lives indicates that Weibull distribution [1] is not suitable to represent the complete life distribution for modern bearings. The predictive capabilities of the model have been used to study the influence of parameters, such as the applied load, the specimen size, etc.

Rolling contact fatigue of nitrided 32CrMoV13 steel

In this paper, dent initiated Rolling Contact Fatigue (RCF) of nitrided 32CrMoV13 steel is studied, both experimentally and numerically, from the indentation process to the RCF failure. First, a method based on nano-indentation testing has been set up to identify the local elastic-plastic behaviour of nitrided 32CrMoV13 steel, hence enabling the characterisation of the micro-yield shear stress profile. Fatigue tests have then been conducted to evidence the influence of different operating conditions on RCF and to compare nitrided 32CrMoV13 steel to a reference material. Last, a numerical study is presented. An indent based endurance limit, H1I is determined. The comparison of H1I with experimental results show that it is a coherent definition of the endurance limit.

A New Methodology to Evaluate the Rolling Contact Fatigue Performances of Bearing Steels With Surface Dents: Application to 32CrMoV13 (Nitrided) and M50 Steels

A new methodology is proposed to evaluate the rolling contact fatigue (RCF) performances of bearing steels in presence of surface dents. The experimental procedure consists in denting the raceway of the test specimen with a hardness machine using spherical diamond tips of different radius, i.e. 200, 400 and 600 μm, and normal loads ranging from 5 to 50 daN. Analysis of various dent geometries yields to an analytical law with five parameters useful to fit experimental profiles for contact simulation. Besides local residual stresses and plastic strains around the dent have been obtained by finite element simulations of the indentation process. RCF tests performed on a two-disk machine have shown better performances of nitrided 32CrMoV13 steel compared to M50 reference steel. The dominating role of sliding has been highlighted and two areas where damage initiates were identified, while the effects of the normal load and hoop stresses are less marked.Copyright