Christophe Jacq

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 Comprehensive Method to Predict Wear and to Define the Optimum Geometry of Fretting Surfaces

This paper presents a fast and robust three-dimensional contact computation tool taking into account the effect of cyclic wear induced from fretting solicitations under the gross slip regime. The wear prediction is established on a friction-dissipated energy criteria. The material response is assumed elastic. The contact solver is based on the half-space assumption and the algorithm core is similar to the one originally proposed by Kalker (1990, Three Dimensional Elastic Bodies in Rolling Contact, Kluwer, Dordrecht) for normal loading. In the numerical procedure the center of pressure may be imposed. The effect of surface shear stress is considered through a Coulomb friction coefficient. The conjugate gradient scheme presented by Polonsky and Keer (1999, Wear, 231, pp. 206-219) and an improved fast Fourier transform (FFT) acceleration technique similar to the one developed by Liu et al. (2000, Wear, 243, pp. 101-111) are used. Results for elementary geometries in the gross slip regime are presented. It is shown that the surface geometry influences the contact pressure and surface shear stress distributions found after each loading cycle. It is also shown that wear tends to be uniformly distributed. This process continuously modifies the micro- and macrogeometry of the rubbing surfaces, leading after a given number of cycles to (i) an optimum or ideal contact geometry and (ii) a prediction of wear.

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

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%.

A Comprehensive Elastic-Plastic Model to Predict Wear and to Define the Optimum Geometry of Fretting Surfaces

In turbine engines the contact between blades and disk may be subjected to fretting fatigue or fretting wear. This is due to the centrifugal force acting on the blade, which causes a relative displacement of the contacting surfaces producing either mixed stick-slip or gross slip.Copyright

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