P. Gilles

Experimental and numerical study of the ploughing part of abrasive wear

Understanding and quantification of ploughing resistance and associated deformation is of primary importance for materials undergoing abrasive wear. The aim of this study is to understand mechanical phenomena induced by a ploughing process. A comparison of numerical simulation with experimental results has been performed in order to validate our numerical model. The process to be studied is the ploughing of a soft flat surface with a rigid sphere. A numerical study with a finite element code has been performed in order to understand the role of different parameters on strain and stress fields. First of all, we have studied the process in terms of applied loads and strained material but the material characteristics seemed to have a larger influence on the process than the boundary conditions themselves. A profilometric study on numerical simulations has shown us that the elastic strain was not totally recovered and gave rise to residual stresses. Moreover, a study of the contact surface and the ploughing friction coefficient has been developed and a comparison to Bowden and Tabor model has been made. It has shown us that strain hardening modeling has an essential influence on the ploughing friction coefficient. Thus, we have performed scratch test on an AISI316L stainless steel in order to validate our numerical results.

Analysis of Residual Stress Induced by Hand Grinding Process

Grinding cup wheel is often used in the case of hand grinding which allows an important material removal rate but with secondary concern of surface integrity. Integrity is strongly affected by the process and consequently influences the surface behaviour in terms of resistivity to stress corrosion and crack initiation. This operation is difficult to master in terms of results on the surface and subsurface due to its manual nature. The paper presents results of an experimental study to investigate the residual stresses induced by this hand grinding process.

Analysis of two simplified methods (R6 & GE-EPRI) for circumferential crack stability in leak-before-break applications

Abstract The Ainsworth method (R6 Option 2) and the GE-EPRI method are compared and analysed. These J-estimation schemes allow us to assess at low cost, crack stability in piping systems, which is an important step in Leak-Before-Break applications. The reliability, flexibility and self-consistency of the two methods are examined by focusing on the effect of geometry, nature of loading and superposition of tension and bending. The GE-EPRI method, based on Finite Element results, is used to check the validity of the hypothesis on which the simplifications in the R6 method for through-wall circumferentially cracked pipes rely. The R6 method appears to be easier to use for the treatment of secondary loads. It is shown that, the Ainsworth Failure Assessment Line, derived in the single load case, is still valid for combined proportional tension and bending loads, provided the appropriate limit load formula is chosen.

3D finite element simulation of TIG weld pool

The aim of this paper is to propose a three-dimensional weld pool model for the moving gas tungsten arc welding (GTAW) process, in order to understand the main factors that limit the weld quality and improve the productivity, especially with respect to the welding speed. Simulation is a very powerful tool to help in understanding the physical phenomena in the weld process. A 3D finite element model of heat and fluid flow in weld pool considering free surface of the pool and traveling speed has been developed for the GTAW process. Cast3M software is used to compute all the governing equations. The free surface of the weld pool is calculated by minimizing the total surface energy. The combined effects of surface tension gradient, buoyancy force, arc pressure, arc drag force to drive the fluid flow is included in our model. The deformation of the weld pool surface and the welding speed affect fluid flow, heat flow and thus temperature gradients and molten pool dimensions. Welding trials study is presented to compare our numerical results with macrograph of the molten pool.

Modelling of Residual Stresses Induced by Turning of 316L

Residual stresses are very important for the lifetime of pieces in their mechanisms. These kind of damages are mainly caused by mechanical, thermal, and metallurgical affectations of the machined material. To control these affectations, we need to link the cutting parameters to the residual stress state observed onto the workpiece surface and depth. These connections can be made with analytical works, experimental works or numerical works. In our case, it has been chosen to work with a numerical support in order to observe and understand precisely the phenomenon involved during cutting operation. While this way of study is really popular, we proposed to model the residual stress generation in a original way by keeping aside the chip formation. This new approach presented in a previous paper was simple and only use a 2D model. This first model moves thermo mechanical loadings onto the workpiece surface to recreate the relative motion between the tool and the workpiece. The new 3D model, presented in this paper, is now improved with an original friction law. It underlines the impact of each passage of the tool onto the others. The physical properties are thermo dependant and the flow stress model is based on a Johnson cook behaviour.

Fast diagnosis and treatment of cracklike defect injuriousness in nuclear power plant equipment

Abstract Increasingly stringent safety requirements governing the nuclear industry have made it essential to gain in-depth knowledge of the injuriousness of cracking phenomena in auxiliary and secondary nuclear power plant systems, and to devise methods of rapidly evaluating potentially injurious flaws. The Defect Injuriousness Diagnosis and Treatment Package (DIDTP) discussed in this paper was developed by Framatome, a French-based PWR builder, with this goal in mind. The first part of this paper gives a general description of the DIDTP, which is made up of tables and nomographs illustrating the injuriousness of flaws liable to be encountered in the most severely loaded regions of plant systems. The basic principles underlying the DIDTP, together with computational methods and application procedure, are detailed. The second part of the paper presents two practical examples illustrating the use of the diagnostic system, one applied to the main steam line, the other to gate valve wedges. The types of results obtained and the usefulness of the system as both a treatment and a diagnostic tool are explored.

Numerical Study of Hand Disc Grinding of Ni-Base Alloy 690

The state of the surface, whatever the metal or alloy used is of paramount importance. Hand disc grinding operation is difficult to master in terms of results on the surface due to its manual nature. From this, comes the great importance to the mastery of the consequences induced by this abrasive process. A previous experimental study on hand disc grinding revealed several consequences on the surface integrity in terms of residual stresses, micro-hardness, hardening of the material etc. Numerical simulation can be a good way to prevent manufacturers of very time consuming experiments for the prediction of residual stresses due to grinding. The purpose of this study is to predict the consequences in terms of induced temperature fields and the state of residual stresses. The action of the disk-grinding wheel on the Workpiece is modeled by a moving heat flux on top of the part surface. All the difficulties lie in the quantification of the heat flux and more precisely in the heat flux density that gives the way the thermal load is distributed in the contact disk grinding/workpiece area. In this paper, an original analytical model for the determination of the heat flux density has been developed. For each step, the thermo-mechanical calculation is performed. Finally, the distribution of temperature and residual stresses will be carried out with the FE software SYSWELD 2010®.

Analytical Method for the Calculation of J Parameter for Surface Cracks in Piping Welds

RSE-M and RCC-MR codes provide flaw assessment methodologies and related tools for Nuclear Power Plant. AREVA, CEA and EDF developed in particular a large set of compendia for the calculation of the parameter J for various components (plates, pipes, elbows, ...) and various defect geometries. The last step of these developments deals with the weld joints : since 2004, the partners are developing a methodology to calculate the J parameter for a defect located in a weld, less conservative than usual methods. This methodology is based on the definition of an equivalent material that leads to the same J value (with same loading conditions and defect geometry) than the bi-material component. The stress-strain curve of this equivalent material is deduced from a combination of the tensile curves of the base metal and of the weld metal. The weigth coefficients applied are specifically defined for the J calculation and generalized to deal with any weld joint geometry.Copyright

Application of the BEREMIN Model for Evidencing the Warm Pre-Stress Effect in an Irradiated Reactor Pressure Vessel Containing a Semi-Elliptical Subclad Defect Under Small and Large LOCA Conditions

The nuclear power plants lifetime is strongly dependent of the guarantee of the reactor pressure vessel (RPV) integrity. Therefore, the RPV integrity has to be demonstrated under the most severe configuration, namely the Pressurized Thermal Shock induced by the Loss of Coolant Accident induced by a large break in the primary loop. For such a transient, the apparent risk of failure is maximum when the load is decreasing; the fracture resistance decreasing rate being stronger. However, such type of loading generates an increase of the fracture resistance as shown by numerous studies (Chell, 1980 – BEREMIN, 1981 – Smith et al., 2004). This is known as the warm pre-stress (WPS) effect. This beneficial effect on the resistance to brittle fracture is not accounted for in the French RCCM and RSEM codes (RCCM, 2000 – RSEM, 2005). EDF has launched several R&D actions with CEA and AREVA as well as with European partners (SMILE, 2001) to validate and model the WPS effect under RPV representative conditions. Proving the existence of this beneficial load history effect (designated as Warm Pre Stress WPS), in the case of a defective RPV in emergency and faulted conditions is the aim of the present paper. The demonstration is conducted in the case of cleavage fracture using an improved version of the BEREMIN model. As opposite to the classical Fracture Mechanics methodology, this approach allows to account for load history effects on cleavage. The study analyzes the behavior of a semi-elliptical under clad crack in the EoL core shell of a 900 MWe RPV for two loading cases: the large break Loss Of Coolant Accident transient and a small break LOCA inducing thermal fluctuations on the vessel inner wall. The WPS effect is evidenced by comparing the plasticity corrected SIF levels of two loadings for the same value of failure probability: the considered WPS loading and a virtual monotonously increasing load applied at the temperature at which the brittle fracture risk is estimated.© 2008 ASME

Engineering Approach for the Reactor Pressure Vessel Integrity in PTS Conditions: Thermal Hydraulic and Fracture Mechanics Studies of a Safety Injection in a Three Loop Power Plant — French Benchmark With EDF and AREVA-NP Tools

For the Reactor Pressure Vessel (RPV) assessment and Lifetime evaluation of the nuclear plants, French Utility applies a series of calculations including thermal-hydraulic, thermo mechanical and fracture mechanics studies in order to study the Pressurized Thermal Shock (PTS) in the down comer caused by the safety injection. Within the frame of the plant life time project, integrity assessments of the French 900 MWe (3-loops) series reactor pressure vessel (RPV) have been performed. We found that the modeling of thermal-hydraulics loads is a source of gain. Considering the length of local 3D calculation and the large number of cases, E.D.F and AREVA-NP decided to share the effort. However the two chains of software differ: EDF uses Code_Saturne (coupled with the thermal solid code Syrthes) and Cuve 1D and AREVA-NP uses Star_CD and CALORI codes respectively for thermal hydraulic and thermo mechanical computations. According to this approach, comparison between the two chains of tools have been performed. Moreover this action contributes to the verification and the validation of each code in accordance with the OECD Best Practise Guidelines (BPG). The study has been achieved by two independent teams from EDF and AREVA-NP. It should be emphasized that this benchmark helped to strengthen the accuracy of CFD and the adapted methodology (working progress). The investigated configuration corresponds to the injection of cold water in the vessel during a penalizing representation of a primary break transient and its impact on the solid part formed by cladding and base metal. Numerical results are given in terms of fluid temperature and velocity fields in the cold legs and in the downcomer. The obtained numerical description of the transient (internal pressure and temperature field within the vessel) is used as boundary conditions for a full mechanical computation of the stresses. This thermal mechanical transient is obtained on a 3D mesh of the vessel, covering the two core shells and their circumferential welds, as well as the internal cladding. The results show that such a complete thermal–hydraulic and mechanic 3–dimensional analysis improves the evaluation of the consequences of the loading on the stress fields and eventually the margins to fast fracture of the RPV. The good agreement observed between EDF and AREVA-NP results and their accordance with the validation computations, confirm the robustness of the approach.Copyright