Eric Feulvarch

A simple and robust moving mesh technique for the finite element simulation of Friction Stir Welding

The simulation of the Friction Stir Welding process is a complex problem which involves physical couplings between mechanics and heat transfer, very large deformations and strain rates in the stirring zone around the pin. To avoid mesh distortions or very large computing time due to the Arbitrary Lagrangian Eulerian technique usually proposed in the literature for the finite element method, a simple but robust moving mesh technique is proposed for the numerical modeling of the FSW process. It is based on a Eulerian formalism and the mesh is composed of 2 parts: a first one which is fixed around the stirring zone and a second one which includes the base material near the tool and moves with a rotational solid motion corresponding to the tools velocity. Therefore, there are no mesh distortions and the Eulerian formalism leads to satisfying computing time. An example clearly evidences the efficiency and robustness of the moving mesh technique proposed for a 3D complex geometry of the tool.

Thermometallurgical and mechanical modelling of welding – application to multipass dissimilar metal girth welds

Abstract The prediction of welding residual stresses and distortions needs to take accurately account of the couplings between heat transfer, metallurgy and stresses–strains. The numerical simulation of multipass welding of dissimiliar metal including ferritic steels is specially difficult as three-dimensional (3D) simulation a priori needs to accurately take into account the complex phenomena in the heat affected zone and in the overlapping regions. The results obtained using a simplified two-dimensional axisymmetric model are discussed according to those resulting from a complete 3D simulation. It is shown that for multipass circular welds, 3D computations are mandatory to analyse overlapping regions but two-dimensional assumptions enable to capture stress distribution in the current region. Comparisons with experimental measurements of stresses using neutron diffraction or deep hole drilling are presented to validate the computed residual stresses.

3D Numerical Prediction of Residual Stresses in Turning of 15-5PH

This study presents the development of a numerical model for the prediction of residual stresses induced in finish turning of a 15-5PH martensitic stainless steel. This methodology uses a hybrid approach combining experimental results (friction and orthogonal friction tests) with a numerical model. The numerical model simulates the residual stresses generation by applying cyclic equivalent thermo-mechanical loads onto the machined surface without modeling the chip removal process. The three-dimensional approach enables to study the influence of the turning passes interactions. It has been shown numerically that the periodicity of loading leads to a significant heterogeneity of material solicitations. Moreover, overlapping of passes accentuates these effects. So, the model highlights the necessity of a multi-passes simulation to reach a constant evolution of residual stresses along the feed direction. In addition, experimental measurements obtained by X-Ray diffraction have been compared with numerical results to validate the model.

A finite element for laminar flow of incompressible fluids with inertia effects and thermomechanical coupling

The Friction Stir Spot Welding (FSSW) process involves large deformations in the neighborhood of the tool. The simulation of this process has to account for a pasty phase in which the material is stirred, and a phase remaining solid. An Arbitrary Lagrangian Eulerian (ALE) approach combined with respectively fluid and solid behaviours in each of those phases may allow to simulate a lot of rotations of the tool into the material while following the boundaries of the sheets. This work focuses on a first stage of this study, the development of a mixed formulation temperature/velocity/pressure of a fluid finite element P1+/P1 in the unsteady case.

Coupled fluid flow and ion transport through intact skin

Skin is the largest organ of the human body. It has a stratified structure consisting of four main layers: the stratum corneum, the viable epidermis, the dermis and the hypodermis. The skin protects the body by preventing fluid loss when exposed to sun, the penetration of undesirable substances in case of pollution or the development of pressure ulcers due to the direct application of external loads linked to clinical problems or ageing or aesthetic treatments. The state of this barrier (intact or damaged) governs its evolutions and its answers. Dryness, microcracks and loss of elasticity are thought to be influenced by fluid flow and the associated changes in ion concentration as a direct result of mechanical stress states. However, these phenomena are complex to understand and to model due to the strong couplings that exist between them and due to the complex behaviour of the different layers of skin tissues which are non-homogeneous, anisotropic, nonlinear viscoelastic materials subjected to a pre-stress in vivo. In addition, the properties of the human skin vary with age, throughout the body and per person. Based on a general phenomenological thermo-hydromechanical and physicochemical approach of heterogeneous media (Jouanna and Abellan 1996), this contribution proposes a model of fluid flow and ion transport through deforming intact skin. A tri-phasic model is adopted, which incorporates a solid phase, a fluid phase and ions. The temperature is supposed to be constant. Although not negligible, electrical effects are not taken into account in this model. The driving forces for transport are the gradients of the chemical potentials of the fluid and of the ions coupled with the stress states of the solids. In the mechanical model, the skin is considered as a stratified material with three layers modelling the three outer layers of the skin: the stratum corneum, the viable epidermis and part of the dermis. All layers are supposed to be made of fluid-saturated materials. Furthermore, each layer is seen as a different solid material within the solid phase and it is described by its own isotropic linear elastic model. An example calculation is given for a specimen of intact skin in contact with a NaCl solution and subjected to an external compressive displacement fixed to 3mm. The obtained numerical results are discussed in terms of fluid flow and ion transport which allow for qualitative understanding of their couplings and of the mechanical behaviour of the solids of each layer separately.

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.

First Steps towards Parametric Modeling of FSW Processes by Using Advanced Separated Representations: Numerical Techniques

Friction Stir Welding (FSW) is a welding technique the more and more demanded in industry by its multiple advantages. Despite its wide use, its physical foundations and the effect of the process parameters have not been fully elucidated. Numerical simulations are a powerful tool to achieve a greater understanding in the physics of the problem. Although several approaches can be found in the literature for simulating FSW, all of them present different limitations that restrict their applicability in industrial applications. This paper presents a new solution strategy that combines a robust approximation method, based on natural neighborhood interpolation, with a solution separated representation making use of the Proper Generalized Decomposition (PGD), for creating a new 3D updated-Lagrangian strategy for addressing the 3D model while keeping a 2D computational complexity

Two 3D thermomechanical numerical models of friction stir welding processes with a trigonal pin

ABSTRACT During the friction stir welding (FSW) process, the behavior of the material is at the interface between solid mechanics and fluid mechanics. This article deals with a comparison of two 3D numerical models of FSW processes with a trigonal pin. The first model is based on a solid formulation and the second one is based on a fluid formulation. Both models use a Norton–Hoff constitutive model with the high temperature sensitivity of the parameters’ value and advanced numerical techniques such as the Arbitrary Lagrangian Eulerian (ALE) formalism. It can be concluded that, basically, these two formulations lead to the same results.

Modeling the Nitrogen Diffusion Enhancement Resulting from a NanoPeening® Treatment on a Pure Iron – Influence of the Grain Morphology

It is well known that nanocrystalline materials have enhanced diffusion properties due to their high grain boundary density which act as fast diffusion channels compared to the lattice. In this paper, we aim at simulating the nitriding process of a pure iron nanostructured by NanoPeening® process. We use a simple diffusional approach taking into account the grain size and the grain morphology resulting from the NanoPeening® treatment. EBSD measurements are carried out to extract morphological parameters which are used in the homogenization method to extract the effective diffusivity distribution. Then a 1D diffusion simulation is performed with this distribution and shows that the grain morphology resulting from the NanoPeening® treatment does not deteriorate the diffusion properties of the material but in fact, improves the nitrogen penetration depth and the diffusion kinetics in addition to the effect of the grain size reduction.

Characterisation of surface martensite-austenite transformation during finish turning of an AISI S15500 stainless steel

During machining, extreme temperature conditions appear in the cutting zone (from 700 to 1,000°C with heating rates around 106 °C/s). Consequently, the metallurgical models used to simulate the impact of the manufacturing process must be adapted to this fast thermal kinetics. Stress-free dilatometry tests have been performed to determine the austenisation kinetics of an AISI S15500 martensitic stainless steel and to identify a phenomenological model. Experimental heating rates vary from 6 °C/s to 11,000 °C/s. The metallurgical model calibrated for high heating rates, has been applied to a typical machining thermal cycle. It has been shown that martensite→austenite transformation does not have the time to significantly occur during the finish turning of AISI S15500 under standard cutting conditions. This result has been confirmed using retained austenite measurements in the machined surface layer.