Jean-Claude Boyer

Mechanical properties of coronary stents determined by using finite element analysis

The mechanical function of a stent deployed in a damaged artery is to provide a metallic tubular mesh structure. The purpose of this study was to determine the exact mechanical characteristics of stents. In order to achieve this, we have used finite-element analysis to model two different type of stents: tubular stents (TS) and coil stents (CS). The two stents chosen for this modeling present the most extreme mechanical characteristics of the respective types. Seven mechanical properties were studied by mathematical modeling with determination of: (1) stent deployment pressure, (2) the intrinsic elastic recoil of the material used, (3) the resistance of the stent to external compressive forces, (4) the stent foreshortening, (5) the stent coverage area, (6) the stent flexibility, and (7) the stress maps. The pressure required for deployment of CS was significantly lower than that required for TS, over 2.8 times greater pressure was required for the tubular model. The elastic recoil of TS is higher than CS (5.4% and 2.6%, respectively). TS could be deformed by 10% at compressive pressures of between 0.7 and 1.3 atm whereas CS was only deformed at 0.2 and 0.7 atm. The degree of shortening observed increases with deployment diameter for TS. CS lengthen during deployment. The metal coverage area is two times greater for TS than for CS. The ratio between the stiffness of TS and that of CS varies from 2060 to 2858 depending on the direction in which the force is applied. TS are very rigid and CS are significantly more flexible. Stress mapping shows stress to be localized at link nodes. This series of finite-element analyses illustrates and quantifies the main mechanical characteristics of two different commonly used stents. In interventional cardiology, we need to understand their mechanisms of implantation and action.

On modelling the growth and the orientation changes of ellipsoidal voids in a rigid plastic matrix

A phenomenological ductile damage model is developed to take into account the growth and the changes of orientation of defects in a undamaged material matrix under large plastic straining. This constitutive model is based on a skew-symmetric tensor-valued function, originally presented by Wang C C (1970 New theorem for isotropic function part 1 and part 2 Arch. Rational Mech. Anal. 36 166–223) and modified for anisotropic functions by Boehler J P (1978 Lois de comportement anisotrope des milieux continus J. Mecanique 17 153–90). A finite element model of a three-dimensional unit cell containing a tilted ellipsoidal void is used to identify the constitutive parameters of the proposed model. Then, this law of orientation change is compared with the two-dimensional model of Bilby B A and Kolbuszewski M L (1977 The finite deformation of an inhomogeneity in two-dimensionnal slow viscous incompressible flow Proc. R. Soc. A 355 335–53) deduced from the original work of Eshelby J D (1957 The determination of the elastic field of an ellipsoidal innclusion and related problem Proc. R. Soc. A 241 376–96). In the proposed ductile damage model, the radius changes of the void are based on the modified version of the Rice and Tracey void growth law presented by Thomason and adapted for the transformation of an ellipsoidal void of any orientation in the sound matrix. The new proposals are checked with different non-linear finite element analyses.

A shear stress dependent ductile damage model

Abstract Prediction of ductile damage is an important challenge for finite element modelling of thermo-mechanical forming processes. Existing plasticity theories intended for the prediction of the void growth are mainly formulated with the normal mean stress, without any influence of the deviatoric stress tensor. In order to take into account shear stress effects during the void growth, a yield function including the mass conservation and its corresponding flow rule is discussed and identified with a modified Rice and Tracey model.

Friction law for hydrostatic mixed lubrication regime

Abstract The accuracy of the finite element simulation of bulk forming processes is connected to the interface friction law. For quasi-static processing operations, the well-known plastic wave friction model, applied to unlubricated contacting interfaces of metallic workpieces with rigid tools, is a versatile link between the Coulomb–Amonton and the constant friction laws. An improvement of the plastic wave friction model is proposed for mixed lubricating conditions when the load is carried by the asperities in direct contact and by the lubricant. The main assumption of this new constitutive relation uses the compressibility of the trapped lubricant in the pockets created inside the tool asperity with the rising plastic wave. This new friction law is used for the simulation of different ring-compression tests carried out by Tan et al. [J. Mater. Proc. Technol. 80–81 (1998) 292].

Numerical simulation of failure prediction for ceramic tools: comparison with forging test results

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code in order to predict the risk-of-rupture of ceramic tools during a forging operation. POLLUX is a software dedicated to the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theory and the statistical theory of Weibull. A surface approach or a volume approach can be retained on the basis of the type of critical flaws in the ceramic. Two different criteria are available in order to characterise the stress state, considering the tensile normal stresses and neglecting the compressive stresses. An identification procedure of the critical flaw type is presented for a particular ceramic material. Statistical parameters of ceramic fracture have been determined experimentally using bending tests performed under environmental conditions close to those of forging. A constitutive equation of the workpiece material has been proposed, issued from torsion tests. In order to validate the model in the case of ceramic tools subjected to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A forging test has then been developed, in which a billet of superalloy is formed in a ceramic tool up to its fracture at the temperature of 1423 K. The experimental distribution of tool fracture, according to the strain of the billet, is in good agreement with fracture predictions computed by the simulation.

A ductile growth model for elasto-plastic material

Abstract In bulk forming processes, the soundness of a manufactured part can be related to the final mechanical properties of its material, which can be locally deteriorated by an induced porosity increase. In order to predict such damage evolution, a void growth model, based upon the Rice and Tracey analysis and suited for thermo-elasto-plastic finite-element modelling is discussed and compared to existing models. The workpiece material is considered as a metallic matrix, following the classical plasticity theory, with microscopic spherical voids. Its slightly compressible macroscopic behaviour is predicted by the Rice and Tracey analysis, the results of a numerical study of a unit cell with or without an incompressible inclusion filling the void showing that the plastic volume changes come mainly from geometric effects. Comparisons between existing growth models and this new model are carried out with the same finite-element software for axisymmetric thermo-mechanical closed-die modelling of a forward extrusion and the collar test.

Failure prediction for ceramic dies in the hot-forging process using FEM simulation

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code, in order to predict the risk of rupture of ceramic tools during a forging operation. The POLLUX code, developed by INSA (Lyon) especially to simulate forging operations, is presented. The fracture model is based on the weakest link theory and Weibull analysis. Two different criteria were chosen in order to characterise the stress state, considering the tensile normal stresses. Comparison between the simulation results and the analytical calculations, in a simple compression case, enables the validation of the numerical model. Applications are presented, in which the design of ceramic forging tools is realized using the failure prediction software. A run-strategy of the program is proposed in order to improve the design of the forging tools.

Drying-Induced Cracking of Abrasive Rings:Risk Prediction and Process Optimisation by Numerical Simulation

ABSTRACT The dependence of the mechanical stresses distribution on the water content and temperature profiles has been numerically investigated in a porous unsaturated hygroscopic abrasive agglomerate of annular shape. The thermophysical, kinetic and mechanical properties of the abrasive agglomerate were determined experimentally. The simulations have been applied to unfired abrasive rings convective drying optimisation by fitting operating conditions in order to avoid cracks formation.

Impact of density gradients on the stress level within a green ceramic compact during drying

Abstract: Internal water content and mechanical stress distributions during convective drying were simulated for a homogeneous and a heterogeneous (with density gradients) annular compact of a green ceramic agglomerate. A diffusive model for water transfer and an elastic model for structural mechanics were applied. Based on experimental measurements, the material apparent density was numerically implemented as a function of spatial coordinates and the key material properties (moisture diffusivity and the Youngs modulus) were implemented as functions of material density. The heterogeneous compact (with density peaks at the top edges) exhibited three times larger circumferential tensile stress at the top external radius corner than that exhibited by the homogeneous one.

An orthotropic specific damage model with inclusion consideration

Abstract A plastic yield function for ductile materials is discussed in order to predict damage during modeling of bulk metal forming processes. The variational formulation used by Rice and Tracey [J. Mech. Phys. Solids 17 (1969) 201] is modified for a cavity in an unit cell in contact with an inclusion. Following Gurson [J. Eng. Mater. Technol. 99 (1977) 2], the influence of the plastic flow on the size and the shape of the void is analysed at the mesoscopic level. Then, the influences of the volume, of the shape of the void and the contact are introduced in a macroscopic plastic potential similar to the specific one proposed by Staub and Boyer [J. Mater. Proc. Technol. 79 (1998) 9]. This dilatational plastic potential and its corresponding plastic flow rule are implemented in a proprietary software and used for the numerical modelling of a workability test.