Hélène Walter-Le Berre

Simulation of the Cold Spray Particle Deposition Process

Cold spray is a rapidly developing coating technology for depositing materials in the solid state. The cold spray particle deposition process was simulated by modeling the high velocity impacts of spherical particles onto a flat substrate under various conditions. We, for the first time, proposed the Couple Eulerian Lagrangian (CEL) numerical approach to solve the high strain rate deformation problem. The capability of the CEL numerical approach in modeling the Cold Spray deposition process was verified through a systematic parameter study, including impact velocity, initial particle temperature, friction coefficient and materials combination. The simulation results by using the CEL numerical approach agree with the experimental results published in the literature. Comparing with other numerical approaches, which are Lagrangian, ALE and SPH, the CEL analyses are generally more accurate and more robust in higher deformation regimes. Besides simulating the single particle impact problem, we also extended our study into the simulation of multiple impacts. A FCC-like particles arrangement model that inspired by the crystal structure was built to investigate the porosity rate and residual stress of deposited particles under various conditions. We observed not only the 3D profiles of voids, but also their distributions and developments during different procedures. Higher impact velocity and higher initial temperature of particles are both of benefit to produce a denser cold spray coating. The compressive residual stresses existed in the interface between the particle and substrate is mainly caused by the large and fast plastic deformation. Another simplified model for multiple impacts was created for the simulation of surface erosion. A severe surface erosion is the result of a high impact velocity, a high friction coefficient and a low contact angle. Two element failure models suitable for high-strain-rate dynamic problems were introduced in this study. For a ductile material as Copper, it followed two fracture modes in our study, which are tensile failure mode and shear failure mode. The former one mainly occurred beneath the substrate surface and the periphery of substrate craters, nevertheless the latter one was found predominately at the surface of craters. Four steps were found during the propagation of crack: void formation; crack formation; crack growth; coalescence and failure. A simple criterion equation was derived based on the simulation results for predicting the initiation of damage, which the erosion velocity v_{ero} is a function of contact angle and erosion velocity for normal impact v_{pi/2}. The equivalent plastic strain could also be a parameter for identifying the onset of damage, identified as being 1.042 for Copper in our study.

Characterization of the Mechanical Properties of the Human Aortic ArchUsing an Expansion Method

Analyzing cardiovascular diseases leads to multidisciplinary problems which require transversal and complementary approaches. This study focuses on the identification of the mechanical properties of the aortic arch. Stereo-correlation technique is used to measure the strain field in the aortic arch during its expansion. The aorta is immersed in water which allows better results in terms of measurement method and to take into account the residual stress and strain effects. Results are obtained at different values of expansion: 9 samples are collected, 7 of them are frozen before the experiment and 2 are fresh. The mean age is 76 years old at the volunteers? time of death. 4 samples did not lead to conclusive results because of the quality of the arterial wall and leaks that happened during the experiment preventing a proper expansion. The horizontal and vertical displacements are relatively homogeneous for all the samples: two preferred radial and longitudinal directions are observed. The strain fields associated with these directions show heterogeneities and have significant differences between fresh and frozen specimen. The final objective is to perform virtual surgical simulation of the whole endovascular stent graft procedure for an aortic aneurysm. This procedure has a high rate of short-term success and its indication compared to open surgery is increasing but it needs to be more reliable and secure. In this context, it is important to identify the mechanical properties of the aorta for further numerical simulations.

Comparison between a generalized Newtonian model and a network-type multiscale model for hemodynamic behavior in the aortic arch: Validation with 4D MRI data for a case study

Blood is a complex fluid in which the presence of the various constituents leads to significant changes in its rheological properties. Thus, an appropriate non-Newtonian model is advisable; and we choose a Modified version of the rheological model of Phan-Thien and Tanner (MPTT). The different parameters of this model, derived from the rheology of polymers, allow characterization of the non-Newtonian nature of blood, taking into account the behavior of red blood cells in plasma. Using the MPTT model that we implemented in the open access software OpenFOAM, numerical simulations have been performed on blood flow in the thoracic aorta for a healthy patient. We started from a patient-specific model which was constructed from medical images. Exiting flow boundary conditions have been developped, based on a 3-element Windkessel model to approximate physiological conditions. The parameters of the Windkessel model were calibrated with in vivo measurements of flow rate and pressure. The influence of the selected viscosity of red blood cells on the flow and wall shear stress (WSS) was investigated. Results obtained from this model were compared to those of the Newtonian model, and to those of a generalized Newtonian model, as well as to in vivo dynamic data from 4D MRI during a cardiac cycle. Upon evaluating the results, the MPTT model shows better agreement with the MRI data during the systolic and diastolic phases than the Newtonian or generalized Newtonian model, which confirms our interest in using a complex viscoelastic model.

Numerical Simulation for Design Evaluation of Thoracic Stent Graft toInvestigate the Migration Phenomena and Type 1a Endoleak of ThoracicAneurysm

Migration and endoleak phenomena are considered to be the principal reasons for Endovascular Aneurysm Repair failure. Wide differences of opinion exist regarding the nature of these critical complications. They occur when there is non-complete and ineffective contact between the endograft ends and the wall of the blood vessel. A major goal of present work is to investigate, using the Finite Element Method, the effect of nitinol stent design on the overall effectiveness of contact and radial force. The specific-patient aneurysmal thoracic aorta are challenging. The optimized stent results show better contact stability to resist the migration. They also show a good compromise of stent design requirements (flexibility and stiffness). Moreover, the new design can also prevent the risk of folding or the collapse of stent struts by mitigating the energy of eccentric deformation caused by high angulation and oversizing.