Raphaël Pesci

Development and mechanical characterization of porous titanium bone substitutes

Commercially Pure Porous Titanium (CPPTi) can be used for surgical implants to avoid the stress shielding effect due to the mismatch between the mechanical properties of titanium and bone. Most researchers in this area deal with randomly distributed pores or simple architectures in titanium alloys. The control of porosity, pore size and distribution is necessary to obtain implants with mechanical properties close to those of bone and to ensure their osseointegration. The aim of the present work was therefore to develop and characterize such a specific porous structure. First of all, the properties of titanium made by Selective Laser Melting (SLM) were characterized through experimental testing on bulk specimens. An elementary pattern of the porous structure was then designed to mimic the orthotropic properties of the human bone following several mechanical and geometrical criteria. Finite Element Analysis (FEA) was used to optimize the pattern. A porosity of 53% and pore sizes in the range of 860 to 1500 μm were finally adopted. Tensile tests on porous samples were then carried out to validate the properties obtained numerically and identify the failure modes of the samples. Finally, FE elastoplastic analyses were performed on the porous samples in order to propose a failure criterion for the design of porous substitutes.

In situ analysis of the influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy

The influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy was studied in this work by in situ microstructural analysis during tensile testing in a chamber of scanning electron microscope. Three types of samples used in this study were obtained by (1) extrusion (as-supplied), (2) I-ECAP and (3) I-ECAP followed by side upsetting. Microstructures, textures and mechanical properties were examined after each processing step. An analytical equation was used to describe flow stress curves of the samples which exhibited various modes of deformation (1) only by slip, (2) dominated by tensile twinning followed by slip and (3) dominated by contraction twinning followed by slip. It was shown that tensile twinning increases strain hardening rate, while the opposite is observed for contraction twinning. The effective Schmid factors for slip in volumes deformed by tensile and contraction twinning were determined in this work using modelling approach as 0.215 and 0.45, respectively. Contraction twinning was also revealed to be responsible for an earlier fracture of the extruded sample subjected to tension, since microcracking was shown explicitly to be initiated within twins.

Inter- and Intragranular Stress Determination with Kossel Microdiffraction in a Scanning Electron Microscope

A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses on the micron scale, using a one cubic micrometer spot. The experimental Kossel line patterns are obtained by way of a CCD camera and are then fully indexed using a home-made simulation program. The so-determined orientation is compared with Electron Back-Scattered Diffraction (EBSD) results, and in-situ tests are performed inside the SEM using a tensile/compressive machine. The aim is to verify a 50MPa stress sensitivity for this technique and to take advantage from this microscope environment to associate microstructure observations (slip lines, particle decohesion, crack initiation) with determined stress analyses.

Strain resolution of scanning electron microscopy based Kossel microdiffraction

Kossel microdiffraction in a scanning electron microscope enables determination of local elastic strains. With Kossel patterns recorded by a CCD camera and some automation of the strain determination process, this technique may become a convenient tool for analysis of strains. As for all strain determination methods, critical for the applicability of the Kossel technique is its strain resolution. The resolution was estimated in a number of ways: from the simplest tests based on simulated patterns (of an Ni alloy), through analysis of sharp experimental patterns of Ge, to estimates obtained by in situ tensile straining of single crystals of the Ni-based superalloy. In the latter case, the results were compared with those of conventional X-ray diffraction and synchrotron-based Kossel diffraction. In the case of high-quality Ge patterns, a resolution of 1 × 10−4 was reached for all strain tensor components; this corresponds to a stress of about 10 MPa. With relatively diffuse patterns from the strained Ni-based superalloy, under the assumption of plane stress, the strain and stress resolutions were 3 × 10−4 and 60 MPa, respectively. Experimental and computational conditions for achieving these resolutions are described. The study shows potential perspectives and limits of the applicability of semiautomatic Kossel microdiffraction as a method of local strain determination.

Estimation of the electron beam-induced specimen heating and the emitted X-rays spatial resolution by Kossel microdiffraction in a scanning electron microscope

A Kossel microdiffraction experimental setup has been developed inside a Scanning Electron Microscope for crystallographic orientation, strain and stress determination at a micrometer scale. This paper reports an estimation of copper and germanium specimens heating due to the electron beam bombardment. The temperature rise is calculated from precise lattice parameters measurement considering different currents induced in the specimens. The spatial resolution of the technique is then deduced.

Effects of the impact of a low temperature nitrogen jet on metallic surfaces

The technology of nitrogen jets impacting surfaces at low temperature has recently been introduced for surface cleaning/stripping. Under the impact of the jet, the material surface undergoes a thermomechanical shock inducing complex transformation mechanisms. Depending on the material and test parameters such as standoff distance, dwell time, upstream pressure, the latter include cleavage, cracking, spalling, blistering, grain fragmentation, phase transformation and ductile deformation. Quite often, these modes are superimposed in the same test, or even in the same material area. In this study, an overview of these mechanisms is proposed for metallic materials. Measurements of thermomechanical variables in the impacted area are presented and the influence of the test parameters on surface transformation is investigated. Grain fragmentation and ultrafast transport of nitrogen in a deep layer below the surface are explored.

Mechanical properties of a nanoporous membrane used in implantable medical devices. Correlation between experimental characterization and 2D numerical simulation

Nanoporous membranes are used for the elaboration of implantable medical devices. In order to guaranty their integrity after implantation in a patient body, it is necessary to characterize the microstructure and the mechanical behavior of such membranes. They present randomly distributed pores around 1µm in diameter at the surface. X-ray nanotomography permits to get the geometry of the pores through the thickness with a reduction of the diameter in the core. A multiscale study is done to characterize the membranes: macroscopic tensile tests permit to get the behavior law of the non porous material and in situ tensile tests are carried on in a Scanning Electron Microscope in order to observe the evolution of pores and cracks during loading. A 2D Finite Element Model is also developed in parallel. The confrontation between experiments and numerical simulations permit to validate the accuracy of the model. The latter is then used to simulate several types of loadings considering various pore distributions and sizes.

In Situ Microstructure Observation Of Steel Grades In The Semi-Solid State For Thixoforging Process By Using Confoncal Laser Scanning Microscopy

It is necessary to well understand the microstructure evolution during high speed heating and forming for steel thixoforging, since it determines the thixotropic flow behavior of materials in the semi-solid state. A new in situ technique - high temperature Confocal Laser Scanning Microscopy (CLSM) - was developed and used for studying the microstructure evolution directly at high temperature where the microstructure in the semi-solid state could not be preserved by quenching experiments for conventional 2D characterization. Several steel grades (C38LTT, 100Cr6 and M2) were investigated during heating from the as-received state to the semi-solid state and finally cooled to the solid state).It has been found that there is a significant difference in diffusion rate of alloying elements between these grades during heating and cooling. In M2, thanks to the high content of alloying elements and their low diffusion rate, the semi-solid temperature range is greater and its microstructure in the semi-solid state could be preserved by quenching or even at a low cooling rate, which means the microstructure of M2 in the semi-solid state can be characterized in room temperature on quenched M2 samples. On the contrary, the microstructure of other steel grades 100Cr6 and C38LTT in semi-solid state can only be revealed by CLSM at high temperature because of the lower volume fraction of alloying elements and their high diffusion rate. It is very interesting to use high temperature CLSM to in situ investigate the microstructure evolution in the semi-solid state, especially at low liquid fraction.

Influence of Temperature on Stress Distribution in Bainitic Steels - Application to 16 MND5-A508 Pressure Vessel Steel

In this study, the internal stress evolution of the ferrite phase of 16MND5-A508 has been determined using X-Ray Diffraction (XRD). The results of in situ tests combined with XRD analyses and performed at different temperatures (-150°C and 22°C) exhibit a difference of about 200MPa between the macroscopic stress and the ferrite one. The stress state in the cementite is determined by a mixture law; it reaches very high values up to 9000MPa. These results highlight the need to analyze the stress directly in the cementite phase by using appropriate tools, since its volume fraction does not allow it using XRD.

Distribution des contraintes dans l'acier bainitique 16MND5. Analyse expérimentale et modélisation polycristalline

La nature biphasee de l’acier bainitique 16MND5 (ferrite/cementite) fait de la Diffraction des Rayons X (DRX) l’outil privilegie pour determiner les etats de contrainte dans la phase ferritique (methode des sin2 ψ). Couples aux observations realisees lors d’essais de traction (surface des eprouvettes et facies de rupture), ces derniers ont permis d’etablir des criteres decrivant le comportement et l’endommagement du materiau a l’echelle cristallographique, aux points bas de la transition fragile-ductile ainsi qu’aux basses temperatures [−196 ◦C;−60 ◦C]. Au cours du chargement, l’endommagement est observe au Microscope Electronique a Balayage, tandis que les contraintes internes sont determinees par DRX : l’etat de contrainte dans la ferrite est inferieur a celui de la bainite (contrainte macroscopique), l’ecart n’excedant pas 150 MPa. Un modele polycristallin a plusieurs echelles est developpe parallelement aux mesures experimentales : une formulation de type Mori–Tanaka est utilisee pour decrire le comportement elastoplastique d’un monocristal ferritique renforce par des precipites de cementite, le passage au polycristal etant realise par une approche autocoherente. La modelisation developpee prend en compte l’influence de la temperature sur les etats de contrainte dans chaque phase et inclut un critere de clivage (valeur critique de la contraite normale aux plans {100}), qui traduit l’endommagement du materiau : elle permet ainsi de predire le comportement reel de l’acier 16MND5 en fonction de la temperature, et de prendre en compte le mode de rupture qui est fragile a partir de −120 ◦C. En outre, il est egalement possible de calculer les deformations des plans diffractants eϕψ, qui peuvent etre comparees a celles mesurees par DRX : cela permet d’evaluer les deformations par orientation cristallographique.