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Tissue engineering technology requires porous scaffolds, based on biomaterials, which have to mimic as closely as possible the morphological and anisotropic mechanical properties of the native tissue to substitute. Anisotropic fibrous scaffolds fabricated by template-assisted electrospinning are investigated in this study. Fibers of electrospun Polycaprolactone (PCL) were successfully arranged spatially into honeycomb structures by using well-shaped 3D micro-architected metal collectors. Fibrous scaffolds present 2 × 4 mm2 wide elementary patterns with low and high fiber density areas. Distinct regions of the honeycomb patterns were analyzed through SEM images revealing different fiber diameters with specific fiber orientation depending on the regions of interest. Tensile test experiments were carried out with an optical observation of the local deformation at the pattern scale, allowing the determination and analysis, at small and large deformation, of the axial and transverse local strains. The honeycomb patterned mats showed significantly different mechanical properties along the two orthogonal directions probing an anisotropic ratio of 4.2. Stress relaxation test was performed on scaffolds at 15% of strain. This measurement pointed out the low contribution of the viscosity of about 20% in the mechanical response of the scaffold. An orthotropic linear elastic model was consequently proposed to characterize the anisotropic behavior of the produced patterned membranes. This new versatile method to produce architected porous materials, adjustable to several polymers and structures, will provide appealing benefits for soft regenerative medicine application and the development of custom-made scaffolds.

Internal biliary stenting during biliary reconstruction in liver transplantation decrease anastomotic biliary complications. Implantation of a resorbable internal biliary stent (RIBS) is interesting since it would avoid an ablation gesture. The objective of present work was to evaluate adequacy of selected PLA-b-PEG-b-PLA copolymers for RIBS aimed to secure biliary anastomose during healing and prevent complications, such as bile leak and stricture. The kinetics of degradation and mechanical properties of a RIBS prototype were evaluated with respect to the main bile duct stenting requirements in liver transplantation. For this purpose, RIBS degradation under biliary mimicking solution versus standard phosphate buffer control solution was discussed. Morphological changes, mass loss, water uptake, molecular weight, permeability, pH variations, and mechanical properties were examined over time. The permeability and mechanical properties were evaluated under simulated biliary conditions to explore the usefulness of a PLA-b-PEG-b-PLA RIBS to secure biliary anastomosis. Results showed no pH influence on the kinetics of degradation, with degradable RIBS remaining impermeable for at least 8 weeks, and keeping its mechanical properties for 10 weeks. Complete degradation is reached at 6 months. PLA-b-PEG-b-PLA RIBS have the required in vitro degradation characteristics to secure biliary anastomosis in liver transplantation and envision in vivo applications.

This work aims at observing the mechanical behavior of the membranous and spongy portions of urethrae sampled on male cadavers in compliance with French regulations on postmortem testing, in accordance with the Scientific Council of body donation center of Grenoble. In this perspective, a thermostatic water tank was designed to conduct ex vivo planar tension tests in a physiological environment, i.e., in a saline solution at a temperature of [Formula: see text] [Formula: see text]. In order to observe the anisotropy of the tissues, the samples were tested in two directions. Tests consisting of a series of load-unload cycles of increasing amplitudes were performed to highlight their viscous behavior. The results were then discussed according to the microstructure of tissue, which was investigated using different staining methods and histological analysis. The observed behaviors were then fitted using an anisotropic hyperelastic or a visco-hyperelastic matrix-fiber model.

International audience; This paper investigates the influence of three different processing parameters on the global mechanical behavior of PVA (Polyvinyl alcohol)/DMSO (Dimethylsulfoxide) hydrogels: the initial concentration of PVA, the DMSO:H2O ratio and the number of freeze/thaw cycle applied to the material. A specific thermo-regulated testing apparatus for hydrophilic materials is presented, along with the performed cyclic and rupture tests. The observed mechanical responses are explained by an in-depth analysis of the cross-linking phenomenon. Using the Neo-Hookean hyperelastic model, the experimental data is fitted and a link between the density of macro-molecular chains in the material and its mechanical behavior is established. Strong differences are observed and discussed.

International audience; Inside the Clausius-Clapeyron regime, transformation stresses during supere-lastic tensile tests of polycrystalline shape memory alloys are linearly dependent on temperature, with coefficients being the slopes of the forward and reverse transformation lines. In this work, experiments are performed to investigate the anisotropy of the slopes of the forward and reverse transformation stress-temperature lines in a NiTi superelastic thin walled tube. The classical Clausius-Clapeyron relation is widely used to model these slopes, although, in a strict sense, this relation is defined at thermodynamic equilibrium. Experimental results disagree with the widely used classical Clausius-Clapeyron relation in two points: (i) that there should be no difference between slopes for forward and reverse transformations and (ii) that the products of the slopes by the transformation strains should not depend on orientation, since the remaining terms (mass density and entropy change) are not orientation dependent. A modified "Clausius-Clapeyron" relation is then proposed, better suited to model the anisotropy of the slopes of stress-temperature transformation lines of forward and reverse in superelastic NiTi. This modification is based on a unified thermodynamic theory of thermoelastic martensitic transformation in which irreversible energies are accounted as a sum of stored elastic energy and dissipated energy. The modified "Clausius-Clapeyron" relation is obtained by expressing that this irreversible energy is temperature dependent and that this temperature dependence is dependent on the orientation.

International audience; In this paper, superelastic behavior of Nickel Titanium thin wires is characterized using the method of dynamic mechanical analysis. Nominal dynamic storage modulus ′ is measured as function of nominal strain and stress during isothermal superelastic tensile tests at three testing temperatures above the reverse martensitic transformation finish temperature. The method brings new information on deformation mechanisms compared to the consideration of only tensile stress-strain curves. It is shown that determination of the elastic moduli E, especially at high strain, requires to calculate the true storage modulus ′. Using ′ and not ′, elastic modulus of oriented martensite is determined equal to 73 GPa of the same order than the elastic modulus of austenite equal to 70 GPa. Two models are then proposed to simulate experimental storage moduli evolution during the tests. A first model explains the ′ evolution during stress plateau by the localization phenomenon ; it leads to express ′ as function of the nominal strain. A second model describes the evolution of ′ after the stress plateau as function of true stress and test temperature. This model permits to determine the Clausius-Clapeyron coefficient of the forward transformation.

International audience; Nickel-Titanium (NiTi) alloys are the most successful Shape Memory Alloys (SMA). They have increasingly been used in the form of thin walled tubes for the fabrication of catheters and self-expanding stents. These tubes are manufactured by a series of hot and cold drawing processes which induce a highly texturized microstructure [1, 2]. The presence of texture leads to strong anisotropic mechanical behavior, which has important consequences in the final behavior of devices manufactured from tubes. This study investigates the anisotropy and the temperature dependence of the superelastic behavior of NiTi thin walled tubes. The anisotropy is experimentally characterized and then analyzed from a thermodynamic point of view [3]. Isothermal superelastic tensile tests were performed at several temperatures above Af on oriented dogbone samples cut in five orientations from the drawing direction. Transformation strains and stresses strongly depend on orientation. Transformation strains are independent of temperature. The temperature dependences of forward and reverse transformation stresses, expressed as forward and reverse Clausius-Clapeyron coefficients, are highly orientation-dependent. The products of the Clausius-Clapeyron coefficients by the transformation strains are not independent of the orientation. All these results are analyzed using the reference frame of equilibrium thermodynamics. The main conclusion is that specific dissipated and stored strain energies during the forward and reverse transformations are dependent on orientation and also on temperature.

International audience; Influence of anisotropy on strain localization phenomenon in tension of superelastic NiTi thin walled tubes Tubes of Nickel-Titanium Shape Memory Alloy (NiTi SMA) are employed to produce 60% of self-expanding cardiovascular stTubes of Nickel-Titanium Shape Memory Alloy (NiTi SMA) are employed to produce 60% of self-expanding cardiovascular stents. Such small-scale devices require tubes with small thickness, which in turn implies a material with highly textured microstructure. Previous works demonstrate that this texture causes significant anisotropy, affecting various thermomechanical key properties. This knowledge is of critical value for the modeling of thermomechanical behavior and design of superelastic devices. As well as thermomechanical properties, the localization phenomenon observed in tensile tests is affected by the anisotropy in a NiTi tube and also needs investigation.The present work aims to address the effect of anisotropy on localization phenomenon through a series of tensile tests performed at three temperatures above Af. Tensile dogbone samples were cut in five orientations from a flattened NiTi thin walled tube (0.165 mm of thickness). The orientations were 0° (drawing direction), 22.5°, 45°, 67.5 and 90° (transverse direction). Using Digital Image Correlation (DIC) technique, strain rate fields were calculated over a zone of interest in the gauge area of the samples, measuring 10 x 2 mm2. Localization characteristics (in particular the angle of the localization bands with respect to the tension direction) were determined from these fields.Localization phenomena were observed for the samples cut at 0°, 22.5°, 67.5° and 90° at the three test temperatures. The samples cut at 45°, however, did not present strain localization at any of the three temperatures; the strain rate fields were very uniform both during loading and unloading. The localization morphologies of the samples cut at 0°, 22.5°, 67.5° and 90° were similar. All showed clear bands inclined from the longitudinal direction of the sample. The paths taken by the bands were essentially the same during loading and unloading, but bands were much more stable during unloading. The influence of the anisotropy on the localization phenomenon was analyzed regarding the band inclination angles from the sample axial direction. The inclination angles were measured for all strain range using the strain rate fields. The inclination angles measured in the samples oriented at 0° and 90° were similar and around 60°. In comparison, the inclination angles measured for the samples 22.5° and 67.5° were smaller, between 50° and 55°. The anisotropic state of the material was analyzed using Hill’s quadratic anisotropic yield function. With the obtained Hill’s anisotropic parameters the inclination of the bands were calculated for each tensile orientation. When confronted with experimental measurements, good qualitative results were obtained from this analysis, allowing to evaluate the relation between the angles of localization bands with samples’ orientation. ents. Such small-scale devices require tubes with small thickness, which in turn implies a material with highly textured microstructure. Previous works demonstrate that this texture causes significant anisotropy, affecting various thermomechanical key properties. This knowledge is of critical value for the modeling of thermomechanical behavior and design of superelastic devices. As well as thermomechanical properties, the localization phenomenon observed in tensile tests is affected by the anisotropy in a NiTi tube and also needs investigation. The present work aims to address the effect of anisotropy on localization phenomenon through a series of tensile tests performed at three temperatures above Af. Tensile dogbone samples were cut in five orientations from a flattened NiTi thin walled tube (0.165 mm of thickness). The orientations were 0° (drawing direction), 22.5°, 45°, 67.5 and 90° (transverse direction). Using Digital Image Correlation (DIC) technique, strain rate fields were calculated over a zone of interest in the gauge area of the samples, measuring 10 x 2 mm 2. Localization characteristics (in particular the angle of the localization bands with respect to the tension direction) were determined from these fields. Localization phenomena were observed for the samples cut at 0°, 22.5°, 67.5° and 90° at the three test temperatures. The samples cut at 45°, however, did not present strain localization at any of the three temperatures; the strain rate fields were very uniform both during loading and unloading. The localization morphologies of the samples cut at 0°, 22.5°, 67.5° and 90° were similar. All showed clear bands inclined from the longitudinal direction of the sample. The paths taken by the bands were essentially the same during loading and unloading, but bands were much more stable during unloading. The influence of the anisotropy on the localization phenomenon was analyzed regarding the band inclination angles from the sample axial direction. The inclination angles were measured for all strain range using the strain rate fields. The inclination angles measured in the samples oriented at 0° and 90° were similar and around 60°. In comparison, the inclination angles measured for the samples 22.5° and 67.5° were smaller, between 50° and 55°. The anisotropic state of the material was analyzed using Hill's

The authors wish to acknowledge the financial support of the French ANR research program ANiM: Architectured NiTi Material (N.2010 BLAN 90201); International audience; The aim of this paper is to present an experimental and modeling study as the first step towards designing and optimizing architectured materials constituted of NiTi tubes. The idea is to combine the intrinsic and novel properties of nickel–titanium shape memory alloys with purposely engineered topologies. By joining thin-wall superelastic tubes via electrical resistance welding, we create regular cellular material demonstrators. The superelastic behavior of two simple architectured materials based on identical tubes, but with two topologies, are experimentally characterized and modeled using finite element approaches. The predicted behaviors are compared by simulating complex loading, exploring the influence of the constitutive material behavior on the effective mechanical properties of cellular materials. The parameters of the constitutive equations are identified on tensile tests performed on small dog-bone shaped specimens, machined from the tubes by spark cutting. The modeling results are finally compared with compression tests performed on these simple architectured NiTi materials. As a further validation of the proposed study, two large cell structures (square and hexagonal stacking) were modeled to gain greater insight into the role of different architectures.

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International audience; This study analyses the thermomechanical tensile behaviour of a cold drawn Ti-50.9at.%Ni wire submitted to heat treatment at 598 K for 30 min, which is below the recrystallization temperature (623 K). Such low temperature heat treatment induces a superelastic loop without a stress "plateau". However, the absence or weakness of peaks on its differential scanning calorimetry prevents the determination of specific latent heat. This is a common effect of nanostructured materials such as superelastic wires. A method using strain and temperature field measurements was developed and used to determine thermal power and thermal energy during superelastic tensile tests through a heat balance. From these results and using a thermodynamic approach, forward and reverse specific latent heat and the martensite fraction are estimated as a function of strain and stress.

This article is an invited paper selected from presentations at the International Conference on Shape Memory and Superelastic Technologies 2013, held May 20-24, 2013, in Prague, Czech Republic, and has been expanded from the original presentation.; International audience; Young’s moduli of superelastic NiTi wires in austenite and stress-induced martensite states were evaluated by three different experimental methods (tensile tests, in situ synchrotron x-ray diffraction, and dynamic mechanical analysis) and estimated via theoretical calculation from elastic constants. The unusually low value of the Young’s modulus of the martensite phase appearing in material property tables (

Cold tube drawing is a metal forming process which enables to produce tubes with high dimensional precision. It consists in reducing tube dimensions by pulling it through a die. Tube outer diameter is calibrated by a die and the tube inner diameter and thickness are calibrated by a mandrel. One of the major concern of metal forming industry is the constant improvement of productivity and product quality. In the aim of pushing the process to the limit the question is how far the material can be processed without occurrence of failure. In the present study, a long conical mandrel with a small cone angle was designed in order to carry out drawing tests up to fracture with experimental conditions very close to the industrial process. The FEM of the process was built in order to access the local stress and strain data. A specific emphasis was put on the friction characterisation. For that purpose force measurement during the conical mandrel experiments enabled to characterise a pressure dependent friction coefficient constitutive law by means of an inverse analysis. Finally, eleven failure criteria were selected to study the drawability of cobalt-chromium alloy tubes. The assessment of failure criteria based on damage variables or damage accumulation variables involved their calibration on uniaxial tensile tests. The experimental studies were completed by SEM fractography which enabled to understand the fracture locus and the propagation direction of the fracture.

This work attempts to develop superelastic TiNi wires with spatially varied shape memory characteristics along their length. A short section of the wire was over-aged by electrical resistance heating, which resulted in the deterioration of its superelasticity, whilst the reminder of the wire length remained superelastic. The tensile stress–strain behaviour of such treated wires is characteristic of two discrete stress plateaus during the stress-induced martensite transformation, corresponding to the original and over-aged sections.

International audience; This paper investigates the effects of temperature on the mechanical properties of silicone rubbers. First, differential scanning calorimetry tests are performed to determine the crystallization and melting temperatures. Second, mechanical tests are carried out at different temperatures above that of crystallization, up to 150 °C. In this temperature range, the silicone rubbers exhibit entropic behaviour. The neo-hookean model is used to fit the mechanical response. Third, the effects of temperature on the hysteresis, the stress softening and the stress relaxation are studied. Strong differences are observed and discussed.

International audience; Soft tissues are mainly composed of organised biological media giving them an anisotropic mechanical behavior. Soft tissues also have the ability to undergo large elastic reversible deformations. Many constitutive models were developed to describe these phenomena. In this chapter, we discuss several varying models and their constitutive equations which are defined by means of strain components or strain invariants. The notion of tangent moduli will be plotted for two well-known constitutive equations, and, we will illustrate how to implement explicitly a structural kinematics constraint in a constitutive law to derive the resulting Cauchy stress tensor.

International audience; One major challenge of biomaterial engineering is to mimic the mechanical properties of anisotropic, multifunctional natural soft tissues. Existing solutions toward controlled anisotropy include the use of oriented reinforcing fillers, with complicated interface issues, or UV-curing processing through patterned masks, that makes use of harmful photosensitive molecules. Here, a versatile process to manufacture biocompatible silicone elastomer membranes by light degradation of the platinum catalyst prior to thermal cross-linking is presented. The spatial control of network density is demonstrated by experimental and theoretical characterizations of the mechanical responses of patterned cross-linked membranes, with a view to mimic advanced implantable materials.

International audience; The superelasticity exhibited by Ni-rich Nickel-Titane shape memory alloys is a widely employed feature in many fields and, particularly, the use of superelastic NiTi tubes is consolidated in the fabrication of a wide range of biomedical stents. NiTi alloys are very often considered as isotropic when modeling the behavior of structures. However, structures like stents manufactured from highly texturized thin tubes will certainly present a marked anisotropic behavior. Therefore, the consideration of isotropy in the design and modeling of devices manufactured from NiTi tubes can lead to serious misuse. In this context the present work aims to quantify the anisotropic mechanical behavior of a thin-walled superelastic NiTi tube. The tube was opened through a one-step shape setting and dogbone samples were cut in five orientations (θ): 0° (drawing direction-DD), 22.5°, 45°, 67.5 and 90° (transverse direction-TD). Samples were subjected to isothermal tensile tests at 40°C, 50°C and 60°C. Strain field was measured in the gauge area by digital image correlation (DIC). Key properties like transformation strain, transformation stress, Clausius-Clapeyron coefficients and hysteresis, accounted as the irreversible energy per superelastic cycle, were then analyzed as function of orientation. Significant variations occurred between the samples oriented at DD and TD to the sample oriented at 45°, reaching around 40% for transformation strain and 70% for hysteresis. Also, although not globally symmetrical, the profiles of the studied properties showed that some directions behave similarly.

International audience; PLA-based biodegradable copolymers are used in many biomedical applications such as temporary implantable devices. Especially, PLA-b-PEG-b-PLA is an excellent candidate for tissue engineering applications. Indeed, it has a good biocompatibility and possesses both mechanical properties of PLA and hydrophilicity of PEG, allowing good properties and degradation time modulation. The main degradation process, for this type of polymers is the hydrolysis of ester links. After the diffusion of water into the polymer bulk, the hydrolysis reaction breaks the polymeric bonds. Modelling of mechanical properties evolution of biodegradable polymers is essential in order to design devices. The aim of this study is to explore and model the viscoelastic properties evolution of a PLA-b-PEG-b-PLA biodegradable copolymer during hydrolytic degradation. The mass decrease, the number average molecular weight and the mechanical properties were studied during 7 degradation weeks. Tensile and relaxation tests in a liquid bath at 37°C were realized at different states of degradation. Stress relaxation is observed, highlighting a viscoelastic behavior for every degradation state. Moreover, the polymer suffers a loss of mechanical properties in the course of degradation. In order to model viscoelastic properties, a generalized Maxwell model is used. This model is first identified on results obtained for non degraded material. Then, based on the invariance of the normalized relaxation curves experimentally observed for the degraded materials, a degradation variable is introduced in the model. Predictions of the model are then compared to experimental results in the course of degradation. The abilities and limits of the model are discussed.

International audience; Most medical applications involving superelastic Ni-Ti shape memory alloys are based on thin wires or tubes. If the tensile behavior of such specimens has been extensively described in the literature, only few studies deal with their mechanical behavior under pure bending and compressive load [1]. In this paper, Ni-Ti wire behaviors were experimentally investigated using pure bending and uni-axial tensile experiments. Uniform or localized behaviors were observed depending on the thermo-mechanical treatment of the wire. Theoretical bending model was proposed. For nonlocalized tests, it allowed to analyze tension-compression asymmetry. Tension and compression constitutive equations were proposed to model localization in bending.

Compression moulded preimpregnated polymer-matrix composites are often porous materials. Rheological studies usually neglect their subsequent compressible complex flow behaviour, together with mouldcomposite friction effects. Therefore the proposed method, including a newly developed compression rheometer, allows the rheology and the compressibility of such materials together with the mould–composite friction phenomena to be characterised without taking into account a priori assumptions on both the rheology of the composite and the form of the friction law. Its validity and usefulness for improving the interpretation of rheological results is demonstrated using a modelling plastic paste and an industrial Sheet Moulding Compound (SMC) during lubricated (or not) compression tests performed at low or high temperatures.

International audience; This study is concerned with the experimental characterization of anisotropy induced by the Mullins effect in a particle-reinforced silicone rubber. Experimental data concerning the influence of type and direction of initial loading on the subsequent stress softening are quite scarce. In this scope, a set of experimental tests were carried out on a filled silicone rubber. Uniaxial tensile tests and bulge tests were used to precondition the samples, i.e., to induce some primary stress softening. In both cases, subsequent uniaxial tensile tests were conducted on preconditioned specimens. The first set of experiments consists of a uniaxial tension path followed by uniaxial tension along different directions. It appears that the stress softening varies from a maximum in the same direction load to a minimum in the orthogonal direction, with respect to the first tensile load direction. Next, the bulge test is proposed as an original way to yield very different biaxial tensile strain-histories for first loading path. The fact that the biaxiality ratio varies from the pole (uniaxial tension) until the bulge border (planar tension), permits to analyze second tensile load curves in a material that experienced a more complex first load path. These experimental data allow to discuss the most appropriate criteria to describe the strain-induced anisotropy phenomenon.

Model composite plates composed of highly conductive slender copper fibres impregnated with a poorly conductive and transparent PMMA matrix were processed with different fibrous architectures, i.e. with various controlled fibre contents and orientations. Their microstructure was characterised using both optical observations and X-ray microtomography. Their in-plane thermal conductivity was measured by using a specific testing apparatus combined with an inverse modelling method. Results point out the strong link between the anisotropy of the in-plane conductivity and of the microstructure. The role of the fibre content on the conductivity is also emphasised. An analytical conduction model which accounts for the influence of the fibre content, the orientation, the aspect ratio and the thermal resistances at fibre-fibre contacts, was proposed and its predictions were compared with the experimental results. Using only one fitting parameter, namely the conductance at fibre-fibre contacts, this model shows a good prediction of all the experiments.

Bulk Moulding Compounds (BMC’s) are short fibre reinforced polymer composites that behave, during their forming, as concentrated fibre suspensions. Their suspending fluid is also a concentrated granular suspension made up of the polymer filled with mineral fillers. In this work, a method is proposed to model their flow. Firstly, the rheology of an industrial BMC was analysed by performing two types of mechanical tests, i.e. lubricated simple and plane strain compression experiments. Experimental results underline the roles of the current strain, the strain rate as well as the mechanical loading on the rheology of the BMC. Secondly, a 3D tensorial rheological model is proposed to reproduce the simple and plane strain compression experiments. Then this model is implemented into a Finite Element code dedicated to the simulation of compression moulding. Simulation results are finally compared with experiments achieved with rather complex flow situations.

International audience; In this paper, the mechanical behaviour of an unfilled silicone rubber is analysed. Firstly, silicone samples were subjected to five homogeneous tests: tensile, pure shear, compression, plane strain compression and bulge tests. During the tests, full-field measurements of the strain on the surface of deformed samples were obtained using a Digital Image Correlation technique. Results show that the Mullins effects and hysteresis, as well as strain rate sensitivity, can be considered as negligible. Results also emphasise the influence of the loading path. Then, five well-known hyperelastic models (neo-hookean, Mooney, Gent, Haines and Wilson and Ogden models) were fitted to the experimental data. Finally, a heterogeneous test was realised by stretching a silicone plate sample containing holes. Finite element simulations of this experiment have been performed with the hyperelastic models. The comparison of experimental and numerical results emphasises the importance of the choice of the hyperelastic modelling in the simulation of strain fields.

International audience; X-ray microtomographs of samples cut from non-deformed and compression moulded Sheet Moulding Compounds (SMC) plates have been analysed using the phase contrast mode. Results emphasise a significant decrease of the porosity after moulding, this phenomenon being enhanced with increasing the initial SMC temperature. They also show that pronounced migration of the polymer-matrix occurs through the thickness of the plates during compression moulding. The substructures of compression moulded plates through their thickness exhibit a core zone sandwiched between upper and lower skins. Inside the skins, the fibre content and the porosity are weaker than in core zones, and fibre-bundles are highly broken up. On the contrary, fibre-bundles remain preserved in core zones. Therein, the significant flattening and widening of bundles, their weak bending as well as their orientation have been quantified.

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International audience; The biodegradable triblock copolymer PLA-b-PEG-b-PLA presents, in its initial state, a non-linear viscoelastic behavior. Its mechanical properties evolves during the in vitro degradation process. Tensile and relaxation tests are performed at 2%, 4% and 6% of load strain for different degradation steps. In order to describe the behavior of the polymer during degradation, an adaptive quasi-linear viscoelastic model is considered. In a first step, the model calibrated on the non-degraded state, perfectly fits the load and relaxation curves for every strain. Then, based on considerations about the preservation of the normalized relaxation curves over degradation time, the adaptive quasi-linear viscoelastic model is adapted to degradation. A degradation parameter that drives the mechanical degradation kinetics is deduced for every tested degradation states. A physically motivated model is finally used to describe the degradation parameter at every degradation step. The whole constitutive model is very accurate to fit themechanical curves at every strain during degradation.

International audience; The aim of this article is to provide an experimental and modeling study of an architectured photosensitive silicone membrane. Mechanical properties are dependent of UV doses used to alter the local cross-link density, resulting in a direct effect on the macroscopic mechanical behavior. A series of mechanical tests were carried out to characterize the mechanical behavior of each phase. Results are presented for various types of loading, including uniaxial, planar, and equibiaxial loading cases, for the UV irradiated and the nonirradiated material. Using the bulge test with an architectured sample, the global stretchability with minimum boundary condition perturbations was investigated. To further explore the unusual properties offered by silicone graded membranes, finite element analysis of graded architecture was performed to try to predict the stress-strain response in the bulge test. Soft-to-hard transition is tested and the macroscopic influence of interface was observed.

International audience; Experimental pure bending conditions are difficult to obtain when large deformations and displacements are required. In this work, a new principle using two universal joints is proposed and developed to enable such pure bending conditions. This principle has been applied to design an apparatus suitable to test small size samples (such as wires of diameter < 1mm) at small curvature radii (≃ 5mm) and to specifically provide small size samples moment-curvature relationship. This article underlines and validates the abilities of this new apparatus by performing and analysing tests on samples made of well-known material.

A spacer including a first part made from a first shape memory material, and a second part made from a second shape memory material.

Highly concentrated planar fibre-bundle suspensions with a transparent PMMA matrix were processed with various initial bundle contents and orientations. They were submitted to simple compression and plane strain compression deformation modes. First rheological measurements are presented. They highlight the role of the bundle content and orientation on recorded stress levels. The transparent matrix allows the observation of fibrous microstructures before and after compressions: The in-plane deformation of bundles (flattening and bending) as well as the evolution of their orientation are analysed and discussed.

Compression mouldings of commercial SMC were performed with an instrumented industrial press under various process conditions. Results underline the influence of process parameters such as the initial SMC temperature, the axial punch velocity and the geometry of the mould on local normal stress levels. They also show negligible fibre-bundle segregation in the principal plane of the moulded parts. Thereby, a one-phase plug flow shell model is proposed as a direct extension of the plug flow model proposed by M.R. Barone and D.A. Caulk [J Appl Mech 53(191):1986;361–70]. In the present approach, the SMC is considered as a power-law viscous medium exhibiting transverse isotropy. The shell model is implemented into a finite element code especially developed for the simulation of compression moulding of composite materials. Simulation and experimental results are compared, emphasizing the role of the SMC rheology on the overall recorded stress levels. Despite the simplicity of the model, rather good comparisons are obtained.

International audience; PLA-b-PEG-b-PLA is a biodegradable triblock copolymer that presents both the mechanical properties of PLA and the hydrophilicity of PEG. In this paper, physical and mechanical properties of PLA-b-PEG-b-PLA are studied during in vitro degradation. The degradation process leads to a mass loss, a decrease of number average molecular weight and an increase of dispersity index. Mechanical experiments are made in a specific experimental set-up designed to create an environment close to in vivo conditions. The viscoelastic behaviour of the material is studied during the degradation. Finally, the mechanical behaviour is modelled with a linear viscoelastic model. A degradation variable is defined and included in the model to describe the hydrolytic degradation. This variable is linked to physical parameters of the macromolecular polymer network. The model allows us to describe weak deformations but become less accurate for larger deformations. The abilities and limits of the model are discussed.

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In the present work, the hysteretic behavior of ferroelastic deformation via martensite variant reorientation in a Ti–50.15 at.% Ni alloy is studied with cycling in shear. The stress hysteresis and the irreversible work dissipated during a cycle are found to decrease with cycling. The stress hysteresis is found to be dependent on the strain span of the loop but to be independent of the apparent strain about which the loop is performed. The mechanical hysteresis is attributed to two contributions: frictional resistance to twin boundary movement and matrix resistance to shape change of polycrystalline matrices.

Ni-rich NiTi shape memory alloys usually undergo a series of thermomechanical treatment in their production and component fabrication processes. Comprehension of property changes and microstructural evolutions induced by these treatments is essential for quality control and improvement of these materials. The effect of controlled ageing for prolonged times (of the order of several hours) has been extensively investigated in the literature. This study investigates the effect of actual production processes of NiTi tubing, which often involve thermomechanical treatment for multiple periods of short durations and which are typical of industrial production routines of devices like stents, on the transformation and deformation behaviour of a Ti–50.8 at%Ni alloy. Evolution of the properties is characterized by DSC measurements and by tensile testing at 310 K.

This paper proposes a mechanistic spring–slider model for simulating the deformation behaviour of polycrystalline NiTi via martensite reorientation. The model is based on the thermodynamic concept of elastic and frictional energies for thermoelastic martensitic transformations and the plasticity concept of grain interior and grain boundary phases. This model is found to be able to describe, in a schematic and qualitative manner, the deformation behaviour of thermoelastic martensite via variant reorientation in polycrystalline matrices. Such a model allows the discussion of several aspects concerning the thermal and mechanical behaviour of thermoelastic martensitic transformations, such as the non-linear recovery, deformation-induced two-way memory effect, strain dependence of mechanical hysteresis and minor loop behaviour of deformation.

The present work investigates the hysteretic behaviour of Cu–Zn–Al single crystal submitted to superelastic shear tests. Major symmetric loops were performed within the shear strain limit of the stress-induced martensitic transformation. Minor loops inside the major loops allowed to analyse more closely the hysteretic behaviour of the Cu–Zn–Al single crystal. Experimental results obey most of the phenomenological observations previously established for polycrystalline Ni–Ti shape memory alloys.

This paper proposes a mechanistic model to simulate the thermal and mechanical behaviour of shape memory alloys. The model is based on the thermodynamic concept of chemical, elastic and frictional energies for thermoelastic martensitic transformations and plasticity concept of grain interior and grain boundary phases. In a thermoelastic martensitic transformation system, a thermally induced transformation and a mechanically induced (stress-induced) transformation require different operating mechanisms from a mechanistic viewpoint. For a thermally induced transformation, the driving force arises from within the matrix and internal stresses are created as a result of frictional movement. For a mechanically induced transformation, the driving force is provided externally and the frictional move- ment occurs when the stress exceeds a critical value. This paper proposes a unified mechanistic model taking into account this difference. The model is able to describe, in a schematic and qualitative manner, the behaviour of a thermoelastic martensitic transformation system in both thermally induced and me- chanically induced processes, including full and partial thermal transformation cycles, stress-induced martensitic transformation, pseudoelastic deformation and ferroelastic deformation via martensite vari- ant reorientation. Such a model allows the discussion of several aspects concerning the thermal and mechanical behaviour of thermoelastic martensitic transformations, such as the non-linear pseudoelas- ticity, deformation-induced two-way memory effect, strain dependence of mechanical hysteresis and minor loop behaviour of deformation.

Architectured membranes (silicone) by geometry or by microstructure are made to present a controlled initial anisotropy. Architectured membranes by geometry present crenels on their external surfaces, and architectured membranes by microstructure have a privilege direction of the chains. The anisotropy is controlled by the architecture. An equivalent model is proposed for both architectured membrane to take into account hyperelasticity, Mullins effect and initial anisotropy. This constitutive equation is extended to viscoelasticity for the architectured membrane by microstructure. Models are validated on experimental data., Des membranes silicones architecturées, par géométrie ou par microstructure, sont élaborées pour présenter une anisotropie initiale contrôlée. Les membranes architecturées par géométrie sont crénelées et celles par microstructure ont une direction privilégiées des chaînes macromoléculaires. L'anisotropie est contrôlée en fonction de l'architecture. Une loi de comportement équivalente pour la membrane architecturée par géométrie prenant en compte l'orientation des créneaux, l'hyperélasticité et l'anisotropie induite par l'effet Mullins du matériau constitutif est développée. Cette loi est implantée dans un code de calculs par éléments finis. Une extension de ce modèle est proposé pour la membrane architecturée par microstructure et permet également de tenir compte la viscoélasticité du matériau. Ces modèles sont finalement comparés à des résultats expérimentaux.

International audience; Electropulse heat treatment is a technique developed to realise fast heat treatment of NiTi shape memory alloys. This study investigates mechanical behaviour of cold worked NiTi wires heat treated with such a technique. It is demonstrated that milliseconds electropulses allow to realise homogeneous heat treatments and to adapt the mechanical behaviour of NiTi wires by controlling the electric energy. The material can be made elastic with different elastic modulus, perfectly superelastic with different stress plateau levels and superelastic with important local residual strain. Due to the short duration and high temperature of the heat treatment, this technique allows to obtain mechanical properties that cannot be obtained with classical heat treatments of several minutes in conventional furnaces such as linear evolution of the final loading and high tensile strength to 1500 MPa for superelastic material or increase of the stress plateau level with cycling for superelastic material.

International audience

International audience; Many applications, especially in the medical field, need the use of highly deformable membranes with required anisotropic properties. The present work is a contribution towards the processing, characterisation and modelling of anisotropic hyperelastic membranes. An unfilled silicone rubber with perfect hyperelastic behaviour is used. The anisotropy is generated by adding orientated crenels on the upper and lower surfaces of thin membranes during their elaboration. The influence of the relative orientation of the crenels on the mechanical response is characterised by performing tensile tests combined with kinematic field measurements by Digital Image Correlation. Two modellings are proposed. First, a simple analytical equivalent membrane model is proposed aiming to represent the behaviour of the architectured silicone membranes without any more parameter than those used in the hyperelastic constitutive equation of the silicone rubber. Second, the effective properties of the membranes are obtained by an a homogenisation approach with multiple scale asymptotic expansions written in the framework of hyperelasticity and by solving localisation problems on Representative Elementary Volumes with a finite element software. Finally, the experimental results are compared with predictions of two modelling, both approaches are equally efficient to describe them.

International audience; Une des principales applications des polymères biodégradables est la fabrication d'implants médicaux. Ceux-ci doivent fournir un maintien mécanique temporaire adéquat et, dans le cadre de l'ingénierie tissulaire, servir de support de régénération pour des tissus lésés avant de se résorber totalement. Le mécanisme de dégradation de ces polymères est l'hydrolyse des liaisons esters. La rupture des chaînes implique alors une évolution des propriétés mécaniques. Nous nous intéressons ici, à l'évolution des propriétés viscoélastiques de copolymères triblocs PLA-PEG-PLA au cours de la dégradation. Abstract: One of the main application of biodegradable polymers is the production of implantable medical devices. They must provide a temporary mechanical bearing or, serve as a regeneration support for damaged tissues in tissue engineering before totally resorb. The degradation's mechanism of these polymers is the hydrolysis of ester links. The bond's cleavage involves an evolution of mechanical properties. In this work, we will study the viscoelastic properties evolution of a PLA-PEG-PLA block copolymers during degradation.

International audience; Implantable polymeric medical devices suffer from a lack of visibility under current clinical imaging techniques. To circumvent this problem, poly(3-caprolactone-co-a-triiodobenzoate-3-caprolactone)s (PCL-TIB) containing from 3.5% to 24% of triiodobenzoate-3-caprolactone units were synthesized as new X-ray macromolecular contrast agents. Physico-chemical and thermal properties of PCL-TIBs were evaluated by 1H NMR, SEC, DSC and TGA. Their potential as radio-opacifying additive for medical devices was evaluated by preparing polymeric blends of PCL-TIB with various (co)polyesters. At first, in vitro X-ray visibility of PCL/PCL-TIB blends was evaluated. A more in depth characterization was then carried out based on PCL/PLA50-PEG-PLA50 blends. The impact of PCL-TIB content on the mechanical properties of blends was evaluated by tensile tests. Stability of X-ray visibility was evaluated by ex vivo implantation of non-degraded blends and of blends degraded for 6 weeks in vitro. Finally,cytocompatibility was assessed by evaluating the proliferation of L929 fibroblasts on the blends.