Michel Coret

Mechanical characterization of liver capsule through uniaxial quasi-static tensile tests until failure

Accidentology data showed that liver is often injured in car crashes; three types of injuries occur: hematoma, laceration and vessel failure. This paper focuses on surface laceration, which involves liver capsule and hepatic parenchyma. Liver capsule behavior has been studied but its failure properties are still unclear, particularly on a local point of view. In the present study, tensile quasi-static tests are run on parenchyma and capsule samples until failure to characterize capsule failure. Normalized load as well as failure properties-ultimate load per width unit and ultimate strain-are determined. Digital image correlation is used to measure the full local strain field on the capsule. Mean values of failure characteristics for hepatic capsule are 47+/-29% for the ultimate local strain and 0.3+/-0.3 N/mm for the ultimate load per width unit. A comparison between human and porcine tissues is conducted based on Mann-Whitney statistical test; it reveals that capsule characteristics are close between these two species; however, freezing preservation significantly affects porcine capsule failure properties. Therefore using porcine instead of human tissue to determine failure characteristics of liver capsule seems satisfactory only on fresh tissues.

Characterization of the nonlinear behaviour and the failure of human liver capsule through inflation tests

This paper aims at describing an inflation test protocol on a human liver capsule using stereo-correlation. The biaxial tension created by the inflation test is comparable to the type of loading the capsule would be subjected to during a liver compression. Confocal microscopy associated to an anti-collagen coloration reveals that the tissue is isotropic at the meso-scale. Stereo-correlation provides the strain field of the capsule during the test. It emphasizes the boundary condition effects on the strain field. The measurement of the shape of the capsule is used to determine the parameters of two hyperelastic (polynomial and exponential) homogeneous models. The ultimate first principal strain before failure is measured locally and its value is 50.5%±10.8%. In this protocol, the light goes throughout the sample and makes the heterogeneities of the material appear as darker grey levels on the pictures. These heterogeneities also appear on the strain fields, so we can assume that they have different material properties.

Characterizing liver capsule microstructure via in situ bulge test coupled with multiphoton imaging.

The characterization of biological tissue at the microscopic scale is the starting point of many applications in tissue engineering and especially in the development of structurally based constitutive models. In the present study, focus is made on the liver capsule, the membrane encompassing hepatic parenchyma, which takes a huge part in liver mechanical properties. An in situ bulge test experiment under a multiphoton microscope has been developed to assess the microstructure changes that arise with biaxial loading. Multiphoton microscopy allows to observe the elastin and collagen fiber networks simultaneously. Thus a description of the microstructure organization of the capsule is given, characterizing the shapes, geometry and arrangement of fibers. The orientation of fibers is calculated and orientation distribution evolution with loading is given, in the case of an equibiaxial and two non equibiaxial loadings, thanks to a circular and elliptic set up of the bulge test. The local strain fields have also been computed, by the mean of a photobleaching grid, to get an idea of what the liver capsule might experience when subjected to internal pressure. Results show that strain fields present some heterogeneity due to anisotropy. Reorientation occurs in non equibiaxial loadings and involves fibers layers from the inner to the outer surface as expected. Although there is a fiber network rearrangement to accommodate with loading in the case of equibiaxial loading, there is no significant reorientation of the main fibers direction of the different layers.

Photobleaching as a tool to measure the local strain field in fibrous membranes of connective tissues

Connective tissues are complex structures which contain collagen and elastin fibers. These fiber-based structures have a great influence on material mechanical properties and need to be studied at the microscopic scale. Several microscopy techniques have been developed in order to image such microstructures; among them are two-photon excited fluorescence microscopy and second harmonic generation. These observations have been coupled with mechanical characterization to link microstructural kinematics to macroscopic material parameter evolution. In this study, we present a new approach to measure local strain in soft biological tissues using a side-effect of fluorescence microscopy: photobleaching. Controlling the loss of fluorescence induced by photobleaching, we create a pattern on our sample that we can monitor during mechanical loading. The image analysis allows three-dimensional displacements of the patterns at various loading levels to be computed. Then, local strain distribution is derived using the finite element discretization on a four-node element mesh created from our photobleached pattern. Photobleaching tests on a human liver capsule have revealed that this technique is non-destructive and does not have any impact on mechanical properties. This method is likely to have other applications in biological material studies, considering that all collagen-elastin fiber-based biological tissues possess autofluorescence properties and thus can be photobleached.

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.

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.

Imaging of the human Glisson's capsule by two-photon excitation microscopy and mechanical characterisation by uniaxial tensile tests

Liver mechanical properties have been well investigated through many studies at the macroscopic level. These studies allowed elaborating more or less sophisticated models providing material data. The rupture phenomenon has been studied as well, for example by Brunon et al. [Brunon et al. 2011] to collect data on how the liver, and more precisely the Glissons capsule, responds to mechanical loading. With the emergence of multi-modal microscopy, these phenomenon have been considered to a whole different scale, as we can now investigate the behaviour of these materials at the microscopic level. This way, we can have a better understanding of damage and rupture mechanisms of these tissues, linking them to microstructure organisation, and thus develop realistic models based on microstructure. Recent studies have therefore focused on imaging the microstructure organisation of biological materials during or after loading. For example Goulam Houssen et al. [Goulam Houssen et al., 2011] have monitored the rat tail collagen fibers behaviour during uniaxial loading using Second-Harmonic Generation (SHG) from TwoPhoton Excitation Microscopy (TPE). Keyes et al. [Keyes et al., 2012] used SHG and autofluorescence from TPE to image pressurised porcine coronary arteries and observed fibers realignment. Thus, the aim of this study is to observe how the different constitutive fibers of the Glissons capsule are organized and how they react when subjected to uniaxial tensile loading, in order to understand how microstructure organisation impacts macro mechanical properties. In a first step, we worked on the imaging of the Glissons capsule by TPE to enforce our knowledge of its components and their state before loading. In a second step, we performed preliminary tensile tests using an in situ micro tensile stage to estimate mechanical parameters such as apparent modulus, ultimate stress and ultimate strain. 2. Methods

Coupled Experimental/Numerical Approach to Determine the Creep Behavior of Zr-4 Cladding Under LOCA Condition

The thermo-mechanical behavior of Zircaloy-4 fuel rods under Loss-Of-Coolant Accident (LOCA) conditions is investigated. A custom experimental setup is dedicated to the high-temperature creep ballooning study of 90mm long cladding samples. Creep tests were performed under an inert environment (argon), for temperatures from 750 to 850°C and internal pressures ranging from 1 to 5 MPa. As the high-temperature creep of metals is strongly influenced by the temperature, the setup allow for a heterogeneous thermal distribution along the specimen. A unique test provides a rich database about the steady-state creep of the alloy. A first campaign is dedicated to bare Stress Relieved Annealed Zr-4.

Geometry of an inflated membrane in elliptic bulge tests: evaluation of an ellipsoidal shape approximation by stereoscopic digital image correlation measurements

Elliptic bulge tests are conducted on liver capsule, a fibrous connective membrane, associated with a field measurement method to assess the global geometry of the samples during the tests. The experimental set up is derived from a previous experimental campaign of bulge tests under microscope. Here, a stereoscopic Digital Image Correlation (DIC) system is used to measure global parameters on the test and investigate some assumptions made on the testing conditions which could not been assessed with microscopic measurements. In particular, the assumption of an ellipsoidal shape of the inflated membrane is tested by comparing the actual sample shape measured by stereoscopic DIC with an idealized ellipsoidal shape. Results indicate that a rather constant gap exists between the idealized and actual position. The approximation in the calculation of a macroscopic strain through analytical modeling of the test is estimated here. The study of the liver capsule case shows that important differences can be observed in strain calculation depending on the method and assumptions taken. Therefore, analytical modeling of mechanical tests through ellipsoidal approximation needs to be carefully evaluated in every application. Here the field measurement allows assessing the validity of these modeling assumptions. Moreover, it gives precious details about the boundary conditions of the bulge test and revealed the heterogeneous clamping, highlighted by strain concentrations.

Compared prediction of the experimental failure of a thin fibrous tissue by two macroscopic damage models

Several models for fibrous biological tissues have been proposed in the past, taking into account the fibrous microstructure through different homogenization methods. The aim of this paper is to compare theoretically and experimentally two existing homogenization methods - the Angular Integration method and the Generalized Structure Tensor method - by adapting them to a damage model for a planar fibrous tissue made of linear elastic and brittle fibers. The theoretical implementation of the homogenization methods reveals some differences once damage starts in the fibrous tissue; in particular, the anisotropy of the tissue evolves differently. The experimental aspect of this work consists in identifying the parameters of the damage model, with both homogenization methods, using inflation tests until rupture on a biological membrane. The numerical identification method is based on the simulation of the tests with the real geometry of the samples and the real boundary conditions computed by Stereo Digital Image Correlation. The identification method is applied to human liver capsule. The collagen fibers Youngs modulus (19±6MPa) as well as their ultimate longitudinal strain (33±4%) are determined; no significant difference was observed between the two methods. However, by using the experimental boundary conditions, we could observe that the damage progression is faster for the Angular Integration version of the model.