Yves Rémond

Structural mechanics of semicrystalline polymers prior to the yield point: a review

The review focuses on the current studies of the deformation response and accompanying structural transformations of thermoplastic semicrystalline polymers subjected to uniaxial tension prior to the yield point. The mechanisms of strain-induced cavitation of amorphous layers and damages of crystalline lamellae are analyzed in line with novel results on the deformation behavior of solid polymers at temperatures exceeding the glass transition point. The coupling of viscoelastic and plastic deformation mechanisms with the small-strain structural transformations is critically discussed on the basis of the advanced theoretical modeling of mechanical properties of semicrystalline polymers.

Bulk modulus and volume variation measurement of the liver and the kidneys in vivo using abdominal kinetics during free breathing

This article presents a method of predictive simulation, patient-dependant, in real time of the abdominal organ positions during free breathing. The method, that considers both influence of the abdominal breathing and thoracic breathing, needs a tracking of the patient skin and a model of the patient-specific modification of the diaphragm shape. From a measurement of the abdomen viscera kinematic during free breathing, we evaluate through a finite element analysis, the stress field sustained by the organs for a hyperelastic mechanical behaviour using large strain theory. From this analysis, we deduce an in vivo Poissons ratio and a homogeneous bulk modulus of the liver and kidneys, and compare it to the ones in vitro available in the literature.

Real time simulation of organ motions induced by breathing: first evaluation on patient data

In this paper we present a new method to predict in real time from a preoperative CT image the internal organ motions of a patient induced by his breathing. This method only needs the segmentation of the bones, viscera and lungs in the preoperative image and a tracking of the patient skin motion. Prediction of internal organ motions is very important for radiotherapy since it can allow to reduce the healthy tissue irradiation. Moreover, guiding system for punctures in interventional radiology would reduce significantly their guidance inaccuracy. In a first part, we analyse physically the breathing motion and show that it is possible to predict internal organ motions from the abdominal skin position. Then, we propose an original method to compute from the skin position a deformation field to the internal organs that takes mechanical properties of the breathing into account. Finally, we show on human data that our simulation model can provide a prediction of several organ positions (liver, kidneys, lungs) at 14 Hz with an accuracy within 7 mm

Modeling and Simulation of the Cooling Process of Borosilicate Glass

For a better understanding of the thermomechanical behavior of glasses used for nuclear waste vitrification, the cooling process of a bulk borosilicate glass is modeled using the finite element code Abaqus. During this process, the thermal gradients may have an impact on the solidification process. To evaluate this impact, the simulation was based on thermal experimental data from an inactive nuclear waste package. The thermal calculations were made within a parametric window using different boundary conditions to evaluate the variations of temperature distributions for each case. The temperature differences throughout the thickness of solidified glass were found to be significantly non-uniform throughout the package. The temperature evolution in the bulk glass was highly responsive to the external cooling rates applied; thus emphasizing the role of the thermal inertia for this bulky glass cast.

Coupling of Nanocavitation With Cyclic Deformation Behavior of High-Density Polyethylene Below the Yield Point

The peculiarities of viscoelastic behavior of high-density polyethylene (HDPE) subjected to the uniaxial cyclic tensions and retractions below the yield point are studied. This required using three different deformation programs including (i) the successive increase in strain maximum of each cycle, (ii) the controlled upper and lower stress boundaries, and (iii) the fixed strain at the backtracking points. The experimental data are analyzed in a framework of the modified structure-sensitive model (Oshmyan , 2006, “Principles of Structural–Mechanical Modeling of Polymers and Composites,” Polym. Sci. Ser. A, 48, pp. 1004–1013) of semicrystalline polymers. It is supposed that increase in the interlamellar nanovoid volume fraction results in speeding-up the plastic flow rate while decreasing cavitation rate. Consequently, a proper fitting of the stress–strain cyclic diagrams is obtained for the applied deformation programs within the common set of model parameters. This makes it possible to reveal evolution of nanovoid volume fraction in HDPE during cyclic deformations.

A Real-Time Predictive Simulation of Abdominal Organ Positions Induced by Free Breathing

Prediction of abdominal organ positions during free breathing is a major challenge from which several medical applications could benefit. For instance, in radiotherapy it would reduce the healthy tissue irradiation. In this paper, we present a method to predict in real-time the abdominal organs position during free breathing. This method needs an abdo-thoracic preoperative CT image, a second one limited to the diaphragm zone, and a tracking of the patient skin motion. It also needs the segmentation of the skin, the viscera volume and the diaphragm in both preoperative images. First, a physical analysis of the breathing motion shows it is possible to predict abdominal organs position from the skin position and a modeling of the diaphragm motion. Then, we present our original method to compute a deformation field that considers the abdominal and thoracic breathing influence. Finally, we show on two human data that our simulation model can predict several organs position at 50 Hz with accuracy within 2-3 mm.

Mechanically-driven bone remodeling simulation: Application to LIPUS treated rat calvarial defects

In this paper we numerically simulate the phenomenon of bone growth in bone defects as driven by external mechanical excitation. Bone growth is accounted for through a continuum model that allows simulation of the filling of a defect. The influence of the model boundary conditions is also discussed. Two and three dimensional simulations are presented, explicitly showing the bone regeneration process inside the cavity on a weekly basis. Numerical results are qualitatively compared with literature experimental data from a rat calvarial defect exposed to low-intensity pulsed ultrasound. The obtained results show trend correlations with the targeted phenomenological observations and allow us to perform a first evaluation of the proposed model parameters to be optimized for clinically relevant situations, even if a systematic experimental campaign is still needed to precisely identify the bio-mechanical parameters involved.

Finite Element Analysis of Temperature and Density Distributions in Selective Laser Sintering Process

In the selective laser sintering (SLS) manufacturing technique a pre-heated layer of material powder undergoes a laser radiation in a selective way to produce three dimensional metallic or polymeric solid parts. Here, we consider sintering of polymer powder. The phase transformation in this process involves the material heat transfer which is strongly affected by the material sintering phenomena. A transient three dimensional finite element model is developed to simulate the phase transformation during the selective laser sintering process. This model takes into account the heat transfer in the material (powder and solid), the sintering and the transient nature of this process. The numerical simulation of the set of equations, describing the problem, is made possible by means of the commercial finite element software Abaqus. A bi-level structure integration procedure is chosen, in which the density is integrated at the outer level and the heat equation is integrated in the inner level. After successfully computing the integration of the density, a material Jacobian representing the thermal phenomena is computed and supplemented the Abaqus Code via an implicit user subroutine material. Results for temperature and density distribution, using a polycarbonate powder, are presented and discussed.

A new multiscale model for the mechanical behavior of vein walls

The purpose of the present work is to propose a new multiscale model for the prediction of the mechanical behavior of vein walls. This model is based on one of our previous works which considered scale transitions applied to undulated collagen fibers. In the present work, the scale below was added to take the anisotropy of collagen fibrils into account. One scale above was also added, modeling the global reorientation of collagen fibers inside the vessel wall. The model was verified on experimental data from the literature, leading to a satisfactory agreement. The proposed multiscale approach also allows the extraction of local stresses and strains at each scale. This approach is presented here in the case of vein walls, but can easily be extended to other tissues which contain similar constituents.

Assessing the three-dimensional collagen network in soft tissues using contrast agents and high resolution micro-CT: Application to porcine iliac veins

The assessment of the three-dimensional architecture of collagen fibers inside vessel walls constitutes one of the bases for building structural models for the description of the mechanical behavior of these tissues. Multiphoton microscopy allows for such observations, but is limited to volumes of around a thousand of microns. In the present work, we propose to observe the collagenous network of vascular tissues using micro-CT. To get a contrast, three staining solutions (phosphotungstic acid, phosphomolybdic acid and iodine potassium iodide) were tested. Two of these stains were showed to lead to similar results and to a satisfactory contrast within the tissue. A detailed observation of a small porcine iliac vein sample allowed assessing the collagen fibers orientations within the medial and adventitial layers of the vein. The vasa vasorum network, which is present inside the adventitia of the vein, was also observed. Finally, the demonstrated micro-CT staining technique for the three-dimensional observation of thin soft tissues samples, like vein walls, contributes to the assessment of their structure at different scales while keeping a global overview of the tissue.