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Dry fabrics comprised of high performance polymeric fibers have been widely used as protection layers in structures submitted to high velocity impacts (HVI). Their outstanding impact energy dissipation ability combined with an high strength-to-weight ratio make them a preferable choice in different applications such as bullet vests or blade containment systems over standard materials. Among the different approaches adopted to study these structures numerical methods assume a central role. Thanks to their reduced costs and the related possibility of evaluating the effects of single phenomena, they are often used to predict the structure ballistic limits or to study the physical events which occur during the penetration. Among the different strategies adopted to model a fabric, mesoscopic models have been largely adopted by different authors. These models assume the yarns as a continuum body while the fabric geometry is explicitly described. Nowadays yarn material models are universally assumed to be linear elastic and orthotropic. This modelling approach mostly focuses on the longitudinal behaviour of the yarn, however fiber-scale analyses and experimental results shows the importance of three-dimensional stress state on the ballistic limit. In order to obtain a three-dimensional description of the yarn strain state during the impact, a novel hyperelastic model for yarn structures here is developed. In a first step, fiber-level preliminary analyses have been performed to obtain the effective behaviour of these structure under the projectile collision. In the second step, the hyperelastic model has been implemented and identified thanks to microscopic elementary tests. Finally, a continuum model of the yarn have been performed. First results show the relevance of the hyperelastic model compared to the fiber-level observation and enhance the limit of the classical linear elastic material model.

The tearing of a muscle-tendon complex (MTC) is caused by an eccentric contraction; however, the structures involved and the mechanisms of rupture are not clearly identified. The passive mechanical behavior the MTC has already been modeled and validated with the discrete element method. The muscular activation is the next needed step. The aim of this study is to model the muscle fiber activation and the muscular activation of the MTC to validate their active mechanical behaviors. Each point of the force/length relationship of the MTC (using a parabolic law for the force/length relationship of muscle fibers) is obtained with two steps: 1) a passive tensile (or contractile) test until the desired elongation is reached and 2) fiber activation during a position holding that can be managed thanks to the Discrete Element model. The muscular activation is controlled by the activation of muscle fiber. The global force/length relationship of a single fiber and of the complete MTC during muscular activation is in agreement with literature. The influence of the external shape of the structure and the pennation angle are also investigated. Results show that the different constituents of the MTC (extracellular matrix, tendon), and the geometry, play an important role during the muscular activation and enable to decrease the maximal isometric force of the MTC. Moreover, the maximal isometric force decreases when the pennation angle increases. Further studies will combine muscular activation with a stretching of the MTC, until rupture, in order to numerically reproduce the tearing of the MTC. [ABSTRACT FROM AUTHOR]

The discrete element method (DEM) presents an alternative way to model complex mechanical problems of silica glass, such as brittle fracture. Since discontinuities are naturally considered by DEM, no complex transition procedure from continuum phase to discontinuum one is required. However, to ensure that DEM can properly reproduce the silica glass cracking mechanisms, it is necessary to correctly model the different features characterizing its mechanical behavior before fracture. Particularly, it is necessary to correctly model the densification process of this material, which is known to strongly influence the fracture mechanisms. The present paper proposes a new and very promising way to model such process, which is assumed to occur only under hydrostatic pressure. An accurate predictive-corrective densification model is developed. This model shows a great flexibility to reproduce extremely complex densification features. Furthermore, it involves only one calibration parameter, which makes it very easy to apply. This new model represents a major step towards accurate modeling of materials permanent deformation with the discrete element method, which has long been a huge challenge in applying this method for continuum problems. [ABSTRACT FROM AUTHOR]

Magnetorheological (MR) fluids are fluids whose properties vary in response to an applied magnetic field. Such fluids are typically composed of microscopic iron particles (r 1 -- 20 fim diameter, 20 -- 40% by volume) suspended in a carrier fluid such as min-eral oil or water. MR fluids are increasingly proposed for use in various mechanical sys-tem applications, many of which fall in the domain of tribology, such as smart dampers and clutches, prosthetic articulations, and controllable polishing fluids. The goal of this study is to present an overview of the topic to the tribology audience, and to develop an MR fluid model from the microscopic point of view using the discrete element method (DEM), with a long range objective to better optimize and understand MR fluid behavior in such tribological applications. As in most DEM studies, inter-particle forces are deter-mined by a force-displacement law and trajectories are calculated using Newton's sec-ond law. In this study, particle magnetization and magnetic interactions between particles have been added to the discrete element code. The global behavior of the MR fluid can be analyzed by examining the time evolution of the ensemble of particles. Micro-scopically, the known behavior is observed: particles align themselves with the external magnetic field. Macroscopically, averaging over a number of particles and a significant time interval, effective viscosity increases significantly when an external magnetic field is applied. These preliminary results would appear to establish that the DEM is a promising method to study MR fluids at the microscopic and macroscopic scales as an aid to tribo-logical design. [ABSTRACT FROM AUTHOR]

When the thermal aspect of sliding contacts is investigated, few models take into account the presence of a third body at the contact interface. Moreover, when the presence of the third body is considered, its rheology is neglected. For this reason, the thermal study of such contact configuration is not fully understood and relies on strong hypothesis or even important simplifications. To fill this lack of knowledge, a thermal model has been developed embedded in a discrete element scheme. Such investigations highlight the key role played by both thermal and mechanical properties of the contact elements. If the third body rheology can affect the localization of the heat generation leading to important thermal asymmetries, the diffusive nature of the first bodies can also strongly control the phenomenon and accentuate or diminish the initial differences of surfaces temperature for the contacting volumes. The goal of this paper is to bring information or complete existing theories (Blok, H. A., 1937, "Theoretical Study of Temperature Rise at Surface at Actual Contact Under Oilness Lubricating Conditions," 1. Mech. E. Conf. Publ., 2, pp. 222-235; Ryhming, I. L., 1979, "On Temperature and Heat Source Distributions in Sliding Contact Problems," Acta Mech., 32; pp. 261-274; Dragon-Louiset, M., and Stolz, C., 1999, "Approche Thermodynamique des Phénomenès liés à l'Usure de Contact," Acad. Sci. Paris, C. R., 327, pp. 1275-1280) but also to bring a new point of view on the differences observed in the past between the numerical predictions and experimental measurements. [ABSTRACT FROM AUTHOR]

The present paper applies a recent continuum theory due to Aranson and Tsimring (2002, "Continuum Theory of Partially Fluidized Granular Flows," Phys. Rev. E, 65, p. 061303) for the dense granular flow of particles in sustained contact to lubrication flows. Such third body granular flow may apply to some solid lubrication mechanisms. The continuum theory is unique in that it addresses solidlike behavior and the transition to fully fluidized behavior. The continuum studies are complemented by a discrete particle dynamics model of Iordanoff et al. (2005, "Numerical Study of a Thin Layer of Cohesive Particles Under Plane Shearing," Powder Technol., 159, pp. 46-54). Three problems are treated: (1)flow due to the gravity of a layer of granular material down an inclined plane, (2) simple shear flow of a layer confined between sliding parallel surfaces, and (3) lubrication flow of a layer confined between a curved surface and a sliding plane. The perspective of this paper is that a continuum model will be more useful than a discrete model in engineering design of solid lubrication systems for the foreseeable future. In the inclined plane problem, the discrete simulations are used to provide material property parameters to the continuum model. In the simple shear problem, for validation, predictions of the continuum model are compared to those of the discrete element computer simulations. Finally, the continuum theory is applied to a more complex lubrication flow [ABSTRACT FROM AUTHOR]