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EN
Thermoforming is one of the new methods for forming of polymer sheets. Free thermoforming is one of the thermoforming methods in which shaping is done with air pressure or vacuum without the plug mold. In this paper, free thermoforming of Poly Methyl Methacrylate (PMMA) has been investigated by experimental tests and finite element simulation. The main purpose of this article is the identification of the real behavior of PMMA during free thermoforming to achieve maximum workable air pressure with respect to initial thickness. For this, at first, tensile and relaxation tests have been done in working temperature (160◦C). Then the process was simulated by Abaqus software with considering four types of the material property: three hyperelastic models (Ogden, Mooney-Rivlin, and Marlow) and a hyperviscoelastic model. After that, experimental tests were done, and the samples final shape were compared with simulation results. Accordingly, the simulation results obtained based on the Marlow hyperelastic model showed the best agreement with the experiments compared to others. After that, maximum workable air pressure versus plate initial thickness and minimum thickness of the deformed plate were achieved by finite element simulation.
EN
The authors tried to identify the parameters of numerical models of digital materials, which are a kind of composite resulting from the manufacture of the product in 3D printers. With the arrangement of several heads of the printer, the new material can result from mixing of materials with radically different properties, during the process of producing single layer of the product. The new material has properties dependent on the base materials properties and their proportions. Digital materials tensile characteristics are often non-linear and qualify to be described by hyperelastic materials models. The identification was conducted based on the results of tensile tests models, its various degrees coefficients of the polynomials to various degrees coefficients of the polynomials. The Drucker’s stability criterion was also examined. Fourteen different materials were analyzed.
EN
A probabilistic finite element (FE) analysis of the L4-L5 and L5-S1 human annulus fibrosus (AF) was conducted to obtain a better understanding of the biomechanics of the AF and to quantify its influence on the range of motion (ROM) of the L4-L5 and L5-S1 segments. Methods: The FE models were composed of the AF and the upper and lower endplates. The AF was represented as a continuous material composed of a hyperelastic isotropic Yeoh matrix reinforced with two families of fibers described with an exponential energy function. The caudal endplate was fully restricted and 8 Nm pure moment was applied to the cranial endplate in flexion, extension, lateral flexion and axial rotation. The mechanical constants were determined randomly based on a normal distribution and average values reported. Results: Results of the 576 models show that the ROM was more sensitive to the initial stiffness of the fibers rather than to the stiffening coefficient represented in the exponential function. The ROM was more sensitive to the input variables in extension, flexion, axial rotation and lateral bending. The analysis showed an increased probability for the L5-S1 ROM to be higher in flexion, extension and axial rotation, and smaller in lateral flexion, with respect to the L4-L5 ROM. Conclusions: An equation was proposed to obtain the ROM as a function of the elastic constants of the fibers and it may be used to facilitate the calibration process of the human spine segments and to understand the influence of each elastic constant on the ROM.
EN
Purpose: The aim of this study is to propose a method to construct corneal biomechanical model which is the foundation for simulation of corneal microsurgery. Methods: Corneal material has two significant characteristics: hyperelastic and viscoelastic. Firstly, Mooney–Rivlin hyperelastic model of cornea obtained based on stored-energy function can be simplified as a linear equation with two unknown parameters. Then, modified Maxwell viscoelastic model of the cornea, whose analytical form is consistent with the generalized Prony-series model, is proposed from the perspective of material mechanics. Results: Parameters of the model are determined by the uniaxial tensile tests and the stress-relaxation tests. Corneal material properties are simulated to verify the hyper-viscoelastic model and measure the effectiveness of the model in the finite element simulation. On this basis, an in vivo model of the corneal is built. And the simulation of extrusion in vivo cornea shows that the force is roughly nonlinearly increasing with displacement, and it is consistent with the results obtained by extrusion experiment of in vivo cornea. Conlusions: This paper derives a corneal hyper-viscoelastic model to describe the material properties more accurately, and explains the mathematical method for determination of the model parameters. The model is an effective biomechanical model, which can be directly used for simulation of trephine and suture in keratoplasty. Although the corneal hyper-viscoelastic model is taken as the object of study, the method has certain adaptability in biomechanical research of ophthalmology.
5
Content available remote Simulation of Energy Dissipation During Impacts with Hyperelastic Elements
EN
The aim of this article is to analyze the possibility of energy dissipation during the impact a pedestrian by a vehicle. The energy created during the impact will be dissipated by element of protection made of a hyperdeformable material. Energy intensive structures are able to take over the kinematic energy during impact, which is equivalent to the work of their destruction (crushing, breaking). This has the effect of increasing pedestrian safety during the accident to fulfillment of the regulation of the European Parliament and Council Regulation (WE) No 78/2009 of 14 January 2009 with all amendments. Ensuring the safety and reduce pedestrian injuries during an accident is very important. Presented in work method of analysis of polyurethane materials, hyperdeformable with using the Finite Element Method can demonstrate how important it is to conduct researches to increase security. Exercising studies of materials using the FEM can verify used materials, and thus, drawing conclusions and present proposals for the use of new materials with improved properties. To describe the properties of the foam will be used previously known theories: hyperelstic materials and properties of elastic-plastic materials, including volumetric plastic deformations. The results of experimental studies will enable determine the model of to the foam and use it in calculations using the Finite Element Method of ABAQUS system. Explicit calculation of the dynamic module of the ABAQUS system is used to simulate impact of a bumper with protective element in the under-bumper beam to the pedestrian, will be analyzed the degree of energy dissipation. Simulation will allow an assessment of the effectiveness pedestrian protection - the operation of protection element.
EN
In modem technology, more widely are used the composites with the gas phase - porous structures with pores closed, filled the gas. Can find here polymeric materials: EPP polyurethanes used in automobile bumpers, CELLASTO polyurethane in suspension systems. To the same group may also include gazars - composites reinforced by gas, and thus structure, in which the gas in the closed pores is subjected to initial pressure. Gazars are made as the structure of metal, usually copper, carbon dioxide or a mixture of gases. The aim of this work is to develop methods of describing the properties of such materials based on knowledge of: basic materials, technologies (gas pressure formed during foaming) using the theory of hyperplastic materials, in particular using Ogden’s model and its modifications. The resulting description can be used for the applicability of hyperelstic models, and therefore in the whole range of deformation of the polymer-based composites and elastic composites of metals (not included plasticity). Verification of the method is carried out through the execution of experimental EPP foam polyurethane derived from car bumpers, enabling a better understanding of materials and will be able to better modify structures, and thus, improve the mechanical properties.
EN
We present a method to determine elasto--mechanical properties of soft biological tissues, and a device able to perform the required measurements in-vivo. The device permits the controlled application of vacuum to small spots of organic tissue and registers the small deformation caused, during the whole measurement process. Deformation is measured with a vision based technique and the grabbed images are processed in real-time to avoid storage problems. We model biological tissue with a hyperelastic quasilinear viscoelastic material law and determine the unknown material parameters via inverse finite element methods.
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