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cited By 1; International audience; The correct characterisation of the articular disc of the temporomandibular joint (TMJ) is key to study the masticatory biomechanics. For the interval from extraction until testing, freezing is the most used preservation technique for biological tissues, but its influence on their behaviour is still unclear. An important error can be committed in the characterisation of such tissues if freezing has any effect on their mechanical properties. Thus, the aim of this study was to determine whether the freezing storage time causes any change in the mechanical properties of the TMJ discs. To check that, the specimens were stored in a −20 °C freezer during different time intervals: 1 day, 1 week, 1 month and 3 months. Fresh specimens, tested right after extraction, were used as the control group. Compressive stress relaxation tests were carried out on the specimens and a quasi-linear viscoelastic (QLV) model was used to fit the experimental curves. A statistical analysis detected significant differences among the groups. Post-hoc tests determined that freezing the specimens more than 30 days may lead to changes in the viscoelastic properties of the tissue. © 2017 Elsevier Ltd

International audience; Low-temperature superconductors are widely used in high-field magnets, mostly within Rutherford-type cables. The run for higher fields leads to greater forces on the conductor, which is pushed closer to its mechanical limit. Managing the higher strain and stress levels on the conductor supposes to perform simulation at the strand level, especially with strain-sensitive superconductors such as Nb3Sn. Three-dimensional models are necessary because inside of a magnet, the conductor is subject to a complex combination of axial and transverse loads. Superconducting cables are anisotropic composite structures that can comprise superconducting strands, insulation materials, stabilizing parts, porosities, etc. They have a multiscale architecture, the performance at the magnet scale being driven by the filament scale. This paper proposes a numerical approach for 3-D finite-element (FE) modeling of Rutherford cables, at the scale of the strand. The 3-D mesh of two reference cables is built with a simplified cable forming model. The accuracy of this geometrical model is assessed by comparing sections of the simulated mesh with tomographic data, using relevant criteria and specific geometry analysis tools. In this approach, the strand is considered as bimetallic, with a copper core, a superconducting bundle ring, and an outer copper ring. After mechanical 3-D FE computation on this bimetallic mesh, the strain/stress state on the local compounds of the strand (including Nb3Sn filaments) is obtained by strain projection. The strain/stress state on the Nb3Sn filaments can be used to estimate the cable current transport capability, thanks to the existing scaling laws.

International audience; The thermo-mechanical properties of a ferritic-steel-based metal matrix composite reinforced with titanium diboride (TiB 2) particles were studied using synchrotron X-ray diffraction. The samples were subjected to in situ heat treatment to investigate the effect of the mismatch in the coefficient of thermal expansion between the matrix and reinforcement. In addition, in situ tensile tests were performed to examine the load partitioning in the composite. Load transfer from the ferritic matrix to the TiB 2 particles was observed at the onset of plasticity in the matrix.

A precise characterization of the articular disc of the temporomandibular joint (TMJ) is essential to study the masticatory biomechanics. The disc is responsible for the load distribution over the articular surface and for absorbing impacts during mastication. The main objective of this work is to characterize the mechanical behaviour of the articular disc under compression, the usual stress state during mastication. A quasi-linear viscoelastic (QLV) model, with a hyperelastic response for the elastic function, is proposed to describe the mechanical behaviour of the articular disc. The validity of that simplified model relies on the independence of their constants with the strain level and strain rate. The independence of the strain level was proved in a previous work. In this paper, different loading rates were tested to fully confirm the validity of the model in the physiological range of loads. Moreover, the strong non-linearity of the stress-strain relation made the exponential strain energy function the most suitable of the different models tried to represent the elastic response of the QLV model.

The temporomandibular joint (TMJ) plays a key role in the distribution of stresses in the mandible during mastication and consequently in the distribution of bone density, due to the interconnection between both variables through bone remodelling. Two finite element models of the mandible were compared to study the influence of the redistribution of stresses produced by the joint: (1) a model without TMJ, but with simplified boundary conditions to replace the joint, as done in previous models; (2) a more realistic model including the articular disc and some ligaments present in the TMJ. The stresses and strains in both models were compared through the strain energy density, used in many bone remodelling models as a measure of the mechanical stimulus. An anisotropic bone remodelling model was used to simulate the behaviour of mandible bone and to estimate its density distribution. The results showed that the TMJ strongly affects the stress distribution, the mechanical stimulus, and eventually the bone density, and not only locally in the condyle, but also in the whole mandible. It is concluded that it is utterly important to include a detailed model of the TMJ to estimate more correctly the stresses in the mandible during mastication and, from them, the bone density and anisotropy distribution.

The aim of this paper is to propose a biomechanical model that could serve as a tool to overcome some difficulties encountered in experimental studies of the mandible. One of these difficulties is the inaccessibility of the temporomandibular joint (TMJ) and the lateral pterygoid muscle. The focus of this model is to study the stresses in the joint and the influence of the lateral pterygoid muscle on the mandible movement. A finite element model of the mandible, including the TMJ, was built to simulate the process of unilateral mastication. Different activation patterns of the left and right pterygoid muscles were tried. The maximum stresses in the articular disc and in the whole mandible during a complete mastication cycle were reached during the instant of centric occlusion. The simulations show a great influence of the coordination of the right and left lateral pterygoid muscles on the movement of the jaw during mastication. An asynchronous activation of the lateral pterygoid muscles is needed to achieve a normal movement of the jaw during mastication.

A finite element model of the temporomandibular joint (TMJ) and the human mandible was fabricated to study the effect of abnormal loading, such as awake and asleep bruxism, on the articular disc. A quasilinear viscoelastic model was used to simulate the behaviour of the disc. The viscoelastic nature of this tissue is shown to be an important factor when sustained (awake bruxism) or cyclic loading (sleep bruxism) is simulated. From the comparison of the two types of bruxism, it was seen that sustained clenching is the most detrimental activity for the TMJ disc, producing an overload that could lead to severe damage of this tissue.

A precise information of the biomechanical properties of soft tissues is required to develop a suitable simulation model, with which the distribution of stress and strain in the complex structures can be estimated. Many soft tissues have been mechanically characterized by stress relaxation tests under unconfined or confined compression. In general, full-thickness samples are extracted to reduce the damage in the tissue as much as possible. However, it is not guaranteed that these samples have a uniform thickness or, in other words, planar parallel faces. In particular, in the articular disc of the temporomandibular joint, many studies can be found testing full-thickness samples for which that thickness is known to be non-uniform, while making the assumption of uniaxial stress state to extract the mechanical properties from those tests. That inaccuracy may have a strong influence in some cases and needs a profound revision. The main goal of this work is to quantify the error committed in that assumption and the influence of the variation of thickness on that error in a particular test: stress relaxation tests under unconfined compression. Based on this error and defining an allowable tolerance, a criterion is established to reject samples depending on their aspect ratio.

The main objectives of this work are: (a) to introduce an algorithm for adjusting the quasi-linear viscoelastic model to fit a material using a stress relaxation test and (b) to validate a protocol for performing such tests in temporomandibular joint discs. This algorithm is intended for fitting the Prony series coefficients and the hyperelastic constants of the quasi-linear viscoelastic model by considering that the relaxation test is performed with an initial ramp loading at a certain rate. This algorithm was validated before being applied to achieve the second objective. Generally, the complete three-dimensional formulation of the quasi-linear viscoelastic model is very complex. Therefore, it is necessary to design an experimental test to ensure a simple stress state, such as uniaxial compression to facilitate obtaining the viscoelastic properties. This work provides some recommendations about the experimental setup, which are important to follow, as an inadequate setup could produce a stress state far from uniaxial, thus, distorting the material constants determined from the experiment. The test considered is a stress relaxation test using unconfined compression performed in cylindrical specimens extracted from temporomandibular joint discs. To validate the experimental protocol, the test was numerically simulated using finite-element modelling. The disc was arbitrarily assigned a set of quasi-linear viscoelastic constants (c1) in the finite-element model. Another set of constants (c2) was obtained by fitting the results of the simulated test with the proposed algorithm. The deviation of constants c2 from constants c1 measures how far the stresses are from the uniaxial state. The effects of the following features of the experimental setup on this deviation have been analysed: (a) the friction coefficient between the compression plates and the specimen (which should be as low as possible); (b) the portion of the specimen glued to the compression plates (smaller areas glued are better); and (c) the variation in the thickness of the specimen. The specimen’s faces should be parallel to ensure a uniaxial stress state. However, this is not possible in real specimens, and a criterion must be defined to accept the specimen in terms of the specimen’s thickness variation and the deviation of the fitted constants arising from such a variation.

Living materials have a very complex behavior and have been a subject of intense research. Bone tissue is a porous, heterogeneous and generally anisotropic material which adapts its microstructure and mechanical properties in accordance to the mechanical environment. The adaptive behavior of its microstructure and properties are a consequence of the process known as bone remodelling.