Mechanical characterization of rubber from one heterogeneous test

International audienceThe present study deals with the mechanical characterization of elastomeric materials. Classically, three homogeneous tests are considered to identify constitutive parameters, namely Uniaxial Tensile (UT), Pure Shear (PS) and Equibiaxial Tensile (ET) tests. Here, a new method is proposed: it consists in inducing the three previous homogeneous tests from only one heterogeneous mechanical test. For this purpose, a conventional tensile machine is used and a new apparatus is designed to be adapted on the machine. The test-induced heterogeneity is discussed related to two criteria based on the existence of UT, PS and ET and on the distribution of the maximal principal elongation at each material points of the sample surface. Experimentally, kinematic fields are provided on the sample surface by an image correlation code suitable for large deformations. Finally, an inverse technique, so-called Virtual Field Method, is used to identify the material parameters in the framework of the Mooney hyperelasticity

Effect of thermal cycles on the deformation state at the crack tip of crystallizable natural rubber

International audienceThis paper deals with the effect of temperature variations on crack tip kinematicsin rubbers, especially in crystallizing rubbers. In such materials, the high deformation levelencountered at the crack tip engenders the formation of crystallites. As a consequence, thecrack tip is reinforced and resists crack growth. However, this phenomenon is signicantly affectedby variations in material temperature. This is classically observed at the macroscopicscale in terms of crack propagation rate and path. In this study, the effect of temperature isstudied at the local scale, by measuring the change in the kinematic eld at the crack tip duringthermal cycles. Results show that, in crystallizable natural rubber, the effect of temperaturedepends on the stretch ratio atained in the zone under consideration. In slightly stretchedzones, the stretch ratio increases with the increase in temperature, whereas it decreases inhighly stretched zones. This highlights the competition between the effects of the variationsin internal energy and in entropy on the thermomechanical response. Moreover, if crystallitesform in highly stretched zones, the increase in temperature leads to crystallite melting,which increases the stretch ratio. This is explained by the fact that crystallites act as llers byconcentrating the stress and therefore by increasing the apparent stiffness of the material

toxines et signalisation

Collection Rencontres en toxinologie ISSN: 1760-6004 ; http://sfet.asso.fr/images/stories/SFET/pdf/EBook-RT17-2009-signets.pdfInternational audienc

Avancées et nouvelles technologies en Toxinologie

Collection Rencontres en Toxinologie ISSN 1760-6004 ; http://sfet.asso.fr/images/stories/SFET/pdf/Ebook-RT18-2010-signets-110322.pdfInternational audienc

The NaV1.7 Channel Subtype as an Antinociceptive Target for Spider Toxins in Adult Dorsal Root Ganglia Neurons

Although necessary for human survival, pain may sometimes become pathologic if long-lasting and associated with alterations in its signaling pathway. Opioid painkillers are officially used to treat moderate to severe, and even mild, pain. However, the consequent strong and not so rare complications that occur, including addiction and overdose, combined with pain management costs, remain an important societal and economic concern. In this context, animal venom toxins represent an original source of antinociceptive peptides that mainly target ion channels (such as ASICs as well as TRP, CaV, KV and NaV channels) involved in pain transmission. The present review aims to highlight the NaV1.7 channel subtype as an antinociceptive target for spider toxins in adult dorsal root ganglia neurons. It will detail (i) the characteristics of these primary sensory neurons, the first ones in contact with pain stimulus and conveying the nociceptive message, (ii) the electrophysiological properties of the different NaV channel subtypes expressed in these neurons, with a particular attention on the NaV1.7 subtype, an antinociceptive target of choice that has been validated by human genetic evidence, and (iii) the features of spider venom toxins, shaped of inhibitory cysteine knot motif, that present high affinity for the NaV1.7 subtype associated with evidenced analgesic efficacy in animal models

A new characterisation method for rubber

International audienceThis paper deals with the mechanical characterisation of elastomeric materials. An original method is proposed to identity the material parameters. It consists of performing only one heterogeneous mechanical test, measuring the displacement/strain field using suitable Digital Image Correlation software and applying an inverse method, namely the Virtual Fields Method, to process the resulting displacement/strain maps. For this purpose, a new apparatus is designed to be adapted to a conventional tensile machine. This apparatus enables us to obtain simultaneously uniaxial tension, pure shear and equibiaxial tension, using only one sample. The heterogeneity of the kinematic fields induced by the test is first discussed in relation to two criteria. The main features of the identification method are then presented, and results provided by a test performed on an elastomeric material are discussed in the context of hyperelasticity

A model to simulate the gravitropic response and internal stresses in trees, considering the progressive maturation of wood

Trees-Struct. Funct. ISI Document Delivery No.: AM5EH Times Cited: 0 Cited Reference Count: 21 Pot, Guillaume Coutand, Catherine Toussaint, Evelyne Le Cam, Jean-Benoit Saudreau, Marc Auvergne Regional Council; European Regional Development Fund This work was supported by a grant from the Auvergne Regional Council and the European Regional Development Fund. Springer New yorkInternational audienceThe developed model of gravitropism takes non-instantaneous maturation of wood into account which enabled to correctly simulate different gravitropic phases and realistic internal stress profiles. A new biomechanical model of tree movement in relation to gravity (gravitropism) is proposed in this study. The modelling of the progressive maturation of wood cells is taken into account, as well as spatio-temporal variations in maturation strains (MS) and mechanical properties. MS were identified using an inverse method that allows the model to fit the gravitropic reaction observed experimentally. For this purpose, the curvature during righting movement, the geometry and the mass distribution of a two-year-old poplar tree was measured. The identified MS are higher than expected, which shows the underestimation of MS by usual measurements. By using the same mechanical parameters and MS as an input, the model gives satisfying results in terms of shape modelling for different trees up to 32 days after tree tilting. The model is able to simulate the latency phase observed in the tree righting movement, and the internal stress profile in the trunk is realistic (low compressive value in the central part of the trunk and zero stress in newly formed cells). The next development of the model will aim to simulate the end of the gravitropic phase in relation with the regulation of MS by the tree

Heat and strain measurements at the crack tip of filled rubber under cyclic loadings using full-field techniques

International audienceThis study aims at characterizing heat sources during the deformation of the crack tip zone in carbon black filled Styrene Butadiene Rubber (SBR). For this purpose, the thermomechanical response of cracked specimens was investigated using coupled full thermal and kinematic field measurements and a suitable motion compensation technique. The kinematic analysis enabled us to define the zone of influence of the crack and to measure the maximum stretch ratio level. The maximum stretch ratio level at the crack tip is higher than that measured at specimen failure during uniaxial tensile tests, which can be explained by considering the maximum chain extensibility. The calorimetric analysis shows that the high heat source gradient zone is very much more confined than the high temperature gradient zone. The heat sources at the crack tip remain positive and small during unloading, which indicates that mechanical dissipation is high and confined to the crack tip. This result highlights that the material behaves very differently in the crack tip zone compared to homogeneous tests. This proves that it not possible to predict the behavior of the crack tip zone from homogeneous tests. Moreover, it is observed that the mechanical dissipation decreases with the number of first cycles, which highlights the fact that the material is increasingly accommodated. This study provides the first accurate measurement of heat sources at the crack tip of rubber, constituting a new experimental tool in the fracture mechanics of rubber

The impact of long-term water stress on tree architecture and production is related to changes in transitions between vegetative and reproductive growth in the ‘Granny Smith’ apple cultivar

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitièresInternational audienceWater stress (WS) generates a number of physiological and morphological responses in plants that depend on the intensity and duration of stress as well as the plant species and development stage. In perennial plants, WS may affect plant development through cumulative effects that modify plant functions, architecture and production over time. Plant architecture depends on the fate of the terminal and axillary buds that can give rise, in the particular case of apple, to reproductive or vegetative growth units (GUs) of different lengths. In this study, the impact of long-term WS (7 years) on the fate of terminal and axillary buds was investigated in relation to flowering occurrence and production pattern (biennial vs regular) in the ‘Granny Smith’ cultivar. It was observed that WS decreased the total number of GUs per branch, regardless of their type. Conversely, WS did not modify the timing of the two successive developmental phases characterized by the production of long and medium GUs and an alternation of floral GUs over time, respectively. The analysis of GU successions over time using a variable-order Markov chain that included both the effects of the predecessor and water treatment revealed that WS reduced the transition towards long and medium GUs and increased the transition toward floral, short and dead GUs. WS also slightly increased the proportion of axillary floral GUs. The higher relative frequency of floral GUs compared with vegetative ones reduced the tendency to biennial bearing under WS. The accelerated ontogenetic trend observed under WS suggests lower vegetative growth that could, in turn, be beneficial to floral induction and fruit set