Philippe Viot

Viscoelastic properties of the human sternocleidomastoideus muscle of aged women in relaxation.

Improving the numerical models of the head and neck complex requires understanding the mechanical properties of the muscles; however, most of the data in existing literature have been obtained from studies on animal muscles. Muscle is hyper-elastic, but also viscoelastic. The hyper-elastic behaviour of the human sternocleidomastoideus muscle has been previously studied. The aim of this study is to propose a characterization of the viscoelastic properties of the same human muscle in relaxation. Ten muscles were tested in vitro. The viscoelastic behaviour was modelled with a generalized Maxwells model studied at the first and second order, using an inverse approach with a subject-specific, finite-element model of each muscle. Based on these models, relaxation times τ (first order: 103s; second order: 18s and 395s) and ratio moduli γ (first order: 0.33; second order: 0.20 and 0.19) were identified. The first-order model provided a good estimate of the relaxation curve (R(2): 0.82), but the second-order model was more representative of the experimental response (R(2): 0.97). Our results provide evidence that the viscoelastic behaviour of the human sternocleidomastoideus muscle can be described using a second-order Maxwells model and that - combined with the previously identified hyper-elastic properties - the response of the muscle in tension and relaxation is fully characterized.

Foaming of amorphous polymers and blends in supercritical CO2: Solubility versus block copolymers addition

Supercritical CO2 (scCO2) is used as a medium for foaming amorphous polymers. Astudy of the solubility of supercritical CO2 in several amorphous polymers (PS andPMMA) and blends is performed, followed by an investigation of the foaming behavior of the polymer–gas systems. Nano-structuring triblock copolymers (styrene-co-butadiene-co-methylmethacrylate SBM and methylmethacrylate-co-butylacrylate-co-methylmethacrylate MAM) were blended as additives to PS or PMMA by extrusion. The addition of these triblock copolymers results in an important weight gain ratio of gas in a wide range of temperatures (from 25 to 80°C), relating this weight gain ratio to the foaming behavior of the blends (CO2 is preferentially located in the micro or nano-domains issued from the structuration of the block copolymer). Foaming is carried out in a batch one-step scCO2 process, keeping constant the saturation pressure and depressurization rate (300 bar and 60 bar/min, respectively). Influence of saturation temperature (25–85 °C) on the final porous structure is shown. In spite of the influence of the terpolymer on the weight gain ratio, the structuration is believed to provide a good control of microcellular foams in PS and PMMA.

Foaming Behaviour and Compressive Properties of Microcellular Nanostructured Polystyrene

A batch foaming process has been employed to obtain microcellular materials from polystyrene plus a SBM copolymer (polystyrene-co-1,4-polybutadiene-co-poly(methyl methacrylate). In the first part of the process, raw materials were mixed and extruded in a proportion 90:10 to obtain the precursor materials, leading to a nanostructured assembly in which SBM self-organizes in the polystyrene matrix. In a second stage, foaming was carried out by means of supercritical CO2 in a single step-process. Foamed samples were produced using a technique based on the saturation of the polymer under scCO2, and final properties were controlled by varying the temperature. The swelling in scCO2 was performed at 300 bar during 16 h, and subsequently releasing the gas with a fixed depressurization rate of 60 bar/min. Temperature was varied from 30 °C to 80 °C, leading to densities from 1.0 g/cm3 to 0.5 g/cm3 and cell sizes from 2 micron to 100 micron. In this part of the work, a comparison between foaming behaviour of neat PS and a nanostructured PS+SBM blend is reported, investigating the role of the nanostructured phase as nucleating agents for microcellular foaming. Finally, low rate compression tests were carried out, analyzing the dependence of mechanical parameters such as elastic modulus, yield stress and densification strain with density.

Block Copolymer-Assisted Microcellular Supercritical CO2 Foaming of Polymers and Blends

The behaviour in supercritical CO2 of block copolymers containing styrenic, butadiene, and methacrylic or perfluroalkyl blocks is studied in view of a specific swelling and foaming by a gas dissolution process. These block copolymers are considered as neat materials or as additives in blends e.g in polystyrene (PS) or polymethylmethacrylate (PMMA) matrices. In both cases (neat or blend) the copolymers may exhibit a structuration at a micro or nano level. The phase separated (nano) structures depend on the block type and the concentration of copolymers in the polymer matrix, so that micelles, vesicles, lamellas, or warm-like structures are generated. Furthermore when one block is chosen as a highly CO2-philic moiety, the nanostructures are able to act as CO2 reservoirs. The result is the possibility to control microcellular foaming, or sometimes nanocellular foaming, of commodity amorphous polymers such as PMMA and PS. Besides, at room temperature, the blocks can be either glassy or rubbery in order to freeze the growth and coalescence of cells during foaming. Different cellular polymers were elaborated by varying either the copolymer type or the foaming conditions (saturation pressure, temperature, depressurization rate). Cell sizes are accessible in a range from 0.2 to 200 μm, and densities from 0.40 to 1 g/cm3. It is also shown that nanostructuring polymers are also efficient to produce polymer foams with oriented / structured voids. This new approach could be used to produce nanocellular or ultra microcellular polymer foams in a simple process, using blending and extrusion.

Quantitative analysis of the deformation of polypropylene foam under dynamic loading

A dynamic crash loading experiment is performed on a polypropylene foam. Several interrupted shocks are conducted, in between which microtomographic acquisitions are made, showing the evolution of the sample during its compression. This data can help construct and validate predictive models, although, because this material is multiscale (consitutive grains at the mesoscopic scale are made of microscopic closed cells), image processing is required to extract useful quantitative measures. Such processing is described here, so as to determine a representative volume for each grain of the sample, in order to associate to each grain and to each stage of the compression values such as grain density. This can help build a predictive model at the mesoscopic scale.

Experimental characterization of post rigor mortis human muscle subjected to small tensile strains and application of a simple hyper-viscoelastic model.

In models developed for impact biomechanics, muscles are usually represented with one-dimensional elements having active and passive properties. The passive properties of muscles are most often obtained from experiments performed on animal muscles, because limited data on human muscle are available. The aim of this study is thus to characterize the passive response of a human muscle in tension. Tensile tests at different strain rates (0.0045, 0.045, and 0.45 s−1) were performed on 10 extensor carpi ulnaris muscles. A model composed of a nonlinear element defined with an exponential law in parallel with one or two Maxwell elements and considering basic geometrical features was proposed. The experimental results were used to identify the parameters of the model. The results for the first- and second-order model were similar. For the first-order model, the mean parameters of the exponential law are as follows: Young’s modulus E (6.8 MPa) and curvature parameter α (31.6). The Maxwell element mean values are as follows: viscosity parameter η (1.2 MPa s) and relaxation time τ (0.25 s). Our results provide new data on a human muscle tested in vitro and a simple model with basic geometrical features that represent its behavior in tension under three different strain rates. This approach could be used to assess the behavior of other human muscles.

Experimental Study of Mouth Guards Response under Impact Loading

Mouthguard seems to be the best protection mean against dental injuries for sport application. The purpose of this study is to compare three different mouth guards under different condition of impacts closed to those observed on real-life shock loading. Specific devices were designed, firstly to pre-load and to form the mouthguard as it specified by the manufacturer; secondly to perform the different impacts on a very specific impact apparatus. Force and displacement were recorded as function of time. Those measurements were supplemented by a high speed video device to visualize the effect of the impact during the loading. A dissipated energy analysis was also performed.

Ex-Situ Study of Polymeric Syntactic Foams Mechanical Response Under Compression Loading: Effects of Foam Microstructure Using Microtomography Techniques

Syntactic foams are widely used in many impact-absorbing applications and can be employed as sandwich core. To improve their mechanical performances, these composite sandwich structures have to be modelled. This approach requires the characterisation of the foam behaviour. Moreover, the microstructure of the syntactic foam has an influence on its macroscopic behaviour; the foam density, the diameter of the porosities, their distribution in the material have to be taken into account.

Modelling of human muscle behaviour with a hyper-elastic constitutive law

Even though numerous studies have been performed assessing muscle mechanical properties, knowledge about this living material is still limited. Many authors have focused on the longitudinal behaviour of muscle by performing tension tests (Anderson et al. 2001; Dresner and Ehman 2001). Muscles tested were mainly animal muscles and muscle behaviour was modelled with rheological elements involving springs and dampers. In order to complete these results, the aim of this study was to assess human muscle behaviour in tension and to derive a hyper-elastic law from the experiments.

Dynamic energy release rate evaluation of rapid crack propagation in discrete element analysis

A numerical procedure for estimating the critical dynamic energy release rate (