International audienceMany soft tissues are naturally made of a matrix and fibres that present some privileged directions. They are known to support large reversible deformations. The mechanical behaviour of these tissues highlights different phenomena as hysteresis, stress softening or relaxation. A hyperelastic constitutive equation is typically the basis of the model that describes the behaviour of the material. The hyperelastic constitutive equation can be isotropic or anisotropic, it is generally expressed by means of strain components or strain invariants. This paper proposes a review of these constitutive equations

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An initially austenitic polycrystalline Ti-50.8 at.% Ni thin-walled tube with small grain sizes has been deformed under tension in air at ambient temperature and moderate nominal axial strain rate. Temperature and strain fields were measured using visible-light and infrared digital cameras. In a first apparently elastic deformation stage, both strain and temperature fields are homogeneous and increase in tandem. This stage is followed by initiation, propagation and growth of localized helical bands inside which strain and temperature increases are markedly higher than in the surrounding regions. During the first apparently elastic stage of the unloading, both strain and temperature fields are homogeneous and decrease. The temperature and strain fields evolutions are then analysed in order to determine the deformation mechanisms (types and extents of phase transformations, variants (de)twinning, macroscopic banding) involved during the homogeneous and heterogeneous stages of deformation throughout the whole tube. The findings have significant implications for the understanding and modelling of superelastic behaviour of NiTi shape memory alloys.

Compression mouldings of commercial SMC were performed with an instrumented industrial press under various process conditions. Results underline the influence of process parameters such as the initial SMC temperature, the axial punch velocity and the geometry of the mould on local normal stress levels. They also show negligible fibre-bundle segregation in the principal plane of the moulded parts. Thereby, a one-phase plug flow shell model is proposed as a direct extension of the plug flow model proposed by M.R. Barone and D.A. Caulk [J Appl Mech 53(191):1986;361-70]. In the present approach, the SMC is considered as a power-law viscous medium exhibiting transverse isotropy. The shell model is implemented into a finite element code especially developed for the simulation of compression moulding of composite materials. Simulation and experimental results are compared, emphasizing the role of the SMC rheology on the overall recorded stress levels. Despite the simplicity of the model, rather good comparisons are obtained.

This study is concerned with the experimental characterization of anisotropy induced by the Mullins effect in a particle-reinforced silicone rubber. Experimental data concerning the influence of type and direction of initial loading on the subsequent stress softening are quite scarce. In this scope, a set of experimental tests were carried out on a filled silicone rubber. Uniaxial tensile tests and bulge tests were used to precondition the samples, i.e., to induce some primary stress softening. In both cases, subsequent uniaxial tensile tests were conducted on preconditioned specimens. The first set of experiments consists of a uniaxial tension path followed by uniaxial tension along different directions. It appears that the stress softening varies from a maximum in the same direction load to a minimum in the orthogonal direction, with respect to the first tensile load direction. Next, the bulge test is proposed as an original way to yield very different biaxial tensile strain-histories for first loading path. The fact that the biaxiality ratio varies from the pole (uniaxial tension) until the bulge border (planar tension), permits to analyze second tensile load curves in a material that experienced a more complex first load path. These experimental data allow to discuss the most appropriate criteria to describe the strain-induced anisotropy phenomenon.

To improve the knowledge on the rheology of sheet molding compounds (SMC) during compression molding, a specific rheometer was designed, allowing to perform homogeneous experiments on SMC specimen under various mechanical loading, strain rates and fiber contents. Results gained during experiments underlined the key role played both by the strain rate and the fiber content. A viscous and transversely isotropic model was then proposed and used to fit stress levels recorded during the experiments. This model that requires few constitutive parameters rather well describes the main features of SMC rheology.