R. Vassoille

Non-linear creep behaviour of PET

The non-linear creep behaviour has been studied on PET films at room temperature. A particular value of the stress, σc, was used to characterize the change between the linear to the non-linear domain. The variations of the elastic modulus, the relaxed modulus and σc revealed great sensitivity to the morphology of the crystallization. A molecular model of non elastic deformation, assuming (i) hierarchical correlated molecular motion, and (ii) nucleation and expansion of sheared-microdomains, was used to analyse the role of stress on anelasticity. To take into account the two-phase structure of semicrystalline films, a phenomenological series/parallel model was applied to express the mechanical coupling between amorphous and crystalline phases. Quantitative agreement was found between theoretical predictions and experimental data for low and high stresses. However, there was a discrepancy in the rate of recovery because the model predicts a strain recovery slower than the experimental behaviour. Consequently, it is proposed to develop further the molecular model mentioned above by specifying the energy profile of a sheared-microdomain and its stress dependence. Then, the difference between creep and recovery strain rate could be explained.

Mechanical properties of a hybrid-oxide glass

Abstract A sol-gel process has been successfully utilized to produced large bulk hybrid-oxide glasses from tetraethoxysilane (TEOS) or tetramethoxy silane (TMOS) incorporating polymeric components of 3-(trimethoxysilyl) propyl methacrylate and methylmethacrylate (MMA). All samples are transparent and without porosity. The morphology of hybrid-oxide glass looks like an oriented fibers composite materials. Mechanical tests show that hybrid-oxide glasses have intermediary behaviors between brittle and plastic materials. Particularly, the plasticity at room temperature of hybrid-oxide glasses has been observed.

Anelastic Behaviour of Ice at Low Temperature Due to Quenched Point Defects

ABSTRACT At low frequencies (~ 1 Hz) and under some conditions, the internal friction peak associated to the relaxation of water molecules in Ice exhibits an irreversibility with the temperature variation rate. This phenomena is described in terms of quenched rotationnal defects out of thermodynamic equilibrium. As a consequence of our results the anomalies observed by several authors on physical properties near 100-130 K are discussed in terms of evolution of defects out of thermodynamic equilibrium.