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Tensile yield stress measurement on polyvinylidene fluoride are presented covering as many as seven decades of strain-rates at temperatures from -50° to 150°C. The data have been analyzed on the basis of four Ree-Eyring processes acting in parallel, three of which have been identified. At high temperatures and low strain-rates, a threshold yield stress is observed. A model is proposed, allowing to describe the yield behavior both at the threshold and in its adjacent strain-rate dependent range. It consists in a modification of the Ree-Eyring theory using an unsymmetrical free energy barrier for the rate process. The data are fully consistent with the above model which implies that yielding occurs only when the stress is higher than a threshold. In this case, a rejuvenation (or de-aging) effect must be assumed in the deformation process which could consist in a melting of a chain segment located between two folds. © 1995 Steinkopff Verlag., SCOPUS: ar.j, info:eu-repo/semantics/published

Doctorat en Sciences, info:eu-repo/semantics/nonPublished

A differential equation for retarded compliance can be derived from a fractal rheological model. The equation can be generalized to account for aging prior to and during creep, and for non-linearity at high extensions. The generalized equation is shown to give an excellent description of aging in semi-crystalline polymers., SCOPUS: ar.j, info:eu-repo/semantics/published

A constitutive equation for viscoelastic behavior containing time derivatives of stress and strain to fractional order is obtained from a fractal rheological model. Equivalence between tree and ladder fractal models at long times is demonstrated. The fractional differential equation is shown to be equivalent to ordinary differential formulations in the case of a simple power-law response; the adequacy of such formulations to describe non-linearity has been demonstrated previously. The model gives a good description of viscoelastic behavior under all stress modes and will be extended in future to include aging effects. © 1994 Steinkopff-Verlag., SCOPUS: ar.j, info:eu-repo/semantics/published

A recent model which was found useful to account for the creep curve of polycarbonate is applied here to the loss curve of this polymer in the α transition range. The deformation behaviour is approached in a twofold manner: an analytical model and a ladder mechanical model both accounting for rejuvenation effects at high strains. The expression of the loss tangent is derived here resulting in a power law of the viscosity with an exponent equal to - 1/3. The viscosity itself is expressed as a function of the temperature Θ characterizing the structural state of the sample. The loss curves obtained on heating the sample at constant frequency up to Tg exhibit a plateau on the left side of the α peak which is attributed to a Θ decrease during the measurements; heating the sample in this temperature range includes an involuntary thermal treatment. The loss curve is rather well modelled here, so the proposed approach seems to be suitable at small strains when the structural state is affected by heating as well as under high stress when the structural state is affected by deformation., SCOPUS: ar.j, info:eu-repo/semantics/published

Two approaches are proposed to account for the creep curve of glassy polymers under high applied stress; one is analytical, the other is in the form of a ladder mechanical model. Both approaches consider that creep deformation induces the rejuvenation of the sample, giving rise to faster kinetics, and they predict an inflection in the creep curve. The related responses were found to fit experimental data better than the stretched exponential law, especially at high strain. The mechanical model must be preferred because it is hierarchical, which is useful for visualization and allows memory effects to be taken into account. © 1992 Steinkopff-Verlag., SCOPUS: ar.j, info:eu-repo/semantics/published

Loss curves at 1 Hz were measured for polycarbonate (PC) samples with various mechanical and thermal histories on heating from -50 to 140°C or on cycling within these limits. The α′ peak, on the left side of the α peak, was found to shift to higher temperatures with decreasing θ values of the samples and even to disappear from the heating loss curve as predicted by the model presented in the first part of this series of papers1. The left side of the α peak is correctly predicted in all cases, but an additional peak, called α″, which was not predicted, appeared to the right of the β peak for samples cooled from temperatures Ta ≧ Tg - 65 K or for quenched samples. The α″ peak was found to vanish on ageing the samples at room temperature and to be erased by plastic deformation. A new definition of ageing and annealing phenomena is advanced using the concept of structural temperature. The α″ and α′ peaks seem to be linked to ageing and annealing, respectively. © 1990., SCOPUS: ar.j, info:eu-repo/semantics/published

The two simplest models that can be put forward to account for the elasticity of composite materials are the Reuss model and the Voigt model in which the constituents undergo, respectively, the same stress or the same strain. Experimental measurements always fall between the values predicted by these models. We propose correcting the Reuss model by stating (Formula Presented.) σf and ϵm being the average stresses undergone, respectively, by reinforcing agent and the matrix. Similarly, we shall modify the Voigt model by supposing (Formula Presented.) σf and ϵm being the average strain undergone, respectively, by reinforcing agent and the matrix. K and L are interrelated tensors which depend on the nature of the reinforcing agent, on its possible orientation, and on the mechanical behavior of the interface and also on the moduli of the constituents. We have developed the equations for determining the tensors with regard to fiber composite, taking into account the characteristics of the fibers (length, diameter, orientation, interface). The evaluation of K and L enables us, therefore, to calculate the modulus or the compliance. Conversly, by measuring the modules or the complience, one can determine K or L and ,in this way, obtain data on the machnism of load transfer from the matrix to the reingforcing agent and thus on the behavior of their interface. The theoretical values of the Young modules calculated from our model are in good agreement with the experemental values obtained by Lees.8 Copyright © 1976 John Wiley & Sons, Inc., SCOPUS: ar.j, FLWNA, info:eu-repo/semantics/published

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The viscoelastic behaviour of three different samples of polycarbonate, with differing pre-treatment, is described here by a generalized non-linear Maxwell system, proposed previously. This model gives the variation of the tensile yield stress related to each sample as a function of temperature and strain rate. The response is checked over a large range of strain rates, at temperatures from 20 to 80° C. The model allows the determination of the damping peak arising from the molecular movements which it expresses. A correspondence is found with the intermediate loss peak location, which occurs for each sample just below the α transition. Results obtained from dynamic mechanical tests and differential scanning calorimetry show that the structure of the samples changes in the range of temperatures where the intermediate loss peak arises at 1 Hz. © 1979 Chapman and Hall Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

The tensile yield behaviour of three forms of polycarbonate differing in their thermal history is investigated, at constant strain rate, over a wide range of temperatures (-140° to + 80°C). The plot of the yield stress versus temperature exhibits a thermal history dependence over the whole range explored and reveals the β transition which is located at the same temperature Tβ for the three forms. The yield behaviour is described assuming that two activated processes α and β requiring additive stresses are involved. The β component of the yield stress is found to be insensitive to thermal history as well as the related loss peak. Only the α process is affected; details concerning this dependence are discussed. © 1983., SCOPUS: ar.j, info:eu-repo/semantics/published

Doctorat en sciences appliquées, info:eu-repo/semantics/nonPublished

The tensile stress-strain curve of a glassy polymer is compared to the response of a rheological system including a generalized non-linear Maxwell model. The treatment assumes that throughout the course of deformation, some structure initially present in the polymer is destroyed and that the initial spectrum of the Maxwell elements is converted to another spectrum. A law of transformation of these spectra as a function of deformation is proposed while their expression is assumed to be analogous to that of the spectrum of a structureless material, experimentally determined. Data are found to be fully consistent with the proposed treatment. © 1978 Chapman and Hall Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

A model is presented to describe the creep behaviour of glassy polymers below the glass transition temperature. It consists of a Hookean spring in series with a non-Newtonian dashpot having an entropy spring in parallel. The shape of the response of this spring is deduced from a master curve, giving the extension as a function of logarithm of time, built from creep data, reported here and obtained on polycarbonate over a wide range of times and temperatures. The model takes into account a number of aspects of creep behaviour and predicts a threshold stress beneath which delayed yielding no longer occurs. Torsional creep data, obtained on Polyvinylchloride by Mallon and Benham are found to be in excellent agreement with the proposed model. © 1975 Chapman and Hall Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

The tensile behaviour of four grades of asphalt is investigated at various temperatures over a wide range of strain rates. It is shown that their ultimate tensile stress in the ductile range can be properly described by the formalism of Eyring's theory of activated rate processes. © 1976 Dr. Dietrich Steinkopff Verlag., SCOPUS: ar.j, info:eu-repo/semantics/published

The loss curve of samples of polycarbonate characterized by different thermal histories was determined in the beta transition range. Peaks were found to be insensitive to thermal histories as was the case for the shear loss curve. The compression yield behavior was investigated at constant strain rate over a wide range of temperatures., SCOPUS: cp.p, info:eu-repo/semantics/published

Both the formation of free volume and the elementary motion characterizing the plastic deformation in the glassy state are described here by the same molecular process, consisting of the transition of a given number of chain segments from fundamental to activated states. Using this basic assumption, a model is proposed which makes use of classical laws of thermodynamics. Five parameters need to be adjusted to take into account the thermodynamic properties related to the free volume: one more is necessary to characterize the molecular process of plastic deformation. The pressure dependence of Tg and the difference between the specific volumes found in the glassy state from isothermal and isobaric measurements can be calculated from the model. Fairly good agreement is found between the theoretical and experimental values of these quantities. The physical meaning of constants belonging to a previously-proposed yield criterion is clarified, and these constants are given as a function of the molecular process. Moreover, the treatment confirms that the glass transition temperature is related to a constant fraction of activated segments and not to a constant value of the free volume. © 1980., SCOPUS: ar.j, info:eu-repo/semantics/published

This paper is concerned with a model which attempts to describe quantitatively, by the same elementary process, the yield behaviour above and below Tg, as well as the effect of annealing on the yield stress. This model links together theories we have previously proposed and relies on the following main assumptions: the deformation processes imply the cooperation of n activated segments and that the free energy increase of an activated segment depends on the structural state of the polymer. A satisfactory agreement is found with yield stress data on polycarbonate (PC), over a very large range of temperatures and strain rates. The correlation between the yield stress and the annealing treatment is also reasonable. © 1984., SCOPUS: ar.j, info:eu-repo/semantics/published

SCOPUS: ar.j, info:eu-repo/semantics/published

SCOPUS: ar.j, info:eu-repo/semantics/published

3e sciences appliquées toutes sections, info:eu-repo/semantics/published, 5

Sixième édition 1981-1982, 3ème Sc. appl. ttes. sect., info:eu-repo/semantics/published, 6

3e en sciences appliquées toutes sections, info:eu-repo/semantics/published, 2

3e en sciences appliquées toutes sections, info:eu-repo/semantics/published, 4

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Tensile creep tests and interrupted tensile tests were performed on poly(vinyl chloride) in order to check some consequences of a rheological model previously proposed to describe the creep behavior of glassy polymers in the linear as well as the nonlinear range of viscoelasticity. The results of the interrupted tensile tests allowed us to measure the stresses borne by the elastic elements as well as the recovery stresses related to the viscous elements of the model. The level found for these stresses did not agree with the values implied by the model. We propose here an improved model consisting of a nonlinear generalized Maxwell system. It takes into account the results of the tests and explains the viscous deformation by the destruction of an initial structure present in the polymer. © 1977, Taylor & Francis Group, LLC. All rights reserved., SCOPUS: ar.j, info:eu-repo/semantics/published

Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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