International audienceLinear viscoelastic material behavior is often modeled using a generalized Maxwell model. The material parameters, i.e. relaxation times and elastic moduli, of the Maxwell elements are determined from either a relaxation or a Dynamical Mechanical Analysis (DMA) experiments. The underlying mathematical problem is known to be ill-posed, which means that uniqueness of the identification is not assured and that small errors in the initial data will conduct to high discrepancies in the identified parameters. The standard technique to remove the ill-posedness is to chose a priori a series of relaxation times and to identify only the moduli. The aim of this paper is to propose two techniques to identify an optimal series of relaxation times. In the case of the relaxation experiment relaxation times will be optimized from the numerical integration of the measured relaxation spectrum. In the case of the DMA experiments we show that mathematical results obtained by Krein and Nudelmann can be used to determine the complete series of relaxation times. The methods are illustrated by identification examples using both artificial and experimental data. The results show that the methods provide a good match of the identified models in term of relaxation or complex moduli
International audienceHighly filled elastomers present strong nonlinear mechanical behavior. This study proposes a biaxial dynamic mechanical analysis (DMA) experiment to study the prestrain induced nonlinearity. This phenomenon has already been observed for uniaxial tests, revealing an increase of the amplitude of the dynamic modulus with prestrain. The novelty proposed here is to investigate the problem under biaxial conditions. For this purpose, a specific apparatus and an appropriate specimen have been designed. Strains and stresses have been measured using localization formulae and compared with measurements from digital image correlation and finite element computations. Biaxial DMA tests were performed on a propellant specimen, for different values of biaxial prestrain. The material is a highly filled elastomer with an important influence of the prestrain on the global viscoelastic behavior. The results exhibit increasing amplitude of the complex modulus with increasing prestrain, as in uniaxial experiments. Moreover, the dependence can be characterized using the second invariant of the prestrain, and the viscoelastic behavior is modeled using a closed-form spectrum of relaxation times
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