Hygrothermal effects and moisture kinetics in a bio-based multi-layered wall:Experimental and numerical studies

International audienceA bio-based multi-layered reference wall has been developed within the framework of the European ISOBIO project. One of the key points of this project was to be able to perform proper simulations of the hygrothermal transfers occurring inside such walls. Previous published investigations, also performed in the framework of this project, have demonstrated that the classic assumption of instantaneous equilibrium between local relative humidity and water content according to the sorption isotherm is not relevant for bio-based porous materials, where, in practice, a rather slow kinetics of sorption occurs. The theoretical background developed in this previous study is used here to determine the kinetic constants of the bio-based construction materials and to perform 1D hygrothermal simulations. The kinetics constants are determined thanks to a 1D cylindrical tool based on the local kinetics approach, validated against several experiments of sorption. Then, heat and hygric transfers recorded on a demonstrator building (The HIVE, Wroughton, UK) are analyzed and are simulated using two modeling tools: TMC based on the Künzel approach and TMCKIN based on the local kinetic approach. From the simulations, the local kinetics improves the small timescale RH dynamics. The comparison with measurements performed in the demonstrator confirms the relevance of the local kinetics approach

Determination of the elongational properties of polymers using a mixed numerical-experimental method

International audienceThe biaxial properties of materials such as rubbers or polymers are often difficult to identify, and generally a simple isotropic behaviour derived from classical 1D tensile test is considered. However, biaxial properties are useful for the simulation of plastic-processing operations such as blow moulding [1] or thermoforming. As previously reported, the rheological behaviour and the mechanical properties of rubbers and polymers can be obtained by using a bubble inflation rheometer [2-4], or multi-axial tensile test [5]. In this work, experimental data provided by optical measurements on tensile tests and bubble inflation tests are coupled with Finite Element Method simulations for identifying the rheological behaviour. This work is actually based on natural rubber and will be extend to thermoplastic (PP and PET) materials. A bubble inflation rheometer has been developed in the laboratory. It allows to blow under r r controlled pressure rubber or thermoplastic membranes [2]: a circular membrane, clamped at the rim, is inflated by applying air pressure to its bottom face (see Fig. 1, left). In the case of thermoplastic, a heating step is necessary before applying the pressure. The heating can be performed by air convection, by conduction (heating cartridge at the rim) and by IR radiation. FIGURE 1. Bubble inflation rheometer (detail, left) and shape contour extraction (right). Two experimental optical techniques based on non-contact measurement by CCD cameras have been developed for in situ measurements: (i) images acquisition of the 2D projection of the bubble is done during the inflation process, giving shape contours versus inflated pressure (see Fig. 1, right); (ii) Digital Image Stereo-Correlation (DISC) [6] is applied using a calibrated stereo rig in order to obtain the three-dimensional description of the strain fields on the surface of the bubble. f To perform DISC directly on the rubber bubble, several difficulties are to be solved (large level of deformation, semi-transparent aspect of the materials, t lighting, etc.). In addition, tensile tests have been performed using DISC due to the high level of strain. Tensile test performed on standard specimen give, on the one hand, the stress/strain curve. On th

Water Transport in Bio-based Porous Materials A Model of Local Kinetics of Sorption—Application to Three Hemp Concretes

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Elastomer biaxial characterization using bubble inflation technique. I: Experimental investigations

International audienceThe biaxial rheological behavior of materials such as elastomers or polymers can be obtained using a bubble-inflation-technique. A circular membrane is clamped at the rim and expanded under gas pressure. The inflation of the circular membrane is recorded using a CCD video camera and the blowing pressure by a pressure sensor. Then, from elongation and curvature radius measurements at the pole of the bubble, one can deduce equibiaxial stress-strain data. This study describes the optimization of a bubble-inflation rheometer. The most sensitive point of the technique is the estimation of the elongation at the bubble pole, deduced from video camera measurements. A direct measurement of the bubble thickness was performed using a magnetic probe in order to validate rheometer results. Such a validation has evidently never been carried out before. Results of quasi-static equibiaxial characterization of elastomers are presented and analyzed

Elastomer biaxial characterization using bubble inflation technique. II: Numerical investigation of some constitutive models

International audienceAn elastomeric material was investigated with a bubble inflation rheometer, and its mechanical behavior was modeled as a rubber-like solid. Classical strain energy functions were considered and the hyper-elastic constants were calculated by a direct identification procedure from simple uniaxial and equibiaxial extension test data, and the results are compared against those obtained by an inverse method from bubble inflation test data. The latter amounted to minimizing a cost function and matching the measured response to a finite element analysis solution, which depended on the unknown material parameters. The optimization employed the Levenberg-Marquardt algorithm and Abaqus software to compute the cost function and its gradients. The constants so obtained were further used in finite dement analysis, and the numerical results were compared with experiments. This study showed that the inverse method, used to estimate the material parameters, is a good alternative to the direct identification, especially since the latter often requires homogeneous strain state, which is very difficult to obtain

Effects of Initial Conditions on Simulations of Hygrothermal Transfers Through a Bio-Based Multi-layered Wall Subjected to a Real Climate

International audienceThis modeling study investigates the effects of the initial conditions on a bio-based multi-layered wall by hygrothermal simulations. Such a study cannot be found in the relevant literature. In a first step, the work consists in numerically generating different climate histories for the studied wall, using two different real climates: a winter and a spring climate. Then, the wall preloaded with these two different climate histories and therefore different hygric and thermal initial conditions is subjected to several cycle of 18 days of the spring climate. This study highlights the significant effects of the initial conditions on the subsequent simulations: strong differences are predicted about relative humidity and water content. Many cycles of 18 days of the spring climate have to be applied to obtain a hygric convergence of the two simulations, i.e., to erase the wall hygric history. The analytic analysis of the results leads to define characteristic times of hygric dependency of about 35–40 days. It appears that the hygric predictions of the simulations become independent of the initial conditions after about three times these characteristic times, i.e., about 3.5 months. Comparatively, the thermal history takes only a few days to be erased. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd

A model of local kinetics of sorption to understand the water transport in bio-based materials

International audienceThe classic models describing the hygric mass transfers inside porous materials seem unsuitable in the case of bio-based materials. They are based on the assumption of instantaneous local equilibrium between relative humidity and water content [1]. These two parameters evolve according to the diffusive fluxes following the sorption isotherms. This study shows that it leads to predict much shorter times of stabilization than those experimentally obtained. A new approach is presented here, it frees from the local instantaneous equilibrium introducing a local kinetics to describe the transformation of water from vapor state to liquid state and vice versa. The local kinetics of sorption is coupled with the well-known hysteresis phenomenon. It is adjusted from bibliographic data [2] giving mass evolution of three hemp concretes under adsorption / desorption conditions. 1D cylindrical simulations allows an excellent fitting on the experiments. Finally, a semi-empirical model is proposed, allowing to determine the kinetics parameters more easily. The effect of the local kinetics model on the hygrothermal transfers occurring through a bio-based wall is then studied