Wavet, a Custom Device Able to Measure Viscoelastic Properties of Wood Under Water-Saturated Conditions Up to 140 deg C (WAVET : Environmental Vibration Analyser for Wood)

This work presents an innovative experimental device conceived to characterize the time-dependent behavior of hygroscopic materials, as wood, at controlled moisture content and temperature. This device, the WAVET, permits the determination of the viscoelastic properties of samples, using harmonic tests at frequencies varying from 5.10-3 Hz to 10 Hz. Conceived to work up to 5 bar, it allows tests in dry or water-saturated conditions to be performed over the temperature range 0\degree C to 140\degree C. In spite of these severe working conditions, the careful device design, together with a rigorous data analysis, allows a rigorous determination of the storage and loss modulus and the loss factor. The results collected for several species of wood emphasize the ability of the WAVET to underline the influence of numerous parameters, namely specie, material direction, anatomical and macromolecular structure, on the rheological properties and notably on the softening temperature. The WAVET also establishes a efficient tool to follow the modifications of the constitutive amorphous polymers of materials submitted to hydrothermal treatments, in relation to the evolution of their viscoelastic properties.Comment: 3

Influence de traitements hygrothermiques sur les propriétés mécaniques de composites à fibres végétales = Effect of hygrothermal treatment on the mechanical properties of natural fibres composites

National audienceCe travail vise à évaluer les performances mécaniques et le vieillissement de biocomposites en vue d'une utilisation potentielle de ces matériaux prometteurs dans des applications structurales. 4 types de composites ont été fabriqués par compression à chaud à partir de matrices thermoplastiques organiques d'origines fossiles et biosourcées : Polypropylène (PP) et Acide Polylactique (PLA), et de fibres synthétiques et végétales : Verre et Chanvre. L'objectif est d'évaluer les performances rhéologiques du biocomposite PLA/Chanvre comparativement aux autres matériaux fabriqués. Les différents composites ont été soumis à des traitements hygrothermiques afin de reproduire un vieillissement environnemental accéléré. Les variations des propriétés physiques et mécaniques ont été suivies à l'aide de plusieurs techniques expérimentales : mesure des masses et des dimensions, essais de traction, analyse mécanique dynamique et observation en microscopie électronique. Le biocomposite PLA/Chanvre présente un pouvoir hygroscopique relativement important avec une teneur en eau à saturation supérieure à 9%, à l'origine de variations dimensionnelles non négligeables. L'eau joue également un rôle plastifiant, conduisant à une diminution des propriétés élastiques du matériau et à l'abaissement de la température de transition vitreuse. Enfin la présence d'eau entraîne la dégradation de l'adhésion fibrematrice, la microfissuration et l'hydrolyse de la matrice biosourcée, l'ensemble se traduisant par une chute sévère des performances mécaniques du matériau

Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the manufacturing oh high performance composites

In this paper, the thermo-mechanical behaviour of hemp fibres (Cannabis sativa L.) is investigated using a Dynamic Mechanical Analyser. Experiments are performed at a frequency of 1 Hz in the temperature range of 20 to 220\degree C. When a periodic solicitation is applied to an elementary fibre, an increase of the fibre rigidity and a reduction of the damping capacity are observed. These evolutions aim at stabilization after an identified number of cycles, traducing a phenomenon of "adaptation". This specific mechanical behaviour certainly involves biochemical and/or structural modifications in the material organisation as microfibrils reorientation. In addition, the behaviour of hemp fibres is affected by temperature. Temperature acts as an activation factor but also as a degradation factor of the viscoelastic properties of fibres. The rigidity and the endurance of fibres are highly affected by thermal treatment at temperature above 150\degree C to 180\degree C. Taking into account these results, polypropylene-hemp fibres composites were manufactured using a specific processing cycle. By respecting the integrity of fibres during manufacturing, it appears that the relatively high level of the specific mechanical properties of composites is really encouraging in sight of applications requiring high mechanical performances

Viscoelastic properties of wood across the grain measured under water-saturated conditions up to 135\degree C: evidence of thermal degradation

In this paper, the viscoelastic properties of wood under water-saturated conditions are investigated from 10\degree C to 135\degree C using the WAVET* apparatus. Experiments were performed via harmonic tests at two frequencies (0.1 Hz and 1 Hz) for several hours. Four species of wood were tested in the radial and tangential material directions: oak (Quercus sessiliflora), beech (Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When the treatment is applied for several hours, a reduction of the wood rigidity is significant from temperature values as low as 80-90\degree C, and increases rapidly with the temperature level. The storage modulus of oak wood is divided by a factor two after three hours of exposure at 135\degree C. This marked reduction in rigidity is attributed to the hydrolysis of hemicelluloses. The softening temperature of wood is also noticeably affected by hygrothermal treatment. After three short successive treatments up to 135\degree C, the softening temperature of oak shifted from 79\degree C to 103\degree C, at a frequency of 1 Hz. This reduction in mobility of wood polymers is consistent with the condensation of lignins observed by many authors at this temperature level. In the same conditions, fir exhibited a softening temperature decreasing of about 4\degree C. In any case, the internal friction clearly raises

Mechanical behavior of hemp fibres : a review of recent works

International audienceThis work proposes a review of literature on the mechanical behaviour of hemp fibres. The paper is focused both on experimental results and modelling

Mechanosorptive creep in single hemp fibres

International audienceThe literature on the time-dependent behaviour of single bast fibres such as flax and hemp is extremely poor. This is however particularly important in view of the development of models able to predict the long-term behaviour of plant fibres and plant fibre composites (PFCs). So, the aim of this study is to characterise the creep behaviour of elementary hemp fibres, and particularly the influence of relative humidity on it. Single hemp fibres are shown to exhibit both instantaneous deformation and delayed, timedependent deformation when tensile loaded. The creep behaviour appears to be a logarithmic function of time with a high deformation rate during the primary creep and a lower and constant one during the secondary creep. The creep rate is also highly influenced by the humidity and humidity variations. Much greater creep in cyclic humidity conditions than in a constant environment at the high-humidity is observed only for high rates of relative humidity variation. This mechanosorptive effect is consistent with sorption-induced stress-gradient explanations

Influence de la température sur les propriétés mécaniques et l'endommagement d'un composite carbone-thermoplastique

International audienceCet article propose une caractérisation thermomécanique d'un matériau composite à matrice organique thermoplastique thermostable renforcé par fibres de carbone. Les caractéristiques thermiques ainsi que les propriétés mécaniques sont étudiées sur une gamme de température s'étendant de l'am biante à 200°C. Un suivi de l'endommagement sous sollicitation thermomécanique est réalisé avec une technique de contrôle non destructif : l'émission acoustique. Cet article met en évidence l'influence de la température sur la résistance ultime et le mode de rupture du matériau sous sollicitations mécaniques dans le sens des fibres. Ces variations sont attribuées à la modification des forces d'adhésion à l'interface fibre/matrice. La ténacité à la rupture interlaminaire est également largement affectée par la température. Le taux de restitution d'énergie critique en mode 1(G1c) diminue d'environ 60% entre l'ambiante et 110°C. Ce travail montre enfin que la signature acoustique des différents modes d'endommagement est sensiblement affectée par la température

WAVET*, a custom device able to measure viscoelastic properties of wood under water saturated conditions (*WAVET : Environmental Vibration Analyser for Wood)

International audienceThis work presents an original experimental device conceived to characterise the viscoelastic properties of wood. Classically, the dynamic mechanical analysis of wood is performed using a commercial apparatus like a DMA (Dynamic Mechanical Analyser). However, when analysing wood with this type of apparatus, many problems related to the hygroscopic behaviour and the orthotropic structure of wood may be encountered. This is why an original apparatus perfectly adapted to the wood features has been developed. The WAVET is able to measure the viscoelastic properties of wood samples under water-saturated conditions, in the temperature range of 5°C to 95°C at frequencies varying between 0.005 Hz and 10 Hz. Samples are tested in a cantilever configuration. The whole experiment has been designed to withstand the severe conditions of temperature and humidity. At the same time, an analytical model based on Kelvin's elements has been developed. This model is able to correct experimental measurements performed close to the resonance frequency. Results obtained for beech samples in radial and tangential directions using the WAVET and a commercial apparatus (DMA 2980 TA Instruments) are compared and discussed. This comparison underlines the relevance of the WAVET device

Online segmentation of acoustic emission data streams for detection of damages in composites structures in unconstrained environments

International audienceAn approach for unsupervised damage detection in ring-shaped Organic Matrix Composites (OMC) under loading based on acoustic emissions (AE) is proposed. It relies on a specific clustering algorithm called Gustafson-Kessel (GK) that manages fuzzy memberships to clusters and complex cluster's shape. A methodology is proposed to 1) make the algorithm robust to initialisation in order to obtain reproducible results and reliable statistical models representing OMC damages, 2) detect and assess AE activity (AEA) over time for AE data mining to emphasize the more relevant AE data in a huge amount of AE hits, 3) adapt the statistical models based on statistical process control using imprecise updating rate automatically tuned

From fibre extraction to the composite manufacturing processes: Which path to adopt to maximise the mechanical properties of natural fibre based composites?

This work proposes to analyse the different aspects that should be taken into account from the fibre extraction from plants to the forming process to achieve correct part forming. The study will focus in a first extent on the impact of the textile operations leading to the yarn manufacturing. The properties of the flax fabric will then be associated to its behaviour during experimental forming and related to the possible appearence of defects such as tow buckling and solutions to prevent its appearance is widely discussed