Vincent Placet

Viscoelastic properties of green wood across the grain measured by harmonic tests in the range 0–95°C: Hardwood vs. softwood and normal wood vs. reaction wood

Abstract The viscoelastic properties of wood have been investigated with a dynamic mechanical analyser specifically developed for wooden materials, the WAVET device. Measurements were carried out on four wood species in the temperature range 0–100°C at frequencies varying between 5 mHz and 10 Hz. Wood samples were tested under water-saturated conditions in the radial and tangential directions. As expected, the radial direction always revealed a higher storage modulus than the tangential direction. Great differences were also observed in the loss factor. The tanδ peak and internal friction were higher in the tangential than in the radial direction. This behaviour is attributed to the fact that anatomical elements act as a function of the direction. The viscoelastic behaviour of reaction wood differs from that of normal or opposite wood. Compression wood of spruce, which has a higher lignin content, is denser and stiffer in transverse directions than normal wood, and has a lower softening temperature (T g). In tension wood, the G-layer is weakly attached to the rest of the wall layers. This may explain why the storage modulus and softening temperature of tension wood are lower than those for opposite wood. We also demonstrate that the time-temperature equivalence fits only around the transition region, i.e., between T g and T g+30°C. Apart from these regions, the response of wood reflects the combined effects of all its constitutive polymers, so that the equivalence is no longer valid.

Concomitant changes in viscoelastic properties and amorphous polymers during the hydrothermal treatment of hardwood and softwood.

The aim of this study was to understand how the molecular structures of amorphous polymers influence wood viscoelastic properties. Wood from oak and spruce was subjected to hydrothermal treatments at 110 or 135 degrees C. Wood rigidity, reflected by the wood storage modulus, showed different modification patterns according to the wood species or the temperature level. Because viscoelasticity is dependent on wood amorphous polymers, modifications of lignins and noncellulosic polysaccharides were examined. Hemicellulose degradation occurred only at 135 degrees C. In contrast, lignins displayed major structural alterations even at 110 degrees C. In oak lignins, the beta-O-4 bonds were extensively degraded and wood rigidity decreased dramatically during the first hours of treatment. Spruce lignins have a lower beta-O-4 content and, relative to oak, the wood rigidity decrease due to treatment was less pronounced. Wood rigidity was restored to its initial value by prolonged treatment, probably due to the formation of condensed bonds in cell wall polymers.

Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations

Abstract The aim of this study is to achieve a better understanding of the nonlinear tensile behaviour of the elementary hemp fibre. This is of great importance in view of the need to develop an efficient predictive tool for the design of natural fibre reinforced composites. This first paper investigates the possible mechanisms responsible for the nonlinear behaviour, using repeated progressive tensile loading with in situ polarised light microscopy. The persistence of residual strain has been confirmed during testing when the tensile load was released. Only a certain fraction of this residual strain is reversible, and the reversibility is time-dependent. Beyond the yield level, the fibre’s rigidity is not deteriorated, but significantly increased as a function of the number of loading cycles and the level of strain. A new scenario involving a stick–slip mechanism, extension and re-orientation of the microfibrils and shear strain-induced crystallisation of the amorphous cellulose is proposed.

Unsupervised Consensus Clustering of Acoustic Emission Time-Series for Robust Damage Sequence Estimation in Composites

This paper suggests a new approach for unsupervised pattern recognition in acoustic emission (AE) time-series issued from composite materials. The originality holds in the development of a clustering ensemble method able to emphasize sudden growths of damages in composites under solicitations. The method combines multiple partitions issued from different parameterizations, initial conditions, and algorithms. A first stage automatically selects multifarious subsets of features based on the entropy of sequences of damages detected by clustering. A polygonal representation of the sequences is suggested to emphasize the kinetics of fracture events. The second stage allows estimating the optimal number of clusters necessary to represent the structure of the AE data stream. The data structure is estimated by consensus clustering with bootstrap ensembles, which allows estimating the uncertainty envelopes of each cluster and giving access to an interval of cumulated loading thresholds necessary to activate a particular damage. A qualitative evaluation phase is proposed on simulated data sets to statistically assess and underline both the robustness and accuracy of the proposed clustering fusion method, comparing

Creep behaviour of single hemp fibres. Part II: Influence of loading level, moisture content and moisture variation

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Diglycidylether of iso-eugenol: a suitable lignin-derived synthon for epoxy thermoset applications

-means, Gustafson-Kessel algorithm, and Hidden Markov models. An application is then presented for the detection of early signs of failure in high-performance carbon fiber-reinforced thermoset matrix composites dedicated to severe operating conditions. Despite the complexity of the configuration (ring-shaped specimens and high emissivity), it is demonstrated that the method emphasizes damage onsets and kinetics (fiber tow breakage, hoop splitting, and delamination) within the unevenly spaced AE time-series recorded during loading.

Development and characterization of a human dermal equivalent with physiological mechanical properties.

This work investigates the tensile creep behaviour of single hemp fibres under constant and cyclic loading coupled to constant or variable moisture content environment. Results show that the primary creep strain rate of such fibres decreases with the increasing stress, while the secondary creep strain rate increases. Load cycling at an average load higher than constant creep load produces a large additional extra creep strain and an increase of the creep rate. Both primary and secondary creep strain rates increase with the increasing moisture content. More creep is also observed in cyclic humidity conditions than in a constant environment at the high-humidity. In agreement with some observations on synthetic fibres, we showed that this accelerated creep is only observed for high moisture cycling rates. This mechanosorptive effect is consistent with sorption-induced stress-gradient explanations proposed in literature.

Experimental Investigations on Viscoelastic Properties of a Shape Memory Polymer

A novel lignin-based synthon, diglycidylether of iso-eugenol (DGE-isoEu) is used as a prepolymer for the preparation of thermosetting resins. DGE-isoEu is synthesized in a two-step procedure with a satisfactory yield from bio-based iso-eugenol (isoEu, 2-methoxy-4-(1-propenyl)phenol) catalytically fragmented from lignin in an organosolv process. DGE-isoEu was fully characterized by NMR, MS and FTIR. Curing of the DGE-isoEu monomer has then been investigated in the presence of several carboxylic acid derivatives hardeners. The thermal and mechanical properties of each material were recorded showing, in particular, a high Tg and instantaneous modulus values in the range of 78–120 °C and 4.6–5.5 GPa, respectively. The lignin derived new materials give very attractive thermo-mechanical properties comparable to that of common BPA-containing epoxy resins.

Creep behaviour of single hemp fibres. Part I: viscoelastic properties and their scattering under constant climate

Different models of reconstructed skin are available, either to provide skin wound healing when this process is deficient, or to be used as an in vitro model. Nevertheless, few studies have focused on the mechanical properties of skin equivalent. Indeed, human skin is naturally under tension. Taking into account these features, the purpose of this work was to obtain a cellularized dermal equivalent (CDE), composed of collagen and dermal fibroblasts.

Industrial Hemp Transformation for Composite Applications: Influence of Processing Parameters on the Fibre Properties

The shape memory polymers (SMPs) are polymeric smart materials which have the remarkable ability to recover their primary shape from a temporary one under an external stimulus. The study deals with the synthesis and the thermo-mechanical characterization of a thermally-actuated SMP, the tBA/PEGDMA, with a special focus on viscoelastic properties. The mechanical characterization is performed using three kinds of tests: quasi-static tensile tests, dynamic mechanical analysis (DMA) and modal tests. The first one allows the identification of the Youngs modulus and the Poisson’s ratio at ambient temperature. Modal analyses are done for various temperature values, and resonance frequencies are measured. In order to validate the time-temperature equivalence on this SMP, a DMA is performed under harmonic loading for different temperatures and a master curve highlights a complementarity of the results. Finally a suitable model for the viscoelastic behavior of the SMP is identified.Copyright