The transverse and longitudinal elastic constants of pulp fibers in paper sheets (original) (raw)
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On the determination of the elastic constants of carbon fibres by nanoindentation tests
Carbon, 2021
Nano-indentation instrumented tests are carried out at shallow depths on PAN-based and MPP-based carbon fibres. Indentation moduli are obtained by performing the tests at ten different measured orientations with respect to the fibre axis. They are used to identify the elastic constants of the fibres, assuming a transversely isotropic behaviour, by minimising a cost function between measured and estimated values. Inconstancies between the identified in-plane shear and transverse moduli and reported literature values are pointed out, and some drawbacks of the nano-indentation method are highlighted. An improved method taking into account the buckling mechanisms of crystallites at stake during the indentation process, and visible in the hysteretic behaviour of force-penetration nanoindentation curves, is proposed. It allows to identify values of elastic constants that are in accordance with literature values. These elastic properties of carbon fibres are in turn used to estimate the elastic properties of epoxy matrix composites containing these fibres. Very good agreement is found with experimentally available values of unidirectional ply properties. An excellent correlation between experiments and Finite Element Analyses of the indentation response of carbon fibres is eventually found.
Elastic properties of cellulose nanopaper
Cellulose, 2012
Nanopaper is a transparent film made of network-forming nanocellulose fibers. These fibers are several micrometers long with a diameter of 4-50 nm. The reported elastic modulus of nanopaper often falls short of even conservative theoretical predictions based on the modulus of crystalline cellulose, although such predictions usually perform well for other fiber composite materials. We investigate this inconsistency and suggest explanations by identifying the critical factors affecting the stiffness of nanopaper. A similar inconsistency is found when predicting the stiffness of conventional paper, and it is usually explained by the effects introduced during drying. We found that the effect of the drying cannot solely explain the relatively low elastic modulus of nanopaper. Among the factors that showed the most influence are the presence of non-crystalline regions along the length of the nanofibers, initial strains and the threedimensional structure of individual bonds.
Cellulose, 2020
A wide variety of wood and non-wood cellulosic fibre sources were used as a feed to produce microfibrillated cellulose (MFC) using a grinding process. Nanopaper was formed using this product, and the tensile index was measured. The hemicellulose content of the feed fibres was measured, and was found to correlate with the production of finer microfibrils and a higher MFC tensile strength. The correlation with tensile strength was improved by the inclusion of a measurement of the MFC particle lengths as measured by a fibre image analyser, with the resulting relation fitting a modified Page Equation. It was hypothesised that the frequency of flaws in the feed fibre cross-section influences the length of the MFC particles produced, and so the zero-span tensile index of the fibres was measured as a proxy for this since it forces cross-sectional fibre breakage. The fibre zero-span tensile index was found to correlate with MFC particle length and so was used in its place in the equation. T...
A technique to measure strain distributions in single wood pulp fibers
1996
AMTRAcr Environmental _-!mL'1£ electron microscopy (ESEM) and digital imase correlation (DIC) ~ used to measure micros1r8in distributions on the surface ofwood pulp fibers. A loadina stage incorporating a fiber lriPPinl system wu desianed and built by the authon. Fitted to the tensile substaae of an ESEM or a Polymer Laboratories MINlMATtester, it provided a reliable fiber straining mechanism. Black spruce lateWood fibers (Picea nwriana (Mill) B.S.P.) of a near-zero microfibril anile displayed a characteristically linear load elonption fonn. ESEM wu able to provide real-time, hi&h mapification images of strainina fibers, crack growth, and complex sinale fiber failure mechanisms. Digital images of sinale fiben ~ also captured and used for subsequent DIC-based strain analysis. Surface displacement and strain maps revealed nonunifonn strain distributions in seemingly defect-free fiber regions. Applied teDsile displacements resulted in a strain band phenomenon. Peak strain (concentration) values within the bands ranIed from O.9CMI to 8.8%. It is hypothesized that this common pattern is due to a combination offacton including the action ofmicrocompressive defects and strainina of amorphous cell-wall polymeric components. Strain concentrations also corresponded well to locations of obvious strain risers such as visible cell-wall defects. Results suaest that the ESEM-bued DIC system is a useful and accurate method to assess and, for the fint time, measure fiber micromechanical properties.
Nanoindentation of bleached and refined pulp fibres
In this study, an attempt has been made to determine the mechanical properties of the wood pulp cell wall after pulping, bleaching and refining by means of nanoindentation in comparison with flax and lyocell fibres. Based on the indents performed on single fibre cell wall cross-sections, it has been found that bleaching and refining processes are causing reduction in indentation modulus and hardness values. It is assumed that the reduction is attributed to the change in lignin content as well as microfibril angle. Surprisingly, refined pulp fibres and lyocell have the same hardness and indentation modulus, which conforms that no residual lignin is present in the refined pulp. The bast fibre, flax, extracted without Kraft cooking revealed higher indentation modulus (23 GPa) and hardness (0.50 GPa) compared to wood pulp fibres.
Full Set of Elastic Constants of Spruce Wood Cell Walls Determined by Nanoindentation
In the last years Nanoindentation (NI) has become a frequently used tool in material science. Since introduced NI in wood science for the characterisation of micromechanical properties, the validity of the obtained data and their significance is a matter of debate. As NI theory was initially developed exclusively for homogenous and isotropic materials, the interpretation of results from heavily anisotropic wood cell walls is not straightforward. Therefore the indentation modulus M (also called reduced modulus) typically determined by NItests is not comparable to the longitudinal elastic modulus obtained with other experimental techniques. As a consequence, NI on wooden cell walls has been mainly used for comparative purposes. In order to overcome this limitation, we present a new approach capable of identifying the stiffness tensor components of the secondary cell wall S2 for quantitative purposes.
Polymer, 2008
Regenerated cellulose fibres of different origin were tested in tension and by means of nanoindentation. Their degree of molecular orientation was characterised with birefringence measurements. An empirical relationship was set up between the degree of orientation expressed by birefringence and the modulus of elasticity parallel to the fibre direction, and the ratio between this modulus and the modulus transverse to the fibre direction, respectively.
Changes in moisture content of single pulp fibers have an immense influence on the behavior of paper and paper products. Here, an atomic force microscopy (AFM)-based method is applied to investigate the viscoelastic properties of pulp fibers at varying relative humidity (RH) in the transverse direction. Pulp fibers have not only anisotropic properties, but also a very rough surface due to their hierarchical structure. For this reason, we have developed a specific load schedule for the AFM-based test method to overcome uncertainties and limitations due to surface roughness of the pulp fibers. The evaluation of the experimental data combines contact mechanics and viscoelastic models which consist of springs and dashpots in series or parallel describing elastic and viscous behavior. Here, it will be demonstrated that the so-called Generalized Maxwell (GM) model yields comparable results for single pulp fibers at five different RH values and in water. The moisture changes lead to a decrease in the elastic modulus but increase in the relaxation effects with increasing RH. All the determined parameters for the elastic and viscous behavior exhibit a gradual decrease with increasing RH from 10 to 75% RH. The elastic moduli decrease by a factor of 10 and the viscosities are decreasing by a factor of 10-20. In water, there is an even more pronounced decrease of the elastic moduli by a factor 100, and the viscosities decrease by at least three orders of magnitude compared to 10% RH. This indicates that the mechanical response of pulp fibers in water is significantly different than in humid air. This is also illustrated by the fact that a GM model of order two suffices to describe the material behavior in humid air but a GM model of order three is necessary to fit the material behavior in water. A possible interpretation is an additional relaxation effect of the pulp fiber wall in water.