Based on a comprehensive set of experimental data it is demonstrated that the moisture properties of aligned hemp fibre yarn/thermoplastic matrix composites are showing low moisture sorption capacity and low dimensional changes. Using a reference humidity of 65% RH, and a common span of ambient humidity levels of 33% and 85% RH, the relative moisture content of composites with a high fibre fraction is ranging from −0.009 at 33% RH and+0.016 at 85% RH, and this lead to relative transverse hygral strains in the range of only −0.007 to+0.011. The axial hygral strain is practically zero. The moisture content of the composites is shown to be well predicted by a mixtures relationship using the measured moisture contents of the constituents. The dimensional changes of the composites are well predicted by micromechanical models of the transverse and axial hygral strains.

Abstract It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1st and the 2nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1st and 2nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 103°C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

Abstract A theoretical study of the amount of moisture held in wood as capillary condensed water in the relative humidity (RH) range of 90–99.9% is carried out. The study is based on idealized geometries of the softwood structure related to micrographs. It is confined to structural elements such as bordered pits and the pointed ends of tracheids. The theoretical amount of water in these elements is found by employing the Kelvin equation. An equal amount of earlywood and latewood cells with different geometries and with different amounts of pits is assumed. The effect of pit aspiration is considered, and different degrees of pit aspiration are assigned to earlywood and latewood. We suggest based on the results that capillary condensation makes only a very small contribution to the equilibrium moisture content. At 99.9% RH the contribution amounts to less than 0.0035 kg water per kg dry wood. This is in line with the experimental results presented in Part 1 of this study.

Abstract Desorption isotherms at 20°C for untreated, acetylated, and furfurylated Norway spruce [Picea abies (L.) Karst.] sapwood were established in the 91.9–99.9% relative humidity (RH) range. Three methods were employed to secure various constant RH levels: saturated salt solutions, climate chambers, and the pressure plate technique. The curve form for the untreated samples did not show an upward bend, except perhaps above 99.5% RH, indicating that – contrary to what has hitherto been assumed – capillary condensation does not play a significant role for water sorption in wood below fiber saturation. Three additional results corroborate this conclusion: (1) calculation of the theoretical contribution of capillary condensation to the moisture content (MC) in wood based on idealized microstructural geometries by means of the Kelvin and Laplace equations resulted in very small contributions to the equilibrium moisture content (EMC), i.e., below 0.35% moisture at 99.9% RH. (2) The ratio between the EMC of acetylated and untreated samples did not show an increasing trend for increasing RH, as would have been the case if capillary condensation had taken place in both untreated and acetylated wood. (3) Low field time domain nuclear magnetic resonance results showed that only the relaxation curves from the furfurylated samples were affected systematically by freezing, indicating that neither untreated nor acetylated wood contained significant amounts of capillary condensed water.

This study presents an experimental investigation of fatigue in wood subjected to tension perpendicular to the grain. The study has been designed with special reference to the influence of the frequency of loading. The investigation reveals an interaction between number of load oscillations and accumulated time under load to failure. This interaction corresponds to frequency dependent fatigue. Current models for damage accumulation and failure modelling are screened with respect to their ability to account for such “two-source” damage. The Damaged cracked viscoelastic material model proved to give a good basis.

The potential of plant fibres in structural materials components is explored by applying textile hemp yarn for fabrication of composites with aligned fibres. This first paper presents a detailed characterisation of the hemp yarn. It is shown that the hemp yarn has a high cellulose content (about 90%), the fibres are evenly dispersed within the yarn with a mean twisting angle of about 11°, the moisture sorption capacity is relatively low (e.g. moisture content of about 8% at 65% RH), and the mechanical properties are good (e.g. tensile ultimate stress of about 660MPa). These findings show that textile hemp yarn is well suited for composite reinforcement. The accompanying second paper is addressing the mechanical properties of the composites.

A duration of load study representing 13 years of testing was recently terminated. Preliminary results have been published over the years. This paper represents the final account of the study, which was focused on the influence of moisture content on time to failure for structural timber subjected to bending under constant load conditions. Two constant moisture conditions (MC = 11 and 20%) and one condition of varying moisture (MC between 11 and 20%) were applied. A total of 816 Norway spruce boards of dimensions 44 × 95 × 1,800 mm 3 were included. Eight groups of non-destructively matched samples were formed. Four groups were subjected to short-term strength tests, and four groups were subjected to long-term tests. Creep and time to failure were monitored. Time to failure as a function of stress level was established and the reliability of stress level assessment was discussed. A significant mechanosorptive effect was demonstrated both in terms of increased creep and shortening of time to failure. The test results were employed for the calibration of four existing duration of load models. The effect of long-term loading was expressed as the stress level SL 50 to cause failure after 50 years of loading. SL 50 was found to be of the order 0.60 for MC = 11%, 0.50 for MC = 20% and 0.44 for MC varying between 11 and 20%. The test results revealed no evidence of a threshold stress level. A reliability based calibration of load-duration factors was performed using probabilistic models of loads and of the short-term and long-term strengths. For permanent and imposed library loads, reliability-based estimation of the load duration factor gave almost the same results as direct, deterministic calibration.

Whereas the first paper is addressing the characteristics of textile hemp yarn, this second paper is addressing the tensile properties of aligned hemp yarn composites under influence of a number of experimental variables: hemp yarn type, fibre volume fraction, thermoplastic matrix type, process temperature, conditioning humidity, and loading direction relative to the fibre direction. Altogether, the results show that aligned hemp yarn composites demonstrate the good potential of plant fibres as reinforcement in composites for structural components. The tensile performance of the hemp yarn composites is excellent. As an example, a fibre volume fraction of 0.48 results in a stiffness of 28 GPa and an ultimate stress of 280 MPa. It is demonstrated that the influence of experimental variables is well understood and can be quantitatively described.

Dislocations are areas of the cell wall in natural fibres where the direction of the microfibrils (the microfibril angle) differs from the microfibril angle of the surrounding cell wall. Such regions affect the quality of natural fibres for industrial use. Dislocations may be studied using a number of techniques, and in this study two of them were compared: scanning electron microscopy (SEM) and polarized light microscopy (PLM). Images are presented that show exactly the same dislocations using these two techniques. The comparison showed that dislocations are more numerous and more clearly visible in images from PLM than in images from SEM, as dislocations are only seen in SEM if they are so large that they result in topographical changes of the fibre surface, i.e. a buckling of the S1 wall or a folding of the whole cell wall. However, for such large dislocations details are seen in the cell wall using SEM, while only a large bright region is seen in PLM.

The strength of wood is reduced when the stressed volume is increased. The phenomenon is termed size effect and is often explained as being stochastic in the sense that the probability of weak locations occurring in the wood increases with increased volume. This paper presents the hypothesis that the lower strength is caused by stress concentrations. The stress concentrations arise from the anisotropic structure of wood, and are therefore deterministic. The hypothesis is substantiated through extensive FEM-calculations and experiments. A reasonable agreement between ultimate stresses determined experimentally and in FEM-computations was found.

The application of non-linear fracture mechanics to wood is a relatively new topic in the area of wood science, however, linear elastic fracture mechanics (LEFM) was first applied to wood in the nineteen sixties. The fracture propagation in wood is mainly governed by two aspects, namely the direction of the principal stresses, and the microstructure. There are six principal crack propagation systems in wood, which are illustrated in Fig 2, see eg. Smith et al. [1] or Reiterer and Tschegg [2]. The usual presumption is that wood is perfectly brittle-elastic (LEFM). When a crack propagates perpendicular to grain, the observed fracture behaviour is quasi-brittle, i.e. a tensile softening branch exists [2]. This indicates that a fracture process zone of some length exists. The LEFM approach is not an adequate approximation if the specimen size is of the same order of magnitude as the length of fracture process zone.

SUMMARY: Modelling of moisture transport in wood is of great importance as most mechanical and physical properties of wood depend on moisture content. Moisture transport in porous materials is often described by Ficks second law, but several observations indicate that this does not apply very well to wood. Recently at the Technical University of Denmark, Department of Civil Engineering, a new model for moisture transport in wood has been developed. The model divides the transport into two phases, namely water vapour in the cell lumens and bound water in the cell walls. The model has earlier been compared to test results and good agreement has been found. However, the previous verifications have been limited as only the total water transport was registered and not the individual components (water vapour, bound water). This paper presents a new type of experiment where the two components are assessed individually. A small RH sensor is placed inside a wood specimen, and a transient moisture transport through the specimen is created. Both the total transport of water and the RH in the vapour are measured. By combining the mathematical moisture transport model with the measurements a separation of the total water transport into the vapour phase and the bound water phase is made possible. The moisture transport model consists of two coupled nonlinear partial differential equations, which have been solved by a 3D Finite Element program. Simulations of the laboratory experiments have been performed, and the numerical and experimental results have been compared. To some extend agreement between the model and the experiments was found.

The problem of describing the migration of dissolved boron in wood is treated with special reference to the commonly used remedial treatment of wooden poles. The governing equations are derived and discussed together with some of the material parameters required. The equations are solved by the finite element method and, finally, results showing the effect of different treatment strategies are presented.