Testing of the fatigue strength along wood fibres at different moisture contents

Testing of the fatigue strength along wood fibres at different moisture contents. The paper determines the effect of wood moisture content on the fatigue strength in compression along fibres. The method of determining the maximum stress at the proportional limit was used for the measurements. Fatigue strength was investigated for three wood species: pedunculate oak (Quercus robur L.), bearded birch (Betula pendula Roth.) and Norway spruce (Picea abies L.), with two moisture contents: close to the absolutely dry state and above the fibre saturation point. The ratio of fatigue strength to short term strength depends on moisture content and is similar for birch (70.3% in the dry state and 72.1% in the wet state), for oak (67.4% and 69.5% in both states) and for spruce (66.6% in the dry state and 68.1% in the wet state). The moisture content of the wood clearly influences the fatigue strength of the wood. On average, the fatigue strength of wood with moisture contents above the fibre saturation point constitutes about 0.20 of the fatigue strength of wood with moisture contents close to 0%. This tendency was found regardless of the tested species. The simplified method for testing fatigue strength at the limit of proportionality has shown its limited usefulness, requiring further analysis and comparison with other methods in order to be thoroughly tested and possibly improved.

Engineering properties of wood under different drying methods

This project work focuses on the comparison of the engineering properties of Pyrus communis under solar and air-drying method. Green samples of the wood were dried simultaneously in both solar kiln and open air for 15 days. The daily moisture content reduction (%MC) under both drying conditions were observed and recorded. Mechanical tests (shear strength, compressive strength, hardness and tensile strength) were conducted. The results showed that open air-dried woods attained fibre saturation point (30-25%MC) within three days. Timbers dried in solar kiln attained lower %MC (12.9, 11.0, 14.3, 12.3) when compared to open-air dried samples (22.1, 18.5, 21.1, 17.1). For all the mechanical tests conducted, solar kiln dried timber had higher mechanical properties than the open air-dried woods.

Useful near infrared spectroscopy model calibrations on solid wood samples of Pterocarpus erinaceus (Poir.) for physical, mechanical and colour properties

Pterocarpus erinaceus (Poir.) (local name: Toti, Tem) is a species endemic to the Guinea-Sudan and Sudan-Sahel zones. It is over-exploited for its wood, which is sold in international trade to China and Indian markets. The selection of high performance wood based on wood properties and their vegetative reproduction represents the best option for the establishment of plantations with the aim of promoting the restoration of degraded natural resources. However, measurements of these characters are lengthy, time consuming and destructive, and difficult to perform in Togo because of the lack of equipment. An alternative to those methods is near infrared (NIR) spectroscopy. The aim of this study was to assess the efficiency and feasibility of near infrared spectroscopy for predicting the physical and mechanical properties and the colour parameters of P. erinaceus wood from Burkina Faso, Niger and Togo. The properties were determined by conventional method and correlated with near infrared spectra using partial least square regression. The partial least square regression models were tested by cross-validation and by external validation. The results of this study showed the possible use of near infrared spectroscopy to estimate the physical and mechanical properties and the colour parameters of P. erinaceus wood. In fact, when validating prediction models for radial and tangential shrinkage, the fibre saturation point (FSP) showed R2 values of 0.70, 0.71, and 0.92, respectively, with ratio performance deviation values of 1.84, 1.87 and 3.57. For the colour parameters L*, a* and b*, the R2 values were 0.83, 0.74 and 0.67, respectively, with ratio performance deviation values of 2.40, 2 and 1.70. The results showed that the basic density and fibre saturation point models were classed as good to very good, respectively. Near infrared spectroscopy can therefore be used to predict efficiently the properties of P. erinaceus wood for a large number of samples, enabling the incorporation of those traits in breeding criteria.

Understanding of Water Sorption Mechanism in Cellulose-Water System: The Perspective of NMR

Water under the fibre saturation point affects many aspects of the performance of the cellulose. Therefore an understanding of the interaction of water with cellulose is essential for the interpretation and prediction of cellulose response. Because of the complicated structure and the different possibilities of the hydrogen-bonds formation, the mechanism of water sorption in water-cellulose system has been studied by different techniques. This paper gave an outline of the structure of cellulose, also an overview of the researches of the mechanism of cellulose-water interaction using nuclear magnetic resonance. Further observations were made aiming at having a deeper understanding of the water-cellulose system.

Experimental study of resistance type wood moisture sensors for monitoring wood drying process above fibre saturation point/ Takistus-tüüpi puidu niiskuse andurite eksperimentaalne uurimine puidu kuivatamise monitooringul niiskussisaldustel üle kiu küllastuspunkti

In wood moisture contents above fibre saturation point (FSP) the measuring accuracy of resistance type sensors starts decreasing and several side effects related to measuring wood moisture begin to occur. These side effects can be taken into account in the measuring process and can be eliminated. This research examined wood charging and discharging in the process of measurements, phenomena related to repeated measurements in both a single electrode insertion spot and different insertion spots in the specimen made of pine (Pinus sylvestris L.) sapwood and black alder (Alnus glutinosa (L.) Gaertn.). The research presented approaching equations separately for the voltage and current of the polarization and depolarization process for the initial phase of the process. Empirical equations were found for wood electrical resistance and electrical capacitance in the polarization process. For black alder possibilities for calibration of a resistance meter and resistance type electrodes were explored. It was found that to improve the calibration accuracy of resistance type measuring electrodes an additional individual calibration with regard to the average moisture content of the specimen may be carried out with the electrodes above FSP. Research results can be implemented in monitoring the wood drying process using resistance type wood moisture sensors, but also in determining the electrical parameters of growing trees.

Experimental study of electrode effects of resistance type electrodes for monitoring wood drying process above fibre saturation point / Elektroodefektide uurimine puidu kuivatamisel niiskussisaldustel üle kiu küllastuspunkti

Due to practical need to improve the accuracy of measurement of wood electrical resistance via creating multivariate calibration models the study of electrode effects was carried out. The experiments were made in DC mode and five types of electrode effects were measured: electrical resistance of the galvanic contact between the measuring electrode and wood, corrosion of measuring electrodes in wet wood, polarization and depolarization of the double layer forming on the surface of the measuring electrodes, residual polarization voltage between measuring electrodes and effect of repeated use of measuring electrodes. For multi-channel measurement mode effect of neighbouring electrodes was measured. Behind each of these factors is a complex transfer mechanism of free and bounded charge carriers from wood to measuring electrodes. Among the factors, polarization and depolarization have an immediate effect (duration of the processes in seconds) while resistance of electrode/wood contact, corrosion of electrodes and residual polarization have a slow effect (duration of processes in hours). For pine sapwood, coefficients of the Stamm formula were found in moisture contents (MC) above the fibre saturation point. It was determined that wood polarization and depolarization indicators are dependent on wood moisture content as is the case with wood electrical resistance. Compared to wood electrical resistance, wood polarization proved approximately four times and depolarization approximately eight times less sensitive to moisture content variation.

Modelling drying processes of fuelbeds of Scots pine needles with initial moisture content above the fibre saturation point by two-phase models

Modelling the drying process of fuel moisture with initial moisture content above the fibre saturation point can be used to determine when fuel will become sufficiently dry (after precipitation) to burn and provide a more accurate prediction of fire potential. Based on analysis of the mechanism by which the drying process occurs, we propose a model comprising two phases distinguished by a moisture threshold of 0.35 g g–1, the fibre saturation point; one phase is controlled by evaporation and the other by diffusion. Each phase has a distinct equation with a different timelag. We compared our two-phase model with a one-phase model (one-timelag model) and another two-phase model by estimating drying of 15 Scots pine (Pinus sylvestris var. mongolica) needle fuelbeds. The results indicate that the two-timelag model improves moisture modelling, thereby reducing mean absolute error by more than 30%, i.e. from 0.0047 g g–1 (one-phase model) to 0.0030 g g–1. The model yields consistent results, further suggesting its potential for improving fuel moisture prediction of fire danger rating systems. The first timelag of the model is affected by fuelbed properties. Equations based on variables that represent fuelbed properties were established, thus saving time when estimating parameters for stand-specific fuel moisture models.

A study of water exchange in wood by means of 2D NMR relaxation correlation and exchange

Two-dimensional T 1-T 2 1H NMR relaxation correlation spectra and T 2-T 2 relaxation exchange spectra for samples of spruce wood across the full moisture content range from 0% to >100% are presented. The T 1-T 2 spectra unambiguously identify water in different environments within the wood that overlap in the corresponding and more traditional one-dimensional T 1 or T 2 experiment. The T 2-T 2 spectra unambiguously reveal the presence of water exchange between the lumen and cell wall above the fibre saturation point in sapwood. An estimate is made of the exchange rate: 1/30 to 1/3 ms-1.