Ciprian Lazarescu

Drying Related Strain Development in Restrained Wood

The objective of this study was to investigate elastic, viscoelastic, mechanosorptive, and plastic strains developed by the drying process in perpendicular-to-grain restrained western hemlock specimens. Drying tests were performed on small, clear wood specimens sawn in such a manner that either the tangential or the radial direction was parallel to their length, where stress driven deformation rates were determined using resistive transducers. Different mechanical restraint schemes were designed by using weights for partly and a load cell for totally restrained specimens. The difference from previous research done in this domain was the restraining technique, which allowed stress to be induced by the drying process, thus, wood was forced to lift the weight as it reached the fiber saturation point. Drying rates were determined gravimetrically on free specimens having similar wood structure. Experiments with free and restrained specimens were carried out simultaneously while drying at 40, 60, and 80°C, down to 17, 12, and 6% target moisture contents. Overall, the magnitude of restrained shrinkage was successfully quantified and separated in free, combined viscoelastic and mechanosorptive and plastic strains. In all these preliminary experiments the dimensional change started at high overall moisture contents, temperature and environmental conditions being key factors. The results can be used to assess the stress indirectly by studying the restrained strain together with moisture content value.

Modeling Shrinkage Response to Tensile Stresses in Wood Drying: I. Shrinkage-Moisture Interaction in Stress-Free Specimens

This article reports on the wood shrinkage during drying in relationship with the temperature and moisture content. All tests were performed perpendicular to the grain on small clear wood specimens of green Western hemlock while drying at 40, 60, and 80°C to 17, 11, and 5% final moisture contents. Overall, wood dimensional changes and moisture loss phenomena were successfully analyzed and interpolated. The shrinkage strain followed a nonlinear pattern with the moisture loss being the driving force and exhibited good correlation with the square value of moisture content in tangential, and linear moisture values could be used to describe shrinkage in radial direction. Both shrinkage intersection points and end of capillary water values increased with temperature; the distinction between the two values could not be made at all times. A nonlinear function containing two regression coefficients (α and β) was found to be a good interpolation of the moisture loss experimental data. Further analyses revealed that β is independent of both target moisture content and temperature, whereas α appears to be influenced by both variables. The correlation between shrinkage and moisture loss rate is intended to be used as a stress prediction tool.

A Novel Method of Measuring Moisture Content Distribution in Timber During Drying Using CT Scanning and Image Processing Techniques

A new method for the nondestructive measurement of moisture content (MC) distribution in timber during drying was developed using X-ray computed tomography (CT) scanning and image processing techniques. The deformed cross section in the CT images due to shrinkage was corrected with the image registration, and the shrinkage was measured by digital image correlation analysis. The pixel-wise MC distributions during drying were measured and visualized successfully. The total timber MC estimated from the MC values of each pixel were strongly correlated with those measured by calculation without geometrical transformation of CT images. The coefficient of determination (R 2) and the standard error of prediction (SEP) were 0.99 and 0.18%, respectively, within the MC range of 19.2–47.3%. In addition, the measured shrinkage distribution during drying was in accordance with the diamonding deformation observed. The results suggest that CT scanning combined with image processing techniques is an effective tool for nondestructive assessment of MC distribution during drying.

Density and Moisture Profile Evolution during Timber Drying by CT Scanning Measurements

Timbers of 105 × 105 mm2 in cross section and 1000 mm long were computed tomography (CT) scanned with a commercial unit in equal time steps during a drying cycle. The study aimed to convert the scanning numbers into wood density values, observe the changes that occur during the dehydration process, and, from these, differentiate moisture content as a function of time and space. The reference for calculating the moisture content of each image pixel was the oven-dry wood scan. Analysis of the results allowed visualizing the water distribution, the mechanism of water removal, as well as check formation. Although only a single drying run was analyzed and a limited number of timbers were scanned, the results revealed trends and thus show promise, so further research is urged because it may lead to a better understanding of water movement and check formation in wood during kiln drying.

Modeling Shrinkage Response to Tensile Stresses in Wood Drying II. Stress–Shrinkage Correlation in Restrained Specimens

This article reports on the effect of tensile stresses, temperature, and target moisture content on shrinkage behavior of short and thin Western hemlock specimens as part of a series of papers dealing with these issues. All tests were performed perpendicular to fiber grain while drying at 40, 60, and 80°C to 17, 11, and 5% final moisture contents at each temperature. Four matched specimens were subjected to different restraints during each drying experiment, namely, zero restraint (free shrinkage), two static restraints of 3 and 6 daN and a dynamic restraint controlled by the drying process. The resulting shrinkage was measured by pairs of resistive transducers located on the middle part of each specimen. Shrinkage percentages, obtained after the tensile stress was released and all the viscoelastic strain recovered, were proven to be a strong function of stress value, moisture, and temperature. The analytical fit yielded high coefficients of determination (R 2 = 0.83–0.85, p < 0.05) for both structural directions tangential and radial, respectively.

Radio-Frequency Heating Kinetics of Softwood Logs

The project assessed the radio-frequency (RF) heating characteristics of logs of two softwood species, namely, Engelmann spruce (Picea spp.) and subalpine fir (Abies lasiocarpa). Sixty logs, equally divided between the two species, were RF heated in two different circumstances—with or without bark—until the temperature sensor from a number of those scattered within each specimen with the lowest reading indicated 70°C. Both species heated in short periods of time (∼60 min), regardless of bark presence or absence, with relatively small energy requirements and without noticeable negative consequences on quality. The sapwood heated up faster, thus reaching higher temperatures because of its high moisture content and better complex permittivity values. Between the two tested species, fir is more prone to RF heating due to its higher ability to convert an electric field into heat.

Modeling Shrinkage Response to Tensile Stresses in Wood Drying III. Stress—Tensile Set Correlation in Short Pieces of Lumber

This article reports on the correlation between tensile stresses, temperature, and target moisture content in short pieces of lumber based on relationships developed on experiments made on small wood strips subjected to different types of restraints. Linear motion position sensors were placed around a tested specimen using a frame connected to a support body. A wireless moisture and temperature monitoring system was used to record wood moisture and temperature changes. All measurements were performed perpendicular to fiber grain while drying at 40, 60, and 80°C in an environment set for a target moisture content of 5%; the experiments were stopped when the average moisture content of the lumber was around 10%. High stress values were obtained for specimens dried at 60°C and a low relative humidity, whereas a high reduction in stress level could be obtained for 80°C. The findings are intended to be used in further studies of the shrinkage process as an indicator of the tensile stresses generated in the early stages of wood drying.

Phytosanitation of mountain pine beetle infected lodgepole pine using dielectric fields at radio frequencies

As an environmentally friendly alternative to chemical treatment, this research aimed to establish whether dielectric heating at high frequency of infested lodgepole pine (Pinus contorta) boards and logs, obtained from the mountain pine beetle devastated forests of British Columbia, can result in wood free of living fungi, nematodes and insects. The sample set contained 230 boards, 50x150 and 50x100 mm2 in cross-section and 20 logs, 200-300 mm in diameter; all tested specimens were roughly one meter long. The intention was to test the efficiency of two temperature/time combinations: 56oC for 30min and 60oC for 15min that were identified in past works as effective phytosanitary combinations. Data showed that both permutations eradicated all infestation levels and types. The electric field power density per treatment cycle ranged from 23 to 50 kW/m3 and the total heating cycle varied from 42 to 116 minutes for all pest and wood type combinations tested.

Color variation of stained wood products in association to near infrared spectroscopy scans

The research aimed to test the hypothesis that near infrared reflectance (NIR) scans could detect surface characteristics associated with uneven coloring before staining red alder (Alnus rubra Bong), cherry (Prunus sp.), pine (Pinus sp.) and maple (Acer sp.) boards. NIR spectra were successfully correlated with average scanned area data of grain angle, density, pre- and post-staining 8-bit color values, and the difference in color induced by staining when combinations of either three, two or one of the tested wood species were used as input parameters. Difficulties were encountered when both cherry and maple were clustered together with the other two species in a generalized model, however, when both were excluded, the model produced very high R2 values (over 0.98).ZusammenfassungZiel dieser Studie war es, die Hypothese zu überprüfen, dass mittels NIR-Messungen bereits vor der Verfärbung Oberflächenmerkmale von Brettern aus Roterlen- (Alnus rubra Bong), Kirsch- (Prunus sp.), Kiefern- (Pinus sp.) und Ahornholz (Acer sp.) bestimmt werden können, die zu ungleichmäßiger Färbung führen. NIR-Spektren wurden mit über Teilflächen gemittelten Werten von Faserwinkel, Dichte, 8-Bit Farbwerten vor und nach der Färbung sowie dem durch die Färbung erzeugten Farbunterschied bei Verwendung von entweder drei, zwei oder einer der untersuchten Holzarten als Eingabeparameter erfolgreich korreliert. Schwierigkeiten traten auf, wenn sowohl Kirsche als auch Ahorn mit den beiden anderen Holzarten in einem umfassenden Modell zusammengefasst wurden. Wenn jedoch beide ausgeschlossen wurden, ergab das Modell sehr hohe R2-Werte (über 0,98).

Wood microfibril angle variation after drying

Abstract The change of microfibril angle (MFA) in wood cell wall was assessed after drying at 60°C and 70°C to a target moisture content (MC) of 8% or 15%. Despite literature contradictions about the effect of drying on MFA, this study showed that drying increased significantly the MFA, possibly as a result of lateral deformation of cellulose microfibrils during water desorption from wood cell walls. Moreover, MFA increased when target MC decreased.