Sound Knots Research Articles

Continuous densification of wood with a belt press: how knot features impact the densification outcome

ABSTRACT Densification, i.e. the transverse compression of sawn timber has been studied and commercialised for well over 100 years but remains an expensive niche product with low annual production volumes. One reason for this is the reliance on time-consuming batch processes in a hot press. To solve this, a continuous densification process using a belt press, capable of densifying full-sized sideboards was developed. However, there is insufficient knowledge about the effect of knots on the densification outcome. The objective of this study was to assess how different knot parameters affect the densified wood in terms of damage and deformation to the knot itself and the surrounding wood material. Multivariate data analysis methods were applied to a dataset of 171 knots, described by 23 variables. The data showed that it is possible to densify knots in a continuous process without causing damage. Especially sound knots are often unproblematic, even at relatively large sizes, while densifying dead knots often resulted in unacceptable damage to the knot or the surrounding wood. From a material selection standpoint, any knots bleeding into the board edge and dead knots greater than 20 mm in diameter should be avoided altogether.

Effect of sawn zone on the quality of lumber in the evaluation of selected pine wood defects

Effect of sawn zone on the quality of lumber in the evaluation of selected pine wood defects. In the course of research on the sawn pine raw material with defined distribution of the defects, a variable level of change in the presence of knots was assessed. Initially, the experimental material was classified in terms of the general-purpose timber, and then the strength classes of wood for structural applications were assigned. The proportion of sound knots increased in case of wood obtained from the middle and top zones. In the case of butt-end logs, an increase in the share of the rotten knots having an average diameter of 2-4 cm was observed. The intensity of the defect’s occurrence corresponded with the zone of origin along the large-sized roundwood length.

Elastic moduli of butt-end logs and the variable knots distribution in Scots pine from Western Poland

In studies on Polish pine wood, the parameters determining its usefulness for processing are highly variable. In contrast, international studies on the optimum utilization of coniferous sawn wood describes its visual features as those indicating the distribution of defects and their impact on the selected mechanical and physical properties of sawn wood. Knottiness is one of the qualitative properties of sawn wood that is essential for wood grading. The objective of this study was to determine a correlation between variable knots and their soundness on the one hand and their selected strength parameters on the other. One of the basic issues of this work includes structural timber optimization. For Polish sawn wood, these indicators are still relatively new. The study material involved edged butt-end log lumber from a 120-year-old forest stand. The results confirmed a correlation between knots frequency and modulus of elasticity. The correlation was strong, indicating that sound knots play a major role in the process of the strength assessments. The study did not demonstrate a significant share of rotten knots and their effect on strength.

Efficiency of radiofrequency-vacuum (RF/V) technology for mixed-species drying of wood disks with inherent defects

Freshly cut 30-mm-thick wood disks of Scots pine, Caucasian elm, narrow-leafed ash, wild cherry, black walnut, and European alder with various natural grade defects were dried together in a radiofrequency-vacuum (RF/V) dryer. A mild four-step drying schedule at the maximum temperature of 45 °C and power density in the range of 1-2 kWm−3 with an “on-off” control of RF radiation at the final step was used. The moisture content of the disks at the end of drying ranged from 10.6% to 13.4%. An almost uniform radial moisture gradient developed in the dried disks. The six wood species, however, exhibited variations in checking. The RF/V drying schedule used here was able to completely inhibit near-bark radial checking in all disks but pine and elm, whereas it failed to efficiently prevent V-cracking in pine and ash with a high mean tangential to radial shrinkage anisotropy (ST/SR). A strong positive correlation was also found between ST/SR and the proportion of V-cracked disks. The occurrence of ring failure within the reaction wood-containing disks of walnut, pine, and elm was almost completely prevented by the drying schedule, but a significant correlation was observed between the pith eccentricity and V-crack length in the compression wood-containing disks of pine. All other natural defects, i.e., scar in cherry, bark pocket, double or triple pithing in walnut and elm, sound knots in pine, and roundness deviation, exhibited no statistically significant effect (p > 0.05) on check formation.

Evaluation of the effect of knots on rolling shear strength of cross laminated timber (CLT)

The study experimentally and analytically investigated the effect of knots on the rolling shear (RS) strength of 3-ply southern yellow pine cross-laminated timber (CLT). Center-point bending tests and two-plate shear tests were conducted on six CLT configurations composed of different types of cross layer laminations based on knots conditions (No Knots, Sound Knots, and Decayed Knots) and pith conditions (Pith, No Pith). The shear analogy method was implemented to evaluate the RS strength values from the bending test results, which were compared against the results from the two-plate shear tests. The cross-layer laminations with either sound knots or decayed knots improved the RS strength of CLT. In general, the RS predicted by the shear analogy method was conservative.

Effect of Knots on Horizontal Shear Strength in Southern Yellow Pine

Knots are inevitable components found in wood that can adversely affect the mechanical properties of the lumber. The objective of this study was to investigate the effect of knots on the horizontal shear strength of southern yellow pine. Knot condition (sound/unsound) and shear plane (radial/tangential face) were studied as the factors of shear strength. The standard ASTM D143-94 (2014) was used to compare 120 pairs of clear shear blocks and shear blocks containing knots. Paired t-test results showed that regardless of the direction of the grain compared with the shear plane (perpendicular or parallel), sound knots increased the shear strength and the unsound knots decreased shear strength. Based on this study, the unsound knot volume was found to be a significant factor in decreasing the shear strength in the radial or tangential face direction. Furthermore, no significant relationship between the knot angle and shear strength was found. Shear failure occurred in the wood when an encased knot sample was tested and shear failure occurred in the knot when an intergrown knot sample was tested.

A model approach to include wood properties in log sorting and transportation planning

ABSTRACTThere is a trend that sawmills are more focused on particular valuable products in their production. This has led to an increased demand for sawlogs that are better adapted to the target products and production efficiency. Depending on the product being produced there are different log properties which are better adapted for certain products than others. Sawmills can require hard constraints on log properties such as length, diameter, internode length and sound knots. Some properties are not required but are desired as they make the production more efficient or increase the frequencies of preferred products. In these cases, we include an added value corresponding to what the industry is willing to pay for improved adaptation of the raw material. To achieve this, we propose an optimization model that integrates logging operations (bucking and forwarding) at harvest areas, transportation planning and flexible description of demand at sawmills. High flexibility for sorting at harvest areas may require additional piles of different properties to be generated. Instead of using a large number of special assortments, we allow many sorting alternatives depending on the requirements used at the industries. The transportation planning decides on the flows between harvest areas and sawmills while considering demand and supply. Even if many potential piles are used in the planning model, only a few may be used in practice. We present computational results based on 16 synthesized geographically distributed harvest areas, each representing all regional variation of mature sample trees from the Swedish National Forest Inventory and a number of sawmills.

Chemical composition of the terpenoids in wood and knots of Abies species

Oily constituents were obtained from sapwood and knots of Abies balsamea and A. alba using steam-distillation, and the chemical compositions were determined by gas chromatography and gas chromatography–mass spectrometry. The oily constituents of sapwood and knots of A. balsamea were mainly oxygenated diterpenes with manoyl oxide (1), epi-13-manoyl oxide (2), manool (3), and epi-13-manool (4). Moreover, 2 and 3 were found to be components in A. alba. The constituents of sound knots were dominated by oxygenated diterpenes, together with 2 and 3. Comparison of the content of oxygenated diterpenes from the material revealed that dead knots of A. balsamea contained a large quantity of 4, which was higher than in sapwood. Knots of both Abies spp. contained large quantities of oxygenated diterpenes that were also higher than in sapwood.

Determination of Crosscutting Safety Zone for Finger-Jointed Pinus sylvestris Furniture Components

Abstract A common problem with finger-jointed wooden furniture components is chipping in the finger joints due to fiber deviations around sound knots. To avoid this, a fixed size safety zone between defects and crosscuts is used, but can lead to an excess of material cut away in the crosscutting operation. To reduce chippings in finger joints while maximizing recovery, an adaptive strategy was developed for setting the safety zone size between sound knots and finger joints in Scots pine (Pinus sylvestris L.) furniture components. The strategy was based upon modeling the risk of chipping the finger joint depending on the knot measurement. The model was used in an adaptive strategy in order to minimize the expected loss due to cutting away material around knots, compared with the cost of rejecting components in later stages due to chipped finger joints. Thus, each knot was assigned a unique safety zone. The strategy was tested using computer simulation of the finger-jointing process, and a sensitivity analysis was performed in order to quantify the effect of variations in the input data. The results show that the adaptive strategy improves recovery by at least 3 percent in the process of turning lumber into finger-jointed furniture components. It is very robust toward variations in knot size measurements (e.g., by scanning equipment), but less robust toward variations in crosscutting precision.

Automatic detection of sound knots and loose knots on sugi using gray level co-occurrence matrix parameters

Knowledge of the occurrence of sound and loose knots on the surface of sawn sugi (Cryptomeria japonica L.f.) is important for its grading and application. This study examined an optical system for detecting sound and loose knots in sugi instead of human being using the combining information of the color and texture features. The proposed system could be conceptually divided into two components: a CCD camera scanning system and a defect detecting algorithm developed by the authors. In the algorithm, the contrast parameter calculated from a gray-level co-occurrence matrix was used to locate the potential defects represented by sound knots and loose knots. The rule-based approach, which was built according to the color feature histograms, was used to identify sound knots and loose knots. A series of samples containing single or multiple sound and/or loose knots were selected at random to verify the efficiency and accuracy of the proposed system. There were 94 sound knots and 86 loose knots on the surfaces of these samples, and the accuracy of locating the positions of sound knots and loose knots was 94.7% and 97.6%, respectively. The accuracies of identifying knots as sound or loose were 96.6% and 98.8%, respectively. The overall detection accuracy of the system was 93.9%. The results indicate that the proposed vision system is an efficient means of detecting sound knots and loose knots.

Identification of Dead and Sound Knots by near Infrared Spectroscopy

This paper examines the identification of dead and sound knots on the wood surface using near infrared (NIR) spectroscopy. Fifty dead and sound knots samples were prepared and NIR diffuse-reflectance spectra were collected from these samples. The spectra of normal wood were also obtained from the neighbourhood of each knot. The wavenumber range from 6600 cm−1 to 5400 cm−1 (1515 nm to 1852 nm) was employed for the multivariate analysis after taking into consideration characteristics of the spectra. The soft independent modelling of class analogies (SIMCA) classification technique was conducted for classification of dead knots, sound knots and normal wood classes. Specific spectral features were found in dead and sound knots at the following absorption bands: 6130 cm−1, 5980 cm−1, 5800 cm−1 and 5700 cm−1, which were related to bark tissue and compression wood. The SIMCA model could identify the differences between dead and sound knots as well as the presence of knots. The loadings and discrimination powers were consistent with the spectral features, showing peaks due to bark tissue and compression wood. The bands provide useful information to determine the effective range of electromagnetic waves for image analysis.

Size and localisation of knots in timber from mountain spruce stands in the Dolomites.

The aim of the study was evaluation of knots in wood of 150-year-old Norway spruce [Picea abies (L.) Karst.] in stands situated at the altitude of 1450– 1740 m above sea level in the Dolomites in Italian section of the Alps. In selected stands, spruce trees were cut down and their length, stem thickness, height to the crown base and stem diameters at every 1 m along the length of merchantable bole were measured. The diameter of knots was measured and they were classified according to their healthiness and the degree of their tightness with the surrounding wood. The relative knot diameters were calculated and the relative height of their location on stem was determined. In total on sample trees there were analysed 1070 knots, of which sound knots (close to 75%) and tight (more than 60%) prevailed. Sound and tight knots had largest diameters, relative diameters and relative heights of location on merchantable boles. The smallest diameters and relative diameters were indicated by rotten and not tight knots that were located at the lowest merchantable bole parts.

Locating and identifying sound knots and dead knots on sugi by the rule-based color vision system

Knowledge of the occurrence of sound and dead knots on the surface of sugi is important for the classification and application of the material. This study examined a color vision system for detecting sound and dead knots on sugi. The system can be conceptually divided into three components: a CCD-camera scanning system, an image-segmenting module, and a rule-based defect identifying module. The results showed that the potential defect regions could be located by Otsu’s threshold algorithm in conjunction with t-test analysis. The accuracies of locating sound knots and dead knots were 92.6% and 97.1%, respectively. The rule-based approach was used to identify sound and dead knots and the identifying accuracies for sound knots and dead knots were 92.0% and 94.1%, respectively. The overall detection accuracy of the system was 87.6%. The results indicated that the rule-based color vision system is an efficient means of detecting sound knots and dead knots on sugi.

Internal distribution of sound and dead knots in Picea abies (L.) Karst.

(L.) Karst. in the south-eastern part of Norway. Distribution of sound and dead knots was evaluated as the mean and the range of the radial extension of sound knots within each whorl. The mean value was found to increase with increasing height in the lower 40% of the tree height followed by an upwards decreasing trend. It was related to diameter at breast height and annual ring width together with site index. The range of sound knot length within a whorl was calculated to 22 mm on average. It was found to be increasing up to 30% of tree height followed by a decrease upwards. The range was related to site index, diameter at breast height, eccentricity and to difference in height to the lowest living branch and the lowest living whorl.

Untersuchungen über die Schnittholzqualität von Tannen- und Fichtenholz aus Plenterbeständen

From three areas of selection forests, the structure of stands is described. Selection logging was performed and the wood quality was investigated. For this purpose the stems were cut in sawmills, each board from the sawn timber was sorted according to the grading rules by determining the defects. The main defects were black and sound knots, cracks, compression wood, resin pockets and peeling off, frequently extending through the whole stem. Differences in timber quality are probably due to former treatment of the forests. Nevertheless, the quality of fir and spruce in selection stands is superior as compared with the average quality of forests cut by compartments; it could be improved by careful tending interventions and pruning.

Schnittholzqualität und Furniereignung von Douglasien aus linksrheinischen Anbaugebieten

Douglas fir sample trees from 9 various stands were cut in sawmills. Every board of the converted timber was graded; all defects in timber were analysed, especially all black and sound knots on the surface of the boards were measured. The percentage of good boards prooved to be comparatively low. From fertile sites the sawn timber quality seems to be inferior compared with wood from poorer and dryer sites. Douglas firs grown single between broadleaved stands showed the lowest timber quality. Throughout the quantity and thickness of black knots were responsible for poor quality. Experiments with production of veneer yielded useful carved veneers when sliced of quartered and clear logs. Branchless logs from 80 years old Douglas firs could be worked up to carved veneers, but are too young for peeled veneers. For full veneer logs diameters of 70 cm and structure of small annual rings is essential.