共查询到20条相似文献，搜索用时 31 毫秒
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C. A. Raymond P. D. Kube L. Pinkard L. Savage A. D. Bradley 《Forest Ecology and Management》2004,190(2-3):187-200
High levels of growth stress are implicated in causing end splitting of logs, deflection during sawing and deformation of boards as stresses are released during sawing operations. Level of stress is a function of strain and the elastic modulus of the wood (MOE). Levels of peripheral strain can be measured on standing trees and, if the MOE is known, stresses can be estimated. The validity of using peripheral strain measurements relies on underlying theoretical models that relate strain to expected patterns of stress distribution and levels of board deflection. This study evaluates these theoretical relationships by determining relationships of stress and strain with board deflection, end splitting and a range of wood properties.
Peripheral strain levels were extremely variable within the bottom log and little evidence was found for consistent patterns of variation, although measurements generally increased with increasing height above ground. Sampling on two sides of the standing tree at breast height appeared to be a suitable strategy, with the mean for these strain readings having a correlation (r) of 0.86 with the average strain in the bottom log.
Growth strain was not a reliable predictor of board deflection and cannot be recommended as a non-destructive sampling method. Overall there was a poor relationship between growth strain and board deflection. No consistent relationships were found between a range of wood properties and growth strain or board deflection across both sites. Stress levels were calculated for each tree as the product of growth strain and modulus of elasticity and the relationship between calculated stress and mean board deflection determined. No relationship was found at either site with correlations being very close to zero.
The underlying theoretical relationships between stress and strain were examined and several questions raised about the validity of such models. 相似文献
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A mill study of 62 trees, in which boards were reassembled into their original logs, permitted the construction of wood quality maps. In this instance stiffness profiles were obtained from butt to upper-top logs, based on machine stress grading of all boards and then averaging values from the 62 trees. Traditionally the butt log has been perceived to be the most valuable log in a tree, because it is bigger and gives a higher recovery of lumber. However, it is shown to contain a wide cone of very low stiffness wood that is confined to the first 2.4–2.7 m above ground level. Above this point stiffness gradients become cylindrical with no noticeable decrease in stiffness up the tree stem. Stiffness in all logs increased radially from pith to cambium with the greatest change being associated with the wood nearest the pith. The low stiffness at the base of the tree suggests that an alternative log bucking strategy should be considered, namely cutting a short 2.4–2.7 m butt log for plywood/LVL or for bolter sawing and only cutting standard length logs above this point.The least stiff logs (lowest 20%) yielded lumber that had an average stiffness that was over 1 GPa less than the average for the population. A case can be made for separating these logs and processing them differently. 相似文献
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Shusheng Pang 《Wood Science and Technology》1996,30(3):165-178
Summary A 2-D mathematical model was developed to simulate moisture movement and heat transfer in width and thickness directions within a softwood board during drying. The model is based on wood physiological features and behaviour observed during drying. In sapwood, liquid water movement is assumed to be a consequence of capillary action between liquid and gas phases inside the cell lumens. However, liquid flow does not occur in wood close to the exposed surfaces because at timber surfaces the wood cells are damaged during the sawing process and consequently the liquid column is broken. In heartwood, liquid flow is negligible since the pits are normally aspirated during the formation of heartwood in the growing tree. Water vapour moves under a partial vapour pressure gradient while bound water diffuses within the wood material due to differences in chemical potential. The model was solved numerically to predict moisture-content profiles. Experiments were undertaken to measure the moisturecontent gradient. Samples were removed from a tunnel dryer at intervals throughout drying, frozen overnight and then cut into slices for moisture-content determination. The experimental results were used to verify the model.This work is supported by the New Zealand Foundation of Research, Science and Technology under contract CO4415 相似文献
6.
The high accuracy of log positioning and the stability of saw blades in breakdown machinery in modern sawmills have reduced
the need to add margins for sawing variations. Oversize green sawing dimensions are still needed, but mainly to allow for
drying shrinkage. This has put a new focus on better adapting green sawing dimensions to the shrinkage behavior of wood. In
this study, a method for optimization of green sawing dimensions using stochastic simulation is presented. Normal distributions
were generated for planed dry dimensions, kerf width, and target moisture content. The minimum share of boards exceeding the
specified dry dimensions was decided, and deformations in boards from all positions in the cross section in a number of logs
were simulated. The simulated shrinkage allowance from stochastic simulations was compared to experimental results from an
industry test and to finite element results based on material data for Norway spruce. The results showed that the green width
of the sawn boards should increase when the number of boards in the center yield increases. The green thickness of boards
should be thinner for center boards and outer boards than for inner boards. 相似文献
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The production of knot-free wood is important since the market demands wood without knots for reasons of both appearance and material properties. This work describes a simulation of the removal of knots from star-sawn and square-sawn timber. The efficiency of the two methods is compared in terms of the length of the knot-free components obtained and the volume yield. The simulation was based on data for trees and logs taken from the Swedish Stem Bank. These data were then used to simulate the sawmill process in a computer program called the Virtual Sawmill. Data related to the boards obtained were used in a MATLAB model simulating the cross-cutting of knots. Simulated star-sawing of logs with a top diameter exceeding 230 mm gave a mean knot-free component length of 417±321 mm, while the mean length of knot-free components for simulated square-sawing of the same logs was 298±122 mm. The volume yield of knot-free components from the two sawing patterns was 91% for star-sawing and 87% for square-sawing. For timber with cross-section dimensions of 38×75 mm2, the mean length and yield of knot-free components from simulated star-sawing were 451±349 mm and 90%, respectively. In simulated square-sawing, the corresponding values were 263±197 mm and 82%, respectively. This shows that star-sawing has potential for the production of knot-free timber. 相似文献
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《Wood material science & engineering》2013,8(3-4):108-115
Abstract The production of knot-free wood is important since the market demands wood without knots for reasons of both appearance and material properties. This work describes a simulation of the removal of knots from star-sawn and square-sawn timber. The efficiency of the two methods is compared in terms of the length of the knot-free components obtained and the volume yield. The simulation was based on data for trees and logs taken from the Swedish Stem Bank. These data were then used to simulate the sawmill process in a computer program called the Virtual Sawmill. Data related to the boards obtained were used in a MATLAB model simulating the cross-cutting of knots. Simulated star-sawing of logs with a top diameter exceeding 230 mm gave a mean knot-free component length of 417±321 mm, while the mean length of knot-free components for simulated square-sawing of the same logs was 298±122 mm. The volume yield of knot-free components from the two sawing patterns was 91% for star-sawing and 87% for square-sawing. For timber with cross-section dimensions of 38×75 mm2, the mean length and yield of knot-free components from simulated star-sawing were 451±349 mm and 90%, respectively. In simulated square-sawing, the corresponding values were 263±197 mm and 82%, respectively. This shows that star-sawing has potential for the production of knot-free timber. 相似文献
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Pedro Ah Shenga Peter Bomark Olof Broman Dick Sandberg 《Wood material science & engineering》2017,12(4):257-262
The sawmill industry is a very important link in the Mozambique forest products value chain, but the industry is characterized by undeveloped processing technology and high-volume export of almost unrefined logs. The low volume yield of sawn timber has been identified as a critical gap in the technological development of the industry. To improve the profitability of the industry, there is thus a need to develop methods and techniques that improve the yield. In this paper, different positioning of logs prior to sawing and the possibility of increasing the volume yield of crooked logs by bucking the logs before sawing have been studied. A computer simulation was used to study the cant-sawing and through-and-through sawing of the logs to determine the volume yield of sawn timber from the jambirre (Millettia stuhlmannii Taub.) and umbila (Pterocarpus angolensis DC.) species. The optimal position, i.e. the position of the log before sawing that gives the highest volume yield of sawn timber for a given sawing pattern when the positioning parameters, offset, skew and rotation, are considered gave a considerable higher volume yield than the horns-down position. By bucking very crooked logs and using the horns-down positioning before sawing, the volume yield can be of the same magnitude as that obtained by optimal positioning on full-length (un-bucked) logs. The bucking reduces the crook of the logs and hence increases the volume yield of sawn timber. 相似文献
10.
鉴于百度试验法时木材干燥特性研究的不全面性,首先利用百度试验法开展了滇产黄毛青冈材干燥特性的初步研究,提出了预报干燥基础,再据此以地板坯料为干燥对象开展干燥工艺的试验研究,以全面归纳分析黄毛青网材的干燥特性。百度法研究表明：黄毛青冈材的初期开裂为4级,内裂为5级,截面变形1—3级,干燥速度为1-2级。地板料干燥工艺研究表明：采用研究提出的预报基准,23mm厚黄毛青冈地板坯料从初含水率52．3％干燥到12．9％,干燥周期为492h,但干燥质量达不到国家标准中对地板料干燥质量的要求,严重变形和过高的含水率偏差是此中主要原因。综合评定后认为,黄毛青冈属难干材,文中还就其干燥工艺的优化提出了建议。 相似文献
11.
《Wood material science & engineering》2013,8(2):76-84
Abstract Wood in general and wooden studs in particular are often distorted owing to uneven shrinkage during the drying process in the sawmill. Twist is often the most detrimental of all types of distortion, and it is caused by spiral grain in combination with variations in moisture content. For sawmills, the objective is to produce dried, straight boards, and one method of dealing with boards with excessive spiral grain is to sort them out and then dry them in a pretwisted position to obtain straight boards after drying. A model using the finite element (FE) method for the simulation of drying twist distortions was first calibrated against laboratory experiments in which boards were dried with and without restraints and pretwists. After the calibration, the FE results were compared with industrial test results for boards that were dried without restraints or with restraints with zero pretwist, i.e. straight restraints. The FE model used an elastic–ideally plastic material model to obtain permanent deformations. The calibration was to set the yield stresses so that there was a good match between FE results and results from the laboratory experiments. The comparison between the industrial test results and the FE results showed that the FE model is capable of realistic simulations of drying boards with and without restraints and presumably also pretwists. 相似文献
12.
Summary Unbarked windthrown softwood timber was stored in water for about two years and during that time the protection effect against
storage decay has been tested at certain intervals. The moisture content of the seperately swimming logs did not change remarkably
in the interior of the stems, only from their ends some water has diffused into the wood. Apart from a small light red-brown
discolouration in consequence of the penetration of tannin from the bark into the wood, which has no economical effect, in
all these logs not any injuries caused by fungi, insects or cracks could be observed. When the logs are piled in more layers,
the stems of the upper layers dried faster and showed all the wellknown storage damages. Provided that timber is sawn soon
after the landing, water storage can be considered as an effective mean to prevent degradation during storage, but if sawing
is retarded, the wood will be attacked by insects and fungi as well as the stems stored in forests after felling.
相似文献
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《Wood material science & engineering》2013,8(1-2):1-11
Abstract Growing markets for chopped firewood have created alternative uses for the by-products of sawmills. Based on empirical data and simulated results, the potential of birch (Betula pendula Roth, Betula pubescens Ehrh.) from commercial thinnings for combined industrial production of sawn timber and firewood billets was investigated. In the simulations, different sawing patterns were used for logs intended to combine production of sawn timber and billets for chopped firewood (‘sawlogs’), and for logs intended only to firewood production (‘firewood logs’). Finally, economical feasibility analysis was done concerning the differences between the sawmills’ traditional business concept and the novel concept combining sawn wood and firewood production. The bucking results for the volume yield of different timber assortments varied only slightly between the different bucking options, i.e. the combinations of timber assortments. The main differences in the volumes of timber assortments were due to the stand type where the birch trees were sampled (planted, naturally regenerated, mixed birch–spruce). In the sawing procedure, the output of sawn timber varied between 24% and 42% of the log volume in the sawlogs, depending on the log diameter class. As the volume yield of sawn timber and firewood billets was counted together in the case of sawlogs, the log consumption was c. 1.75 m3 of roundwood per 1 m3 of sawn timber and firewood billets. In the case of the firewood logs, the log consumption rate was considerably lower, only c. 1.35. The economic calculations showed that using the firewood approach in sawing may increase the net added value of products by €1.9–5.4 m?3 of logs, depending on their diameter class. As a conclusion, parallel production of sawn timber and firewood from logs from the first and second commercial thinning of birch-dominated stands is a concept that could work as an alliance between a sawmiller and a firewood entrepreneur. The concept could be competitive compared with both traditional sawmilling and production of chopped firewood. 相似文献
14.
C. Foley 《Wood Science and Technology》2001,35(5):453-465
Knowledge of the three-dimensional orthogonal directions of wood material at any position within a tree is necessary for
the understanding of strength reducing effects of knots and essential for the continuation of research in areas which relate
small clear wood specimen behavior to the behavior of full size structural timber. A complete three-dimensional paradigm describing
the geometry of knots and related fiber distortion, initially derived to predict the strength-reducing behavior of knots in
structural timber of Norway Spruce with the finite element method, is presented in this article. Besides strength prediction
analyses, it is believed that the paradigm may be useful in other areas of research on structural timber that are effected
by fiber orientation, such as drying and form change of structural timber. The paradigm generates fiber orientation in any
position within a log or lumber from assumed fiber patterns in planes parallel to the longitudinal direction of the original
tree. Fiber patterns in the radial and tangential directions are derived from physical restraints related to fiber production
within the annual increase surfaces of the tree and from theories of knot formation. The adaptability of the paradigm allows
practically any softwood knot to be modeled with an accuracy that is limited only by input-data. The knot-axis may be non-linear,
and the knot cross-section oval with its vertical and horizontal axis increasing from the pith of the stem at chosen rates.
Spiral grain may also be included in the paradigm and vary with the annual growth layers. Investigations presented in this
article showed that generated fiber orientations for Picea abies complied well with measured fiber distortions, and that the general trends of fiber orientation, explained by the applied
knot formation theory, is reflected in the measured specimens.
Received 12 May 1999 相似文献
15.
In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that
may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation
capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological
model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep.
The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood,
the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time.
The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load
for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever
under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic
element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep
tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore
predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process. 相似文献
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Guillermo Riesco Muñoz Andrés Remacha Gete Manuel Gasalla Regueiro 《Annals of Forest Science》2013,70(7):695-706
Context
The commercial feasibility of sawmilling depends on the expected volume and value of sawn planks. Models that predict the volume of sawn timber of a particular quality and produced from logs of known characteristics are therefore very useful.Aims
The objectives were to study variation in sawing yield and to obtain models that predict lumber volume and grade recovery on the basis of easy-to-measure predictor variables of saw logs.Methods
Forty-six oak trees growing in Galicia (NW Spain) were felled and cut into logs. The logs were visually graded and sawn mainly into quartersawn planks, which were dried, planed and visually graded for structural purposes.Results
The total volumetric sawing yield was 47.6 %. The sawing yield for planks of structural dimensions (cross-section, 70?×?120 or 70?×?170 mm) was 43.4 %, but decreased to 8.4 % for structural sized and quality grade beams because of wane and biotic damage in many pieces. Log grade did not significantly affect sawing yield in the sample analysed, despite the wide range of diameter over bark at the smallest end in the sampled logs (22–77 cm). The sawing pattern affected total sawing yield (F?=?4.913; p value?=?0.001) and the sawing yield for structural planks (F?=?6.142; p value?=?0.0002); radial sawing with one cut and live sawing of half logs provided the highest yields. Three models were proposed for estimating sawn volume in timber products, with the small-end log diameter over bark as the predictor variable and R adj 2 between 0.31 and 0.78 (p value?<?0.01).Conclusion
For the purpose of producing oak timber destined for structural use, the presence of bark and sapwood in planks must be reduced in the sawing process; this would decrease the total lumber recovery but increase the timber value yield. Air drying must be accelerated to reduce biotic damage in sawn planks. Geometric mean diameter over bark at the smallest end (d) outperforms other measures as a predictor variable for total or structural sawn timber volume. 相似文献17.
Steam conditioning of softwood boards after kiln drying is of critical importance for relief of residual drying stresses
and to improve distribution of final moisture content. The conditioning practice in New Zealand includes two steps: immediately
after high temperature (HT) drying the load is cooled until the core wood temperature is 75 to 90°C, and then the stack is
steam conditioned for a period of 1 to 4 hours depending on the lumber thickness and moisture content after drying. In this
work, experimental and theoretical studies were performed to better understand the conditioning process and to investigate
factors which influence its effectiveness. In the experiment, 50 mm thick Pinus radiata sapwood boards were first dried at 120/70°C for 11, 12, 13, 16 and 18 hours, respectively, to varying moisture contents,
and then cooled and steam conditioned for 1 hour. To assess the effectiveness of conditioning, moisture pick-up, moisture
gradient, and transverse residual drying stress (indicated by cup and strain) were measured. It was found that drying wood
to a low moisture content (below 6%) increased the conditioning effectiveness. A separate matched stack was conditioned for
4 hours after 13 hours drying which showed better results than 1 hour conditioning. A mathematical model for wood drying was
extended to include both the cooling and conditioning phases. The model was numerically solved to examine the wood temperature
and moisture content changes during the whole process of drying, cooling and final steam conditioning. Increase in wood temperature,
moisture pickup and moisture gradient during steam conditioning were predicted and validated by the experimental data. This
information is currently being used at the New Zealand Forest Research Institute in simulation of stress development and relief
for drying of Pinus radiata lumber.
Received 6 July 1998 相似文献
18.
Magnus Fredriksson Olof Broman Fredrik Persson Ann Axelsson Pedro Ah Shenga 《Wood material science & engineering》2014,9(1):31-39
With the development of scanning technology in sawmills, it is possible to optimise log rotational position when sawing. However, choosing a different rotational position than horns down might be detrimental for the board shape after drying, especially for curved logs. Thus, there is a need to investigate at what level of log curve it is possible to freely rotate logs without causing board warp. This study was carried out through a test sawing that was conducted at a sawmill situated in the middle of Sweden. The tests were made on 177 Norway spruce logs, with varying amount of curve. Half of the logs were sawn in the horns-down position, half were sawn rotated perpendicular to horns down. Log shape and warp of the dried boards were measured. The results indicated a relationship between board spring, log curve and choice of rotational position. Furthermore, board bow was related to log curve but not rotational position. It can be concluded that for straight logs, with a bow height of less than 15 mm, an unconventional rotational position does not cause excess spring in the boards. Bow and twist are not affected by the rotational position at all. 相似文献
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《Wood material science & engineering》2013,8(3-4):87-97
Abstract An integrated simulation tool, formed by integrating the InnoSIM sawing simulation system with the RetroSTEM simulator, was used to convert available wood raw materials from final felling into sawn timber, allowing for calculation of the three-dimensional wood properties of individual stems (stem geometry, heartwood formation, knottiness) as well as the volume, quality and value of sawn timber in a Norway spruce stand with different thinning regimes (unthinned, normal and intensively thinned). Based on the input data of sawing patterns, the simulations indicate that there are relatively small differences (<8%) in the volume yield (m3ha?1) of sawn goods resulting from sawlogs available from final felling with different thinning practices. However, intensive thinning yielded the largest stem diameters and the greatest volumes (m3ha?1) of large-sized centre goods (thickness: 50, 63, 75 mm) of rather poor quality. Normal thinning yielded the largest volume of A-grade side boards and centre goods (m3ha?1), as well as the best total value ([euro]ha?1) of sawn timber. Differences observed in sawn timber quality distribution can contribute to even more significant variation in value yields, if pricing mechanisms of timber products change to favour higher grade timber products. 相似文献
20.
Henrik R. Hallingbäck Karl-Anders Högberg Harald Säll Johan Lindeberg Marie Johansson Gunnar Jansson 《European Journal of Forest Research》2018,137(4):553-564
In breeding Norway spruce, selection for improved growth and survival is performed at age 10–15 years in order to optimize genetic gain per year. We investigated whether a selection based on wood traits such as density and grain angle, measured under bark in the field at the same age would be informative enough with respect to structural quality traits of sawn boards. To achieve this objective, a sawing study was conducted on the butt logs of 401 trees from a 34-year-old Norway spruce progeny trial situated in southern Sweden. Stem discs were excised from the top of the logs and radial profile data of grain angle, and wood density was recorded for specific annual rings. The sawn and dried boards were assessed for structural traits such as twist, board density, bending stiffness (static modulus of elasticity, sMoE) and bending strength (modulus of rupture, MoR). Additive genetic correlations (ra) between single annual ring density measurements and board density, sMoE and MoR were consistently strong (ra>?0.7) for annual rings 5–13. Genetic correlations of similar magnitude between grain angle and board twist were estimated for all investigated annual rings (from 2 to around 26 under bark). Consequently, it was found that indirect selection for wood density and grain angle at the tree age 10–16 years would result in more genetic gain per year than selection at later ages. This makes it feasible to perform simultaneous selection of progeny in the field for both growth and wood traits at similar ages. 相似文献