Microstructure of wood char

Summary The exposure of wood treated with a commercial fire retardant chemical to fire conditions resulted in different patterns of char and fissure development than are seen in untreated whole wood. In addition microstructural observations demonstrated the existence of particulate fire retardant residues in char from treated wood. The source and mode of growth of these particles is considered as well as effects of fire retardants on crack growth, char development and overall appearance of chars from treated wood.The authors wish to acknowledge the general support of the University of California Berkeley Fire Research Group (NSF-RANN-S-22053, NFPCa-S-22584), University of California Berkeley Electron Microscopy Lab. (NSF-GB-38359) and the University of California, Davis Department of Botany (NSF-GB-29653). In addition we would also like to thank Dr. Arno P. Schniewind, University of California Forest Products Laboratory and Dr. R. Falk, University of California, Davis, Department of Botany     

A review of residual stresses and tensioning in circular saws

Summary The origin and measurement of residual stresses and their effect on the transverse stability of circular saws are discussed, with emphasis placed on nondestructive stress analyses, their limits of application, and their possible adaptation to the measurement of residual stresses in circular saws. Saw stability variations can be computed once the stress distribution is known. Evaluation of the X-ray diffraction technique and the ultrasonic and magnetic methods were considered for this purpose.Alternatively, saw stability can be related to resonance and bending stiffness measurements in specific modes. However, the state of stress cannot be inferred from these tests. For saw stability prediction, measurement of the state of stress is more essential than are direct stiffness measurements.The authors would like to express their gratitude to Professor R. M. Bragg, Department of Material Science and Engineering, U. C. Berkeley, and to Dr. R. L. Gause and Mr. W. N. Clotfelter, Marshall Space Flight Center, Huntsville, Alabama, for valuable suggestions and permission to use X-ray and ultrasonic equipment. The authors are also grateful for the financial support of the project from the University of California Forest Products Laboratory; the California Cedar Products Company, Stockton; the California Saw, Knife and Grinding Company, San Francisco; Sun Studs, Roseburg, Oregon; Weyerhaeuser Company, Tacoma, Washington; and McIntire-Stennis Funds.     

Friction coefficient as a guide to optimum rake angle in wood machining

Summary In spite of the obviously important role of friction in the wood cutting process, there has been little application in wood machining practice of true friction coefficients obtained from rubbing tests. Friction coefficients were measured during slow rubbing of incense-cedar blocks at two moisture contents on surfaces of saw steel, high speed steel and tungsten carbide. The coefficients were used in an orthogonal model of cutting to estimate the optimum rake angle for the planing of incense-cedar, and some evaluation of the estimate was made using data from cutting experiments reported elsewhere.The author wishes to thank Charles Berolzheimer, Director, The California Cedar Products Company Research Department, for his support of this work, and Richard Lemaster, University of California Forest Products Laboratory, for provision of facilities and assistance in that laboratory     

Development of kiln drying schedule severity indices for degrade control

Summary Severity of kiln schedules used to dry wood has traditionally been determined after the fact. As an alternative, analytical indices using integral calculus have been developed to rate kiln schedule severity. Theoretical justification is presented for the general form of an index as well as for several specific cases. Possible uses of this type of index are also indicated. The indices take into account the dry and wet bulb temperatures used throughout a schedule as well as the duration of application and points of application of each combination of temperature and humidity used.The assistance of the California Cedar Products Company, Stockton, California, and, in particular, Charles Berolzheimer and John Rhemrev, is gratefully acknowledgedThe authors are, respectively, Research Associate and Project Leader, Forest Products Laboratory, University of California, Richmond     

Development of kiln drying schedule severity indices for degrade control

Summary A previous article presented the theoretical development of analytical indices to rate the severity of kiln schedules used to dry wood. These indices take into account the dry and wet bulb temperatures used during a kiln schedule together with the points of application and duration of application of each combination of temperature and humidity used. Experimental data for several such indices are presented for collapse development during the kiln drying of incense cedar pencil slats. While promising, the results were inconclusive as to the use of a severity integral for prediction of the effect of different schedules or schedule changes on drying degrade. A lack of homogeneity in the test material confounded the results.The assistance of the California Cedar Products Company, Stockton, California and, in particular, Charles Berolzheimer and John Rhemrev, is gratefully acknowledgedThe authors are, respectively, Research Associate and Project Leader, Forest Products Laboratory, University of California, Richmond     

Differential calometric analysis of wood and wood components

Summary The prolysis of cellulose, hemicelluloses, lignin preparations, and wood was studied by differential calorimetric analysis (DCA) for the range of 25° to 800° C. The test samples included powdered and filter paper celluloses; hardwood xylan; softwood galactoglucomannans, compression wood galactan, and arabinogalactan; a synthetic (DHP), sulfuric acid, Björkman, Brownell, and cellulase lignins; and unextracted and extracted hardwoods and softwoods. Heats of reaction were determined from the DCA thermal transition areas. Distinct differences were found between the thermograms of each hemicellulose and lignin sample. Although wood species could not be separated thermally, hardwood and softwood thermograms differed because of the hemicellulose degradation pattern.Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the U.S. Department of Agriculture.Formerly Research Technologist, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture. The Laboratory is maintained at Madison, Wis. 53705, in cooperation with the University of Wisconsin. Present address: The Pennsylvania State University, University Park, Pennsylvania 16802.     

Dynamic behavior of wood strip over elastic underlayment composite flooring subjected to light impact loads

Summary The dynamic behavior of composites consisting of wood flooring strips placed over rubber sheeting was investigated. A light impact load was applied to the center of the strips and impact force, contact duration and contact area were measured. The energy transfer from the impacting mass to the composite was estimated by measuring the restitution coefficient. Frequency analysis was conducted in order to examine the vibration characteristics of the composites.This study was carried out at the Forest Products Laboratory, University of California at Berkeley while the first author was a visiting scholar supported by the Science and Technology Agency, Japan     

Measurements of compression creep of wood at humidity changes

Summary Uniaxial compression creep tests of tubular spruce specimens loaded in uniaxial compression are reported. The relative humidity in the interior of the tube is varied in a controlled manner. The results indicate that creep is greatly increased in magnitude by simultaneous humidity changes of either sign.Thanks are due to Dr. John Zahn of Forest Products Laboratory, Madison, Wisconsin, for various helpful advice, as well as for arranging the manufacture of test specimens in his laboratory. Thanks are also due to Forest Products Laboratory of the U.S. Department of Agriculture, Madison, Wisconsin, for funding a large part of this work under subcontract FP-81-0389.     

Constitutive equation of wood at variable humidity and temperature

Summary Theoretical analysis of the effects of variations in moisture content and temperature on the creep of wood is presented. Thermodynamics of the processes of diffusion of water in wood microstructure is discussed and distinction is drawn between macrodiffusion and microdiffusion. The constitutive relation for steady states of moisture content and temperature is formulated on the basis of Maxwell chain model whose viscosity coefficients depend on moisture content and temperature. It is shown that the apparent acceleration of creep due to simultaneous drying (or wetting) as well as heating (or cooling) may be modeled as additional, stress-induced shrinkage (or swelling) and stress-induced thermal expansion (or contraction), described by shrinkage and thermal expansion coefficients that depend on the absolute values of the rates of pore humidity and temperature. Certain other sources of irreversibility of creep are also discussed.Thanks are due to Forest Products Laboratory of the U.S. Department of Agriculture, Madison, Wisconsin, for funding a large part of this work under subcontract FP-81-0389. Thanks are also due to John J. Zahn, research engineer from this laboratory and the subcontract monitor, for his numerous valuable discussions during the progress of the work.     

Detection of lignans in western hemlock by radiography

Summary Evidence of the detection by radiography of -conidendrin and hydroxymatairesinol, major lignans of western hemlock wood, is presented. Advantages of this new technique to wood chemists, as well as implications to wood technologists in terms of erroneous measurement of the specific gravity of unextracted wood by X-ray densitometry, are discussed.The authors thank R. M. Kellogg of the Western Forest Products Laboratory for his helpful discussions. This research was supported in part through grants to J.H.G. Smith, University of British Columbia, Faculty of Forestry, from the National Research Council of Canada (Grant No. A-2077) and the British Columbia Forest Service.     

Theoretical models for creep slip of nailed joints between wood and wood-based materials

Summary Applying modern methods of analyzing floor, wall, and connection subsystems in light-frame wood buildings requires information on the stiffness of nailed joints under long-term loads. Because this information can best be derived by testing nail-joint specimens under constant loads, theoretical procedures were developed that use test data for constant loads to predict stiffness under variable in-service loads; five nonlinear, viscous-viscoelastic models were develpoed on the basis of existing formulations of creep and mechanisms of load transfer between nails and wood. The models incorporated the modified superposition and strain-hardening principles in describing responses to discrete or continuous loading functions. Tests have shown that the models closely predict creep slip of typical nailed joints.This research was jointly supported by the Forest Research Laboratory, Oregon State University, and the Cooperative State Research Service, U.S. Department of Agriculture, Special Grant 85-CRSR-2-2553. This is Paper 2288 of the Forest Research LaboratoryFormerly Graduate Research Assistant Forest Research Laboratory     

Changes in structural and chemical components of wood delignified by fungi

Summary Cerrena unicolor, Ganoderma applanatum, Ischnoderma resinosum and Poria medulla-panis were associated with birch wood that had been selectively delignified in the forest. Preferential lignin degradation was not uniformly distributed throughout the decayed wood. A typical white rot causing a simultaneous removal of all cell wall components was also present. In the delignified wood, 95 to 98% of the lignin was removed as well as substantial amounts of hemicelluloses. Scanning and transmission electron microscopy were used to identify the micromorphological and ultrastructural changes that occurred in the cells during degradation. In delignified areas the compound middle lamella was extensively degraded causing a defibration of cells. The secondary wall, especially the S₂ layer, remained relatively unaltered. In simultaneously white-rotted wood all cell wall layers were progressively removed from the lumen toward the middle lamella causing erosion troughs or holes to form. Large voids filled with fungal mycelia resulted from a coalition of degraded areas. Birch wood decayed in laboratory soil-block tests was also intermittently delignified. Selective delignification, sparsely distributed throughout the wood, and a simultaneous rot resulting in the removal of all cell wall components were evident. Scanning electron microscopy appears to be an efficient technique for examining decayed wood for fungi with the capacity to selectively delignify wood.The authors would like to thank Kathy Zuzek for technical assistance and Dr. M. Larsen, Forest Prod. Lab., Madison, for identifying the sporophores of Poria medulla-panis. This research was founded in part by a grant from the USDA Forest Service, Forest Products Laboratory and from the Graduate School, University of Minnesota     

The morphological variability of the bordered pit membranes in gymnosperms

Summary The membranes of bordered pits were investigated in 120 species of 47 genera of the gymnospermous families, using light and electron microscopic techniques. A great variability in the morphological structure of the pit membranes has been established. The differences in structure are pronounced and depend mainly on the genus involved.A classification of six morphological structure types is presented. They do not necessarily correspond with the chemical composition and the functional behaviour. The results on the variability of the pit membrane have shown the necessity of an extended definition for the torus.Dedicated to Prof. Dr. A. J. Stamm on the occasion of his 75 th birthday.Gratitude is expressed to Prof. Dr. R. W. Kennedy and Dr. R. W. Meyer, Forest Products Laboratory, Vancouver, B. C., Prof. R. D. Graham, Prof. Dr. D. Hermann and Prof. Dr. R. Krahmer, School of Forestry, Corvallis, Oregon, and Prof. Dr. V. La Marche, University of Arizona, Tucson, for their support in obtaining wood samples. Thanks are also due to Mrs. H. Berndt for her careful technical assistance.     

不同温度下喷施油菜素内酯对香椿幼苗的影响

为了探究外源油菜素内酯(BR)对不同温度(8、28、38℃)下香椿幼苗生长的调节作用,以盆栽5月龄香椿幼苗为试材,以喷施清水为对照,研究外源油菜素内酯对不同温度下香椿幼苗生物量、叶绿素含量、光合特性、叶绿素荧光特性的影响。结果表明;外源BR处理可提高不同温度下香椿幼苗叶绿素的含量,显著提高香椿幼苗生物量和干物质积累量,使香椿幼苗叶片组织加厚,叶片相对含水率(RWC)提升,并且提高了香椿幼苗的净光合速率(P_n)、气孔导度(Gs)、最大光化学效率(F_v/F_m)、实际光化学效率(ΦPSⅡ)和光化学猝灭系数(qP),降低了胞间CO_2浓度(C_i)、蒸腾速率(T_r)、初始荧光(F_o)、非光化学猝灭系数(qN)。综上所述:喷施油菜素内酯可以促进适温下(28℃)香椿幼苗植株的生长,提高香椿幼苗在温度胁迫下的缓解作用和适应能力。     

Utility pole decay

Summary The Basidiomycetes associated with decay in pine, Douglas-fir, and cedar utility poles within various geographic regions of North America were investigated. On the basis of 313 isolations from these poles, 9 fungi appeared to be of major importance in internal pole decay in the United States. These fungi were the following: Lentinus lepideus, Lenzites saepiaria, L. trabea, Peniophora A., P. gigantea, and Poria radiculosa in pine, and L. lepideus, Poria carbonica, P. monticola, and P. xantha in Douglas-fir poles. Lentinus lepideus was overwhelmingly predominant in pine poles, whereas Poria carbonica was similarly predominant in Douglas-fir poles. Lenzites trabea was the fungus most often isolated from cedar poles; however, it is believed to be associated primarily with shell rot of cedar poles and to be of little significance, therefore, in causing internal decay of cedar. The fungi associated with western red-cedar and red and jack pine poles in Canada are listed, although the frequency of their occurrence is not included.This work was in cooperation with the Navy Department, Naval Facilities Engineering Command.The author is indebted to the following for furnishing cultures and pole sections or both for culturing or for information on the identities of the species of decay fungi found in Canadian poles: Joe Clark and John Kulp, U. S. Forest Products Laboratory, Madison, Wisconsin; Robert Graham and John Mothershead, Forest Research Laboratory, Oregon State University, Corvallis, Oregon; O. Floyd Hand, Bonneville Power Administration, Vancouver, Washington; John Shields, Canadian Forest Products Laboratory, Ottawa, Ontario; and John Roff, Canadian Forest Products Laboratory, Vancouver, British Columbia. The author is especially indebted to members of the Forest Disease Laboratory, Laurel, Maryland, for their invaluable aid in identifying representative cultures of many of the fungi discussed in this investigation.The Laboratory is maintained at Madison in cooperation with the University of Wisconsin.     

The validity of the use of SEM/EDAX as a tool for the detection of UF resin penetration into wood cell walls in particleboard

Summary The interpretation of data from scanning electron microscopy (SEM) combined with energy dispersive analysis of X-rays (EDAX) is dependent on the size of the excitation volume, the magnitude of which for organic materials such as wood has been little studied. From the analysis of a model system comprised of a thin layer of gold sandwiched between two layers of an epoxy plastic, it was shown that about 90% of all excitations arise from a volume about 6 m wide and 4 m deep. However, these dimensions vary with the orientation of the gold relative to the specimen surface, and the specimen tilt. Theoretical predictions based on these data support, but not conclusively, the previously published SEM/EDAX evidence of the penetration of urea formaldehyde resins into the wood cell wall in particleboard manufacture.This paper was written while A. J. Bolton was Heritage Visiting Scientist at the Forest Products Laboratory, U.S. Department of Agriculture, and Visiting Professor at the Department of Forestry, University of Wisconsin, both at Madison, Wisconsin, U.S.A. This support is gratefully acknowledged. The authors would also like to acknowledge the assistance of E. Edwards in machining the SEM specimens     

Predicting the load capacity of wood composite I-beams using the tensor polynomial strenght theory

Summary The tensor polynomial strength theory for anisotropic materials was coupled with finite-element analyses to predict the ultimate load capacity of several wood-composite I-beams. Small-specimen tests with the constituent materials provided elastic constants for the finite-element computations and ultimate strengths for development of strength tensors.Ultimate load capacity, failure modes, and sites of failures in wood composite I-beams were predicted. Analytical results were in excellent agreement with data from full-scale I-beam tests and demonstrated the influence of web joints and material strengths on failure characteristics.The generous assistance of the Trus Joist Corporation, the Blandin Forest Products Company, and the Potlatch Corporation, and the financial support of the Cooperative State Research Service, USDA Forest Service, and the Alabama Agricultural Experiment Station at Auburn University are gratefully acknowledged. This is Paper 2316, Forest Research Laboratory, Oregon State University, Corvallis, Oregon     

A reexamination of some deviations from Darcy's Law in coniferous wood

Summary A number of workers have noted a marked decrease of wood permeability with increasing specimen length, and to date a model proposed by Bramhall has been used to explain the phenomenon. This describes the effect of random blockages on the permeability of an anisotropic porous medium. However, the model does not allow for the fact that transverse flow may allow longitudinal flow paths to circumvent blockages to some extent, so some deviation from it can be expected. The issue is an important one if laboratory experiments are to be used as a basis for the prediction of the behaviour of wood during impregnation or drying on a commercial scale. A modified model which allows for transverse flow is therefore developed. New experimental evidence which cannot be explained by the Bramhall model, but which can be explained by the new model, is presented.This paper was written while the author was Heritage Visiting Scientist at the Forest Products Laboratory, U.S. Department of Agriculture, and Visiting Professor at the Department of Forestry, University of Wisconsin, both at Madison, Wisconsin, USA. This support is gratefully acknowledged. The author would also like to thank Messrs. J. Ambler, M. C. Breese and S. H. Turner, who carried out some of the experimental work, and staff at the Commonwealth Forestry Institute, Oxford, who provided the Juniperus procera material tested     

Chemical mechanism of fire retardance of boric acid on wood

It is commonly accepted that the fire retardant mechanism of boric acid is a physical mechanism achieved by the formation of a coating or protective layer on the wood surface at high temperature. Although a char-forming catalytic mechanism has been proposed by some researchers, little direct experimental support has been provided for such a chemical mechanism. In this paper, new experimental results using thermal analysis, cone calorimetry (CONE), and gas chromatography–Fourier transform infrared spectroscopy (GC–FTIR) analysis are presented and the fire retardant mechanism of boric acid on wood is discussed. Basswood was treated with boric acid, guanylurea phosphate (GUP), and GUP–boric acid. Treated wood was then analyzed by thermogravimetry (TG/DTG), differential thermal analysis (DTA), CONE, and GC–FTIR analysis. Thermogravimetry showed that the weight loss of basswood treated with boric acid was about three times that of untreated or GUP-treated wood at 165°C, a temperature at which GUP is stable. The DTA curve showed that boric acid treated basswood has an exothermal peak at 420°C, indicating the exothermal polymerization reaction of charring. CONE results showed that boric acid and GUP had a considerable synergistic fire retardant effect on wood. The GC–FTIR spectra indicated that compounds generated by boric acid treated wood are different than those generated by untreated wood. We conclude that boric acid catalyzes the dehydration and other oxygen-eliminating reactions of wood at a relatively low temperature (approximately 100–300°C) and may catalyze the isomerization of the newly formed polymeric materials by forming aromatic structures. This contributes partly to the effects of boric acid on promoting the charring and fire retardation of wood. The mechanism of the strong fire retardant synergism between boric acid and GUP is due to the different fire retardant mechanisms of boric acid and GUP and the different activation temperatures of these two chemicals.The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.     

Estimating the influence of knots on the local longitudinal stiffness in radiata pine structural timber

A theoretical model is developed to evaluate the negative effect of knots on the local longitudinal stiffness (elastic modulus) in radiata pine structural timber. The parameters in the compliance matrices for knotwood in branches are estimated by extending the mechanical formula for clearwood also to knotwood, assuming a structural similarity for stem and branch. The stiffness of knots in the longitudinal direction of sawn boards is then obtained by coordinate transformation. The effective local longitudinal stiffness in a mixture of wood (a combination of knotwood and stem wood) is estimated in terms of the elastic moduli and volume fractions of all phases using a modified rule of mixtures. The effects of branch angle and volume fraction of a knot on the local longitudinal stiffness of structural timber are simulated under different scenarios. Experimental observations also demonstrate similar trends to those shown in the simulations. Received 5 December 2000 The author is grateful to Prof. John Walker (The New Zealand School of Forestry), Dr. Huawu Liu (Wool Research Organisation of New Zealand), Dr. Ian Cave (Department of Chemistry, Canterbury University) and Dr. Dave Cown (New Zealand Forest Research) for their comments and advice during the preparation of this paper.