Showing papers in "Wood Science and Technology in 1972"

Tree growth stresses ? Part V: Evidence of an origin in differentiation and lignification

Abstract: An important relationship has been shown between measurements of expansion of the radial dimension of cells during the lignification phase of differentiation, widely reported swelling of the thickness of cell walls during lignification, the fundamental mechanics of strain interactions in the three cardinal directions in materials generally, and a theory linking anisotropic behaviour of wood to its microfibril arrangement and the effect of encrusting substances in the cell wall. As a consequence, it has been deduced that tangential, longitudinal and radial stresses of substantial magnitude are generated in wood tissues as a result of lignification. In comparisons with measured growth stresses, it has been concluded that they are probably identical with the stresses that would develop as a result of lignification of the cell walls.

The morphological variability of the bordered pit membranes in gymnosperms

Abstract: 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.

A review of surface and interfacial adhesion in wood science and related fields

Abstract: The study of adhesion in wood and wood-based products becomes increasingly important as work continues toward greater utilization of our total forest resources. The present quality of adhesively bonded wood products will be improved in order to achieve bond quality that harnesses the full strength of wood and, more importantly, leads to greatly enhanced bond durability. These goals will primarily be reached through developing a chemically-anchored bond interface and resin systems that are more flexible and compatible in the cured state.

Wood as a linear orthotropic viscoelastic material

Abstract: Measurements were made of the principal components of the creep compliance tensor in the radial-longitudinal and the tangential-longitudinal planes of Douglas-fir at 10 percent moisture content. Extensional creep compliance measurements at angles to the grain were also made. The results show that creep parallel to grain occurs at an increase in volume, and that creep at angles to the grain can be predicted from standard transformation equations. It is concluded that wood can be regarded as a linear orthotropic viscoelastic material.

The influence of microfibril angle on the longitudinal shrinkage-moisture content relationship

Abstract: Incremental longitudinal shrinkage has been measured on 44 samples of Pinus radiata at each 5% increment of moisture content from 0 to 25%. The samples range in mean microfibril angle from 10° to 40°. The data is presented in the form of a family of curves, of incremental shrinkage against microfibril angle, for each moisture content. This family of curves compares very closely with those derived theoretically by Barber [1968] and Cave [1972] based on considering the cell wall as a fibre composite of cellulose microfibrils embedded in a matrix which swells on wetting and whose shear modulus is a function of moisture content.

A theory of the shrinkage of wood

Abstract: A relation describing the shrinkage properties of a fibre-composite is applied to a multilayered thin-walled model of wood. With the theory developed it is possible to specify shrinkage with respect to moisture content, as well as with factors involved in cell-wall geometry and composition. However, until more is known of the properties of the water-reactive matrix component of the composite it is only practicable to describe shrinkage with respect to the geometrical and compositional factors at the moisture-content of the stress free state.

Orthotropic elastic constants of wood

Abstract: Wood tracheids are essentially tubular structures but wood cross sections are characterized by large numbers of triple points or junctures of wall segments from three adjacent cells. A symmetric triple point is taken as an approximation to the basic unit of wood structure. This element is analysed as a linearly elastic, isotropic body. It is shown that bending effects enhance the deformations arising from simple strains so that the overall response of the element is anisotropic. The resulting stiffnesses are ordered $$E_L \user2{ > }E_R \user2{ > }G_{LR} \sim G_{LT} \user2{ > }E_T \user2{ > }G_{RT} $$ for what are considered to be fairly typical element geometries. It is shown that for all geometries the longitudinal Youngs modulus is proportional to the volume fraction of cell wall material.

The characterization of wetwood in western hemlock

Abstract: One of the problem areas in the kiln drying of western hemlock lumber is the wide variation in final moisture content of the wood. This variation in moisture content is due to the presence of sinker or wetwood in the heartwood. The features of wetwood which differentiate it from the normal heartwood include higher specific gravity, higher extractives content, and lower permeability. The apparent higher specific gravity can be fully accounted for by the higher extractives content. The principial extractive is α-conidendrin. The wetwood in western hemlock often occurs together with ring shake and under these circumstances the white deposit on the shake surfaces is also α-conidendrin and not matairesinol, the substance usually associated with ring shake in western hemlock.

Swelling of a fibre reinforced composite in which the matrix is water reactive

Abstract: The swelling behaviour of an elastic fibre reinforced composite consisting of an inert fibrous phase embedded in a water reactive matrix is described. It is supposed that the addition of water causes the matrix to swell and become more compliant while the fibres passively resist the swelling stresses exerted by the matrix. A concise relation for the shrinkage or swelling strain is derived in terms of moisture content, the volume proportions of the phases, the overall elastic constants of the composite, the elastic constants of the matrix, and the moisture content at which the reinforcing elements are stress free.

Some aspects of the rheological behaviour of wood. Part IV : Non-linear behaviour at high stresses in bending and compression

Abstract: The incidence of non-linearity with stress in the rheological behaviour of wood in bending was shown earlier to occur at an unexpectedly low stress. This has been further confirmed by the study of an additional species, alpine ash.

The behaviour of particle board and hardboard beams during moisture cycling

Abstract: The effects of creep at constant conditions of moisture content and the effects of deformations occurring during cycling of moisture content have been studied in loaded beams of particleboard and hardboard. The variations in stiffness in loaded and unloaded beams of particleboard during moisture cycling were also determined. Contrary to the findings of some workers but in agreement with the findings of another, the increases in the deflections of loaded beams of particleboard during cycling of moisture content were due mainly to desorption of moisture. Qualitatively similar behaviour was found for particleboard, hardboard and wood but quantitatively the effects were greatest in particleboard and hardboard. It appears that the basic mechanism of the transient effects in bending, arising with the simultaneous action of load and change in moisture content, may be similar in wood, particleboard and hardboard.

The fibre saturation point of various morphological subdivisions of Douglas Fir and Aspen wood

Abstract: In both Douglas-fir and aspen wood, the fiber saturation point of early wood is greater than that of late wood, solvent extraction lowers the fiber saturation point, and the fiber saturation point for reaction wood is less than that for whole wood.

Review of the sufficiency of research on the swelling pressure of wood

Abstract: Terms used in relation to swelling pressure are first defined. The present status of knowledge of the swelling pressure of wood under various conditions is then briefly discussed.

The ultrastructure of intercellular passageways in vessels of yellow poplar (Liriodendron tulipifera, L.) part I: Vessel pitting

Abstract: The newly matured intervessel pit membranes of yellow poplar contain distinct openings. With increasing age the membranes become progressively occluded with incrustations which obstruct the small openings. Openings were not detected in the vessel-to-ray parenchyma and vessel-to-fiber pit membranes at any age.

Osmotic stress in wood — part II: on the computation of drying stresses in wood

Abstract: Plastic stress arising in wood during drying is calculated according to the theoretical model developed earlier. The mechanism of stress reversal and the type of resudual stress corresponding to different values of material constants are shown. The results are in qualitative agreement with experimental evidence.

Fibrillar angle variation in red pine determined by Senarmont compensation

Abstract: Fibrillar angles were determined in the secondary wall layers of three rings across the stem of a mature red pine. The Senarmont compensation method was adopted for this study since it gives results for all three wall layers. It involves the determination of birefringence in a series of sections cut at various angles to the long axis of the tracheids. Maxima and minima in the resultant plots of birefringence versus section angle correspond to fibrillar orientations. In the “early latewood” of each ring, the S2 layr was found to contain a single Z helix of small fibrillar angle and the S1 dual S and Z helices of much flatter orientation. Results for S3 were found to be so close to those for the corresponding S2 layers that there was some doubt as to their validity. In each layer a slight decrease in fibrillar angle was observed between rings 4 (juvenile wood) and 12 (transition zone); a sharper decrease was seen between ring 12 and the mature ring 20. This trend corresponds inversely to tracheid length differences for the three rings. Correlations of tracheid wall thickness and overall cross-sectional dimensions with the fibrillar angle variations were less readily apparent.

Tyloses in fibre tracheids

Abstract: Since the discovery of tyloses in the secondary xylem of angiosperms, their occurence has been noted only for vessels, although parenchyma cells necessary for their formation are also associated with fibriform elements. Sporadic remarks about tyloses in tracheids related to gymnosperm species, and the only observation on tyloses in angiosperm tracheids has been identified as an error. An anatomical investigation of various magnoliaceous timbers has now demonstrated that a development of tyloses in fibre-tracheids can also be regarded as a part of the normal transition from sapwood to heartwood in angiosperms. More than half of the 22 species studied were characterized not only by tyloses in the vessels but also in the fibretracheids. All the main genera with the exception of Liriodendron show the development of tyloses in the two different xylem elements. Distribution, frequency, and type of tyloses in the fibretracheids are comparable with those in the vessels.—Further investigations on other species of Magnoliaceae are undertaken in order to clarify possible anatomical relations regarding the development of tyloses.

Measurement of mean microfibril angles of wood tracheids

Abstract: During a recent investigation into wood fracture, a need arose to measure the mean microfibril angles of those tracheids which had been split in the fracture. The following technique, which results in the removal of an undisturbed surface layer of tracheids from the specimen, enables this to be done. The specimen is delignified by boiling in a suitable solution, (for example a 2 : 1 mixture of 100 volumes hydrogen peroxide and glacial acetic acid). After lignin removal, the wood block retains its original shape, although it is very fragile. I t is then washed, dried, and glued to a glass microscope slide, with the surface containing the split tracheids against the slide. The glue should be quite stiff when applied, as it must penetrate the specimen no further than the first layer of tracheids (Fig. 1). \"Araldite\" epoxy resin is suitable.

Exothermic heat released during catalytic polymerization of basswood-methyl methacrylate composites

Abstract: Oven dried basswood was vacuum impregnated with methyl methacrylate monomer containing various amounts of free radical catalyst and trifunctional crosslinking agent. The wood-monomer composite was polymerized at 60°C in an air oven and the rate of heat generated was followed by means of a thermocouple placed in the center of the sample.

Osmotic stress in wood — Part I: The analogy between thermal and swelling stress

Abstract: The analogy between thermal stress and swelling (drying) stress in wood makes it possible to give mathematical formulae for calculating the drying and swelling stresses in many situations. The models allow for viscoelastic and plastic phenomena.

Dielectric measurement of moist wood in a sealed system

Abstract: Dielectric measurements were carried out on moist wood at 25° C in ac fields from about 4500 rms V/cm and over a frequency range from 100 Hz100 kHz During the measurements the wood specimens were hermetically sealed in a special type of sample holder With this new measurement technique it was found that the dissipation factor and dielectric conductivity were field-dependent at low frequencies

Tree growth stresses—Part IV: Visco-elastic strain recovery

Abstract: Experimental methods for the determination of visco-elastic strain recoveries in timber are described. Intensive studies were carried out on Eucalyptus regnans F. Muell., to determine the characteristics of visco-elastic, longitudinal strain recovery, and its distribution along radial limes, in wood from butt, middle and top logs of trees; and also to investigate the effect on strain recovery of different diameters of trees and tree ages. Much less extensive tests, made on three other eucalyptus species and on Pinus radiata D. Don, were undertaken to explore the possibility of strong species differences.

Protection of Wood Particle Board against Termites

Abstract: Particle boards are of increasing economic importance. Without special treatment they are destroyed by termites. Deterioration of unprotected material as well as possibilities and present knowledge of preservation against termite attack are briefly described. Recent laboratory experiments with particle board treated to make it resistant to humidity and basidiomycete fungi showed that only one organic preservative out of four protected the boards also against termite attack; one of two water-soluble preservatives gave promising results. Heterotermes indicola (Wasmann) from India proved to be more aggressive and resistant to the preservatives than Coptotermes amanii (Sjostedt) from Kenya and Nasutitermes nigriceps (Haldemann) from Guatemala. Kalotermes flavicollis (Fabr.) from Italy was also very tolerant to the fungicidal preservatives. Further development of treatments of particle board against termites is required.

Maximum effective vessel diameters of hardwoods

Abstract: Measurements of the gas pressure required to displace water and allow gas bubbles to start continuously passing through the void structure of hardwood cross sections varying in thickness from 01 100 cm have been used to calculate the maximum effective opening diameters These diameters decrease with an increase in the thickness of the cross sections, as is the case for softwoods Extrapolation of diameter—thickness plots to zero thickness gives the maximum vessel diameters, which agree well with microscopically determined values The relatively small decrease in the maximum effective opening diameters with increasing thickness of the cross sections for most of the sapwood samples tested is due to irregularities in the vessel diameters and presence of perforation plates at the ends of the vessel segments The decrease in the maximum effective opening diameter with an increase in the thickness of the cross sections is greater for the heartwood than for the sapwood The ratio of the sapwood to the heartwood values tends to become constant between cross section thicknesses of 25 and 50 cm This ratio was small for tyloses free northern red oak but appreciable for the hardwoods containing tyloses in the heartwood Air drying followed by oven drying and resaturating of the specimens with water caused only small variable changes in the maximum effective opening diameters that were within the range of experimental accuracy of the measurements (approximately 5%)

The effect of shear and poisson's ratio in the static bending of wood beams

Abstract: New displacement equations for the static bending of six common types of simple, finite anisotropic or orthotropic beams are presented. The effect of shear and Poisson's ratios in the determination of Young's modulus by static bending is discussed.