Wood and Fiber Science (E-Journal)
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The Ultrastructure of Pits in Paulownia Tomentosa
Get PDFThe pitting between various cell elements in Paulownia tomentosa was studied at the ultrastructural level. Plasmodesmata are more numerous in the pit membranes of ray parenchyma than in the longitudinal parenchyma. The edges of the pit borders in the latter are curved, while those of the former are more angular in shape. Numerous blind pits were found in ray parenchyma cells, but they occurred less frequently in fiber tracheids. The tylosis wall with a "compound middle lamella equivalent" layer archs over the half-bordered pit pair area, sometimes showing a recess at the half-bordered pit pair area. The pit membrane of all pitting types is a continuation of the compound middle lamella. Pit borders of vessel pits show a variety of forms and shapes. In bordered pit pairs, the S1 of the pit border ends at the rim of the pit chamber
Modeling The Nonlinear Moment-Rotation Relationship Of A Nail Plate Connector1
Get PDFThis paper presents a mechanics-based procedure for modeling the nonlinear moment resistance of multiple-dowel wood connections. The lack of a consistent methodology for predicting rotational resistance of multiple-dowel joints was identified as one of the barriers to adoption of a ready-to-assemble (RTA) wood framing system. Integral to the RTA system is a nail plate connector (NPC) that consists of a metal plate with multiple dowel and is used to assemble RTA framing members into complete structural systems. The principle of energy conservation is used to derive the model. The proposed procedure is formulated such that the nonlinear response of the nails and plate bearing are explicitly included in the model to accurately predict the moment-rotation relationship over a wide range of deformations. Therefore, the model provides the information on both the ultimate strength and deformation capacity needed to establish safety margins and to perform serviceability checks, respectively. The method requires input of the lateral load-displacement relationships for an individual nail and plate bearing on the end grain of wood framing member. These relationships can be readily measured using lateral test procedures or determined analytically. The formulation showed excellent correlation with test results (R2 = 0.98). The proposed model presents an engineering and research tool and has the potential to promote the use of timber frames assembled with multiple dowel joints
The Bordered Pit Membrane in Differentiating Balsam Fir
No full textBordered pit membranes in the cambial zone of balsam fir [Abies balsamea (L.) Mill.] were studied via surface replica and transverse section. An initially solid membrane is gradually perforated at tracheid maturity to yield a peripherally permeable margo bounded by a denser annulus and centrally thickened torus. Large, radially directed microfibrils in the margo are present at an early stage of pit development and do not arise as a result of pit aspiration. Relevance of findings to recent literature and viewpoints is discussed
Modeling the Effect of Out-of-Plane Fiber Orientation in Lumber Specimens
Get PDFA method is presented to account for the effect of three-dimensional fiber orientations near knots in a two-dimensional lumber tensile strength prediction model. Data we have collected show that grain angles dive from 15 to 90 degrees out of the wide face plane of flat-grained lumber within a region of about one knot radius from the visual edge of a knot. The diving nature of the grain angles is accounted for in a two-dimensional model, called GASPP+, by transforming a three-dimensional material compliance matrix, and extracting the appropriate coefficients for use in a two-dimensional compliance matrix. Failure criteria are modified to reflect the decreased strength associated with nonzero dive angles. These modifications led to accurate tensile behavior predictions, as evidenced by load-displacement plots and ultimate load measurements of lumber specimens. It is shown that consideration of dive angles is important in predicting the tensile strength and failure mode of thin lumber specimens. Lumber specimen thickness and the manner of loading influence the magnitude of the dive effect on strength
Effect of Density and Polymer Content on the Hygroscopic Thickness Swelling Rate of Compression Molded Wood Fiber/Polymer Composites
Get PDFThe effects of polymer content and board density on the hygroscopic thickness swelling rate of compression-molded wood fiber/polymer composites were investigated in this study. A swelling model developed by Shi and Gardner (2005) was used to study the thickness swelling process of wood fiber/polymer composites exposed to water vapor conditions in which a parameter, KSR, was used for the comparison of the swelling rates. The polymer materials used to process the wood fiber/polymer composites were from a reclaimed automobile plastic mixture, also called polymer fluff. Polymeric diphenylmethane diisocyanate (pMDI) resin was used as a binder. Six polymer contents (0, 15, 30, 45, 60, and 100%) and four target specific gravities (0.55, 0.75, 0.90, and 1.00) were evaluated in the experiments using the swelling model. It was shown that the swelling model successfully fit the empirical swelling rate data as impacted by different board densities and polymer contents. Board density has a significant effect on the swelling rate of the composites. The swelling rate increased linearly as board density decreased. The effect of polymer content on the swelling rate depends partially on board density. Polymer content did not show a significant effect on swelling rate at an oven-dry density of 900 kg/m3. It was also confirmed from this study that the accuracy of the swelling model prediction is a function of the magnitude of the swelling rate parameter. The lower the thickness swelling rate of the composites, the more accurate the prediction obtained from the swelling model
Longitudinal Water Permeability of Western Hemlock II. Unsteady-State Permeability
Get PDFA mathematical model was developed and used as the basis for constructing two experimental apparatuses to investigate the permeability of western hemlock to water. When unsteady-state and steady-state permeability were compared, the unsteady-state permeability of both sapwood and normal heartwood was found to be higher than the steady-state permeability, but that of wetwood was generally lower than the initial steady-state permeability and was of the order of the final steady-state permeability. The permeability of western hemlock to water, regardless of whether it is measured by steady-state or unsteady-state techniques, is the highest for sapwood, followed by wetwood and normal heartwood.Under unsteady-state conditions, sapwood permeability is time-independent, but wetwood exhibits time-dependent behavior, probably caused by blocking of the openings on pit membranes by movable extractives when water flows through the cell lumen. Storing western hemlock wetwood in water at room temperature reduces its water permeability. Wet pockets that form when wetwood of western hemlock is subjected to kiln-drying have lower permeability than the dried portion of the lumber
Orthotropic Strength and Elasticity of Hardwoods in Relation to Composite Manufacture Part III: Orthotropic Elasticity of Structural Veneers
Get PDFStructural veneers approximately 3.2 mm (1/8 in.) in thickness are widely used as basic constituents in structural composites such as plywood, laminated veneer lumber (LVL), and parallel strand lumber (PSL). The veneer processing operation (peeling) may adversely alter the mechanical properties of the wood substance by introducing compression-set, cracks, and splits, etc. The modulus of elasticity (MOE) in tension of five hardwood species, which are potential raw materials for composite manufacture, was investigated in veneer form. The experimental work included dynamic MOE determination using ultrasound stress wave timing and static MOE measurements for comparison purposes. The orthotropy of MOE in the longitudinal-tangential (LT) plane was also a target of the investigation. Theoretical models were fitted to experimental data that may predict the MOE of the constituents according to their position within the consolidated composites. Experimental and analytical results indicated that a combined model including the Hankinson's formula and an orthotropic tensorial approach is the best estimator for MOE of veneers having inclined grain orientation. Furthermore, the relationship between static and dynamic MOE values may be obtained by second-order polynomial models
Genetic Variation in Wood Mechanical Properties of Calycophyllum Spruceanum at an Early Age in the Peruvian Amazon
Get PDFCalycophyllum spruceanum (Benth.) Hook. f. ex Shum. is an important timber species of the Peruvian Amazon Basin. Due to overexploitation in natural populations, users are turning to young trees of potentially lower quality. Therefore, variation in juvenile wood properties should be investigated to determine whether wood quality can be maintained or, if necessary, improved by breeding. A provenance/progeny test was established to evaluate genetic variation in growth and wood properties of young trees, the strength of their genetic control, as well as their interrelationships both at the genetic and phenotypic levels. This paper presents results obtained for ultimate crushing strength (σL), the static compliance coefficient (S11) in longitudinal compression, the dynamic s11 in the longitudinal direction (determined by ultrasound), and air-dry density at 39 months. Results indicate that the mechanical properties of juvenile wood of this species are adequate for structural uses. There was significant variation in all wood properties due to families within provenances, and in all but dynamic s11 due to provenances. Families accounted for a larger percentage of the total phenotypic variance than provenances. Heritability estimates were higher for σL and static s11 than for dynamic s11 and density. Genetic correlations indicate that selecting trees with denser wood and/or faster growth would have a positive effect on some mechanical properties. A non-destructive ultrasonic method appeared suitable for estimating juvenile wood strength and stiffness of this species
Image Correlation Analysis of Multiple-Bolt Wood Connections
Get PDFDisplacement beneath the bolts in multiple-bolted wood connections was studied using digital image correlation. This method combines digital image analysis and image correlation to calculate surface displacements from a set of digitized video images of an object under an applied load. Double-shear connections constructed of clear, straight-grained yellow-popular were tested in compression parallel to grain. Five different bolt patterns were used to analyze the effect of number of bolts in a vertical row and number of bolts in a horizontal column on displacement distribution among bolts. It was discovered that for multi-bolt patterns in a vertical row, parallel to load displacements below the outer bolts, are higher than those below the center bolt(s) but not equal in magnitude as previously assumed and the surface displacements perpendicular to the load beneath the bolts split along the centerline of the bolts. Variation in material properties, rigid body motion, eccentric loading and/or wood failure beneath a bolt are detectable with digital image correlation and may influence the results if not carefully considered in experimental design