Mechanical response of wood perpendicular to grain when subjected to changes of humidity

This paper describes the deformations caused by stress and humidity interaction, mechano-sorption, in the cross grain directions of wood and the relaxation or accumulation of internal stresses caused by these deformations. Long-term tests on small clear specimens in cyclic climates with both tensile and compressive loads were carried out. The development of internal stresses in timber was measured indirectly at different times during the adsorption and desorption processes. Released deformations were measured from cross-sections after cutting them to small slices. These deformations were used to estimate the internal stresses caused by the humidity variations. Tests with constant loads and multiple humidity cycles show a mechano-sorptive strain that is ten times higher than the elastic strain. It is shown that existing models for describing mechano-sorption perpendicular to grain are inaccurate when applied to multiple humidity cycles. The present results demonstrate that if the mechano-sorptive behaviour and the moisture gradients in wood can be accurately described, it is possible to predict the stress distribution in a timber cross-section by knowing the climate history.     

Micromechanics of wood subjected to axial tension

Summary The behaviour of a small group of wood fibers of Sitka spruce during tensile loading is investigated. The load-extension curves for both early and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield point is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is increased further, the curve exhibits the third segment showed by a significant change in slope. These curves look different from those obtained on thick specimens. In this respect, the behaviour of a thin wood specimen subjected to cyclic type tensile loading along its longitudinal direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Young's modulus of a wood fiber during tensile loading. The model considers wood as an assembly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S₂ layer made of a composite material consisting of a lignin and hemicellulose matrix reinforced by helical microfibrils along the fiber. Furthermore, it is assumed that the microfibril angle a in the S₂ layer is not uniform along the fiber axis and matrix degradation occurs in the zones where the microfibril angles are bigger. The validity of this assumption is verified by using holographic interferometry to visualize the displacement field of the specimen's surface under tension.The work reported in this paper is supported by a grant from the Swiss National Science Foundation, and its support is gratefully acknowledged     

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     

Application of modal analysis by transfer function to nondestructive testing of wood I: determination of localized defects in wood by the shape of the flexural vibration wave

This study was intended to detect nondestructively some defects such as knots and grain deviations in wood using modal analysis. The shapes of flexural vibration waves at the first mode generated by the tapping of wooden beams were determined using the transfer function. The wave shapes obtained were compared with the theoretical wave shape for a uniform material; and the possibility of detecting defects in wood was examined. The results are summarized as follows: (1) The shapes of flexural vibration waves at the first mode of wooden beams free of defects coincided almost completely with the theoretical wave shape. (2) The shapes of flexural vibration waves of wooden beams containing defects such as knots clearly differed from the theoretical wave shape, especially near the defect. (3) Based on these results, it should be possible to detect the presence of defects and to determine their location in wood.Part of this report was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo, April 1999. This article is translated from the Japanese edition published inMokuzai Gakkaishi Vol. 47, No. 4, 1988     

Shape stability of modified engineering wood product subjected to moisture variation

Abstract

A modified softwood product would enable the utilization of softwood in new areas. Densification is an old modification method to improve wood properties such as hardness and resistance to abrasion. A major problem with densified wood is, however, its ability to retain its original dimensions under the influence of moisture. Therefore, this study investigated the influence of surface to bottom layer thickness ratio on the shape stability of a modified and three-layered cross-laminated engineering wood product (EWP) subjected to moisture variations. The study describes a simple solid wood densification technique based on compressing a clear solid piece of softwood with vertical annual rings in the radial direction by restraining the tangential expansion. Three-layered cross-laminated EWP was manufactured with the densified wood as a surface layer. The recovery of the densified wood in the surface layer was then reduced to movements in the same level as the other two layers of unmodified wood. The EWP was subjected to climatic variations in order to investigate its shape stability. The results disclosed that an appreciable degree of shape stability was obtained by an increase in the surface to bottom layer thickness ratio of the EWP.     

An experimentally validated fatigue model for wood subjected to tension perpendicular to the grain

This study presents an experimental investigation of fatigue in wood subjected to tension perpendicular to the grain. The study has been designed with special reference to the influence of the frequency of loading. The investigation reveals an interaction between number of load oscillations and accumulated time under load to failure. This interaction corresponds to frequency dependent fatigue. Current models for damage accumulation and failure modelling are screened with respect to their ability to account for such “two-source” damage. The Damaged cracked viscoelastic material model proved to give a good basis.     

Digital image analysis of radial shrinkage of fresh spruce (Picea abies L.) wood

Contact-free digital image analysis was performed of the radial shrinkage of fresh, fully saturated small spruce wood beams. An experimental test set-up was developed to ensure constant distance from the charge-coupled device camera to the sample surface as well as constant climate and light conditions during the whole experiment. Dimensional changes were observed immediately after the drying process began. An unexpected distinct effect could be observed which could not be explained by drying surface layers only. After a fast initial radial shrinkage a slowing down of the dimensional changes occurred at high mean moisture contents. A complete interruption of any dimensional changes followed. Finally, a recovery from shrinkage was even observed. It is assumed that strong negative pressure occurred in the fully saturated capillaries owing to dehydration which led to additional dimensional changes. As a consequence, the break of the water column and aeration in these capillaries finally resulted in a recovery period in the shrinkage rate due to the pressure release. After this effect, the dehydration was characterized by a phase of fast and almost linear shrinkage due to drying surface layers. Finally, the shrinkage slowed down to zero when reaching equilibrium moisture content.     

Effect of thinning on the development of compression wood in stems of Corsican pine

This study considered the effects of thinning on the development of compression wood in stems of 35-year-old stand of Corsican pine (Pinus nigra L.). Part of the stand had been thinned at 5-yearly intervals and part left unthinned. Twenty trees each from the thinned and unthinned stands were randomly selected and felled. Measurements were made on tree height, stem diameter, stem slenderness and canopy depth. Wood samples were removed from the central part of the main log and cross-sectional measurements made on ring width, basic density and compression wood content. Cross-sectional area of compression wood was found to be three time higher in stems from the unthinned trees in comparison with those from the thinned trees. No significant differences in mean radial ring width or basic density were found between treatments. Correlations indicated that, with increasing in stem diameter, compression wood content increased in the unthinned trees, while a decline in compression was observed in the thinned trees. Tree height was also positively correlated with compression wood content in unthinned trees, while no equivalent relationship was observed in thinned trees. Observations from this study, while not conclusive, suggest that phototropic stimulus may be producing stem inclinations in the unthinned stand as trees compete for space in the canopy, whereas crown competition has been largely eliminated in the thinned stand; and that this is responsible for compression wood levels recorded in this study.     

Mechanical analysis of the strains generated by water tension in plant stems. Part II: strains in wood and bark and apparent compliance

Tree stems shrink in diameter during the day and swell during the night in response to changes in water tension in the xylem. Stem shrinkage can easily be measured in a nondestructive way, to derive continuous information about tree water status. The relationship between the strain and the change in water tension can be evaluated by empirical calibrations, or can be related to the structure of the plant. A mechanical analysis was performed to make this relationship explicit. The stem is modeled as a cylinder made of multiple layers of tissues, including heartwood, sapwood, and inner and outer bark. The effect of changes in water tension on the apparent strain at the surface of a tissue is quantified as a function of parameters defining stem anatomy and the mechanical properties of the tissues. Various possible applications in the context of tree physiology are suggested.     

Influence of loading point on the static bending test of wood

We conducted three-point bending tests by changing the condition at the loading point and then examined the influence of the loading point on the test data. Yellow poplar (Liriodendron tulipfera L.) was used for the tests. First, using loading noses with various radii, static bending tests were conducted by varying the depth/span ratios. Deflections were measured from the displacement of the cross head and at the point against the loading nose: Young's and shear moduli were obtained from the modified Timoshenko's bending equation proposed in a previous paper. Then a similar testing procedure was undertaken by inserting cushion sheets of Teflon between the specimen and the nose. After the measuring these moduli, bending strengths were measured using the loading noses and cushion sheets. The following results were obtained: (1) When the deflection was measured from the displacement of the cross head, the radius of the loading nose had an influence on the additional deflection when the depth/span ratio was high, causing the dependence of the shear modulus on the radius. In contrast, the radius had little influence on the measurement of Young's modulus. By placing cushion sheets between the nose and the specimen, the effect of the radius was moderated. When the deflection was measured at the point against the loading nose, the radius of the nose had little influence on the additional deflection; hence the loading nose had little influence when obtaining Young's and shear moduli. This tendency was commonly observed regardless of whether the cushion sheets were in place. (2) When the specimen had a high depth/span ratio, the bending strength increased with the increase in the radius of the loading nose. However, the influence of the radius was small when the specimen had a low depth/span ratio. There was no significant effect of the cushion sheets used here on the measurement of bending strength.Part of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

Erratum to: Comparative analysis of a wood-adhesive bondline

The wood–adhesive interface was analyzed using five methods with the objective of quantitatively assessing penetration of adhesive into the porous wood network. Methods included fluorescence microscopy, scanning electron microscopy, backscatter electron imaging, wavelength dispersive spectroscopy, and X-ray microtomography (XMT). Each method provided a visual inspection, and all of the analysis methods were applied to the same field of view. XMT was the primary technique of interest. Rubidium hydroxide was used in place of sodium hydroxide in the formulation of phenol–formaldehyde adhesive. Rubidium was found to increase the X-ray attenuation of the adhesive. However, rubidium migrated beyond the adhesive interphase during specimen preparation, thus reducing its effectiveness for image contrast enhancement. The wood species studied included red oak (Quercus rubra), Douglas-fir (Pseudotsuga menziesii), and hybrid poplar (Populus deltoides × Populus trichocarpa). All techniques used for this study were useful, but each presented some limitations for bondline analysis. Despite the problem with rubidium migration, XMT for this application was promising.     

尼龙轮在木材装卸索道中应用

用尼龙轮代替木材装卸架杆中走行轮和起重轮的铸铁或铸钢材质轮,解决了铸铁或钢滑轮对钢丝绳的磨耗大寿命短的难题,可提高钢丝绳寿命。具有较大的使用价值和明显的经济效益。