Mechanical properties assessment of Cunninghamia lanceolata plantation wood with three acoustic-based nondestructive methods

The objectives of this study were to establish the method of evaluating wood mechanical properties by acoustic nondestructive testing at standing trees and at logs of a Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantation, and to compare three acoustic nondestructive methods for evaluating the static bending modulus of elasticity (MOE), modulus of rupture (MOR), and compressive strength parallel-to-grain (σ_c) of plantation wood as well. Fifteen Chinese fir plantation trees at 36 years of age were selected. Each tree was cut into four logs, for which three values of dynamic modulus of elasticity, i.e., E _sw, of the north and south face based on stress waves to assume the measuring state of the standing tree, E _fr, longitudinal vibration, and E _us, ultrasonic wave, were measured in the green condition. After log measurements, small specimens were cut and air-dried to 12% moisture content (MC). Static bending tests were then performed to determine the bending MOE and MOR, and compressive tests parallel-to-grain were made to determine σ_c. The bending MOE of small clear specimens was about 7.1% and 15.4% less than E _sw and E _us, respectively, and 11.3% greater than E _fr. The differences between the bending MOE and dynamic MOE of logs as determined by the three acoustic methods were statistically significant (P < 0.001). Good correlation (R = 0.77, 0.57, and 0.45) between E _sw, E _fr, and E _us and static MOE, respectively, were obtained (P < 0.001). It can be concluded that longitudinal vibration may be the most precise and reliable technique to evaluate the mechanical properties of logs among these three acoustic nondestructive methods. Moreover, the results indicate that stress wave technology would be effective to evaluate wood mechanical properties both from logs and from the standing tree.     

Bending creep behavior of wood under cyclic moisture changes

This study examined the bending creep behavior in the longitudinal direction of six species under cyclic moisture content (MC) changes. For each species, tests were made at 20°C with five cyclic relative humidity changes between 65% and 95%, beginning from moisture adsorption. A load corresponding to 25% of short-term breaking load of the species was applied to the radial section of each specimen with four-point bending. The effect of MC change on instantaneous compliance was also investigated under the same condition. The quantitative relation between mechano-sorptive (MS) compliance and MC change was examined, and the material parameter KM for the relation in specific sorption was determined. Results indicated that the total compliance in the six species with different behavior increases with sorption time. As an integral part of total compliance, instantaneous compliance changes linearly with MC and influences to a greater or lesser extent the total compliance behavior. In general, with increasing MC change, the MS compliance linearly increases during the first adsorption and all desorption and decreases slightly during subsequent adsorption. The material parameterK _M varies markedly not only with species but also with specific sorption. The first adsorption causes the largest deformation, followed by desorption.This paper was presented at the 48th annual meeting of the Japan Wood Research Society in Shizuoka, Japan. April 1998     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

An investigation of the influence of selected factors on the properties of spruce wood

Thirty Norway spruce trees (Picea abies (L.) Karst.) from the forest district of the ETH Zurich were tested for bending MOR, static MOE of bending and dynamic MOE (calculated from eigenfrequency and sound velocity). The specimens were clear and were sampled from the whole of the stem. Their correlations to density, annual ring width, height in the tree, distribution over the stem diameter and the percentage of compression wood were statistically analysed. All three elasticity modules and the maximal stress can be very well predicted from a linear function of the sample density with a common gradient across the compression wood values but with different intercepts that decrease with increasing compression wood content. The other variables have highly significant impacts on the response variables too, however, this is largely irrelevant for the goodness of fit. Further, a clear increase of density, of MOE and of bending MOR was measured from pith to bark and similarly with decreasing annual ring width. Concerning the height of the stem, no distinct trend for the mechanical properties could be found.     

竹材在压缩大变形下的力学行为Ⅱ.微观变形特征

以毛竹小试件为对象,使用显微加栽装置研究了竹材在三个方向的压缩大变形下微观结构的变化特征。研究表明,竹材在径向受压和弦向受压时具有相同的屈服极限,这主要取决于薄壁基本组织在横向具有相同的屈服行为。由于竹材基本组织是传递荷栽的优良基体,竹壁在径向压缩和弦向压缩下具有相同的宏观力学行为,可视为两相纤维复合材料;在轴向压缩大变形下,竹材承栽的主体是竹纤维,轴压屈服极限是横压屈服极限的3倍。     

Outdoor exposure tests of structural laminated veneer lumber I: evaluation of the physical properties after six years

To investigate the durability of structural laminated veneer lumber (LVL), outdoor exposure tests have been conducted since 1990 at a field-testing site at the Forestry and Forest Products Research Institute. This paper is an interim report on the results after 6 years' exposure. Seven kinds of structural LVL with no preservative treatment were subjected to the tests. These specimens were sampled at the testing site each year and then stored for more than 1 year in a testing room conditioned at 20°C and 65% relative humidity. We then measured the modulus of elasticity (MOE) by longitudinal vibration frequency, the penetration depth by the Pilodyn method, weight loss, color difference (E ^*) by the CIEL ^* a ^* b ^* system, swelling, compression strength, and bending-shear strength. Deterioration caused by outdoor exposure was obvious in the color, weight, MOE, and compressive strength of LVLs, but not in the penetration depth by the Pilodyn method or the bending-shear strength. The retention values of MOE and compressive strength after 6 years of exposure were 78% and 77%, respectively. The difference in durability among material species was not significant in general, except that heavy decay by brown rot fungi took place on some of the grand fir specimens. It should be noted that no significant delamination occurred in any of the adhesive layers, although slight checks developed on the surface of the specimens.     

Influence of temperature and steam environment on set recovery of compressive deformation of wood

Low-density hybrid poplar wood (Populus deltoides?×?Populus trichocarpa) was densified by mechanical compression under saturated steam, superheated steam, and transient conditions at temperature levels of 150, 160, and 170°C. Furthermore, compression of wood under saturated steam conditions at 170°C, followed by post-heat-treatment at 200°C for 1, 2, and 3?min, was performed. To determine the influence of compression treatment on the set recovery, specimens were subjected to five cycles of water soaking and drying. Modulus of rupture (MOR) and modulus of elasticity (MOE) of specimens compressed under saturated steam conditions at 170°C and post-heat-treated at 200°C were determined in the dry condition and after five soak/dry cycles. Higher temperature of the compression treatment resulted in lower equilibrium moisture content, while the steam conditions during the treatment and the post-heat-treatment did not have significant effect. Furthermore, the highest degree of densification was obtained in specimens compressed under saturated steam conditions at 170°C and post-heat-treated at 200°C. The steam condition and temperature influenced the set recovery of compressive deformation. Reduced hygroscopicity does not necessarily imply reduced set recovery. The results established that considerable fixation of compressive deformation can be obtained by compressing the wood in a saturated steam environment and by post-heat-treatment at 200°C. The short heat-treatment had no influence on MOR or MOE, but soaking/drying treatments caused a decrease in the MOR and MOE.     

Strength properties of glued laminated timber made from edge-glued laminae I: strength properties of edge-glued karamatsu (Larix kaempferi) laminae

The object of this study was to investigate the strength properties of edge-glued laminae and to propose a suitable grading method based on the lamina modulus of elasticity (MOE). Edge-glued laminae composed of lumber with similar MOEs (uniform laminae) and edge-glued laminae produced by randomly gluing lumber independent of MOE (random laminae) were made from karamatsu (Larix kaempferi) lumber having the same thickness and length, but various widths. For both the uniform and random laminae, there was a strong correlation between MOE values measured using the longitudinal vibration technique, the static bending test, and a grading machine. The average values of bending, tensile, and compressive strengths of the uniform laminae were similar to those of the random laminae. On the other hand, the average strength of laminae without end joints was significantly higher than that of finger-jointed laminae for both uniform and random laminae. Finger-joints and knots played a significant role in the failure of specimens, but the edge-gluing and the difference in MOE within an edge-glued lamina did not appear to affect the strength properties. The bending, tensile, and compressive strengths of edge-glued laminae were strongly correlated to the lamina MOE.     

Use of transmittance near-infrared spectroscopy to predict the mechanical properties of 1- and 2-year-old transgenic aspen

The investigation of genetically modified trees requires rapid and reliable techniques to measure the mechanical properties at an early age in order to give timely feedback to forest geneticists. In this study, transmittance near-infrared (NIR) spectroscopy was used to predict the green mechanical properties of 1- and 2-year-old transgenic and wild-type aspen. Green modulus of elasticity (MOE) in three-point bending and green ultimate compression strength (UCS) parallel to the grain were predicted from the NIR spectra of dry wood meal pellets. Green UCS had strong correlation (R ² = 0.91) and green MOE had good correlation (R ² = 0.78) with the spectra. The latter could be explained by the moderate correlation of MOE with the lignin content of the transgenic samples, suggesting that besides chemical composition MOE also depends on anatomical properties. The ratio of performance to deviation value suggested that the calibration model of both UCS (2.94) and MOE (1.91) could be used for screening.     

Comparative study on three dynamic modulus of elasticity and static modulus of elasticity for Lodgepole pine lumber

The dynamic and static modulus of elasticity （MOE） between bluestained and non-bluestained lumber of Lodgepole pine were tested and analyzed by using three methods of Non-destructive testing （NDT）, Portable Ultrasonic Non-destructive Digital Indicating Testing （Pundit）, Metriguard and Fast Fourier Transform （FFT） and the normal bending method. Results showed that the dynamic and static MOE of bluestained wood were higher than those of non-bluestained wood. The significant differences in dynamic MOE and static MOE were found between bulestained and non-bluestained wood, of which, the difference in each of three dynamic MOE （Ep. the ultrasonic wave modulus of elasticity, Ems, the stress wave modulus of elasticity and El, the longitudinal wave modulus of elasticity） between bulestained and non-bluestained wood arrived at the 0.01 significance level, whereas that in the static MOE at the 0.05 significance level. The differences in MOE between bulestained and non-bluestained wood were induced by the variation between sapwood and heartwood and the different densities of bulestained and non-bluestained wood. The correlation between dynamic MOE and static MOE was statistically significant at the 0.01 significance level. Although the dynamic MOE values of Ep, Em, Er were significantly different, there exists a close relationship between them （arriving at the 0.01 correlation level）. Comparative analysis among the three techniques indicated that the accurateness of FFT was higher than that of Pundit and Metriguard. Effect of tree knots on MOE was also investigated. Result showed that the dynamic and static MOE gradually decreased with the increase of knot number, indicating that knot number had significant effect on MOE value.     

Compression of wood under saturated steam, superheated steam, and transient conditions at 150°C, 160°C, and 170°C

In this paper, the compressive deformation of hybrid poplar wood (Populus deltoides?×?Populus trichocarpa) at high temperature (150, 160, and 170°C) and under various conditions of steam pressure was studied. Temperature and conditions of steam environment affected the relative density change and creep deformation during compression, as well as properties of the resulting densified material. While the temperature significantly affected the compression deformation of specimens compressed under transient and superheated steam conditions, temperature within the range studied had little effect on the compressive deformation in saturated steam. In all tested conditions, compression deformation was achieved without cell wall fractures. Higher temperature of compression, regardless of steam condition, resulted in lower equilibrium moisture content. In specimens compressed under saturated steam, the modulus of rupture (MOR) and modulus of elasticity (MOE) were increased proportionally to the increase in density, while the compression under superheated steam produced lower increase in the MOE and MOR than expected based on the increase in density. Compression in transient steam conditions at 170°C produced densified wood with higher MOE and MOR than expected based on the increase in density.     

培育措施对人工林樟子松木材力学性质的影响

研究了不同培育措施(初植密度、间伐强度、坡向、坡位)对人工林樟子松(Pinus sylvestris var.mongolica)木材的密度和力学性质(横纹抗弯强度、抗弯弹性模量、顺纹抗压强度、冲击韧性、端面硬度)的影响。初植密度(1.5m×1.0m,1.5m×2.0m和1.5m×2.5m)对木材密度和抗弯弹性模量有显著的影响。初植密度为1.5m×1.0m时,木材主要力学指标值最大。适当间伐可提高木材的抗弯弹性模量和顺纹抗压强度,但重度间伐则会降低木材的力学强度。坡向(阳坡和阴坡)对木材的抗弯弹性模量影响显著,阳坡高于阴坡。坡位(上坡位和下坡位)对除端面硬度外的木材力学性质指标影响都显著,各项力学指标值均是坡下高于坡上。表5参9。     

Fatigue and creep in chipboard

Summary Matched samples of chipboard were loaded in four-point bending under either a 7 hours loaded/17 hours unloaded cycle, a 17 hours loaded/7 hours unloaded cycle, or constant load in order to asses the effect of cyclic loading on creep deformation. Tests were carried out at three humidity levels (30, 45 and 90% RH) and at three stress levels (30, 45, and 60% of the short term strength). The creep rate of samples under constant load was greater than under cyclic load, with the 7 hours loaded/17 hours unloaded samples giving the lowest creep rate. Analysis of the data on an accumulated time-under-load basis showed that the law of superposition did not apply to the 7 hours loaded/17 hours unloaded samples at the 30% and 45% stress levels, with these samples giving lower deflection than for the other loading conditions. The 17 hours loaded/7 hours unloaded samples also did not appear to obey the law of superposition, although the difference between them and the constantly loaded samples was slight. Considerable scatter in data accumulated at the 60% stress level did not allow any differences in the superposition analysis of cyclic and constant loaded samples to become apparent. Increasing the stress level applied to samples produced an almost linear increase in relative creep for all loading regimes. The 7 hours loaded/17 hours unloaded samples gave consistently lower relative creep values at all levels of stressing, but particularly at the 30 and 45% stress levels. Increasing the relative humidity from 30% to 65% RH had an almost negligible effect on relative creep of all samples, but increasing the humidity to 90% RH caused a marked increase in relative creep.     

Moisture content and strain relation in wood by Bragg grating sensor and unilateral NMR

The paper describes a low-invasive experimental apparatus designed for the accurate determination of mechanical deformation and moisture content (MC) relationship on macroscopic wood samples. The device is particularly indicated for monitoring wooden handwork whose mechanical deformation is especially critical in relation to its role such as, for example, works of art or architectural works. The MC of wood is measured with a portable single-sided NMR probe and mechanical deformation by a fiber Bragg grating optical sensor. The data obtained are of high accuracy, despite the dimensions of the sample. The methodology provides an effective tool for investigating the dynamic relation between environmental relative humidity, MC, and shrinking–swelling of wood. Adsorption results collected for longitudinal deformation in silver fir (Abies alba Mill.) are presented to show the sensitivity of the optical sensor. Interesting findings include the detection of two different mechanisms of elongation and the time evolution of water mobility versus hydration and strain.     

Characterization of major,unused, and unvalued Indonesian wood species I. Dependencies of mechanical properties in transverse direction on the changes of moisture content and/or temperature

Mechanical property changes due to the moisture content (MC) and/or temperature changes were examined for 15 Indonesian wood species. A static bending test was carried out at 20°C, 65% relative humidity (air-dry), and water-saturated at 20°C (wet-20) and 80°C (wet-80). For individual test conditions, modulus of elasticity (MOE) and modulus of rupture (MOR) increased linearly with specific gravity regardless of wood species; however, maximum deflection did not correlate with specific gravity for any MC or temperature conditions. The relative values of MOE and MOR measured in wet-20 to air-dry conditions were variously affected from slightly to strongly depending on the wood species. However, the relative values always decreased markedly when saturated in water at 80°C, regardless of wood species. The relative MOE, MOR, and maximum deflection values due to the change in MC or MC and temperature combined were independent of specific gravity but may be dependent on wood type: softwood or hardwood.     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Genetic variation in wood stiffness and strength properties of hybrid larch (Larix gmelinii var. japonica × L. kaempferi)

Genetic parameters for wood stiffness and strength properties were estimated in a 29-year-old hybrid larch stand (Larix gmelinii var. japonica × Larix kaempferi). The study included 19 full-sib larch families from Hokkaido, northern Japan. Implications of these genetic parameters in wood quality improvement are subsequently discussed. Traits included in the analyses were the dynamic modulus of elasticity of green logs (E _log), the modulus of elasticity (MOE), the modulus of rupture (MOR), compression strength parallel to the grain (CS) in small clear specimens, wood density (DEN), and diameter at breast height (DBH). DEN had the lowest coefficients of variation and MOE the highest. The narrow-sense heritability estimates of E _log, MOE, MOR, and CS were 0.61, 0.44, 0.60, and 0.43, respectively, and those of DEN and all mechanical properties increased from an inner to outer position within the stem. E _log and DEN had high positive phenotypic (0.52–0.83) and genetic (0.70–0.92) correlations with MOE, MOR, and CS. The mechanical properties of the inner position of the stem had rather high phenotypic and genetic correlations with those of the outer position and overall mean. The predicted gains in wood stiffness (E _log and MOE) were higher than those of the strength properties (MOR and CS). The predicted correlated responses in MOE, MOR, and CS when selecting for E _log and DEN were 72.6%–97.8% of a gain achievable from direct selection of these traits. DBH showed an insignificant correlation with all mechanical properties, although selection of this trait had a slightly negative effect on the mechanical properties.     

Effect of size and geometry on strength values and MOE of selected hardwood species

The total hardwood timber stock of German forests is fast growing. The lack of knowledge concerning test standards, product standards and sorting criteria makes it difficult to expand the processing and marketing of hardwoods into the field of construction usage. Strength and stiffness data derived from small, defect-free specimens do mostly exist, but in order to be able to insert hardwoods into building applications, data derived from real size specimens is needed. Subsequently, the results of these two different specimen categories need to be correlated and the so-called size effect needs to be quantified and qualified. This paper aims to analyze the size effect of defect-free compression, bending and tensile specimens for the six European hardwood species maple (Acer spp.), birch (Betula pendula), beech (Fagus sylvatica), ash (Fraxinus excelsior), oak (Quercus spp.) and lime (Tilia spp.). They are tested exclusively parallel to grain. Regarding the compression strength for maple, birch and ash, the specimen dimensions did not influence the compression strength value. For beech, oak and lime, it was observed that compression strength increased as the specimen volume was increased. The bending strength of all species decreased as the specimen dimensions increased. Concerning the tensile strength, a clear statement on whether dimensions influence the tensile strength value is not possible. Further research with adjusted specimen sizes, specimen shapes and machine set-ups is needed. Regarding the compression and bending MOE, in most cases, the dimensions did not influence the MOE values. In tensile testing, MOE values differed significantly for the different specimen sizes. Whether these differences were due to slightly different test set-ups in the different sizes or a true size effect could not be answered conclusively.     

Influence of accessory substances,wood density and interlocked grain on the compressive properties of hardwoods

Wood samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected to perform moisture sorption tests associated with parallel-to-grain and tangential compression tests using a multiple step procedure at 25°C. Cold-water and hot-water extractives, sequential cyclohexane (CYC), acetone (ACE) and methanol (MET) extracts, ash content (ASH), wood density and interlocked grain (IG) were evaluated on matched samples too. Wood density corrected for the accessory substances was by far the major factor positively affecting the compressive properties of tropical hardwoods. The total amount of accessory substances is required in order to establish better relationships between physico–mechanical properties and density of tropical hardwoods. For a given wood density, the ultimate stress in parallel-to-grain compression was higher in tropical hardwoods than in temperate hardwoods. However, the compliance coefficients for both types of woods were quite similar. Sequential extraction with organic solvents was the most suitable method for evaluating the effect of extractives on compressive properties of tropical hardwoods. The CYC and ACE fractions did not contribute to variation in these mechanical properties. The substances dissolved in MET affected positively the compliance coefficient s ₁₁ in parallel-to-grain compression and negatively the compliance coefficient s ₃₃ in tangential compression. The IG decreased the compliance coefficient s ₁₁ but also decreased the ultimate stress in parallel-to-grain compression. Finally, variations in compressive properties that were due to changes in equilibrium moisture content (EMC) were clearly influenced by wood density; denser woods were more sensitive to changes in EMC than lighter woods.     

Physical and mechanical properties of Pinus leucodermis wood

Abstract

The present work reports on the main physical and mechanical properties of Pinus leucodermis mature wood, one of the least studied coniferous species in south-east Europe. Pinus leucodermis heartwood specimens were found to have average density values of 0.73 g cm^?3 at equilibrium moisture content of 11.5% and average density of 0.64 g cm^?3 under oven-dry conditions. The overall tangential shrinkage was 3.4% and the radial shrinkage was 1.9%. The modulus of rupture was on average 77 N mm^?2, while the static modulus of elasticity averaged 7087 N mm^?2. The hardness of P. leucodermis heartwood using the modified Janka test was 33.4 N mm^?2 in the transverse direction and 48.0 N mm^?2 in the longitudinal direction, while its compression strength parallel to grain was approximately 41.6 N mm^?2.