Estimating mechanical properties of clear wood from ten‐year‐old Melia azedarach trees using the stress wave method

The objective of this study was to examine the potential of stress wave velocity (SWV) as a rapid and non-destructive method to estimate the mechanical properties of Melia azedarach wood. The SWV, dynamic modulus of elasticity (MOE_d), modulus of elasticity (MOE), modulus of rupture (MOR, bending strength) and density were determined on ninety 20 ? 20 ? 320 mm clear wood specimens, obtained from stems of three ten-year-old M. azedarach trees, and tested at environmental equilibrium in 20°C, 60?% relative humidity (a moisture content of approximately 12?%). There was a statistically significant (0.1?% level) but weak correlation (R²?=?0.23) between the SWV and MOE, but no statistically significant correlation was found between the SWV and MOR. Much better results for prediction of static properties of M. azedarach wood were obtained when SWV and wood density (WD) were used together through calculation of MOE_d in the air-dry condition (MOE: R²?=?0.76, MOR: R²?=?0.47), although in the case of MOR a model based on WD alone is slightly better (R²?=?0.58), and WD is also almost as good as MOE_d for predicting MOE. It is concluded that SWV coupled with WD can be employed as a predicting parameter to evaluate the mechanical properties of M. azedarach wood during the manufacturing process, although WD alone is also effective. The SWV alone would not be useful due to MOE being almost directly proportional to WD at this moisture content.

     

Influence of drying temperature upon some mechanical properties of beech wood

The paper presents the results obtained in an experimental study concerning the influence of drying temperature upon the mechanical properties of beech wood (Fagus sylvatica L.). Sound wood samples without red heart were cut from white (unsteamed) beech timber parts, dried at different temperatures: 20 °C, 80 °C, 90 °C, 100 °C, 115 °C and same relative air humidity: 50%. After performing classical tests for evaluation of some selected mechanical properties, the following conclusions could be drawn: all bending properties (static bending strength, modulus of elasticity and impact bending strength) increased with increasing temperature, confirming thus the benefiting effect of heat upon wood plasticity. The tensile strengths, both parallel and perpendicular to grain, increased with increasing temperature, but only in the range below 100 °C; as soon as the temperature exceeded this value, the tensile strengths began decreasing. As far as compression strength parallel to grain, shearing strength and splitting resistance is concerned, no significant influence of temperature could be established. However, it seems that these properties are negatively affected by kiln-drying, as even with low kiln-drying temperatures these strengths are much lower than in case of air-drying.     

Tensile strength properties of particle boards at different temperatures and moisture contents

The equilibrium tensile properties of urea-formaldehyde (UF)- and phenol-formaldehyde (PF)-bonded particle boards have been studied at moisture contents between 1 and 33% and at temperatures between ?15°C and +45°C. These conditions may occur e.g. during exterior use of the boards. The tensile strength and the modulus of elasticity decrease slightly while the strain at rupture increases slightly between 1% and 7% moisture content. These effects are similar for UF- and PF-bonded boards. Above 7% moisture content the tensile strength and the modulus of elasticity decrease markedly while the stretch at rupture increases with increasing moisture content. These changes are more pronounced for UF-bonded boards. The bending stiffness is less affected than the modulus of elasticity since the board thickness increases with increasing moisture content. The tensile properties are not significantly affected by the temperature at low moisture contents, but at high moisture contents the combined effects are more pronounced. The effect of moisture is generally stronger than that of temperature, in the range studied.     

Effect of relative humidity on some physical and mechanical properties of different types of fibreboard

Equilibrium moisture content and strength properties as a function of relative humidity (RH) were measured in three types of commercial fibreboards with different densities. The measurements were made after specimens had been conditioned to equilibrium at 35, 50, 65, 80 and 95% RH and 20 °C. It was shown that the modulus of elasticity decreased slightly between 35 and 65% RH and markedly – at above 65% RH for all types of boards. A very strong decrease of modulus of elasticity was observed at about 80% RH (capillary condensation). A similar decreasing trend was observed in bending strength. Overall, high RH had a noticeably detrimental effect both on the MOE and the bending strength for all tested boards. Adequate correlation exists between bending MOE and bending strength with moisture content. The lower the density of boards the higher their equilibrium moisture content.     

Der Einfluss der Temperatur auf die Biegefestigkeit und den Elastizitätsmodul bei verschiedenen Holzwerkstoffen

The influence of temperature on the bending strength (MOR) and the modulus of elasticity (MOE) of eight different wooden materials (2 MDF, 1 OSB, 2 particle boards, beech and spruce plywood, solid wood panel) was tested at temperatures between -20 °C and +60 °C. Thereby a reduction of MOR of 12–39% and of MOE of 14–46% was measured in the range of 20 °C and 60 °C. The work to maximal load varied within a large range. Depending on the wooden material there was an increase of up to 48% or a reduction of up to 31% between 20 °C and 60 °C. Between 20 °C and -20 °C the MOR increased 5–22% and the MOE 3–27%. Only the MOE of the solid wood panel in fibre direction decreased by 6%. The work to maximal load ranged between a reduction by 19% and an increase by 10%.     

Swelling restraint of thermally modified ash wood perpendicular to the grain

The paper reports on creep of ash wood (Fraxinus excelsior L.) thermally modified at 180 and 200 °C, and subsequently subjected to compression in tangential and radial directions and simultaneously wetted, from the moisture content (MC) of 6% to above the fibre saturation point (FSP). The compressing load made 0.00, 0.25, 0.50 and 0.75 of impact stress at the proportional limit (R_c). The compression stress needed to restrain the swelling of wood, the so-called swelling pressure, was indirectly determined from isochrones of mechano-sorptive creep. The most important finding was that thermal modification reduces the strain of ash wood subjected to compression perpendicular to the grain to a degree proportional to the mass loss. The compression stress needed to restrain the swelling of thermally modified wood is ca. 10 and 20% smaller in the tangential and radial directions, respectively. This effect leads to a reduction in the anisotropy of swelling pressure of thermally modified wood perpendicular to the grain. Moreover, although upon thermal modification the mass loss of wood takes place, at the MC of 6% it shows practically the same modulus of elasticity (MOE) and Rc as the unmodified wood. After wetting to MC higher than the FSP, the thermally modified wood at 200 °C shows significantly higher MOE and Rc than the wood modified at 180 °C and untreated wood. Reduction of wood hygroscopicity, an inevitable effect of thermal modification, also reduces the range of changes in mechanical properties of wood caused by the increase in its MC to the FSP.     

Effect of tannin on increasing UF adhesive performance at high temperature investigated by TMA and TGA analysis

Tannins have been largely studied as wood additives and wood adhesives. In a perspective of a desirably increased utilization of natural raw material in the wood timber industries, this study provides further and specific information on the use of condensed tannins to improve the heat resistance of urea–formaldehyde (UF) resins. The results obtained by thermomechanical and thermogravimetric analyses show that high percentage (33 and 50 %) of quebracho tannin in UF-tannin blend dramatically improves the overall UF thermal resistance, but 50 % of tannin decreases the maximum value of modulus of elasticity (MOE) of the bonded joint. The best balance between mechanical performance and decreased degradation at high temperature is obtained with 33 % of quebracho tannin. The MOE value of UF + 33 % of quebracho tannin at 260 °C is seven times greater than MOE of UF control. Thermograms show that quebracho tannin has a beneficial effect on increasing the resistance of glue-mix to degradation induced by temperature. When the content of tannin in the glue-mix is increased up to 50 %, the resin degrades completely at about 670 °C, i.e. more than 50 °C higher than neat UF resin. This improvement is related mainly to the resorcinol-type ring structure of tannin and the alkaline pH of UF + quebracho tannin glue mix that confer a higher resistance against joint degradation to UF even at elevated temperatures.     

Strength reduction of spruce wood through slow freezing

Some selected mechanical properties of spruce wood (Picea abies L.) were determined after freezing green timber boards under different conditions. The influence of the freezing rate and the time of exposure to negative temperatures were evaluated by applying three different freezing conditions which may occur in winter when green timber is stored in an open yard. It was found that a high freezing rate (?10 °C/h) does not affect wood strengths at all, while slow freezing (by ?1 °C/h) significantly reduces all mechanical wood properties, especially MOR, MOE and the compressive strength (by 20…30 %), the Janka hardness (by 18 %), and also the tensile strength (by 10 %). A longer time of exposure, involving repeated freezing and thawing due to natural temperature variations led to further reduction of MOR, MOE and the compressive strength (up to 37 %), but without further affecting the tensile strength, shear strength and hardness. The obtained results may be useful to industrials with respect to a more careful planning of green timber purchase and storage in wintertime.     

Influence of moisture content of wood on sound velocity and dynamic MOE of natural frequency- and ultrasonic runtime measurement

This study comprises the analysis of the influence of the moisture content of wood on the dynamic properties—sound velocity and dynamic MOE (considering the density)—of green and dry sawn timber. The main objective was to find applicable functions or factors for the adjustment of the determined properties on the reference condition (moisture content u=12%) for the moisture content range below and above fiber saturation. Therefore, two dynamic measurement methods—measurement of the natural frequency and ultrasonic runtime—were considered. Possible differences in the results, pros and cons of both methods should be identified on the one hand. On the other hand, the applicability of already published moisture adjustments to the given data of both methods should be proved.     

Relationship between timber quality and the influence of moisture content above fibre saturation on mechanical properties in beams of fast-growing Argentinean Eucalyptus grandis

The present paper reports on the results of an investigation regarding the relationship between timber quality and the influence of moisture content above fibre saturation on bending strength and stiffness in structural-sized beams of fast-growing Argentinean Eucalyptus grandis. An empirical research project with a sample containing 96 pairs of structural-sized beams (seasoned and unseasoned) was carried out. Results showed that the influence of moisture content on mechanical properties was related to timber quality and it increased with timber quality but this relationship did not clearly emerge from the results obtained by visually strength grading the sample with the most important strength and stiffness reducing growth characteristic (the presence of pith), whereas it was better revealed by machine grading the sample with the most important single parameter (modulus of elasticity). Results also showed that decisions related to structural design with timber of high quality exposed to service class 3 according to European standards or installed near fibre saturation are situated on the unsafe side if the strength and stiffness values are obtained by multiplying those corresponding to seasoned timber by a constant factor representing the average behaviour of this material. Even though this average behaviour compared relatively well with the results obtained by applying the criteria adopted by standards of overseas and Latin-American countries, which do not consider timber quality for this purpose, the reliability required by structural design accounts for the convenience of taking into account the relationship between timber quality and the influence of moisture on mechanical properties where timber of high quality is concerned.     

Mechanical and physical properties of thermally modified Scots pine wood in high pressure reactor under saturated steam at 120, 150 and 180 °C

Scots pine sapwood and heartwood were thermally modified under saturated steam at 120, 150 and 180 °C in a high pressure reactor. Mechanical properties such as dynamic and static modulus of elasticity (MOE), static modulus of rupture (MOR), Brinell hardness and impact toughness were evaluated. The static MOE for sapwood did not decrease substantially (approximately 1 %), not even with a high mass loss of more than 12 %, when the wood was modified at 180 °C. Static MOE of the wood increased approximately 14 %, when modified at 150 °C. Surprisingly, MOR increased by 15 %, when modified at 150 °C with mass loss of 2.3 %. Whereas impact strength and hardness decreased somewhat, when modified at 180 °C. Moreover, high anti-swelling efficiency values were obtained (60 % for sapwood and 52 % for heartwood) when modified at 180 °C.     

Effect of thermal treatment on the physical and mechanical properties of phyllostachys pubescen bamboo

Thermal treatment of Phyllostachys pubescen bamboo was performed in a dry oven at seven temperature levels (100–220 °C) for four duration times (1–4 h). The results showed that mass loss increased with increasing temperature and duration, and the maximum reduction reached 29.0 %. The color of heat-treated bamboo was darkened and all three color parameters (L^*a^*b^*) were significantly changed. Modulus of elasticity (MOE) was affected slightly when samples were heat-treated below 200 °C, even a slight increase compared with control samples; but it decreased quickly when samples were treated above 200 °C and the maximum reduction was 20.2 %. However, the Modulus of rupture (MOR) and the contents of holocellulose and α-cellulose reduced significantly with increasing temperature and duration when samples were heat-treated above 160 °C, they both strongly correlated with mass loss. It could be confirmed that thermal treatment on bamboo shows an interesting potential to improve bamboo quality.     

Influence of temperature of thermal treatment on surface densification of spruce

Spruce (Picea abies L. Karst) wood lamellae, thermally treated at 170, 190, 210 and 230 °C were surface densified by compression at a temperature of 150 °C to three degrees of compression. Immediate springback, set recovery, mechanical properties in 3-point flexure, Brinell hardness and density profiles measurements were used to determine the effect of thermal treatment on the properties of surface densified wood. The highest immediate springback occurred in wood specimens thermally treated at the highest temperature (230 °C) and decreased with decreasing thermal treatment temperature. The untreated samples had the highest set recovery, which decreased with the temperature of thermal treatment. The surface densification increased hardness and bending strength. The highest increase was in the case of untreated wood and decreased with the temperature of thermal treatment. The modulus of elasticity (MOE) and modulus of rupture (MOR) of surface densified wood decreased with increasing thermal treatment temperature. The trend was similar for specimens which were thermally treated but not surface densified. Surface densification increased the density of the specimens in the first few millimetres below the surface. The highest density was achieved in untreated specimens and the lowest in specimens thermally treated at the highest temperature.     

Bending creep performance of modified timber

The present study is a supplement to Epmeier and Kliger (2005), which investigated the effect of three different modification methods (acetylation, modification with methylated melamine resin and heat treatment in vegetable oil) on four material properties (density, modulus of elasticity (MOE), creep deflection and relative creep) and their inter-relations. In this study, another modification method (furfurylation) and four additional material parameters (“relative stiffness”, i.e. MOE/density, moisture content, change in moisture content and anti-creep efficiency) and their correlations are included. In addition, the creep curves were analysed to assess the development of bending creep performance. The properties were assessed by experimental work on 132 specimens of Scots pine sapwood with dimensions of 45×70×1100 mm³. It was found that modification leads to significant changes in material properties. Furfurylation increases density and reduces creep deflection, relative creep and moisture content. Modified timber tends to deflect and creep significantly less than untreated timber. However, the extent of reduction in relative creep appears not to be related to the extent of reduction in creep deflection. Initial deflection and/or modulus of elasticity are suitable for predicting the creep deflection of untreated and modified timber.     

Zur dynamischen Torisonselastizität von Holz

The changes in torsional elasticity of untreated and fire retardant impregnated beech and spruce specimens under cyclic temperature load were determined in torsional vibration tests. A temperature of 60°C already caused a permanent 25% reduction in torsional modulus of elasticity which remained unaffected by further temperature rises and by repeated heating and cooling cycles up to 200°C. Above 200°C oxidation presumably occurred with wood strength deteriorating visibly. Also, previously water saturated specimens showed greater losses in torsional elasticity than air-dried or dried ones. Fire retardant treatment with acid salt mixtures reduced torsional modulus of elasticity by 35% already at room temperature and subsequently led to a rupture of specimens at 200°C. Apart from fluctuations in absolute values both wood species reacted similarly to temperature load and protective treatment.     

Zum Einfluß der Strömungsgeschwindigkeit auf die Hochtemperaturtrocknung von Schnittholz

To determine the influence of flow velocity (w) on high-temperature drying of lumber, drying tests with green spruce board specimens 25 mm thick were carried out at 115 °C dry-bulb temperature and 65 °C wet-bulb temperature (115/65 °C), at 115/95 °C, 130/65 °C, 130/95 °C and, for comparison, also at 90/70 °C, in each case atw=3 m/s andw=8 m/s. The relative reduction in drying time from 80 to 10% wood moisture content, obtained by increasing the flow velocity from 3 to 8 m/s, proved in high-temperature drying tests (22.4 to 24.9%) to be about twice as high as in the case of drying at 90/70 °C (11.1%). The increase in dry-bulb temperature from 115 °C to 130 °C in each case slightly intensified the influence of flow velocity on the average drying rate; a raising of wet-bulb temperature from 65 °C to 95 °C, however, showed practically no effect.     

Untersuchungen zur Anwendung der Schallgeschwindigkeitsmessung für die Ermittlung der elastomechanischen Eigenschaften von Spanplatten

Sound speed transmission in laboratory and industrial particleboards was studied. The boards had different densities and adhesive content. Bending strength (MOR) and module of elasticity (MOE) were determined according to DIN-standards. On the same test pieces sound speed transmission parallel to board surface was determined. A correlation between MOR and MOE with the speed of the sound transmission was calculated. An important effect of board density and amount of adhesive on the transmission was found.     

Temperature control in a solar kiln

Realizing the effect of venting on temperatures in a solar kiln, average kiln temperatures were measured in a solar kiln with automatic vent control. These temperatures were found to be less than 2°C above ambient with the wood above fibre saturation point and increased to about 10°C when the wood reached 12% moisture content. The effect of these relative small differences between kiln and ambient temperatures on the drying rate were investigated as well as methods to increase the average temperature in a solar kiln.     

Density and some mechanical properties of densified and heat post-treated Uludağ fir,linden and black poplar woods

The effects of thermo-mechanical densification and heat post-treatment on air-dry density, modulus of elasticity (MOE), bending strength (MOR), and compression strength parallel to the grain (CS) of Uluda? fir (Abies bornmulleriana Mattf.), linden (Tiliagrandifolia Ehrh.), and black poplar (Populus nigra L.) wood samples were investigated. Samples were densified with compression ratios of 25 and 50%, and at 100 and 140?°C. Then, the heat treatment was applied to the samples at 185 and 212?°C for 2 h. According to the results of the study, density of all wood samples increased together with the increase of compression ratio. Regarding compression temperature, the highest density increase was obtained at 100?°C. Mechanical strength (MOE, MOR, and CS) in densified samples increased depending on compression ratio and increase of density. The highest strength increase was in black poplar samples and the lowest was in linden samples. After heat post-treatment, mechanical strength of samples decreased depending on increase in treatment temperature. However, strength values (MOR except) of samples which are 50% compressed and heat-treated were found higher than control samples.     

Dynamic mechanical thermal analysis (DMTA) of aqueous phenol formaldehyde (PF) resin modified by nano copper oxide (CuO)

To improve the bio-resistance of engineered wood composites products via gluing process, aqueous phenol formaldehyde (PF) resin was modified using nano CuO containing alkane surfactant and polyvinyl alcohol (PVA) 17-99. The modified PF system was analyzed by dynamic mechanical thermal analysis, and the mechanical properties of the bonded plywood panels including tensile strength, modulus of rupture (MOR), modulus of elasticity (MOE) and shear strength under five test conditions were also evaluated. The results indicated that the addition of nano CuO incorporating PVA 17-99 separated the gel point and vitrification point in the curve of tan δ, which is related to the delaying of moisture loss in modified PF resin during the curing process. The modification showed adverse effect on tensile strength but only a minimal influence on MOR and MOE. Additionally, PVA 17-99 reduced the water resistance of cured PF resins. However, with the test conditions of dipping in 100 °C water for 6 h, then drying for 20 h at 63 °C in air, followed by dipping in 100 °C water for 4 h, PVA consolidated the re-curing effect on the PF resin and compensated the strength loss from hydrolysis. Thus, the modified PF system not only guaranteed bio-resistance of glued wood composites via CuO, but also has the potential for developing self-curing wood composites being applied as structural construction materials.