Abstract
The objective of this research was to determine the influence of wood species (Fagus sylvatica L. and Populus tremula L.), thickness (4, 6, 10, 18 mm), and degree of densification (0%, 10%, and 20%) on the impact bending strength (IBS) and Brinell hardness (BH) in the radial direction. Three-factor analysis of variance confirmed that the difference in IBS was significantly related to the wood species and wood thickness. Wood densification did not have a significant effect on IBS. In addition, beech wood exhibited higher IBS values than aspen wood. The IBS values increased proportionally with increasing thickness. All factors affecting Brinell hardness were statistically significant, although thickness had the smallest influence overall. The Brinell hardness values were substantially higher in beech wood than aspen wood, and in some cases were more than three times greater. On the other hand, densification exhibited a more positive effect on increasing Brinell hardness for aspen wood than beech wood.
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Impact Bending Strength and Brinell Hardness of Densified Hardwoods
Miroslav Gašparík,a,* Milan Gaff,a Lenka Šafaříková,a Carlos Rodriguez Vallejo,b and Tomáš Svoboda a
The objective of this research was to determine the influence of wood species (Fagus sylvatica L.and Populus tremula L.), thickness (4, 6, 10, 18 mm), and degree of densification (0%, 10%, and 20%) on the impact bending strength (IBS) and Brinell hardness (BH) in the radial direction. Three-factor analysis of variance confirmed that the difference in IBS was significantly related to the wood species and wood thickness. Wood densification did not have a significant effect on IBS. In addition, beech wood exhibited higher IBS values than aspen wood. The IBS values increased proportionally with increasing thickness. All factors affecting Brinell hardness were statistically significant, although thickness had the smallest influence overall. The Brinell hardness values were substantially higher in beech wood than aspen wood, and in some cases were more than three times greater. On the other hand, densification exhibited a more positive effect on increasing Brinell hardness for aspen wood than beech wood.
Keywords: Impact bending strength; Brinell hardness; Beech; Aspen; Densification; Thickness
Contact information: a: Department of Wood Processing, Czech University of Life Sciences in Prague, Kamýcká 1176, Prague 6 – Suchdol, 16521 Czech Republic; b: Department of Forestry Engineering, Campus de Rabanales s/n, University of Cordoba, 14071 Cordoba, Spain;
* Corresponding author: gathiss@gmail.com
INTRODUCTION
Wood is generally recognized as one of the most important renewable resources and among the most versatile and widely used materials. On the other hand, much of its uses depend on the species, because while some tree species are used almost everywhere, others have limited application. The application of wood is directly influenced by its physical and mechanical properties.
European beech (Fagus sylvatica L.) is a native wood that grows throughout Europe (Eilmann et al. 2014). Beech is one of the most commonly used hardwoods in Europe (Pöhler et al. 2006; Gryc et al. 2008) for furniture, floors, toys, veneer products, and musical instruments, as well as for the production of stairs, cladding, and glued load-bearing elements in construction (Ohnesorge et al. 2010; Aicher and Ohnesorge 2011; Guntekin et al. 2014). On the other hand, European aspen (Populus tremula L.) wood has only occasional uses. In the woodworking industry, aspen is used in the manufacturing of wood-based materials (plywoods, particle, and flakeboards), in the furniture industry for underlying veneers or surface veneers for backside or nonvisible surfaces (Kärki 2001), and for the facings of ceilings or saunas where high strength or hardness are not necessary (Möttönen et al. 2015). In the past, aspen was primarily used for the production of matches; currently, it has application in the production of biomass fuel, paper, and pulp (Kärki 2001; Heräjärvi and Junkkonen 2006).
The uses for these wood species are closely related to their mechanical properties. Mechanical properties differ by wood type because they depend not only on the type of loading (tension, pressure, and bending), but also on the loading’s character (static or dynamic loading) (Bal and Bektaş 2012). In general, wood can resist static loading to a greater extent than dynamic loading. Static loading is characterized by an increasing loading force over time, while dynamic loading is where a maximal force acts over a very short duration or instantly (Bal and Bektaş 2012).
The mechanical properties of wood can be altered in various ways, depending on the requirements. Wood densification is one of the most common methods for the modification of mechanical properties. It works with the principle that these properties are directly dependent on changes in wood density and has been confirmed by a number of authors dealing with either surface densification (Lamason and Gong 2007; Gong et al. 2010; Rautkari et al. 2009, 2011; Laine et al. 2013, 2014) or volumetric densification (Navi and Girardet 2000; Kamke 2006). The final densification effect also depends on other conditions and their mutual combinations, such as the use of plasticizing, temperature, moisture content, presence of chemical substances, etc.
One of the most important properties for dynamic loading is the impact bending strength (IBS). It is the ability to resist immediate maximal loading, which means absorbing and dissipating energy through impact bending (Požgaj et al. 1997; Bal and Bektaş 2012). IBS refers to the numerical expression for the amount of work consumed in breaking (cracking) wood under given conditions (Bal 2016). Wood with a high IBS is referred to as tough. On the other hand, if the impact bending strength is low, wood is described as brittle (Bučar and Merhar 2015).
Wood quality can be characterized according to the type and shape of the fracture after breaking. Tough wood creates a fibrous, spiky fracture. Brittle wood usually produces blunt, non-fibrous, stepped fractures. In brittle fractures, the deformation is relatively low and the fracture happens suddenly. Certain wood species can have relatively high strength but still be brittle in terms of their IBS (Kollmann 1967). Impact bending strength is influenced by various factors, such as density, fiber orientation, moisture content, and temperature.
Hardness refers to the ability of wood to resist the penetration of another object into its structure (Heräjärvi 2004; Kurt and Özçifçi 2009). Hardness is important not only when machining the wood with cutting tools (sawing, milling, peeling, etc.) (Grekin and Verkasalo 2013), but also for wood products that are subject to scratches or abrasion (floors, wooden stairs, etc.) (Rautkari et al. 2013). The main disadvantage of hardness is that its value is markedly influenced by the testing method and its associated conditions (Niemz and Stübi 2000; Hirata et al. 2001). To determine wood’s hardness, only the Brinell and Janka methods are widely used. While the Janka method is used almost exclusively in North and South America, the Brinell method is most widely used in Europe (Grekin and Verkasalo 2013).
This research focuses on examining the IBS and Brinell hardness of beech and aspen wood while testing perpendicular to the grain in the radial direction.
The main goal was to determine the effects of densification and wood species on the IBS and Brinell hardness values.
EXPERIMENTAL
Materials
European beech (Fagus sylvatica L.) and European aspen (Populus tremula L.) woods were used for preparing the samples. Samples of four thicknesses (4, 6, 10, and 18 mm) and 35 mm in width were produced. Sample length was 300 mm for IBS and 150 mm for Brinell hardness. Samples were conditioned to an equilibrium moisture content (EMC) of 8% (ɸ = 60 ± 3% and t = 20 ± 2 °C). The EMC represented the final moisture content of furniture and wooden joinery (flooring and cladding) for interior use, according to EN 942 (2007) and ČSN 91 0001 (2007). The samples of both wood species were divided into two groups: the first group was designated for densification and the second group consisted of non-densified (reference) samples. The investigation involved 288 total samples.
Methods
Densification
All samples designated for densification were cold-pressed in a UPS 1000 hydraulic press (RK MFL Prüfsysteme GmbH, Germany) without prior plasticizing. Pressing was carried out in three phases: The first phase consisted of closing the press and gradually densifying the samples to the required thickness value over 5 min. During the second phase, the samples were pressed for 2 min. The final phase consisted of gradually opening the press and unloading the samples over 3 min. Subsequently, the samples relaxed for 5 min. Table 1 contains the values of the pressing force used for densification of the individual sets of test samples.
Table 1. Pressing Force for the Densified Samples
For both wood species, the densities of the non-densified, as well as the densified samples, were evaluated (Table 2).
Table 2. Average Density Values for the Individual Groups
Values in parentheses are the standard deviations
Impact bending strength (IBS)
The IBS was determined on a pendulum impact machine (impact head weight 20 kg), based on Charpy’s principle and in accordance with ISO 3348 (1975). Charpy’s principle can be briefly described as follows: a hammer falls along a circular trajectory from height h1; if the hammer has no obstacle it reaches height h0; it applies h0 < h1 because of friction resistance; if the hammer hits the experimental sample, it also reaches the left side but only to the position h2; the work necessary for breaking the sample is recorded on the apparatus’ dial (Fig. 1).
Fig. 1. Charpy’s principle of the impact bending strength test
The samples were positioned so that the pendulum head would act in the radial direction, i.e., on the tangential surface. The test was carried out with a constant span between the support centers of 240 ± 1 mm to allow for monitoring the influences of various sample thicknesses. The results of IBS for the non-densified samples were compared with the results of the samples densified at 10% and 20%.
Brinell hardness (BH)
Brinell hardness was determined in the radial direction (on the tangential surface of the sample) at three locations in the center of the sample’s width (parallel to the sample’s length), according to EN 1534 (2010), with some modifications. The measurement of hardness was performed using a DuraVision-30 hardness tester (Struers, Denmark) with a steel (carbide) indenter. The hardness tester automatically captured the loading force, measured the depth and diameter of the indentation, and subsequently calculated the hardness value from this data. The maximum loading force was reached at 10 s, held for 10 s, and then the force was released over the period of 10 s. The parameters for the BH measurement are given in Table 3.
Table 3. Parameters of Brinell Hardness
Evaluation and Calculation
The IBS and BH values were evaluated using MANOVA, specifically utilizing Fisher’s F-test in STATISTICA 13 software (Statsoft Inc., Tulsa, Oklahoma, USA). The results were evaluated using 95% confidence interval which reflects a significance level of 0.05 (P < 0.05).
The IBS was calculated in accordance with ISO 3348 (1975) and Eq. 1,
(1)
where Aw is the IBS at the moisture content during the testing (J/cm2), Q is the energy required for fracture of the sample (J), b is the width of the sample (cm), and h is the thickness of the sample (cm).
The IBS values were converted to the moisture content of 12%, according to Dubovský et al. (2003) and Eq. 2,
(2)
where A12 is the IBS at the moisture content of 12% (J/cm2), Aw is the IBS at the moisture content during the testing (J/cm2), w is the sample moisture content during the testing (%), and αis the correction coefficient for moisture content, which was equal to 0.02 for all wood species.
Brinell hardness was calculated using a hardness tester, according to EN 1534 (2010) and Eq. 3,
(3)
where HBW is the BH of wood (MPa), F is the maximum load force (N), D is the diameter of the carbide ball (mm), and d is the diameter of the residual indentation (mm).
The BH values were subsequently converted to the moisture content of 12%, according to Dubovský et al. (2003) and Eq. 4,
(4)
where HBW12 is the BH at the moisture content of 12% (MPa), HB is the BH at the moisture content during the testing (MPa), w is the sample moisture content during the testing (%), and α is the correction coefficient of moisture content for hardness perpendicular to the grain, which was equal to 0.025 for all wood species.
The wood density was determined during testing according to ISO 13061-2 (2014) and Eq. 5,
(5)
where ρw is the density of the sample at moisture content w (kg/m3), mw is the weight of the sample at moisture content w (kg), and Vw is the volume of the sample at moisture content w (m3).
The moisture content of the samples was determined according to ISO 13061-1 (2014) and Eq. 6,
(6)
where w is the moisture content of the samples (%), mw is the weight of the sample at moisture content w (kg), and m0 is the weight of the oven-dry sample (kg). Oven-drying was carried out according to ISO 13061-1 (2014).
RESULTS AND DISCUSSION
Impact Bending Strength
Table 4 presents a statistical evaluation of the influence of factors on IBS. Wood species and material thickness were statistically significant (P < 0.05). The degrees of densification, as well as the interaction of all factors, did not significantly influence the IBS.
Table 4. Statistical Evaluation of the Factors Influencing the Impact Bending Strength
The mean IBS value for aspen samples (4.6 J/cm2) was approximately 31.3% lower than that of beech samples (6.7 J/cm2; Fig. 2a). This difference was caused by the different densities of the wood species. Aspen wood exhibited a lower density, ranging from 25.1% to 28.6%, for the individual groups in comparison with beech wood (Table 2).
Fig. 2. a) Influence of wood species and b) degree of densification on the impact bending strength
In general, densification increased the mechanical properties of wood, which were directly dependent on rising density. The present results indicate that increasing the degree of densification did not have a significant influence on the IBS (P = 0.658) and resulted in a slight numerical decrease (Fig. 2b). Generally, wood densified at 10% exhibited a 6.8% lower IBS than non-densified wood. Densification at 20% exhibited a 5.1% decrease in IBS. Table 2 shows wood densities before and after densification. These differences possibly resulted from variability in the density of the tree trunks from which the samples were cut. The most notable differences were found in aspen wood. Heräjärvi and Junkkonen (2006), and Kärki (2001) found that aspen wood density within the trunk changes markedly in the direction from the pith to the cambium and also with increasing distance from the stump.
Fig. 3. Influence of the material thickness on the impact bending strength
The IBS values rose in a statistically significant manner (P = 0.001) with increasing material thickness (Fig. 3). The most marked increase of 29.8% was observed between the thicknesses of 10 mm and 18 mm. The lowest increase of 8.8% was observed between the values of the samples with a thickness of 6 mm and 10 mm. Different thicknesses results in a change of cross sectional areas of samples, thereby affecting the amount of energy required for its reassignment during the investigation of IBS.
Fig. 4. Influence of the material thickness, densification, and wood species on the impact bending strength
The IBS of beech wood exhibited a different pattern than aspen wood (Fig. 4). Beech wood exhibited variable IBS values, which were not directly proportional to the sample’s thickness. On the other hand, aspen wood exhibited IBS values that were positivity associated with its thickness. Densification had no clear influence on beech and aspen wood, which was an expected result. This can be explained by the fact that densification only concerns a certain surface layer of wood, which has generally little effect on the whole cross-section of the sample and is directly related to the energy required for breaking.
The IBS of non-densified beech wood was 7.6 J/cm2 for a thickness of 10 mm (Table 5). Samples with this thickness are most suitable in terms of comparison with other results, because their cross-sectional area is closest to the area (4 cm2) given by the standard ISO 3348 (1975). Previous studies presented similar results; for example, Bal and Bektaş (2012) reported an IBS for Eastern beech (Fagus orientalis Lipsky) of 7.2 J/cm2, and Lokaj and Vavrušová (2010) determined a slightly lower IBS value of 6.9 J/cm2. On the other hand, slightly higher IBS values were determined in other research. Skarvelis and Mantanis (2013) investigated the mechanical properties of beech wood from different locations in Greece and found IBS values of 7.8 J/cm2for Eastern beech (Fagus orientalis Lipsky) and European beech (Fagus sylvatica L.). Bektaş et al. (2002) determined an IBS of 8.5 J/cm2 for Eastern beech (Fagus orientalis Lipsky) and Wagenfür (2000) stated the highest IBS values for beech wood of 10 J/cm2.
Table 5. Mean Values of the Impact Bending Strength
Values in parentheses are standard deviations
The IBS of aspen wood retained similar characteristics for all cases. Although the IBS slightly increased with increasing thickness, densification exhibited an insignificant effect on IBS.
In this study, the mean IBS value of non-densified aspen wood was 4.6 J/cm2 at a thickness of 10 mm. Slightly lower values of IBS were identified in previous studies, such as Požgaj et al. (1997) (3.8 J/cm2 for aspen wood species) and Wagenfür (2000) (4 J/cm2). Makovická–Paulínyová et al. (2006) and Barcík et al. (2008) reported IBS values that were approximately 30% lower (3.2 J/cm2) than the present study.
Brinell Hardness
For BH, all factors and their combined effects were significant (P <0.05; Table 6). The effect of material thickness was the least significant effect (P = 0.041).
Table 6. Statistical Evaluation of the Influence of Factors on the Brinell Hardness
As expected, the BH of beech wood was much greater than that of aspen wood (Fig. 5a). The mean difference in BH was approximately 240%. On the other hand, beech wood density was approximately 200 kg/m3 higher than that of aspen, and a similar difference in density was also observed upon densification (Table 2). Higher total density and less variability in the density between individual growth rings can contribute to a greater BH in beech wood.
In this case, gradually increasing the densification resulted in a proportionate increase in BH (Fig. 5b). While the mean BH of non-densified wood was approximately 35 MPa, wood densified at 10% was 7.1% greater in hardness, whereas wood densified at 20% increased in hardness by 17.1%.
Fig. 5. a) The influence of wood species and b) the degree of densification on Brinell hardness
The influence of material thickness on the BH had the lowest statistically significant influence (P= 0.041) (Fig. 6). Although hardness increased with sample thickness, this increase was not proportional, and the differences in thickness reached an asymptote at 5.7%. To some extent, the thickness factor could be affected by the plane anvil of the hardness tester during the measurement. When the indenter was pressed into the sample, the wood surrounding the indentation was deformed and densified within its volume. Material thickness influences unequal densification to a certain degree. Completely removing this effect would ensure that the hardness of the wood would not significantly vary with the change in thickness.
Fig. 6. Influence of material thickness on the Brinell hardness
As already mentioned above, the BH of beech wood was several times greater than aspen wood (Fig. 7). In certain situations, its BH values were as much as four times those of aspen wood. The effect of material thickness showed no clear trend, and changes in hardness were quite variable. As expected, the strongest influence on BH was achieved through wood densification. The wood layer beneath the surface was densified the most, and the indenter was pushed into this layer when measuring BH. Although both beech and aspen are diffuse-porous wood species, beech wood has a higher initial density and therefore was less influenced by densification than aspen wood. During gradual densification, the wood density increased to the values representing the density of cell walls (1,500 to 1,540 kg/m3), which is similar for all woods (Gibson and Ashby 1999). Although beech started from a higher initial density (its libriform fibers are thin, thick-walled, and have a smaller lumen), further densification was not as intensive as in aspen. Lower-density aspen wood (with wider, thin-walled libriform fibers, with a larger lumen) can be densified more intensively (Požgaj et al. 1997).
Fig. 7. Influence of material thickness, densification, and wood species on the Brinell hardness
In this research, the mean value of BH in the radial direction of non-densified beech wood was 55.5 MPa, which was notably higher than that of other studies. In comparison, Wagenfür (2000) found a BH value of 34 MPa in the direction perpendicular to the fibers for European beech. Pelit et al. (2015) investigated the influence of thermo-mechanical densification and heat treatment on beech veneers and found a mean BH value of 31.9 MPa for Eastern beech (Fagus orientalis L.). Some studies reported even lower BH values. Lo Monaco et al. (2015) examined the technical properties of beech wood from two areas in Central Italy and determined that the BH values were 29.8 and 27.7 MPa, respectively.
Non-densified aspen wood exhibited a BH value of 14.5 MPa in the radial direction, which was slightly higher than the literature. Wagenfür (2000) reported a BH value in the direction perpendicular to fibers of 11 MPa for European aspen. A similar value of BH (12 MPa) was reported by Fang et al. (2012) in research investigating the influences of densification and oil-heat treatment on aspen veneers. On the other hand, Cloutier et al. (2008) found a higher BH value of 17 MPa when studying the effect of densification and heat treatment on aspen veneers.
The hardness of wood is primarily influenced by the method of measurement and its associated processing conditions (Kúdela 1998; Niemz and Stübi 2000; Hirata et al. 2001). Different hardness values are mainly caused by different processing conditions (measuring time, loading force, etc.) that arise from different methods of study.
Table 7. Mean Values of Brinell Hardness
Values in parentheses are standard deviations
CONCLUSIONS
- The IBS of wood was primarily influenced by the material’s thickness and wood type. As expected, beech wood achieved higher IBS values in all instances (mean of 31%), whereas aspen wood exhibited lower IBS values. Material thickness exhibited no clear influence on IBS for beech, while the opposite was true for aspen. The IBS value of aspen wood increased with increasing thickness. The effect of wood densification was not statistically significant (P > 0.05) and resulted in lower IBS values ranging from 5.1% to 6.8%.
- Brinell hardness was primarily influenced by the degree of densification and wood type, while the effect of material thickness had the least significant influence (P = 0.041). Wood densification was closely associated with increasing density and exhibited the highest effect on aspen wood. The hardness of aspen wood, densified at 20%, increased by 24% to 53%, depending on the thickness. Densified beech wood achieved only slightly higher BH values, ranging from 1% and 10%.
ACKNOWLEDGMENTS
The authors are grateful for support of the University-Wide Internal Grant Agency (CIGA) of the Faculty of Forestry and Wood Science at Czech University of Life Sciences Prague, Project 4308-2016.
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Article submitted: July 1, 2016; Peer review completed: August 10, 2016; Revised version received and accepted: August 15, 2016; Published: August 29, 2016.
DOI: 10.15376/biores.11.4.8638-8652