Bending Characteristics of Laminated Wood Composites Made of Poplar Wood and GFRP

• In this study, 4 layers of 5 mm thick slats obtained by sawing method from poplar wood were used. Plain woven GRFP with low density and grammage of 100 g/m 2 (Type 1) and plain woven GRFP with high density and grammage of 200 g/m 2 (Type 2) were placed and glued between each layer. Polyvinyl acetate (PVAc-D4), Polyurethane (PU) and dual-component Epoxy (L285-resin and H285-hardener) adhesives were used for gluing the layers. Strength values (bending and modulus of elasticity) between the obtained layers were investigated. As a result of the study, it was determined that epoxy glue has higher strength than polyurethane and polyvinyl acetate glues; Type 2 plain woven fabric has higher strength than Type 1 plain woven fabric; and parallel load to the glue line results in higher performance than perpendicular load to the glue line.


INTRODUCTION 1. UVOD
The value of forest products is also increasing due to the continuous decrease in forest resources and the increase of costs in the world. Due to the increase in furniture consumption, it will be possible to meet the demand for forest products by processing the forests in accordance with the scientifi c principles and to use the cut trees most effi ciently. Lamination technique is used for effi cient use of wood materials, removal of defects and formation of diagonal fi bers in curved formations. With the developing technology, it is used as glued The black poplar wood (Populus nigra) used as a solid wood in the preparation of the test specimens was obtained entirely randomly from lumber mills in Usak province. The choice of wood material was to ensure perfect timber, smooth fi bers, without knots, with normal growth, no reaction wood, and no fungus and pest damage. The slats were cut from black poplar by a circular sawing machine with the dimension of 5 mm × 70 × mm × 1000 mm. The slats were stored until reaching a moisture content of 12 % in an air conditioning room with a temperature of (20 ± 2) °C and a relative humidity of (65 ± 5) %.

Glass fi ber fabric (GFRP) 2.2. T kanina ojačana staklenim vlaknima (GFRP)
It is produced from glass fi ber materials such as fi berglass, silica, colemanite, aluminum oxide, soda. Glass fi ber is the most known and used among fi ber reinforced composites (Dost Kimya, 2017). Plain woven GFRP with low density and grammage of 100 g/m 2 (Type 1) and plain woven GFRP with high density and grammage of 200 g/m 2 (Type 2) were used between slats to improve the mechanical properties of timber structural elements and are shown in Figure 2.

Ljepilo
Polyvinyl acetate (PVAc-D4), Polyurethane (PU-D4) and Epoxy (L285 resin + H285-hardener) adhesives were used for bonding slats. Polyvinyl acetate adhesive (PVAc-D4) is an adhesive with advantageous properties such as odorless, easy application, quick curing, cold application and non-fl ammability (Polisan Ltd. Şti., 2017). Polyurethane adhesive (PU-D4) is a one component, polyurethane based, fast and laminated timber material, which has an important place in today's design world as a contemporary material that allows to reach the smallest part of the wood by applying small pieces of wood with the help of glue, allowing wider openings and any desired shape.
Regarding solid wood materials, laminated wood materials, which are superior in terms of aesthetic, economic, and technological properties, have been suggested to be preferred in skeletal elements that require strength, particularly in LVL (Laminated Veneer Lumber) furniture production (Eckelman, 1993). Laminated wood materials are used in columns and beams as building elements, and in furniture which is exposed to high static and dynamic forces, in particular by sticking the plaque coating plates hot or cold under prestress under high pressure, fl at or inclined (Dongel, 1999).
Tests with glass fi ber reinforced materials were fi rst carried out by Wangaard (1964) and Biblis (1965). In these initial experiments, both researchers made experiments using epoxy resin-treated one-way glass fi ber on different types of solid wood materials. The fi rst experiment on laminated beams was carried out by Theakson (1965). Experiments were carried out using water-based adhesive and epoxy adhesive with glass fi ber woven, glass fi ber felt and one-way untreated glass fi ber in various shapes. The highest performance was achieved with one-way glass fi ber. In recent years, some researches have been carried out on mechanical reinforcement of glass fi ber and wood based structural materials (Pidaparti and Johnson, 1996 The elements (beams) under the infl uence of the bending force are divided into two groups as "horizontal laminar elements" and "vertical laminar elements" according to the applied force direction (Baird and Ozelton, 1990). When the load is applied perpendicular to the glue line, it is called horizontal, and when the load is applied parallel to the glue line it is called vertical element. Examples of horizontal and vertical laminar elements are given in Figure 1.
The aim of this study was to bond and thicken layers between 5 mm thick Populus nigra slats using polyvinyl acetate (PVAc-D4), polyurethane (PU-D4) and dual-component epoxy resin (L285-resin and H285-hardener) to determine the bending strength and modulus of elasticity of laminated wood materials produced in 4 layers by placing low-density and high density glass fi ber fabric (GFRP) in order to strengthen the layers. .....Nuri Yildrim, Karaman Th e test specimens were prepared according to the standard "TS EN 408:2010 + A1:2014-04 Timber and Glued Laminates -Determination of Some Physical and Mechanical Properties". During the production of the test specimens from air-dried 5 mm thick solid materials, 4 solid layers of PVAc-D4, PU-D and Epoxy glues and plain woven GFPR with low and high density interlayer materials and samples without interlayer (as a control) were produced. For interlayer samples, 3 layers of glass fi ber material were used for intermediate support between solid layers. In the lamination process, the dimensions of slats are 5×100×1000 mm and 4 layers are bonded. In the case of samples of interlayer materials, the glue solution was applied to the solid bonding surfaces with a brush and glue spread of 180-200 gr/m². In the bonding process, the surfaces are glued and kept for 5-6 minutes (open time). The cold laminating process was carried out by setting the pres-sure to 1.2 N/mm², cold in the hydraulic press with a pressure gauge suitable for hot and cold preseason for 8 hours (closed time).The laminated material obtained after pressing is prepared with woodworking machines according to the standard. The prepared samples are shown in Figure 3.
By using 2 plain woven types (control, Type 1 and Type 2), 3 glue types (PVAc-D4, PU-D4, and epoxy), 1 wood type (poplar) and 2 load types (bending strength and modulus of elasticity), a total of 90 samples (3×3×1×2×5) were prepared with 5 replicates for each parameter. Prior to testing, all specimens were stored in a conditioning room maintained at (20±2 °C) and 65 % RH until moisture equilibrium was achieved. The prepared test samples were tested, according to the 4-point bending principle, in parallel and perpendicular direction to the glue line using the SHIMADZU universal testing machine placed in the laboratory of Karabuk University Safranbolu Vocational School.
The loading speed of the test machine is 5mm/ min. The bending strength, modulus of elasticity and load carrying capacity of the specimens placed with the (1) and h is thickness (mm).

Modulus of elasticity 2.5.2. Modul elastičnosti
The modulus of elasticity was determined as (Eq. 2.): Where E m·g4 is modulus of elasticity (N/mm²), λ is length measured for identifi cation of the modulus of elasticity, b 1 is width -dimension in tangential direction (cm), h 1 is height -dimension in radial direction (cm), a is distance between loading point and nearest spam (mm), F 2 -F 1 is increase of the load ratio on the correct line of the load-defl ection curve (N), and W 2 -W 1 is increase in deformation corresponding to F 2 -F 1 (mm).

Evaluation of data 2.6. Evaluacija podataka
Statistical results (arithmetic mean X, standard deviation SS and coeffi cient of variation %V) of the data obtained in the experiments were calculated. In order to determine the test results, multiple variance analysis (ANOVA) was used to determine the effect of factors on the values obtained for all groups. The Duncan test was used to indicate the signifi cance level of the interaction of the factors (p <0.05) with 5 % error. Under the name of homogeneity group (HG), groups that differ were designated as A, B, C, etc.

Čvrstoća na savijanje paralelno sa sljubnicom
According to the normality analysis test result, the regions show normal distribution. The statistical evaluation results of bending strength of laminated composite material and solid wood materials are given in Table 1 and results of multiple variance analysis are given in Table 2.
The effect of glue type and glass fi ber fabric type on the bending strength parallel to the glue line was signifi cant (p <0.05). The double interaction of the glue type and glass fi ber fabric type (p <0.05) was negligible with respect to the error. Duncan test results applied to determine which groups are different are given in Table 3 by glue type and Table 4 by glass fi ber fabric type.
According to Table 4, the highest bending strength was obtained from Type 2, while the lowest bending strength sample was obtained from control samples.  The statistical evaluation results of bending strength of laminated wood material and solid wood are given in Table 5, and the results of multiple variance analysis are given in Table 6.
The effect of the type of glue and glass fi ber fabric type on the vertical bending strength of the glue line was signifi cant (p <0.05). The double interaction of the glue type and glass fi ber fabric type (p <0.05) was negligible with respect to the error. Duncan test results applied to determine which groups are different are given in Table 7 by glue type and Table 8 by glass fi ber fabric type.
According to Table 7, the highest bending strength was obtained from epoxy glue, while the lowest bending strength was obtained from polyurethane glue (PU-D4).   Table 8, the highest bending strength was obtained from Type 2 glass fi ber fabric samples, followed by Type 1 glass fi ber fabric and control samples.

Modulus of elasticity parallel to glue line 3.3. Modul elastičnosti paralelno sa sljubnicom
The statistical evaluation of the results of laminated wood material and modulus of elasticity parallel to glue line is given in Table 9, and the results of multiple variance analysis are given in Table 10.
The effect of the type of glue and glass fi ber fabric type was signifi cant with the margin of error (p <0.05) in the modulus of elasticity parallel to the glue line. The double interaction of the glue type and glass fi ber fabric type (p <0.05) was negligible with respect to the error. Duncan test results applied to determine which groups are different are given in Table 11 by glue type and Table 12 by glass fi ber fabric type.
According to Table 11, the highest modulus of elasticity was obtained from epoxy glue and the lowest modulus of elasticity value was obtained from polyurethane (PU-D4) glue.
According to Table 12, it can be seen that the highest modulus of elasticity was obtained from Type 2 glass fi ber fabric samples, while the lowest value was obtained from control samples.

Modulus of elasticity perpendicular to glue line 3.4. Modul elastičnosti okomito na sljubnicu
The statistical evaluation results of modulus of elasticity of laminated wood material and solid wood are given in Table 13, and the results of multiple variance analysis are given in Table 14.
The effect of the type of glue and glass fi ber fabric type was signifi cant with the margin of error (p<0.05) in the modulus of elasticity perpendicular to the glue line. The double interaction of the glue type and glass fi ber fabric type (p<0.05) was negligible with respect to the error. Duncan test results applied to determine which groups are different are given in Table  11 by glue type and Table 12 by glass fi ber fabric type. Duncan test results applied to determine which groups are different are given in Table 15 by glue type and  Table 16 by glass fi ber fabric type.
The highest modulus of elasticity value was obtained with epoxy glue, followed by polyvinyl acetate grafted (PVAc-D4) and polyurethane grafted (PU-D4), respectively.
According to Table 16, it can be seen that the highest modulus of elasticity was obtained from Type 2, while the lowest value was obtained from control samples.
As a result, it was determined that epoxy glue has higher strength than polyurethane and polyvinyl acetate glues; Type 2 plain weaving woven fabric has higher strength than Type 1 plain weaving woven fabric; and laminates parallel to the glue line have higher performance than those perpendicular to the glue line. In recent years, some research has been carried out on the mechanical reinforcement of glass fi ber and wood-based structural materials (Basterra et al., 2012;Mıstak, 2013;Bal and Ozyurt, 2015;Guntekin 2015). In these studies, the researchers generally made experiments on the bending properties and adhesion strength of the reinforced material. Premrov et al., (2003) investigated the mechanical strengths of wooden structural elements reinforced with carbon fi ber, and they obtained a 50 % higher strength in the bending strength of laminated elements. Gaff and Gaffrik (2015) also examined the effect of bending strength of laminated beech wood on densifi cation. As a result, the bending strength value was found to

ZAKLJUČAK
In this study, the modulus of elasticity of the laminated wood material supported by various glass fi ber materials was investigated from 4 aspects. To this purpose (Populus nigra.) wood, which is widely used in the manufacture of furniture and building elements in our country, was made of 4 layers of glass fi ber elements placed in porous structure between slats and bonded with epoxy, polyvinyl acetate (PVAc-D4) and polyurethane (PU-D4). As a result of the test, control samples and samples supported by glass fi ber fabric were statistically evaluated according to the glue type, glass fi ber type and load type. Based on the results, epoxy adhesive showed the highest bending strength parallel and perpendicular to the glue line, while polyurethane (PU-D4) glue showed the lowest bending strength. According to the glass reinforcing fi ber type, Type 2 showed the highest bending strength, while the control samples showed the lowest bending strength. Epoxy + Type 2 showed the highest bending strength combined with glue type and glass reinforcing fi ber type. Epoxy adhesive showed the highest modulus of elasticity strength parallel and perpendicular to the glue line, while polyurethane (PU-D4) glue showed the lowest bending strength. Regarding the type of glue, epoxy adhesive showed the highest modulus of elasticity perpendicular to the glue line, while polyurethane (PU-D4) glue showed the lowest modulus of elasticity value. Epoxy + Type 2 showed the highest modulus of elasticity combined with glue type and glass reinforcing fi ber type.
Based on the experimental results obtained, it was determined that the bending strength and the modulus of elasticity increased the strength of the support materials as compared to the control example. Since there is a signifi cant increase in the strength properties of the material when using the intermediate fi ller material in the laminated materials, its use may be preferred in furniture and building properties. In the literature, it is stated that the performance will increase as a result of the increase of layer thickness in pine samples. As a continuation of this study, researchers are recommended to focus on different layer symmetry and testing of laminated samples with different fi lling materials.