PEER-REVIEW ARTICLE PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3367 Assessment of Torque Performance during Screwing in Beech Plywood Reinforced with Glass Fiber in Phenol- Formaldehyde Resin Gökhan Kayacık ,a and Önder Tor ,b This study aimed to develop plywood, which is generally used as a building material in outdoor areas, by adding low-cost glass fiber and to examine some physical and mechanical properties of the developed plywood. For this, beech veneers (600 × 600 mm dimension, 2 mm thickness in 3% to 5% humidity) were glued with phenol-formaldehyde adhesive at the levels of 10%, 20%, and 30% by adding powdered e-type glass fiber in two different sizes (25 µm and 10 µm) to produce seven-layer beech plywood. Density, equilibrium moisture content, thickness swelling, tensile- shearing, and tensile-perpendicular to the surface were evaluated on test samples cut from the beech plywood in 50 × 50 mm dimension, and screwing torque values were carried out on test samples cut in 500 × 50 mm dimensions. Two-factor analysis of variance (ANOVA) was performed separately for the tests above. Results indicated that in cases where interactions between the glass fiber size and ratio were significant at the 0.05 significance level, the least significant difference value (LSD) analysis of the interaction was performed. According to LSD results, the tensile strength perpendicular to the surface decreased as the glass fiber ratio was increased. DOI: 10.15376/biores.20.2.3367-3385 Keywords: Screw; Seating torque; Maximum torque; Tensile-shear resistance; Tensile resistance; Perpendicular to the surface Contact information: a: Directorate General For Prisons and Detention Houses, Ministry of Justice, Namık Kemal Neighbourhood. Milli Müdafaa Street No:22 Floor:1 No:106 Kızılay, Çankaya, Ankara, TÜRKİYE; b: Department of Forest Industry Engineering; Faculty of Forestry, Kastamonu Üniversity, Kuzeykent Mahallesi Orgeneral Atilla Ateş Paşa Caddesi No:19 Posta Kodu: 37150 Merkez, Kastamonu, TÜRKİYE; * Corresponding author: ondertor@kastamonu.edu.tr INTRODUCTION The use of forest products continues to increase due to the rapid increase in the world population, urbanization, technological developments, social, cultural and economic developments. Existing raw materials can no longer meet the industry's demand. It has become more difficult to obtain raw wood materials due to the industry’s growth in wood consumption, highlighting the need for the most cost-effective wood-use approach (Çakıroğlu 2012). In recent years, studies have attempted to obtain more valuable products by combining materials such as glass, steel, stone, plastic, wood, and concrete in different shapes or proportions. On the other hand, the development of carbon, high-strength glass, boron, and aramid fibers continues, and these developed materials were previously used in high-risk applications, but later they were also used in structural applications (Ekinci 2004). mailto:ondertor@kastamonu.edu.tr https://orcid.org/0009-0005-8720-8406 https://orcid.org/0000-0002-9405-1081 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3368 A study reported some mechanical and physical properties of layered veneer timbers glued with phenol formaldehyde adhesive reinforced with glass fiber textile using poplar veneers (Özyurt 2015). As a result of this research, the glass fiber woven support significantly increased the boards’ shock resistance, density, and modulus of elasticity. Also, it reduced the tensile and shear resistance, thickness swelling, and water absorption percentages (Özyurt 2015). Bal (2014) investigated the effect of glass fiber textiles on some physical and mechanical properties of laminated wood obtained from poplar veneers; the results showed that the mechanical resistance of reinforced laminated wood is higher than that of ordinary laminated wood. In another study where the bending strength properties of red pine wood materials reinforced with glass fiber and steel plate were investigated, the bending strength increased by 14% in reinforcement with glass fiber and by 24% in reinforcement with steel plate (Güntekin and Aydın 2015). The bending strength of glass fiber fabric-reinforced wooden laminated materials was examined in another study; where 100 and 200 g/m2 of glass fiber fabric were added to the samples prepared from chestnut (Castanea sativa) wood to examine any increase in their strength. According to the results, the samples parallel to the glue line showed higher strength than those perpendicular to the glue line (Karaman and Yıldırım 2018). One of the critical mechanical properties is the screwing performance in wood and wood-based materials. There are some studies about the factors affecting the screw-driving torques in wood-based materials. In a study, screw resistances were investigated on glass-fibre- strengthening wood material (Bal and Efe 2015). For this purpose, layered veneer timbers obtained from poplar and layered veneer timbers reinforced with glass fiber textiles were used. Based on the results, the glass fiber reinforcement increased screw pullout resistance, screw head pullout resistance, and screw lateral pullout resistance (Bal and Efe 2015). The penetration depth and critical torques, including screwdriver air pressure, seating, and maximum torques, were assessed in a study examining the variables influencing the amount of vertical driving force supplied to the screws during the screwing process on particleboard face orientation. The results demonstrated that when the pilot hole was not drilled, the vertical forces used during screwing had a substantial impact on the critical screwing torque magnitude; however, when the pilot holes with a diameter of 3.2 mm were drilled, this effect was not significant. Additionally, it was reported that in boards without drilled pilot holes, the air pressure of the drill during screwing had no discernible impact on the magnitude of the critical screwing torques; however, if pilot holes were drilled, increasing the drill's air pressure from 0.45 to 0.62 MPa resulted in increases in the average maximum torques. (Tor et al. 2019). In a study investigating the effects of countersink hole and screwing direction, pilot hole diameter and pilot hole depth on screwing torque values in medium-density fiberboards were obtained by joining medium-density fiberboards face to face. According to the results, the quadruple interaction between screwing direction, pilot hole diameter and depth, and the condition of the countersink hole is significant (Tor 2019a). Measurements of seating torque and maximum torque were made in a study examining the effects of the pilot hole diameter drilled on medium-density fiberboard on screwing torques. The statistical analysis indicated that the pilot hole diameter significantly impacted the seating and maximum torque values in medium-density fiberboard. According to the statement, the maximum torque values ranged from 1.18 to 2.68 N.m. In contrast, the average seating torque ranged between 0.37 and 1.23 N.m when screws were tightened on the medium-density fiberboard side surface. The maximum torque values for PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3369 surface samples ranged from 2.89 to 5.41 N.m, while the average sitting torque values ranged from 0.57 to 1.56 N.m (Tor 2019b). In this study, the effects of mechanical properties on screwing torques of plywood, which is generally used as a building material in outdoor areas, were improved by adding low-cost glass fiber. Features of glass fiber reinforced composites such as low cost, easy availability, ease of production, and high strength are the main reason these composites have been used in many areas from past to present (Ekiz 2013). This study aimed to develop a seven-layer plywood material with beech veneers glued with the phenol-formaldehyde adhesive by adding powdered e-type glass fiber. Some physical and mechanical properties (density, equilibrium moisture content, thickness swelling, tensile-shearing, tensile perpendicular to the surface, and screw-driving torques) were carried out on test plywood samples. The effects of the glass fibre size and ratio in phenol-formaldehyde adhesive were evaluated in beech plywood. EXPERIMENTAL Materials and Methods The beech veneers used in the study were supplied by a local company producing plywood in Kastamonu, dried to 3% to 5% moisture and in dimensions of 600 × 600 mm. Toby phenol formaldehyde adhesive, widely used in the plywood manufacturing industries, was obtained from Polisan Kimya Sanayi A.Ş. The phenol-formaldehyde adhesive was mixed with the glass fiber in various different ratios of 10%, 20%, and 30% and added e-type glass fiber in powder form in two different sizes, 25 µm and 10 µm in a room temperature until homogeneously mixed. a) b) Fig. 1. Beech veneers (a) and powdered e-type glass fibers of 25 µm and 10 µm (b) The beech plywood was produced in seven layers based on the TS 1250 (1974). Beech veneers of dimensions 600 × 600 mm were pressed for 10 min in a single-layer hydraulic press set at 120 °C and 12 kg/cm² press pressure. Test samples of the 7-layered plywood were prepared in the Wood Culture Research and Application Center at Kastamonu University. The pilot holes with a diameter of 3.5 mm, equal to the total thickness of the samples, were drilled in the test samples where the screwing torque values were to be determined. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3370 The sheets were produced in the Impregnation Laboratory within the Department of Forest Industry Engineering, Faculty of Forestry, Kastamonu University. The average thickness of the 7-layer plywood produced was measured as 14.28 mm. Physical Properties Equilibrium moisture content The equilibrium moisture content of the samples was determined according to the standard of TS EN 322 (1999) and is shown in Table 1. The moist weights of test samples, each measuring 50 × 50 mm, were weighed on an analytical balance with an accuracy of ±0.01 g. They were kept in an oven at 103±2 °C until they reached a constant weight, and their total dry weight was weighed on an analytical balance with an accuracy of ±0.01 g. Table 1. Standards and Sample Quantities Used in Physical Property Tests Test Standard Number of Samples Density TS EN 323/1 (1999) 20 Equilibrium Moisture Content TS EN 322 (1999) 10 Swelling to Thickness TS EN 317 (1999) 10 Swelling to thickness The thickness swelling of the samples was determined according to the standard of TS EN 317 (1999). Ten of the test samples obtained were used for each test group. Mechanical Properties Tensile-shear resistance Tensile-shear resistance values of the samples were determined according to a standard of TS EN 314-1 (1998) and is shown in Table 2. The experiment was conducted on a Shimadzu brand universal testing machine at Kastamonu University, Forest Industry Engineering, Wood Mechanics and Technology Laboratory. Table 2. Standards and Sample Quantities Used in Mechanical Property Tests Test Standard Number of samples Tensile-Shear Resistance TS EN 314-1 (1998) 10 Tensile Strength Perpendicular to the Surface TS EN 319 (1999) 10 Tensile strength perpendicular to the surface Tensile strength perpendicular to the surface of the samples was determined according to the standard of TS EN 319 (1999). The experiment was conducted on a Shimadzu brand universal testing machine at Kastamonu University, Forest Industrial Engineering, Wood Mechanics and Technology Laboratory. Determination of screwing torque values As wood material, pilot holes with a diameter of 3.5 mm were drilled at 50 mm intervals with a column drill on the test samples cut in 500 × 50 mm dimensions from the boards produced with veneers obtained from beech logs according to the standard TS 1250 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3371 (1974). The pilot hole depths were adjusted to be the total thickness of the test samples. Screwing torque values were tested by placing 10 mm thick metal plates countersunk in the middle of the test samples that was fitted the screw head. Wood screws with Phillips head were 4 × 30 mm in dimension and made out of steel were used in the study. (a) (b) (c) Fig. 2. Torque screwdriver (a) 0.1 < 1.2 N.m, (b) 1.2 < 3.0 N.m, (c) 3.0 < 6.0 N.m The test samples were screwed with a pilot hole drilled using a torque screw-driver (Fig. 2.). The torque value at the moment when the screw fully contacts or entirely sits on the material surface is obtained as “seating torque”, and after the seating torque is obtained, the torque value just before the screw strips in the sample was “maximum torque” value in N.m. Thirty pieces from each test group were used to determine the screwing torque values. Data Analysis The data of the tests conducted for physical and mechanical properties were statistically evaluated separately by two-factor analysis of variance (ANOVA) (Table 3). According to the ANOVA results, in cases where interactions were significant at the 0.05 significance level, the least significant difference (LSD) analysis of the interaction was performed. In cases where the binary interaction was insignificant according to the ANOVA results, the LSD analysis was performed on the factors separately, and intra-factor comparisons were made. RESULTS Using beech plywood with seven and nine layers, pilot hole diameters of 3.0 and 3.5 mm, pilot hole depths of 60% and 80%, and metal plate thickness of 7.5 and 10 mm, the factors affecting screwing torques were investigated in the study. The average seating torque values ranged between 0.31 and 0.69 N.m, while average maximum torque values were between 0.50 and 4.7 N.m. In this study, in 7-layer poplar plywood samples using 3.5 mm pilot hole diameter and 10 mm metal plates, the average seating torque was determined to be 0.505 N.m, and the maximum torque average was 1.295 N.m in samples with 60% pilot hole depth. In samples with 80% pilot hole depth, the average seating torque was determined to be 0.320 N.m, and the maximum torque average was 1.150 N.m (Tor et al. 2020). In this study, 7-layer beech plywood was created by applying powdered e-type glass fiber in two different sizes, 25 µm and 10 µm, to the adhesive at 10%, 20%, and 30% levels. Tests were carried out for screwing torque values by placing 10 mm thick metal plates PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3372 countersunk in the middle of the test samples in which pilot holes were drilled with a diameter of 3.5 mm and as much as the plate thickness. For the test samples mixed with 10-micron size glass fibers, the average seating torque was 0.402 N.m, and the maximum torque was 4.72 N.m; in samples with a 10% glass fiber ratio, the average seating torque was 0.415 N.m, and the maximum torque was 4.98 N.m in samples with 20% glass fiber ratio. The maximum torque is 4.98 N.m in samples with a 30% glass fiber ratio with 25 µm glass fiber size. The average seating torque was 0.37 N.m, and the maximum torque was 4.46 N.m. For the test samples mixed with 25-micron size glass fibers, the average seating torque was 0.432 N.m, and the maximum torque was 4.98 N.m; in samples with a 10% glass fiber ratio, the average seating torque was 0.442 N.m, and the maximum torque was 5.32 N.m in samples with 20% glass fiber ratio. The maximum torque was 5.32 N.m in samples with a 30% glass fiber ratio. The examples with 0.44 N.m average seating torque and maximum torque of 5.16 N.m. In addition, in the control samples, the average seating torque was determined as 0.347 N.m, and the maximum torque was 4.35 N.m. Physical Properties Table 3 shows statistical results for physical properties. Table 3. F- and P-Values of Physical Properties Physical Properties Source F-Value P-Value Density Glass Fiber Size (µm) 4.590 0.0337 Glass Fiber Ratio (%) 6.890 0.0002 Glass Fiber Size (µm) x Glass Fiber Ratio (%) 1.460 0.2278 Equilibrium Moisture Amount Glass Fiber Size (µm) 1.600 0.2085 Glass Fiber Ratio (%) 3.690 0.0133 Glass Fiber Size (µm) x Glass Fiber Ratio (%) 0.650 0.5831 Swelling to Thickness (2 s) Glass Fiber Size (µm) 11.840 0.0010 Glass Fiber Ratio (%) 3.050 0.0340 Glass Fiber Size (µm) x Glass Fiber Ratio (%) 2.290 0.0856 Swelling to Thickness (24 s) Glass Fiber Size (µm) 1.140 0.2886 Glass Fiber Ratio (%) 1.530 0.2150 Glass Fiber Size (µm) x Glass Fiber Ratio (%) 3.090 0.0323 Density In the samples with 10-micron glass fiber size, the highest density average was determined as 10% and 30%, and in the samples with 25-micron glass fiber size, the highest density average was detected in the samples with a 10% glass fiber ratio (Table 4). According to the ANOVA results, the interaction between glass fiber size and glass fiber ratio was insignificant, as the P-value was 0.2278 at the 0.05 significance level. Based on the LSD analysis (Tables 5 and 6), the density in samples with a glass fiber size of 10 microns was statistically higher than that of 25 microns. The results showed that 10% and PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3373 30% samples were statistically denser than the control samples and samples with a 20% glass fiber ratio. Table 4. Descriptive Statistical Information for Density Experiment Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (g/cm³) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 20 0.726 0.016 2 10 20 0.730 0.013 2 20 20 0.716 0.011 2 30 20 0.730 0.018 2 25 0 20 0.726 0.016 2 10 20 0.726 0.020 3 20 20 0.712 0.009 1 30 20 0.717 0.012 2 Table 5. Comparison of Average Density Values According to Glass Fiber Size Glass Fiber Size (µm) 10 25 0.726 A* 0.721 B LSD = 0.0047 * Means followed by a common letter are not significantly different at the 5% level Table 6. Comparison of Average Density Values According to Glass Fiber Ratio Glass Fiber Ratio (%) 0 10 20 30 0.726 A* 0.729 A 0.715 B 0.724 A LSD = 0.0066 * Means followed by a common letter are not significantly different at the 5% level Equilibrium Moisture Amount In samples with a 10 µm glass fiber size, the highest average moisture content was detected in samples with a 10% glass fiber ratio. In samples with 25 µm glass fiber size, the highest average moisture content was detected in samples with 10% glass fiber ratio. According to the ANOVA results (Table 7), the two-way interaction between glass fiber size and glass fiber ratio was insignificant, as the P-value was 0.5831 at the 0.05 significance level. Therefore, the effect of each factor was examined separately. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3374 Table 7. Descriptive Statistical Information for the Equilibrium Moisture Content Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (%) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 20 9.100 0.575 6 10 20 9.218 0.934 10 20 20 8.630 1.152 13 30 20 8.469 0.687 8 25 0 20 9.100 0.575 6 10 20 9.335 1.930 21 20 20 9.233 1.182 13 30 20 8.588 0.620 7 According to LSD analysis, there was no statistical difference between the MC in samples with glass fiber sizes of 10 microns and 25 microns (Table 8 and 9). When compared according to the glass fiber ratio, there was no statistical difference between the control and plywood samples with 10% and 20% glass fiber ratios. In addition, there was a statistical difference between the control samples and the plywood samples with 10% glass fiber content and the plywood samples with 30% glass fiber content. Table 8. Comparison of Average MC Values According to Glass Fiber Size Glass Fiber Size (µm) 10 25 8.854 A* 9.064 A LSD = 0.3284 * Means followed by a common letter are not significantly different at the 5% level Table 9. Comparison of Average MC Values for Glass Fiber Ratio Glass Fiber Ratio (%) 0 10 20 30 9.010 A* 9.276 A 8.931 AB 8.529 B LSD = 0.4644 * Means followed by a common letter are not significantly different at the 5% level Swelling to Thickness In samples with 10 µm glass fiber size after 2 h of immersion in water, the highest average swelling thickness was detected in samples with a 30% glass fiber ratio. In samples with 25 µm glass fiber size, the highest average swelling thickness was detected in samples with a 30% glass fiber ratio. According to the ANOVA results (Table 10), the two-way interaction between glass fiber size and glass fiber ratio was insignificant, as the P-value was 0.0856 at the 0.05 significance level. Therefore, the effects of each factor were examined separately. According to the LSD analysis (Table 11 and 12), there PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3375 was a statistical difference between swelling thickness in samples with glass fiber sizes of 10 µm and 25 µm. When compared according to the glass fiber ratio, there was statistically no difference in thickness swelling between the control and plywood samples with a 10% glass fiber ratio. Table 10. Descriptive Statistical Information for the Thickness Swelling Test in Water Immersion for 2 h Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (%) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 10 2.931 1.089 37 10 10 2.966 0.956 32 20 10 4.265 1.567 37 30 10 4.354 1.693 39 25 0 10 2.931 1.089 37 10 10 2.498 0.806 32 20 10 2.637 0.799 30 30 10 2.992 0.454 15 Table 11. Comparison of Average Values of 2 h Thickness Swelling for Glass Fiber Size Glass Fiber Size (µm) 10 25 3.629 A* 2.765 B LSD = 0.5009 * Means followed by a common letter are not significantly different at the 5% level Statistically, there was no difference for swelling in thickness between plywood samples with 20% glass fiber content and plywood samples with 30% glass fiber content. Additionally, there was a statistical difference in swelling thickness between plywood samples with 20% and 30% glass fiber content and plywood samples with 10% glass fiber content. Table 12. Comparison of 2 h Thickness Swelling Average Values for Glass Fiber Ratio Glass Fiber Ratio (%) 0 10 20 30 2.931 BC* 2.732 C 3.451 AB 3.673 A LSD = 0.7083 * Means followed by a common letter are not significantly different at the 5% level In samples with 10-micron glass fiber size after 24 h of immersion in water, the highest average swelling thickness was detected in samples with a 20% glass fiber ratio. In PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3376 samples with 25 µm glass fiber size, the highest average swelling thickness was detected in samples with a 30% glass fiber ratio. According to the ANOVA results (Table 13), the two-way interaction between glass fiber size and glass fiber ratio was significant, as the P- value was 0.0323 at the 0.05 significance level. According to the LSD analysis (Table 14 and Fig 3), there was no statistical difference in thickness swelling between plywood samples with a glass fiber size of 10 µm and 25 µm with a 10% glass fiber ratio. There was a statistical difference in thickness swelling between plywood samples with a glass fiber size of 10 µm and 25 µm with a 20% glass fiber ratio. No difference in thickness swelling was observed between the above plywood samples with with a 30% glass fiber ratio. When compared according to the glass fiber ratio, there was no statistical difference between plywood samples with a glass fiber size of 10 µm and a glass fiber ratio of 20% and 30%. Statistically, there was no difference in swelling thickness between the control with a glass fiber size of 10 µm and the plywood samples with a 10% glass fiber ratio. Table 13. Descriptive Statistical Information for the Thickness Swelling Test in Water Immersion for 24 h Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (%) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 10 5.988 0.902 15 10 10 5.643 0.259 5 20 10 6.783 0.699 10 30 10 6.352 0.927 15 25 0 10 5.988 0.902 15 10 10 6.063 0.826 14 20 10 5.758 0.582 10 30 10 6.218 0.845 14 Table 14. Comparison of 24 h Thickness Swelling Average Values for Glass Fiber Size in each Glass Fiber Ratio Glass Fiber Ratio (%) Glass Fiber Size (µm) 10 25 Control 5.988 A* 5.988 A 10 5.643 A 6.063 A 20 6.783 A 5.758 B 30 6.352 A 6.219 A LSD = 0.689 * Means followed by a common letter are not significantly different at the 5% level PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3377 Fig. 3. Comparison of 24-h thickness swelling average values for glass fiber ratio. (* Means followed by a common letter are not significantly different at the 5% level) (LSD:0,689) There was a statistical difference in thickness swelling between plywood samples with 10 µm glass fiber size and 20% and 30% glass fiber ratio and plywood samples with 10% glass fiber ratio. There was no statistical difference in thickness swelling between plywood samples with a glass fiber size of 25 microns. Mechanical Properties F- and P-values for mechanical properties of the sources are given in Table 15. Table 15. F- and P-Values Based on Mechanical Properties Mechanical Properties Source F-Value P-Value Tensile-Shear Resistance Glass Fiber Size (µm) 1.990 0.1627 Glass Fiber Ratio (%) 2.270 0.0877 Glass Fiber Size (µm) × Glass Fiber Ratio (%) 0.690 0.5587 Tensile Strength Perpendicular to the Surface Glass Fiber Size (µm) 0.000 0.9466 Glass Fiber Ratio (%) 3.530 0.0191 Glass Fiber Size (µm) × Glass Fiber Ratio (%) 0.180 0.9096 Seating Torque Glass Fiber Size (µm) 21.030 <0.0001 Glass Fiber Ratio (%) 27.730 <0.0001 Glass Fiber Size (µm) × Glass Fiber Ratio (%) 4.350 0.0053 Maximum Torque Glass Fiber Size (µm) 17.950 <0.0001 Glass Fiber Ratio (%) 18.340 <0.0001 Glass Fiber Size (µm) × Glass Fiber Ratio (%) 3.560 0.0151 0 1 2 3 4 5 6 7 8 9 10 Control 10 20 30 T h ic k n e s s s w e ll in g ( % ) Glass Fiber Ratio (%) Glass Fiber Size 10 µm Glass Fiber Size 25 µm A AB BC C A A A A PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3378 Tensile-Shear Resistance In samples with a 10-micron glass fiber size, the highest average tensile-shear strength was detected in samples with a 10% glass fiber ratio (Table 16). In samples with 25 µm glass fiber size, the highest average tensile-shear strength was detected in samples with a 10% glass fiber ratio. According to the ANOVA table for the tensile-shear strength test, the bilateral interaction between the glass fiber size and glass fiber ratio was insignificant at a significance level of 0.05, so the effects of each factor were examined separately. Table 16. Descriptive Statistical Information for Tensile-Shear Test Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (N/mm²) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 10 2.254 0.248 11 10 10 2.264 0.380 17 20 10 2.093 0.397 19 30 10 1.827 0.308 17 25 0 10 2.254 0.248 11 10 10 2.275 0.542 24 20 10 2.271 0.562 25 30 10 2.135 0.339 16 Table 17. Comparison of Glass Fiber Size Average Tensile-Shear Strength Values Glass Fiber Size (µm) 10 25 2.234 A* 2.109 A LSD = 0.1758 * Means followed by a common letter are not significantly different at the 5% level According to the LSD analysis (Table 17 and 18), there was no statistical difference between the tensile-shear strength of the samples with a glass fiber size of 10 and 25 µm. Table 18. Comparison of Glass Fiber Ratio Average Tensile-Shear Strength Values Glass Fiber Ratio (%) 0 10 20 30 2.255 A* 2.270 A 2.182 AB 1.981 B LSD = 0.2486 * Means followed by a common letter are not significantly different at the 5% level Compared to the glass fiber ratio, there was no statistical difference in tensile-shear strength between the control and plywood samples with 10% glass fiber ratios. Statistically, PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3379 there was no difference in tensile-shear strength between plywood samples with 20% glass fiber content and plywood samples with 30% glass fiber content. In addition, there was a statistical difference in tensile-shear strength between control samples and plywood samples with 10% glass fiber content and plywood samples with 30% glass fiber content. Tensile strength perpendicular to the surface In samples with a 10 µm glass fiber size, the highest average tensile strength perpendicular to the surface was determined in samples with a 10% glass fiber ratio (Table 19). In samples with 25 µm glass fiber size, the highest average tensile strength perpendicular to the surface was determined with a 10% glass fiber ratio. Based on ANOVA results, the two-way interaction between glass fiber size and glass fiber ratio was insignificant, as the P-value was 0.9096 at the 0.05 significance level. Therefore, the effects of each factor were examined separately. The LSD analysis (Tables 20 and 21) showed that there was no statistical difference between the tensile strength perpendicular to the surface in the samples with a glass fiber size of 10 and 25 µm. Compared to the glass fiber ratio, there was statistically no difference in tensile strength perpendicular to the surface between the control and plywood samples with 10% glass fiber ratios. Table 19. Descriptive Statistical Information for Tensile Strength Perpendicular to the Surface Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (kg/cm²) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 10 1.509 0.369 24 10 10 1.385 0.401 29 20 10 1.226 0.406 33 30 10 1.149 0.191 17 25 0 10 1.509 0.369 24 10 10 1.285 0.352 27 20 10 1.274 0.182 14 30 10 1.180 0.436 37 Table 20. Comparison of Glass Fiber Size Average Tensile Strength Values Perpendicular to the Surface Glass Fiber Size (µm) 10 25 1.31726 A* 1.31200 A LSD = 0.1561 * Means followed by a common letter are not significantly different at the 5% level Statistically, there was no difference in tensile strength perpendicular to the surface between plywood samples with 20% glass fiber content and plywood samples with 30% glass fiber content. In addition, there is a statistical difference in tensile strength PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3380 perpendicular to the surface between the control and plywood samples with 20% and 30% glass fiber content. Table 21. Comparison of Glass Fiber Ratio Average Tensile Strength Values Perpendicular to the Surface Glass Fiber Ratio (%) 0 10 20 30 1.5090 A 1.3350 AB* 1.2498 B 1.1648 B LSD = 0.2208 * Means followed by a common letter are not significantly different at the 5% level Determination of screwing torque values In samples with 10 µm glass fiber size, the highest average seating torque was detected in samples with a 20% glass fiber ratio (Table 22). In samples with 25 µm glass fiber size, the highest average seating torque was detected in samples with a 20% glass fiber ratio. According to the ANOVA results, the two-way interaction between glass fiber size and glass fiber ratio was significant, as the P-value was 0.0053 at the 0.05 significance level. As per the LSD analysis (Table 23 and Fig 4), the glass fiber size was 10. There was a statistical difference in seating torque between 10 and 25-micron plywood samples with 10% glass fiber content. Table 22. Descriptive Statistical Information for Seating Torque Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (N.m) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 30 0.347 0.047 14 10 30 0.402 0.061 15 20 30 0.415 0.066 16 30 30 0.370 0.058 16 25 0 30 0.347 0.047 14 10 30 0.432 0.048 11 20 30 0.442 0.046 10 30 30 0.440 0.050 11 There was no statistical difference in seating torque between plywood samples with a glass fiber size of 10 and 25 µm with a 20% glass fiber ratio. There was a statistical difference in seating torque between two plywood samples with a glass fiber size of 10 and 25 µm with a 30% glass fiber ratio. Compared to the glass fiber ratio, there was no difference in seating torque between plywood samples with 10 µm glass fiber size and 10% and 20% glass fiber ratio. Statistically, there was no difference in seating torque between the control with a glass fiber size of 10 µm and the plywood samples with a 30% glass fiber ratio. There was a statistical difference in seating torque between the control samples with a glass fiber size of 10 µm, the plywood samples with a 30% glass fiber ratio, and the PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3381 plywood samples with a 10% and 20% glass fiber ratio. There was a statistical difference in seating torque between plywood with a glass fiber size of 25 microns and a glass fiber ratio of 10%, 20%, and 30% and the control samples. Table 23. Comparison of Average Seating Torque Values According to Glass Fiber Size Glass Fiber Ratio (%) Glass Fiber Size (µm) 10 25 Control 0.343 A* 0.343 A 10 0.402 B 0.432 A 20 0.415 A 0.442 A 30 0.370 B 0.440 A LSD=0.0271 * Means followed by a common letter are not significantly different at the 5% level Fig. 4. Comparison of average seating torque values according to glass fiber ratio in each glass fiber size. (* Means followed by a common letter are not significantly different at the 5% level) (LSD:0.0271) In samples with a 10 µm glass fiber size, the highest maximum torque average was detected in samples with a 20% glass fiber ratio (Table 24). In samples with a 25 µm glass fiber size, the highest maximum torque average was detected in samples with a 20% glass fiber ratio. According to the ANOVA results, the two-way interaction between glass fiber size and glass fiber ratio was significant, as the P-value was 0.0151 at the 0.05 significance level. The LSD result (Table 25 and Fig 5) was examined because the two-way interaction between the glass fiber size and glass fiber ratio was significant at 0.05. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Control 10 20 30 Glass Fiber Ratio (%) Glass Fiber Size 10 µm Glass Fiber Size 25 µm B* A A A B A A B S e a ti n g T o rq u e (N .m ) PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3382 Table 24. Descriptive Statistical Information for Maximum Torque Glass Fiber Size (µm) Glass Fiber Ratio (%) Number of Samples Average (N.m) Standard Deviation (𝜎) Coefficient of Variation (%) 10 0 30 4.350 0.552 13 10 30 4.717 0.665 14 20 30 4.975 0.589 12 30 30 4.458 0.755 17 25 0 30 4.350 0.552 13 10 30 4.975 0.638 13 20 30 5.317 0.517 10 30 30 5.158 0.428 8 According to the LSD analysis, there was no statistical difference in maximum torque between plywood samples with a glass fiber size of 10 and 25 µm with a 10% glass fiber ratio. There was a statistical maximum torque difference between plywood samples with a glass fiber size of 10 and 25 µm with a 20% and 30% glass fiber ratios. When compared according to the glass fiber ratio, there was no difference in maximum torque between plywood samples with a glass fiber size of 10 µm and a glass fiber ratio of 10% and 20%. There was a difference in maximum torque between plywood samples with a glass fiber size of 10 microns and a 20% and 30% glass fiber ratios. A 10% glass fiber ratio seems to represent an optimal amount of fiber in the composite material where there is sufficient reinforcement from the fibers without overloading the matrix. At this ratio, the fibers are likely well-distributed within the polymer matrix, allowing for strong interfacial bonding between the fibers and the matrix material. This leads to higher tensile-shear strength, as the fibers contribute to load transfer effectively. Statistically, there was a maximum torque difference between the plywood samples with a glass fiber size of 10 µm and a 20% glass fiber ratio and the control samples. There was a statistical maximum torque difference between the plywood samples with 20% and 30% glass fiber content, with a glass fiber size of 25 µm, and the control samples. There was a statistical maximum torque difference between plywood samples with 20% and 30% glass fiber content, with a glass fiber size of 25 µm, and plywood samples with a 10% glass fiber ratio (Fig. 5.). Table 25. Comparison of Average Maximum Torque Value by Glass Fiber Size in Each Glass Fiber Ratio Glass Fiber Ratio (%) Glass Fiber Size (µm) 10 25 Control 4.350 A* 4.350 A 10 4.717 A 4.975 A 20 4.975 B 5.317 A 30 4.458 B 5.158 A LSD=0.3022 * Means followed by a common letter are not significantly different at the 5% level PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3383 Fig. 5. Comparison of average maximum torque value according to glass fiber ratio in each glass fiber size (* Means followed by a common letter are not significantly different at the 5% level) (LSD = 0.3022) CONCLUSION AND RECOMMENDATIONS 1. The physical and mechanical characteristics of beech plywood with a 20% glass fiber ratio and a 25 µm glass fiber size were evaluated in this study. Samples with a 10% and 30% glass fiber ratio had a slightly greater density average than the control samples. Samples with a 10% glass fiber ratio also had the greatest moisture average, with an average that was higher than the control samples. 2. Samples with a 30% glass fiber ratio showed the highest average swelling thickness after 2 h of submersion in water. Additionally, samples with a 10% glass fiber ratio had the highest average tensile-shear resistance, which was on average higher than that of the control samples. The average tensile-shear resistance dropped as the glass fiber ratio rose. 3. Of all the glass fiber ratios used in the test, samples with a 20% had the highest maximum torque averages and the highest average seating torque averages. The study showed that by adding inexpensive e-type glass fiber to the glue improved maximum torque and seating torque. 4. The findings of this study can help to understand prevent health issues such as cumulative damage to connective soft tissues and injuries to the hand, wrist, forearm, shoulder, and neck brought on by repetitive motions, powerful movements, and restricted or fixed postures related to torque. Accessing adequate 0 1 2 3 4 5 6 7 8 Control 10 20 30 M a x im u m T o rq u e ( N .m ) Glass Fiber Ratio (%) Glass Fiber Size 10 µm Glass Fiber Size 25 µm C* B A A AB B A AB PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Kayacık and Tor (2025). “Screw torque in plywood,” BioResources 20(2), 3367-3385. 3384 and high-quality raw materials is made more difficult by the daily growth in the number of industrial production enterprises. 5. Working with the plywood made of different classes of glass fibers and tree species will result in the economic benefit of increasing its class of life span through improved resistance values and screwing torque value. This development also means utilizing and valorizing raw material that comes from forests in an effective and efficient manner. ACKNOWLEDGMENTS The authors are grateful for the support of the Wood Culture Research Center at Kastamonu University to prepare the test samples. REFERENCES CITED Bal, B. C. (2014). “Some physical and mechanical properties of reinforced laminated veneer lumber,” Construction and Building Materials 68, 120-126. DOI: 10.1016/j.conbuildmat.2014.06.042 Bal, B. C., and Efe, F. T. (2015). “The effect of reinforcement with glass fiber fabric on some screw strengths of laminated veneer lumber,” Düzce University Faculty of Forestry Journal of Forestry 11(2), 40-47, Düzce University Bal, B. C., and Özyurt, H. 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Article submitted: January 29, 2025; Peer review completed: February 21, 2025; Revised version received and accepted: March 6, 2025; Published: March 14, 2025. DOI: 10.15376/biores.20.2.3367-3385 https://bioresources.cnr.ncsu.edu/wp-content/uploads/2019/07/BioRes_14_3_6645_Tor_YDHZ_Factors_Critical_Screw_Driving_Torques_Particleboard_15636.pdf https://doi.org/10.17475/kastorman.626376 https://doi.org/10.1590/01047760201925012608