Impact of selected factors on the bending forces at the proportionality limit and yield point in laminated veneer lumber

Svoboda, T., Sikora, A., Záborský, V., and Gaffová, Z. (2019). "Impact of selected factors on the bending forces at the proportionality limit and yield point in laminated veneer lumber," BioRes. 14(2), 2491-2512.

Abstract

Knowledge of the force required to overcome deformation at the proportionality limit, yield point, as well as knowledge of the effect of selected factors on the characteristics during bending stress, have scientific and practical significance. They are the basis for designing tools for bending and accurately determining the stresses to which products and their parts may be subjected during use. This study analyzed the effect of selected factors on the selected characteristics, including the forces at the proportionality limit (FE) and yield point (FP). The chosen factors of this study were the wood species (Fagus sylvatica L. and Populus tremula L.), non-wood component (carbon and fiberglass), non-wood component position in the composition matrix (up and down), material thickness (6 mm, 10 mm, and 18 mm), and adhesive used (polyvinyl acetate and polyurethane), as well as their combined interaction. The results contributed to the advancement of knowledge necessary for the study and development of new materials with specific properties for their intended use. The results could improve the innovative potential of wood processing companies and increase their performance and competitiveness in the market.

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Impact of Selected Factors on the Bending Forces at the Proportionality Limit and Yield Point in Laminated Veneer Lumber

Tomáš Svoboda,^a Adam Sikora,^a,* Vladimír Záborský,^a and Zuzana Gaffová ^b

Knowledge of the force required to overcome deformation at the proportionality limit, yield point, as well as knowledge of the effect of selected factors on the characteristics during bending stress, have scientific and practical significance. They are the basis for designing tools for bending and accurately determining the stresses to which products and their parts may be subjected during use. This study analyzed the effect of selected factors on the selected characteristics, including the forces at the proportionality limit (F_E) and yield point (F_P). The chosen factors of this study were the wood species (Fagus sylvatica L. and Populus tremula L.), non-wood component (carbon and fiberglass), non-wood component position in the composition matrix (up and down), material thickness (6 mm, 10 mm, and 18 mm), and adhesive used (polyvinyl acetate and polyurethane), as well as their combined interaction. The results contributed to the advancement of knowledge necessary for the study and development of new materials with specific properties for their intended use. The results could improve the innovative potential of wood processing companies and increase their performance and competitiveness in the market.

Keywords: Product innovations; Laminated wood; Force at the proportionality limit; Force at the maximum limit

Contact information: a: Department of Wood Processing and Biomaterials, Czech University of Life Sciences in Prague, Kamýcká 1176, Prague 6 – Suchdol, 16521 Czech Republic; b: Department of Forest and Wood Products Economics, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521 Czech Republic; *Corresponding author: sikoraa@fld.czu.cz

INTRODUCTION

From an environmental point of view, wood is a renewable material, which makes it important. Wood has always been present in the lives of people, and it is still widely used today. A great advantage of wood is its elasticity in relation to its density. However, it is characterized by a low flexibility and difficulty in molding, especially in bigger material sizes. These negative properties motivate the development and application of lamination (Aydın et al. 2004).

Wood-based laminated veneer lumber (LVL) materials consist of several components, but the basic component is wood. The composition design and production of LVL materials is primarily geared to the intended use of the material. Therefore, to create LVL, it is necessary to understand the proposed composition under the influence of various factors. The individual layers of this material can be modified or unmodified. The properties of these materials can be monitored and assessed on the basis of a force-deflection diagram, in which the forces required to achieve deflection at the proportionality limit and yield point can be determined (Fig. 1).

Fig. 1. Force-deflection diagram during bending (A) and determining the proportionality limit and yield point (modulus of rupture) (B) (Svoboda et al. 2017)

The force at the proportionality limit F_E can be defined as the force required to achieve deflection at the proportionality limit, and the force at the yield point F_P is characterized as the force required to achieve deflection at the yield point. In a deeper analysis of the force-deflection diagram up to the proportionality limit, it can be found that deflection only causes elastic deformation (Gaff et al. 2016; Gaff et al. 2017a; Babiak et al. 2018; Hýsek et al. 2018). Once the proportionality limit has been reached and the external forces are released, the deformation returns to its original state. From this assumption it can be concluded that only elastic deformation occurs and stress is evenly distributed in the wood up to the proportionality limit. Stress above the proportionality limit is no longer even (Požgaj et al. 1997) and plastic deformation occurs (Gaff et al. 2017b; Gaff and Babiak 2018a,b; Sikora et al. 2018).

The strength and stiffness of wood can be increased by combining it with non-wood components based on different materials and also by obtaining adhesive joints (Blomberg and Persson 2007; Corigliano et al. 2016). In the 1960s, technologists and manufacturers became interested in improving the mechanical properties of wood, which led to the production of composite fibers. Composite materials include carbon fiber, aramid fiber, basalt fiber, fiberglass, asbestos fiber, polyvinyl alcohol (PVA), and others (Plevris and Triantafillou 1995; Redon et al.2001; Sviták and Ruman 2017). An important factor in the use of these materials, in addition to their mechanical properties, is the location in the laminate material composition (Mosallam 2016). Experimental research has shown that by correctly locating the non-wood component on the stressed tensile zone, it is possible to significantly improve bending properties of layered materials (Raftery and Harte 2013).

The literature points to increasing requirements for wood quality and methods of joining individual layers (Florek et al. 2009; Sandoz 2009; Steiger and Gehri 2010; Stöd and Heräjärvi 2010). Another important factor when gluing laminated wood-based materials or laminated materials based on wood and non-wood components is the type of adhesive used. Glued structural elements are characterized by improved properties compared with solid wood (Lorenzo 2010; Vallée et al. 2015). As a result of poor spreading of the adhesive, improper pressing, or an uneven surface of the glued components, the load bearing capacity may decrease because of incorrect bonding. Wood, as a hygroscopic material, continuously absorbs and releases moisture from and into the atmosphere, which results in changes in the moisture stress, and hence dimensional changes. These changes may cause the adhesive to separate from the wooden element. This can be prevented by using a suitable adhesive for the specific purpose of the bonded element (Saracoglu 2011; Khorasan 2012).

The aim of this study was to determine the effect of the laminated wood structure (wood type, non-wood component type, non-wood component position in the structure, material thickness, and adhesive type) on the main strength characteristics of the force-deflection diagram (forces at the proportionality limit and yield point).

EXPERIMENTAL

Materials

The experiments were conducted on lamellas with a thickness of 6 mm, 10 mm, and 18 mm (thickness of non-wood component was 0.15 mm), width of 35 mm, and length of 600 mm (Fig. 2). Lamellas were made from beech wood (Fagus sylvatica L.) and aspen wood (Populus tremula L.) from the Polana region of Slovakia. Each test group was represented by 20 replicate samples.

Fig. 2. Test samples: beech (A) and aspen lamellas (B)

The effect of the non-wood component on the monitored characteristics was evaluated by comparing the values measured using carbon or fiberglass cloth. The effect of the non-wood component position within the lamella structure was also studied. The non-wood component was positioned as the top (orange color for carbon and red color for fiberglass in Fig. 3) or bottom layer (dark blue for carbon and light blue for fiberglass in Fig. 3). The effect of the adhesive was evaluated by comparing the test results obtained from the lamellas glued using polyvinyl acetate (PVAc) (green in Fig. 3) and polyurethane (PUR) adhesive (yellow in Fig. 3). The samples were conditioned to a moisture content of 8% in a climate chamber (ED, APT Line II, Binder, Tuttlingen, Germany) at a relative humidity of 40% and temperature of 20 °C.

Methods

Determination of the selected characteristics

During testing, the support span length for three-point bending was set to 20 times the thickness (i.e., the support span length was changed in relation to the thickness of the lamellas). The samples were subjected to flexural testing using an FPZ 100 universal testing machine (TIRA, Schalkau, Germany), according to EN 310 (1993), Eilmann et al. (2014), and ISO 13061-2 (2014).

Fig. 3. Arrangement of the test specimens

The cross-head speed was set to 3 mm/min so that the duration of the test did not exceed 2 min. The forces that exceeded the proportionality limit and yield point of the samples were recorded using an ALMEMO 2690-8 datalogger (Ahlborn GmbH, Braunschweig, Germany).

Evaluation and calculation

To determine the influence of the individual factors on the bending characteristics, an analysis of variance (ANOVA) and Fischer’s F-test were performed using Statistica 12 software (Statsoft Inc., Tulsa, OK, USA).

RESULTS AND DISCUSSION

Table 1 shows the average values and variation coefficients (in parentheses) of the forces at the proportionality limit (F_E) and yield point (F_P) of the aspen laminated materials reinforced with non-wood components (carbon fiber and fiberglass) and glued with PVAc and PUR adhesives. The largest force at the proportionality limit (1094 N) was measured with the 18-mm-thick test specimens with fiberglass on the convex side glued with PUR adhesive. The largest force at the yield point (2112 N) was measured with the test specimens that had a total thickness of 18 mm and carbon fibers glued on the convex side with PUR adhesive. The importance of the quality of the bottom layer of laminated wood has been examined in detail in the work (Raftery and Harte 2013). The effect of the non-wood component was verified after comparison with the results of Svoboda et al. (2017), who determined the forces at the proportionality limit and yield point for laminated materials without non-wood components.

Table 1. Mean Values of the Forces at the Proportionality Limit and Yield Point, and the Coefficient of Variance of the Aspen Wood

WS – wood species; NWC – non-wood component; T – thickness; F_E – force at the proportionality limit; F_P – force at the yield point; A – aspen; CA – carbon; LA – fiberglass; U – up; D – down

Table 2 shows the average values and variation coefficients (in parentheses) of the forces at the proportionality limit (F_E) and yield point (F_P) for the laminated beech materials reinforced with non-wood components and glued with PVAc and PUR. The largest force at the proportionality limit (1419 N) was measured in the test specimens with an overall thickness of 18 mm and carbon fibers glued with PVAc on the convex side relative to the load direction. The largest force at the yield point (2405 N) was measured on the 18-mm-thick test specimens with fiberglass glued on the convex side with PVAc adhesive. After comparing the results with those of Svoboda et al. (2017), the same conclusions were reached as for the laminated aspen materials. The effectiveness of the type of adhesive is determined by the surface structure of the bonded surface, as has been shown in the research by Vallée et al. (2015). The results of this research were used to verify the effects of the adhesive on all the characteristics observed for both beech and poplar layered wood.

Table 2. Mean Values of the Forces at the Proportionality Limit and Yield Point, and Coefficient of Variance of the Beech Wood

WS – wood species; NWC – non-wood component; T – thickness; F_E – force at the proportionality limit; F_P – force at the yield point; B – beech; CA – carbon; LA – fiberglass; U – up; D – down

All of the measured data was statistically evaluated using a single-factor analysis, in which the test specimen type was chosen as the default factor. The evaluation was based on the significance level p, which was less than 0.005.

Tables 3 and 4 show the statistical evaluation of the effect of the test specimen type on the force at the proportionality limit in the aspen laminated materials with the non-wood component placed on the top or bottom side with respect to the direction of the stress. The results clearly showed that the test specimen type had a significant effect on the force at the proportionality limit.

Table 3. Statistical Evaluation of the Effect of the Factors and their Interaction on the Force at the Proportionality Limit for Aspen and NWC Down

NS – not significant, *** – significant at p < 0.005

Table 4. Statistical Evaluation of the Effect of the Factors and their Interaction on the Force at the Proportionality Limit for Aspen and NWC Up