Shear and Tensile Strength of Narrow Glued Joint ... - ScienceDirect

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ScienceDirect Procedia Engineering 172 (2017) 292 – 299

Modern Building Materials, Structures and Techniques, MBMST 2016

Shear and tensile strength of narrow glued joint depending on the grain direction of plywood plies Girts Frolovsa*, Karlis Rocensa, Janis Sliserisa a

Riga Technical University Faculty of Building and Civil Engineering Department of Structural Engineering and Reconstruction, Kalku st. 1, Riga LV-1001, Latvia

Abstract In this research, the shear and tensile strength of birch plywood edge-to-surface were investigated with specially made specimens for determination of these properties for narrow glued joint. Various thicknesses (4,0; 6,5; 9,0) of plywood and the fiber orientation of surface layer (0° and 90) were investigated. The average results showed that shear strength is 7.11 MPa and the tensile strength is 3.39 MPa. It was found that the orientation of plywood surface layer has minor influence on the shear and tensile strength of edge-to-surface joint. © Published by Elsevier Ltd. This ©2017 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: Glued joint; plywood; Tensile strength; Shear strength.

1. Introduction Recently, much effort has been devoted to increase the use of renewable resources and wood is one of them. Since the plywood is the most used transformed wood-based material [1] [2] and it is widely used in building constructions, the expansion in application is necessary. The properties of standard plywood have been improved by optimization of stacking sequence [3] or use of ribbed panels [4]. Although the phenol-formaldehyde resin adhesive is mostly used for plywood manufacturing [5] and ribbed plywood plate applications [6] PVA glue could be used for several materials. Several types of ribbed plates exist to increase specific strength -plywood plates with core consisting of standard straight ribs [7] [6] [8]or plates with core consisting of modified ribs [8] [4]. By increasing thickness of ribbed plates the material savings increases but at the same time thickness to span ratio increases that induces the increase the shear influence of the plate. For these plates relative thin ribs are glued to skin material and the shear of the plate could become dominant for the determination of plate’s load-bearing. As a result, * Corresponding author. Tel.: +371 27598115 E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016

doi:10.1016/j.proeng.2017.02.117

Girts Frolovs et al. / Procedia Engineering 172 (2017) 292 – 299

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interface adhesion should be analyzed to determine the behavior of joints between plywood edge and skin surfaces. For simulation of these plates with nonlinear FEM analysis the required input data should be determined. The main factors that affect the strength and quality of the bond line is the pressure needed for the moisture content of wood influences both bond formation and bond performance and the third factor is sawing accuracy. This approach could be used not only for wood applications but also for laminated glass design [9] or crack developing [10] etc. 2. Experimental procedure The experimental study consists of two stages: the laboratory tests and simulations of determination and input data validation with experimental investigation. The achieved results afterwards could be used for designing ribbed plywood plates. In the first stage of the experimental study the effect of the level of pressure on the strength and stiffness and characteristics of the edge-to-surface joint [11] was investigated. Uniform adhesion and adequate quality of glue line are often obtained by using high compression pressures (more than 0.5 MPa) during curing time. This means heavy and expensive hydraulic pressing devices in process if it is involved in manufacturing of ribbed plates. For adequate strength and stiffness, the bond pressure 0.5 MPa was chosen. Although it showed that for more reliable results the investigations should be done in a considerable number of specimens in particular series therefore investigated in this paper. Nonlinear behavior of glued joint is simulated by using the cohesive finite element that takes into account the crack development in glue layer. It is realized with INTER20X finite elements with initial zero thickness in ANSYS environment that simulates separation process in glued joint. In calculations the corresponding interfacial separation δ (displacement jump across the interface) is defined with division in normal δn and tangential (shear) δt separation. The ultimate normal σmax and ultimate tangential stresses τmax. The cohesive zone model could be described with exponential material behavior (TB,CZM,,,,EXPO) with the required input data (TBDATA,1,σ_max,δ*n,δ*t,,) or bilinear material behavior (TB,CZM,,,,BILI) with the required input data (TBDATA,1,σmax,δn,τmax,δt. Exponential law curve shape helps with nonlinear convergence. The experimental investigations were made to determinate the required input data for calculation model. In both cases (to determinate the tension and shear properties) special specimens were made that describes the shear and tension behaviour of glued joint between plywood surface and plywood edge. 2.1. Materials Various thicknesses of birch plywood (RIGA PLY of manufacturer A/S Latvijas Finieris) were taken [5]. The modified wood material properties should be taken as a result of pressing in manufacturing process [12], [13] and [14] For all edge-to-surface joints the polyvinyl acetate (Vincents Polyline PVA D3) glue was used that provides water resistant connection of class D3 according to EN 204 [15]. 2.2. Test specimens Several shear standards exist to determine the shear properties of composite sandwiches and glued joints [16] [17]. Single lap joint is mostly used for determination of glued joint shear properties, although some problems remain like eccentricities or complicated joints when designing a joint with cut-ins. In this case, the double lap joint was chosen ASTM D3528-96 [18] that provides two possibilities of double lap glued joint to be used on tension machines with grips for shear tests. These standards for lap joints are mostly made for surface-to-surface glued joint but relatively narrow joint for ribbed plates is made and it could have different stress-strain curve and shear/tensile strength. For ribbed plates edge-to-surface double T joint is made that consists of straight rib part of a plywood with outer fibers in longitudinal direction and curved rib parts that could be straight but due to technology reasons for easier manufacturing were made in transversal directions. In this case both directions of fiber orientation were investigated. For the specimens the tension grips were used to apply force. Displacement (of grips) - load curve was recorded. Special types of specimens were created to determine the properties of glue line in edge-to-surface joints. The double lap specimen was created to determine the shear strength of specimen with 5 mm gap between both rib elements (Fig.

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1 A). The overlap was created symmetrically on both sides of specimen. The series of specimens differ to each other with the thicknesses (4.0; 6.5 and 9.0 mm) and the fiber orientation of surface layer (0° and 90°) of ribs and skins. Based on the ASTM standard E122-00 [19]the sample size was equal to specimens. The results of [11]was (mean shear strength was 7.7 MPa and coefficient of variation 15 %). Were used in the calculation of sample size. Although for sample sizing 10 specimens in one sample should be taken according to the ASTM E 122-00, at least 10 specimens in each sample were investigated. For most used cases when the edge plywood outer fibers are in longitudinal direction to joint number of specimens were increased to 80 specimens for edge 4,0mm and 140 for edge 6.5mm. The special series of specimens (Fig. 1 B) were made to determine the tensile strength of specimens with various orientations of the outer fiber of plywood and various thicknesses of plywood. Both types of specimens (tensile and shear) were tested in tension by using loading machine.

Fig. 1. Specimens for determination of ultimate shear and tensile strength of glued joint: (a) specimen for shear strength determination of glued joint; (b) specimen for tensile strength determination of glued joint.

2.3. Preparation of specimens All plywood elements were machined by circular-blade table saw. Specimens for determination of shear properties were prepared by fixing vertical position of edge plywood and a gap of 5 mm between them. The pressure was produced by using a hydraulic press or by weights. In this research only cold press was used although it is recommended to use hot press for efficiency manufacturing of ribbed plates. Good glue line depends strongly on the straightness and smoothness of the wood edges and surfaces. The surfaces and edges should be cleaned from dust, oil and dirt. Continual quality control of dimensions is essential during the manufacturing. The glue was applied on plywood edges. The surface plywood was positioning that the overlap is symmetrical on both sides of both edge plywood elements. The specimens were kept under a pressure of 0.5 MPa at 20°C and RH 65% for 24 h. Afterwards the pressure was taken off and the specimens were kept for at least 72h to provide strength that is closer to final strength of joint. Similar procedure was made for specimens for determination of tensile properties. In this case the edge plywood was fixed in vertical position and applied glue on its edge. Then the surface plywood was positioning on this edge plywood and other edge plywood with already applied glue on its edge was positioning on the other side of surface plywood directly against the first edge plywood. 3. Results and discussion For shear strength determination more than 300 double lap specimens were investigated and for tensile strength more than 50 specimens were investigated both will be discussed separately.


3.1. Shear and tensile strength Mean shear strength was determined 7.11±0.12 MPa, mean displacement at fracture was 0.64±0.01 mm (Fig. 2). When the outer fibers of edge plywood are in the same direction as the shear plane the difference from mean value is lower than 5% and it is 6.92 MPa but when the outer fibers of edge plywood are in transversal direction to the shear plane it showed that the mean value is about 6% higher.

Fig. 2. Shear strength depending on grain direction of a joint.

The mean tensile strength for plywood edge to surface was determined 3.39±0.18 MPa and the mean displacement at fracture was 0.15±0.01 mm. The tensile strength depending on material is shown in (Fig. 3)

Fig. 3. Tensile strength depending on grain direction of a joint.

The higher strength of narrower joint could hypothetical explained due to influence of squeezed glue of a joint – for narrow joint it increases glue are more comparing to wider joint. Meanwhile when the most of the layers are of edge plywood are in transversal direction to joint, the strength results showed about 3% higher results. These higher results for cases when edge plywood has more layers in transversal direction to joint could be explained that as the

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most of the fracture was through the longitudinal fibers of edges or skins then theses transversal layers needs more load or fracture is at skin plywood in larger area. The distribution that is close to the normal distribution was determined by using statistical analysis of specimen series (Fig. 4). For case A (in brackets value for displacements): standard deviation 1.05 (0.11); coefficient of variation - 14.77 (16.43); asymmetry - 0.13 (0.21); excess - 2.96 (3.00); correlation to the normal distribution – 98.34%. (98.67%) For case B: standard deviation - 0.76 (0.03); coefficient of variation - 22.57 (22.23); asymmetry – 0.07 (0.03); excess – 2.61 (2.93); correlation to the normal distribution – 91.88% (92.34%).

Fig. 4. Histograms of plywood edge-to-surface glued joint strength (a) probability distribution of ultimate shear strength; (b) probability distribution of ultimate tensile strength.

The shear (Fig. 5 A) and tensile (Fig. 5 B) strength of good quality glue line (wood failure of longitudinal fibers of fibers in most of the cases of ribs 80% or skins 15%) was determined mostly influenced by sawing accuracy which in this case was ±0.05mm.

Fig. 5. Different cases of failure modes (a) for specimens of shear properties determination; (b) for specimens of tensile properties determination.


For the specimen of shear strength determination was technologically simpler to produce in higher level of accuracy so the results of these specimens could be suggested as more confident to take as input data although for ANSYS the tensile properties are determinant. 3.2. Joint behavior under shear and tensile load The displacements of joints were recorded and characteristic Stress-displacement curves showed in (Fig. 6) Mean shear displacements at fracture for specimens of shear properties determination was 0.64±0.01 mm and mean tension displacements at fracture for specimens of tensile properties determination was 0.15±0.01 mm. Dashed lines show the upper and lower boundaries of achieved results from experimental investigations.

Fig. 6. Stress-displacements characteristic curves of glued joints for determination of edge-to-surface properties (a) shear strength (for series with edge with thickness of 6,5 mm and skin with thickness of 4,0 mm with fiber direction same as applied force direction) (b) tensile strength (for series edge with thickness of 6,5 mm and skin with thickness of 4,0 mm with fiber direction orthogonal to applied force direction).

The purpose of this study was to find out the properties of narrow edge-to-surface glued joint with PVA glue. Furthermore, the aim was to determine the constants for ANSYS input data and to investigate statistics and dispersion of these joints. The main criteria for the acceptable gluing were adequate shear and tensile strength. PVA adhesives were used, because they do not need high compression pressures and could be used in both cold and hot press. Gluing requires quality specifications and continual quality control of dimensions is essential during the manufacturing. Limiting conditions where the gluing is still satisfactory in order to develop working and quality control instructions for industrial scale gluing. 4. FEM calculations 4.1. ANSYS Input Unfortunately, these values could not be taken direct because the stress and displacements of glue line not the specimen is required. The input constants for ASNYS was determined by using numerical experiments and applying the most adequate results for 0.100 mm for tensile gap and 0.125 mm for shear slip. (Fig. 7)

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Fig. 7. Stress-displacements curves of glued joint (a) Normal stress-gap curve for CZM model, (b) Tangential stress-slip curve for CZM model.

To relieve the calculations and number of equations, for the joints only half of the specimen was modelled. SOLSH190 elements were used for simulations with layered properties as they are for plywood principles. 4.2. ANSYS Results The results show that non-linear solution is adequate for glued joint simulations and show that the specimen deforms in the area of upper and lower results from experimental investigations (Fig. 6) and deformed plot in load step before the fracture is showed in (Fig. 8).

Fig. 8. ANSYS result plot (due to symmetry only one half of specimen showed) (a) Tensile separation; (b) Shear slip.

It could be summarized that the values achieved is reliable for edge-to-surface joint design and use of achieved constants and behaviour in other finite element simulations to determine e.g. simulations of plates. Although it is more recommended that both shear and tensile properties of edge-to-surface joint is defined separately or in this case the shear properties to take more determinant as the tensile properties. Short-term and long-term creep behaviour of this type glued joint should be investigated in future for adequate design of ribbed plates.


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5. Conclusions Various thicknesses (4,0; 6,5; 9,0) of plywood and outer layer grain orientations (0 and 90) showed that the highest shear and tensile strength of edge-to-surface is when the most of the layers of edge plywood are in transversal direction to joint, although the difference is acceptable to assume constant values of input data for all cases. For the specimens of shear strength determination, the normal distribution (98.34%) and for the specimens of tensile strength determination the distribution that is close to normal distribution (91.88%) was achieved. Input constants were determined for use in Cohesive Zone Modeling (CZM) – Mean shear strength 7.11 MPa with mean displacements 0.64 mm and mean tensile strength 3.39 MPa with mean displacements 0.15 mm. Acknowledgements The research leading to these results has received the funding from Latvia state research programme under grant agreement "INNOVATIVE MATERIALS AND SMART TECHNOLOGIES FOR ENVIRONMENTAL SAFETY, IMATEH”. References [1] Vital Forest Graphics, 2009. [Online]. Available: http://www.unep.org/vitalforest/Report/VFG_full_report.pdf. [Accessed 15 March 2015] [2] Production of Wood–based Panels, [Online]. Available: http://eippcb.jrc.ec.europa.eu/reference/BREF/WBP_online.pdf. [Accessed 15 March 2015]. [3] J. Brauns and K. Rocens, Topology optimization of multi-layered composite structures, Civil Engineering 11 Proceedings 2011, 103 - 108. [4] G. Frolovs, K. Rocēns and J. Šliseris, Comparison of a load bearing capacity for composite sandwich plywood plates, Environment. Technology. Resources. Proceedings of the 10th International Scientific and Practical Conference, June 18-20, 2015 Rezekne, Latvia, vol. 1, pp. 39-45. [5] Plywood handbook, Riga: Latvijas Finieris, 2005. [6] Kerto-Ripa-Rippen-und-Kastenelemente," 2015. [Online]. Available: http://www.metsawood.com/global/Tools/MaterialArchive/MaterialArchive/Kerto-Ripa-Rippen-und-Kastenelemente.pdf. [7] E. Labans and K. Kalninsh, Experimental validation of the stiffness optimisation for plywood sandwich panels with rib-stiffened core, Wood Research Slovak Forest Products Research Inst. 59(5) (2014) 793-802. [8] A. Kukule and K. Rocens, Prediction of Moisture Distribution in Closed Ribbed Panel for Roof, 2nd International Conference "Innovative Materials, Structures and Technologies" Riga, Latvia, vol 96, 2015. [9] J. Sliseris and K. Rocens, Optimal design of composite plate with discrete varying stiffness, Composite Structures 98 (2013) 15-23. [10] T. Serafinavicius, A. K. Kvedaras and G. Sauciuvenas, Bending behavior of structural glass laminated with different interlayers, Mechanics of Composite Materials 49(4) (2013) 437-446. [11] A. Pupurs, J. Varna, M. Loukil and D. Mattsson, 16th European Conference on Composite Materials, ECCM 2014, in Damage development and stiffness reduction in laminates in out-of-plane loading, Seville; Spain; 22 June 2014 through 26 June, 2014. [12] G. Frolovs, K. Rocens and J. Sliseris, Glued Joint Behavior of Ribs for Wood-Based Composite Plates, 2nd International Conference „Innovative Materials, Structures and Technologies” Riga Latvija, vol. 96, p. 012048, 2015. [13] L. Pereligin, B. Ugolev, П. (Л.М. and У. Б.Н., Лесная промышленность, Wood Science (In Russian) 1971, p. 288. [14] K. A. Rocens, Technoligical regulation of wood properties, Riga: Zinatne, 1979, p. 220. [15] E. Labans, K. Kalnins and O. Ozolins, Experimental and Numerical Indentification of Veneers Mechanical Properties, Construction Science 1 (2010) 38-43. [16] PVA D3, 22 June 2015. [Online]. Available: http://www.polyline.lv/lv/docs/PDF/pva_d3_apraksts.pdf. [17] BS 5350-C5:2002 Methods of test for adhesives. Determination of bond strength in longitudinal shear for rigid adherends. [18] EN 1465:2009 Adhesives. Determination of tensile lap-shear strength of bonded assemblies. [19] ASTM D3528-96(2008), Standard Test Method for Strength Properties of Double Lap Shear Adhesive Joints by Tension Loading, ASTM International. [20] ASTM E122-00, Standard Practice for Calculating Sample Size to Estimate, With a Specified Tolerable Error, the Average for Characteristic of a Lot or Process, ASTM International.