Moistureinduced stresses in engineered wood flooring with OSB substrate
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Abstract
Engineered wood flooring (EWF) is a multilayer composite flooring product. The cross layered structure is designed to give good dimensional stability to the EWF under changing environmental conditions. However, during winter season in North America, the indoor relative humidity could decrease dramatically and generate an important cupping deformation. The main objective of this study was to characterize the interlaminar stresses (σ _{33}, σ _{13} and σ _{23}) distribution at freeedges in EWF made with an OSB substrate. A threedimensional (3D) finite element model was used to predict the cupping deformation and to characterize stresses developed in the EWF. The finite element model is based on an unsteadystate moisture transfer equation, a mechanical equilibrium equation and an elastic constitutive law. The physical and mechanical properties of OSB substrate were experimentally determined as a function of the density and moisture content. The simulated EWF deformations were compared against the laboratory observations. For both simulation and experimental results, the cupping deformation of EWF was induced by varying the ambient relative humidity from 50 to 20% at 20°C. A good agreement has been found between the numerical and experimental EWF cupping deformation. The stress distribution fields generated by the model correspond to the delaminations observed on the OSB substrate in the climate room. Delamination in EWF can occur principally under the action of the tension stress or a combination of tension and shear stresses. Finally, simulated results show that the levels of interlaminar stresses are maximal near the freeedges of EWF strips.
Keywords
Layered structure Delamination Finite element analysis (FEA) Wood testingIntroduction
Engineered wood flooring (EWF) is a cross layered structure designed to give good dimensional stability under changing environmental conditions. However, during winter season in North America, the indoor relative humidity could decrease dramatically and generate an important cupping deformation. The magnitude of EWF deformation depends on the physical and mechanical properties of each layer and its behavior following changes in moisture content (MC) and temperature near the top surface.
Barbuta et al. [1] developed two types of OSB (Oriented Strand Board) as a substitute for Baltic Birch (Betula pendula Roth) plywood (BBP) used as a substrate for EWF. The specialty OSB panels were manufactured from two types of wood strands: a mixture of 90% aspen (Populus tremuloides Michx.) and 10% paper birch (Betula papyrifera Marsh.) and 100% ponderosa pine (Pinus ponderosa Dougl. ex Laws.). In a second study, Barbuta et al. [2] evaluated the performance of EWF prototypes manufactured with OSB substrate in conditioning room. They found that there was no significant difference, in terms of cupping deformation, between the EWF manufactured with aspen/birch OSB and BBP substrate. However, after the evaluation test in conditioning room, delaminations were observed in the specialty OSB substrate. These delaminations are the result of the high level of interlaminar stresses distribution at freeedges in EWF.
The finite element method is a useful tool to predict the behavior of EWF [3, 4]. Blanchet et al. [5] developed a threedimensional (3D) finite element model of the hygromechanical cupping in layered wood composite flooring. The model was based on two sets of equations: (1) the threedimensional equations of unsteadystate moisture diffusion and (2) the threedimensional equations of elasticity including the orthotropic Hooke’s law which takes into account the shrinkage and swelling of each layer. Blanchet et al. [6] used this finite element model to support the design of EWF. They first demonstrated that at the macroscopic level, the hypothesis of elastic linearity is acceptable. Geometrical parameters and wood species properties were then used to assess different EWF construction. They found that the thickness and mechanical properties of the core layer are the most important design parameters controlling cupping deformation of EWF in service. They also indicated that the backing layer has a smaller, although still significant, impact on cupping.
The woodadhesive interface was characterized by Belleville et al. [7] to determine its impact on EWF hygromechanical behavior. The finite element analysis has shown that better results are obtained when the woodadhesive interface is considered in the model.
Deteix et al. [8] have investigated the impact of the adhesive layer on the dimensional stability of layered wood composites and how it should be modeled by the finite element method. They showed that the use of a finite element mesh with a single layer of elements in the adhesive layer can lead to significant errors. This is particularly true for low values of the adhesive effective diffusion coefficient.
The objectives of the current study are to determine the physical and mechanical properties of the OSB substrate and to characterize stresses leading to delamination at the freeedge of EWF with an OSB substrate by the finite element method.
Materials and methods
Physical and mechanical properties of EWF components
Finite element model parameters
Parameters  Material  

Surface  Substrate  Adhesive  
Sugar maple  OSB  PVA I  
d _{b} (kg/m^{3})  597^{a}  1178^{i}  
d _{n} (kg/m^{3}) D (m^{2} s^{−1})  500^{f}  650^{f}  800^{f}  4.18 × 10^{−12i}  
D _{L} (m^{2} s^{−1})  2.20 × 10^{−9b}  –  –  –  
D _{T} (m^{2} s^{−1})  1.80 × 10^{−11c}  –  –  –  
D _{R} (m^{2} s^{−1}) T (°C)  1.80 × 10^{−11c}  2.63 × 10^{−11} 20^{f}  1.33 × 10^{−11f} 20  6.68 × 10^{−12f} 20  
RH (%)  50  20  50  20  50  20  
D _{R} (m^{2} s^{−1})  1.80 × 10^{−11c}  2.63 × 10^{−11f}  1.33 × 10^{−11f}  6.68 × 10^{−12f}  
M _{0} (%)  8.6  6.2  –  6.2  –  6.2  –  8.6 
M _{1} (%)  5.8  –  3.4  –  3.4  –  3.4  5.8 
h (kg m^{−2} s^{−1} %^{−1}) β (m m^{−1} %^{−1})  3.20 × 10^{−4b}  3.20 × 10^{−4} ^{b}  3.20 × 10^{−4} ^{b} 3 × 10^{−3i}  
β _{L} (m m^{−1} %^{−1})  1.50 × 10^{−4d}  2.36 × 10^{−4f}  2.07 × 10^{−4f}  1.75 × 10^{−4f}  
β _{T} (m m^{−1} %^{−1})  3.30 × 10^{−3d}  9.18 × 10^{−4f}  9.14 × 10^{−4f}  9.79 × 10^{−4f}  
β _{R} (m m^{−1} %^{−1}) E (Pa)  2.10 × 10^{−3d}  2.86 × 10^{−3f}  3.57 × 10^{−3f}  5.00 × 10^{−3f}  1.27 × 10^{+10j}  
E _{L} (Pa)  1.38 × 10^{+10e}  6.72 × 10^{+9f}  7.00 × 10^{+9f}  9.50 × 10^{+9f}  9.83 × 10^{+9f}  1.23 × 10^{+10f}  1.30 × 10^{+10f}  
E _{T} (Pa)  6.78 × 10^{+8e}  9.27 × 10^{+9f}  9.84 × 10^{+8f}  1.54 × 10^{+9f}  1.61 × 10^{+9f}  2.15 × 10^{+9f}  2.21 × 10^{+9f}  
E _{R} (Pa) G (Pa)  1.31 × 10^{+9e}  7.50 × 10^{+7f}  8.50 × 10^{+7f}  1.26 × 10^{+8f}  1.51 × 10^{+8f}  1.47 × 10^{+8f}  1.70 × 10^{+8f}  4.72 × 10^{+9k} 
G _{LR} (Pa)  1.01 × 10^{+9e}  1.54 × 10^{+8f}  1.62 × 10^{+8f}  2.24 × 10^{+8f}  2.29 × 10^{+8f}  2.45 × 10^{+8f}  2.44 × 10^{+8f}  
G _{TR} (Pa)  2.55 × 10^{+8e}  7.70 × 10^{+7f}  8.20 × 10^{+7f}  1.06 × 10^{+8f}  1.36 × 10^{+8f}  1.24 × 10^{+8f}  1.47 × 10^{+8f}  
G _{LT} (Pa) ν  7.53 × 10^{+8e}  3.37 × 10^{+9g}  1.12 × 10^{+9g}  1.63 × 10^{+9g}  1.99 × 10^{+9g}  2.57 × 10^{+9g}  2.68 × 10^{+9g}  0.35^{l} 
ν _{LT}  0.500^{e}  0.183^{h}  
ν _{RT}  0.820^{e}  0.019^{h}  
ν _{TL}  0.025^{e}  0.161^{h}  
ν _{RL}  0.044^{e}  0.013^{h}  
ν _{TR}  0.420^{e}  0.312^{h}  
ν _{LR}  0.460^{e}  0.364^{h} 
Finite element modeling
The cupping deformation was generated by varying relative humidity (RH) from 50 to 20% at 20°C. This corresponds to a decrease in moisture content from 8.6 to 5.8% for the sugar maple surface layer and PVA adhesive and from 6.2 to 3.4% for OSB substrate. The water vapor desorption occurred by free convection at the top of the surface layer. All the other surfaces were assumed impervious.
The theoretical model of hygromechanical EWF deformation used in this study is based on the model developed by Blanchet et al. [5] and improved by Deteix et al. [8] and Belleville et al. [7]. The governing equations used in the model are threedimensional moisture conservation (3 and 4), threedimensional equation of mechanical equilibrium (5) and Hooke’s law for orthotropic materials which takes into account the shrinkage of each layer (6).
Each layer of EWF is assumed to be elastic. It is also assumed that the principal material directions of the sugar maple surface layer and the three sublayers of the OSB substrate are perfectly oriented with the strip. The experimental data obtained for unidirectional OSB panels were linearly extrapolated to calculate the physical and mechanical properties of OSB substrate function of the vertical density profile (VDP) and the ΔM. For FE simulations, the initial moisture content M _{0} was set at 8.6% for the sugar maple surface layer and PVA adhesive and 6.2% for OSB substrate. This was due to the different sorption isotherm of these two materials. The equilibrium moisture content M _{ ∞ } was as follows: 5.8% for surface layer and PVA adhesive and 3.4% for OSB substrate. It was assumed that moisture transfer occurs only through the top of the surface layer.

for the surface and the adhesive layers

for the OSB substrate
The moisture content at equilibrium was:

for the surface and the adhesive layers

for the OSB substrate
For the mechanical part of the problem, the conditions were:
The numerical simulations were performed using a constant time step of 0.1 s. The total time of simulation was 28 days. At each time step, the moisture content variation (ΔM) was used to calculate the corresponding displacements (u). The ΔM was also used to update the mechanical and physical properties of OSB substrate. To reduce the computation time and the data generated by the model, the mechanical calculations were realized at only 20 time steps.
To compare the simulated EWF cupping deformations with the experimental deformations, EWF prototypes were produced. The manufacturing parameters are the same as those used by Barbuta et al. [2] for EWF prototypes with an OSB substrate. After a conditioning period at 50% RH and 20°C, twelve EWF specimens of 86 mm wide and 50 mm long were placed in a conditioning room at 20% RH and 20°C for 4 weeks. Before the test in climate chamber, the EWF specimens were sealed with aluminum foil on the bottom face and the edges to impose moisture transfer by the top surface only. The cupping deformations were measured over the width of the strip with a dial gauge as described by Blanchet et al. [18]. The coordinates of measurement points (A, B, C) were introduced in the finite element model to calculate the simulated cupping deformations (Fig. 2). The cupping deformation was defined as the difference in vertical displacement between the points A and B and the central point C. The minus sign was used to indicate that the cupping is a convex deformation of the EWF strip.
Results and discussion
Engineered wood flooring cupping deformation
Even though the cupping simulated curve obtained from a coarse mesh is closer to the experimental one, a finer mesh is necessary for an accurate description of the interlaminar normal and shear stresses. For both experimental and numerical results (Fig. 5), approximately 91% of the maximum cupping deformation occurred in the first 10 days of the experiment. The slopes of the simulated curves in the first days are influenced by the properties of the surface layer. Blanchet et al. [5] showed that the variation of the transverse water diffusion coefficient has an important effect on the curve slope. After that period, the cupping deformation curve turns into a relatively flat plateau. It is important to indicate that the numerical simulations were performed using the mean values of the material properties without considering their variability. The average coefficients of variation (CV) for physical and mechanical properties of clear wood are between 15 and 25% [19], whereas the average measured CV values for OSB are approximately 30%. Parametric studies indicate that the maximum cupping deformation is influenced by the modulus of elasticity in parallel and perpendicular to the surface grain of OSB and by sugar maple shrinkage coefficient in tangential direction. The variation of these parameters with ±15% leads to a modification of the simulated maximum cupping deformation of approximately ±19%.
Stress characterization
Internal bond strength and shear strength of unidirectional OSB with flat density profile as a function of nominal density at 20°C and 20% RH
Parameters  Unidirectional OSB nominal density  

500 kg/m^{3}  650 kg/m^{3}  800 kg/m^{3}  
IB (MPa)  0.583  0.829  1.054 
S _{LR} (MPa)  2.714  3.898  4.708 
The comparison shows that only the simulated tensile stress σ _{33} is higher than the internal bond strength (Fig. 6). It seems that the model overestimates the stress field in the vicinity of the EWF strip freeedges. The use of an elastic model may explain this overestimation of the stress. Moses and Prion [20] also used an elastic finite element model to predict the strength of laminated veneer lumber (LVL). They found that shear stresses obtained from the model were approximately 1.7 times higher than nominal values.
The stress fields (Fig. 8) obtained from the model are in accordance with the observation made on the OSB samples after conditioning at 20°C and 20% RH. Seven out of twelve samples presented microdelamination: five between the glue line and the tongue and six in the groove zone.
Conclusions
The main objective of this study was to characterize the interlaminar stress responsible for delamination at freeedges in EWF. A threedimensional finite element model was used to predict the cupping deformation and to characterize stresses developed in the EWF. The physical and mechanical properties of the OSB substrate required in the model were determined experimentally.
Good agreement has been found between the numerical and experimental EWF cupping deformation. Compared to the experimental measurement, the finite element solution overestimates the maximum deformation by 7%. Parametric studies indicate that the maximum cupping deformation is influenced by the MOE in parallel and perpendicular directions of the OSB substrate and by the shrinkage coefficient in tangential direction of the sugar maple. The variation of these parameters with ±15% leads to a modification of the simulated maximum cupping deformation by approximately ±19%.
The stress distribution fields generated by the model are in accordance with the delaminations observed on the OSB substrate after conditioning at 20°C and 20% RH. Delamination in EWF can occur principally under the action of the tension stress or a combination of tension and shear stress. Finally, simulated results show that the levels of interlaminar stresses are the highest near the freeedges of EWF.
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