Master thesis in Forestry and Wood Technology. Title: Bending properties of commercial wood-based panels by NDT methods

Transcription

1 Master thesis in Forestry and Wood Technology Title: Bending properties of commercial wood-based panels by NDT methods Author: Francesco Poggi Supervisors: Stergios Adamopoulos and Sheikh Ali Ahmed Examiner: Jimmy Johansson Course code: 5TS01E Semester: Spring 2017

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3 Abstract This thesis work focuses mainly on the application of non-destructive testing (NDT) methods on wood-based panels (WBP) in order to estimate the bending properties. To prove the accuracy and applicability of these methods on WBP, their results are correlated with results from a standardized static bending test. The behavior in different climate conditions and the application on panels of larger sizes is also questioned to provide an indication about strong points and boundaries of NDT methods applied on WBP. The bending properties are of major importance, especially for materials suited to bear loads. Bending stiffness, represented by the modulus of elasticity (MOE), is an expression of the deflection rate of a material under load. The bending strength, represented by the modulus of rupture (MOR), is an expression of the maximum load withstood by a material before rupture. Before testing, the material is acclimatized in three climate conditions: dry (20 C, 35% RH), standard (20 C, 65% RH) and wet (20 C, 85% RH), to understand the bending properties variation and how the NDT methods are affected by the variation in moisture content. The materials used are seven types of WBP, in particular four types of particleboards (PB), one type of high-density fiberboard (HDF), one type of dual density PB (with high and low density areas along the production direction) and one type of lightweight panel (Board-on-stiles, a composite panel of HDF, PB and paper honeycomb). To test the bending properties the following NDT methods are considered: transversal resonance vibration and longitudinal resonance vibration with the use of the BING system and the time-of-flight with the use of Fakopp Ultrasonic Timer and Silvatest Trio. The resonance vibration methods, transversal and longitudinal, are based on the relation between resonance vibration properties and bending properties of a material. The relation with bending properties also exists for the stress wave velocity (SWV) through a material, calculated with the time-of-flight method. The dynamic MOE resulting from these tests is then correlated with the static MOE and MOR from the static bending test. The NDT methods resulted to be reliable on WBP, with generally high levels of correlation between dynamic MOE and static MOE and MoR. The highest correlation value for MoE is with the transversal resonance vibration while the highest for MOR is with the longitudinal resonance vibration. The results of the dynamic MOE for all the NDT methods are higher than the static MOE, as confirmed also in the literature; the average ratio between the dynamic and the static MOE is, for example, up to 1,6 for WBP in standard climate condition, tested with Fakopp U.T.. These results are extremely higher than values suggested by previous studies. Moreover, the ratio increases with increasing relative humidity of the climate condition. The results from the tests on larger sizes suggest a possible application in this field. The time-of-flight method is suitable for in-plane uniform materials, like the PB and HDF, while the transversal resonance methods give also a good representation of the properties of the dual density PB and the light-weight panel.

4 Keywords Wood-based panels; Non-destructive testing; Bending properties; Resonance vibration; Time-of-flight; Modulus of elasticity, Modulus of rupture, Stress wave velocity; Relative humidity.

5 Table of Contents 1 INTRODUCTION WOOD-BASED PANELS Particleboards Fiberboards Lightweight panels WOOD-WATER RELATION BENDING PROPERTIES Testing methods Static bending test Non-destructive tests Resonance 5 6 Time-of-flight 7 2 AIM HYPOTHESIS 9 3 MATERIALS AND METHODS COMMERCIAL PANELS SAMPLING CLIMATE CONDITIONS CALCULATION OF MC AND DENSITY STATIC BENDING TEST NON-DESTRUCTIVE TESTS BING system Time-of-flight Fakopp Ultrasonic Timer Sylvatest Trio LIMITATIONS 19 4 RESULTS AND DISCUSSION MOISTURE CONTENT AND DENSITY STATIC BENDING TEST NON-DESTRUCTIVE TESTS Stress wave velocity Tests on the same samples General results on WBP Tests on full size panels 36 5 CONCLUSIONS 37 6 REFERENCES 38

6 1 Introduction 1.1 Wood-based panels Wood-based panels (WBP) are valuable raw materials suitable for construction and furniture making. The family of WBP is very large and it includes all the composite products made of veneers, particles, fibers or solid wood bonded together with glue in the shape of a panel (Fig. 1). (a) (b) (c) Fig 1. In order from the top: particleboard (a), oriented-strand board (b) and mediumdensity fiberboard (c) (WBPI, n.d). The natural characteristics of the wood material are mitigated by the processing during the production. The required final properties define the parameters of production, for instance the type of particles, the type of glues, the thickness of the panel, etc. The result is a range of panels that can serve many purposes and consequently widely differ from each other. Even so, there are some common general properties that give wellacknowledged advantages: - The material is in-plane uniform, with a flat, smooth surface and defects are limited - The costs of production are moderate - The industrial production allows to have sizes (especially width) rare or unattainable with solid wood - The properties of the panels can be modified according to the final purposes - The mechanical properties are mediocre but the high consistency makes it a reliable material (Thoemen et al. 2010). The possibility to play with so many factors gives the realistic opportunity to produce panels with reduced weight but still fulfilling the physical and mechanical requirements 1

7 of the market. The reduction in weight is a factor of interest since it brings economical and sustainability advantages: the material consumption is reduced together with the costs and impact of transportation of raw material and finished products. Moreover, the WBP industry is often fed by low-grade logs, unsuitable for the solid wood industry. This optimizes the use of the material and it also results in a further reduction of production costs. Some of the most common panels are particleboards, fiberboards and, especially for furniture making, lightweight panels Particleboards Particleboards (PB) are composed of wood particles together with a synthetic glue and pressed into a panel at high pressures and temperatures. The dimension of the particles defines the different types of PB: chip, flakes and strands are the most common. In Europe it is common to use the name particleboard for the chipboard (Thoemen et al. 2010), which can create confusion. In this study, the word particleboard is used to refer to chipboard and not used as the generic term for these materials. For PB, one of the first official pieces of information dates back to 1936/37 when it was patented, but the idea took over only several years later (Kollmann et al., 1975). The most common type has a three-layer structure, with high density and fine particles on the top and the bottom layer, and low density and big particles in the core layer. This structure provides good physical and mechanical properties, given by the stiff and consistent surface layers. The average weight is reduced by the porosity of the coarse core layer. Some factors that differentiate PB are: - Thickness: Different thicknesses can be related to mechanical properties but generally it is decided according to the dimensional requirements of the final product. - Density: Density is linearly correlated to the mechanical properties of wood (Hein et al. 2011, Missanjo and Matsumura 2016) and, as a general trend, of WBP (Thoemen et al. 2010) so it is adjusted according to the mechanical requirements in the final application. - Type of glue: Urea-formaldehyde (UF) is the most commonly used for PB for indoor applications because it is relatively cheap and provides good strength, even though it is sensitive to humidity. The poor moisture resistance of UF can be improved with melamine-urea-formaldehyde (MUF) or phenol-formaldehyde (PF) (Thoemen et al. 2010). One alternative to UF is the PMDI (Polymeric diphenylmethane diisocyanate) which offers high performances, thus the possibility to reduce the glue amount in panels, at a higher price (Thoemen et al. 2010) Fiberboards Fiberboards, as for particleboards, are the result of hot pressing of processed wood material together with a synthetic glue. In this case, ligno-cellulose fibers are the main components (Thoemen et al. 2010). According to the density obtained, the fiberboard can be low- (LDF), medium- (MDF) or high-density (HDF). The HDF is usually thinner compared to LDF and MDF and the final purposes are consequently different. 2

8 In Europe the first fiberboard was patented in 1772 (Kollmann et al. 1975) but the first MDF industry, using a similar production process as today, was built only in 1965 (Thoemen et al. 2010) Lightweight panels Lightweight panels are composites of different materials generally characterized by a strongly reduced weight compared to standard panels but still good mechanical performances. The group can be very wide including different structures and materials. One type commonly used in the furniture industry is the paper honeycomb core panel. The honeycomb consists of an array of open cells, formed from very thin sheets of material attached to each other. Usually the cells form hexagons... [that] closely resemble the bee's honeycomb found in nature (Bitzer 1997). Paper honeycomb has its origin as a decoration material in China 2000 years ago (Bitzer 1997), but now it is widely used for furniture making. 1.2 Wood-water relation As mentioned before, the transformation of wood material into panels mitigates the natural characteristics of wood but only to a certain extent. For instance, the hydrophilic behavior is maintained, therefore the material has the capacity to absorb and release water, being in dynamic equilibrium with the surrounding environment. The moisture content (MC) reached once in equilibrium with a certain climate condition (temperature and relative humidity), is called equilibrium moisture content (EMC). The EMC results to be lower in WBP compared to solid wood (Kollmann et al. 1975, Thoemen et al. 2010) and this can be attributed to the presence of the glue (usually below 10% of the weight) or to the thermal treatment that the wood material undergoes during the production of the panels, such as drying, hot pressing and eventual defibration (Thoemen et al. 2010). There is a threshold value for the MC in wood called fiber saturation point (FSP) and it is generally considered around 30%. Below it, the water in wood is hydrogen-bonded to the wood cell wall polymers; above it, the water in wood is free and fills the micro and macro cavities (Thoemen et al. 2010). Both in wood and WBP, the variation of EMC below the FSP is the most interesting since it has a strong influence on all physical and mechanical properties of wooden materials (Thoemen et al. 2010). Shrinkage and swelling are dependent on the variation of the MC below the FSP. It is proven for WBP that the swelling in the direction of the plane is slightly higher while the thickness swelling is way higher compared to solid wood of the same species in the same condition (Thoemen et al. 2010). The enhanced swelling can be explained by the densification process that the material undergoes when it is hot pressed into a panel: the general trend in the panel industry is to reach a density 50% higher than the density of the raw material (wood). The panel is then more susceptible to have spring back (thus swelling) with variation of the EMC (Thoemen et al. 2010). It is generally accepted that the mechanical properties of wood and WBP reduce at a higher MC (Thoemen et al. 2010). Other studies proved that the mechanical properties of WBP reduce with increased temperature (Bekhta and Marutzky 2006, Ayrilmis et al. 3

9 2010, Kojima et al. 2016). A study from DeXin and Östman (1983) measured the modulus of elasticity (MOE) of two types of PB in three humidity conditions (dry, normal, wet) and at three temperatures (-15, 20 and 45 C). The results were that the MOE decreased with increasing MC: the MOE of urea-formaldehyde (UF) boards decreased by 45% with MC going from 7% to 17% at 20 C, while for phenolformaldehyde (PF) boards the MOE decreased of 60% with the MC going from 7% to 33% at 20 C. Temperature in general has a lower influence on the mechanical properties than the MC. The combination of both high temperature and a high MC was proved to have the greatest impact on the mechanical properties, a statement confirmed by Yu et al (2013). They found a strong reduction of the MOE and the modulus of rupture (MOR) for several WBP at 30 C and 90% RH climate condition compared to the standard European condition (20 C, 65%), for example, a reduction of MOE by 30% for PB and by 39% for HDF and a reduction of MOR by 8% for PB and by 18% for HDF The variation of temperature and relative humidity in the air is something that happens continuously, driven by many variables like the time of the day, the weather conditions and the period of the year or the climate of the area. This change happens outdoor as well as indoor and consequently our wooden products have a constant variation of MC to be in equilibrium with the surrounding air. This means that every wooden product has to maintain its performance requirements within certain limits, under the condition of changing climate conditions. The use of different climate conditions during the tests on industrial products or for research gives a better picture of the potential of applicability of a certain material. 1.3 Bending properties To optimize the use of a material, it is necessary to understand its properties, behaviors and limitations. In 1975, Kollmann et al. reported a list of 35 properties for particleboard, grouped in: general properties, physical properties, mechanical (elastic and strength) properties, technological properties and resistance against destruction. Two of the most important properties of wood are the bending stiffness and bending strength. They are especially important if the material has a load bearing function. Indeed the bending stiffness indicates how much (in this case) a panel would deflect under load, and the bending strength indicates how much load would provoke the fracture of the panel Testing methods Static bending test The most acknowledged way to determine those properties is by the destructive static bending test. The standard procedure for WBP is the three-point bending test (Figure 2), according to the European standard EN 310:1993, Wood-based panels - Determination of modulus of elasticity in bending and of bending strength (CEN, 1993a). 4

10 Fig. 2 three-point bending test (Rusmee 2007) This test is based on the theory of the bending of the simple beam, supported at the ends and loaded in the center, perpendicularly to the longitudinal direction (Hoadley 2000). The result is a load-deflection curve, obtained by correlating the load and the deflection withstood by the sample, until rupture (Figure 3). The straight slope of the curve, where load and deflection are linearly correlated (elastic range), shows the stiffness of the beam; from this part of the curve the MOE can be calculated. The maximum load shows the strength of the beam and it can be used to calculate the MOR (Hoadley 2000). Fig 3. Load-deflection curve for a beam of white pine. Load and deflection are expressed in pound (lb) and inches (in) respectively (Hoadley 2000). The method described is only one of the several suitable to estimate the properties of wood and wood-based materials Non-destructive tests Many others methods are part of the large family of the non-destructive evaluation (NDE) and testing (NDT) methods. Non-destructive evaluation of wood - Second edition (Ross 2015) defines non-destructive evaluation as [...] the science of identifying the physical and mechanical properties of a piece of material without altering its end-use capabilities [...]. In the same book, a list of techniques, based on different methods is presented (Figure 4). 5

11 Fig 4. Non-destructive techniques (Ross 2015). The use of visual characteristics to estimate the properties of wood is widely diffuse. For example, the number and the dimension of the knots are good indicators of the quality of a log. The NDE/NDT methods were developed first in the attempt to assess the properties of structural lumber in North America (Ross 2015). Further developments led to understanding the potentials of many non-destructive techniques, applicable on wood and wood-based materials (Ross 2015). Moreover, given their non-destructive nature, these methods are repeatable and applicable on-site with standing trees and onservice structures. The field of non-destructive methods is therefore in constant growth and evolution, with many commercial testing equipment available and increasing accuracy over the years. The interest of this study towards stiffness and strength in bending, led to the choice of the resonance method and the time-of-flight method, based on vibrational properties and wave propagation respectively. The theory supporting these methods is that the dynamic MOE estimated with non-destructive methods have a good correlation with the MOE calculated with the static destructive method; this correlation is largely proved for solid wood (Ross et al. 1991; Ilic 2001; Wang et al. 2001; Green et al. 2004). Since the development of NDT started from solid wood, knowledge and literature in this field are greater compared to WBP. The first attempts were to estimate the applicability on WBP. Dunlop (1989) suggested a possible applicability for the production line monitoring but the accuracy was too low to replace or complement the standard destructive bending tests in other applications. Over the years NDT methods improved in accuracy and increased the range of applicability and the range of measurable properties. Resonance The resonance method is based on the relation between the static MOE and the dynamic MOE predicted by using resonance vibration frequencies in transversal or longitudinal direction (Hein et al. 2011). The dynamic transversal MOE is estimated using one or more frequencies from the transversal (or flexural or bending) vibration, and it represents the stiffness under bending stress. The dynamic longitudinal MOE is 6

12 estimated using the first frequency from longitudinal (or axial) vibration, and it represents the stiffness under compressive strength (Hein et al. 2011). Transversal vibration method is used mainly for the stiffness estimation of lumber, while the longitudinal vibration method can be applied on lumber and logs (Legg and Bradley 2016). In 2011 (Olsson et al.) the flexural and longitudinal dynamic MOE were calculated on boards of Norway spruce. For the flexural dynamic MOE, the resonance frequency of the first bending mode was used while for the longitudinal dynamic MOE, the first axial resonance frequency was used. The results showed that the dynamic flexural and longitudinal MOE have both a good correlation with the static MOE, even though it was stronger for dynamic flexural MOE. A strong correlation between dynamic MOE and static MOE was found again by Hein et al. (2011), who confirmed the accuracy of the resonance method to estimate the bending properties of large pieces of wood by testing small samples. A strong correlation was also found between stress wave speed propagation (obtained with longitudinal vibration frequencies) and elastic and mechanical bending properties for particleboard and fiberboard. The coefficients of determination R 2 resulted to be from 0.93 to 0.98 (Bombadilla et al. 2012). In 2015, Guan et al. tested three types of WBP of production full-size with the free-free resonance transversal vibration method. The boards were supported at the nodal vibration points (Figure 5) and the first natural vibration frequency was used to calculate the dynamic MOE. Fig 5. First vibration mode under free-free support condition. L is the total length of the sample L corresponds to the nodal vibration points (Guan et al. 2015). A strong correlation of R=0,923 resulted between the dynamic MOE from full-size boards and the static MOE from samples. The dynamic MOE was on average 6,2% higher than the static MOE. The knowledge about applicability of the resonance method to wood-based light-weight panels is missing. Time-of-flight The time-of-flight (TOF) method is based on the high correlation that was found to exist between the static MOE and the wave speed propagation (stress wave velocity) in a material (Ross 2015; Baar et al. 2011). For solid wood, the wave speed propagation in longitudinal direction increases with higher stiffness, higher density and lower microfibril angle (Legg and Bradley 2016). The TOF can be measured mainly in two ways, with the sound impulse generated by a hammer or by an ultrasonic transducer. The former is displayed in Figure 6, where the sound impulse goes from one spike to the 7

13 other and the time that it takes is recorded. This time is called time-of-flight and together with the distance from the spikes is used to calculate the stress wave velocity. Fig. 6. TOF testing equipment with spikes and hammer (Legg and Bradley 2016). This method is particularly suitable to measure the properties of standing trees. The method with ultrasonic transducers generates an ultrasonic impulse instead of using the hammer. The impulse is sent through the wood and the time that it takes to get from one transducer to the other one is measured by the machine. The dynamic longitudinal MOE can be calculated using the time-of-flight, the distance between the transducers and the density of the material. The ultrasonic vibration method (time-of-flight) was tested in a study from Haines et al. (1996) on spruce and fir together with the free-free resonance transversal vibration method (free support condition at nodal vibration points) and longitudinal resonance method. The dynamic MOE from the resonance transversal vibration method was highly correlated with the static MOE and the other two methods gave higher results, with the ultrasonic vibration method being the highest. Similar results were obtained by Baar et al. (2011), who tested wood from various species with ultrasounds and longitudinal resonance method. The dynamic MOE resulting from the ultrasonic method was on average 6% higher than that from the longitudinal one. Many different types of particleboards were tested in 1980 (Dunlop) by measuring the time-of-flight with an electric signal impulse. The correlation of the acoustic velocity with the static MOE and MOR was reasonably good, with R=0.73 and R=0.65 respectively. Due to the wide range of material used, the correlation resulted to be a little lower than some other studies. In another study with particleboards, Ross and Pellerin (1988) reached a correlation of 94% between stress wave speed and MOE. The correlation for MOR was about 90%. In 2005, the reliability and accuracy of the ultrasonic technique to estimate the MOE of particleboards was confirmed again (Najafi et al. 2005). The knowledge about accuracy 8

14 and applicability on PB is wide, but no studies are done on material like HDF and panels with uneven density distribution like DDPB or BOS. 2 Aim The aim of this study is to measure the bending properties of different wood-based panels in three climate conditions and test several NDT methods on these materials. Using static bending tests as reference, applicability and accuracy of NDT methods is questioned and the potential application of these methods on full size panels and light weight panels is investigated. 2.1 Hypothesis Based on the aim of the study, the hypothesis can be stated as it follows: - More humid climate conditions reduce bending stiffness and strength of WBP - NDT methods confirm to be applicable and reliable on WBP, with higher values compared to static bending tests. Moisture content and other parameters of the material have influence on the final results of the tests - Some of the NDE methods are also applicable on full size boards, including light-weight panels 9

15 3 Materials and methods 3.1 Commercial panels A wide range of WBP is tested in this study. The details of the materials are summarized below (Table 1). Table 1. Details of the materials Code Type of WBP Characteristic Type of glue Thickness (mm) Purpose Additional info PB1 PB Standard UF 16 Furniture PB2 PB High density UF 20 Furniture PB3 PB PMDI 19 Construction PB4 PB PMDI 19 Construction Load bearing Non-load bearing, dry areas Load bearing, damp areas DDPB Dual density PB UF 18 Furniture HDF HDF UF 2.5 Furniture BOS BoS Light-weight panel UF 18 Furniture HDF, PB and paper honeycomb PB = particleboard, HDF = high-density fiberboard, BoS = Board-on-stiles, UF = Urea formaldehyde, PMDI = Polymeric diphenylmethane diisocyanate. The PBs, from PB1 to PB4, have a common three-layer structure with density distribution higher on the surface and lower in the core. The density is uniform in the directions of the plane. The fifth PB, is a special product that keeps the 3-layer structure but has strategic high and low density lines along the direction of production. The amount of material in the core layer defines the high and low density lines. This alternation is found in the board perpendicularly to the direction of production (Figure 7). 10

16 Fig.7. DDPB density pattern. H = High density, L = Low density, the arrow indicates the direction of production. Dimensions: a 5 cm and b 29 cm. The BoS (Board-on-stiles) is a combination of HDF, paper honeycomb and stiles of PB. The HDF constitute the two surfaces and three stiles of PB are placed on the edges and in the middle, all in correspondence of future stress-bearing areas. The rest of the room between HDF and PB stiles is filled with honeycomb (Fig.8). Fig. 8. Board-on-Stiles (Biele Group, n.d.) 3.2 Sampling The production direction of the boards is identified from the sanding signs on the surface and for each board two set of samples are cut: perpendicular and parallel to the direction of production, according to the European standard EN326-1:1994 (CEN 1994). For the dual density PB the sampling is done only along the density lines, so that the samples resulted having uniform density. For the HDF the sampling is done randomly, without specification of the direction of production. For the BoS the sampling was done parallel to the direction of production so that the samples resulted to be composed of PB and HDF. As specified on the European standard for the static bending test (CEN 1993a), at least six samples in parallel direction and six in perpendicular direction per type of panel are 11

17 needed. In this case 36 samples are cut from every board (six plus six samples per every climate condition). For the non-destructive test, the samples are acclimatized in order dry, standard and wet climate condition. The same set of samples is used for all the testing. The samples cut from every board type are five in parallel and five in perpendicular direction. The boards used for the full size board test have different dimensions, with sides between 50 and 90 cm. 3.3 Climate conditions The climate conditions considered are: - Dry (20 C, 35% RH) - Standard (20 C, 65% RH) - Wet (20 C, 85% RH) The MC determination and the bending properties (static and dynamic) determination is done on samples previously acclimatized at these three climate conditions. Density is calculated only at 20 C, 65% RH, as specified by the standard. The samples are considered acclimatized if the difference between two weighing, 24 hours from each other, is smaller than 0,1% of the mass of the sample, as stated in the European standards EN310:1993 (CEN 1993a), EN322:1993 (CEN 1993b) and EN323:1993 (CEN 1993c). Since the temperature is the same for these three climate conditions, what is worth mention is the variation of RH. For practical reasons, in the Results and discussion the value of RH (35%, 65% or 85%) is used to indicate the related climate condition. 3.4 Calculation of MC and Density To calculate MC and density the European standards EN322:1993 (CEN 1993b) and EN323:1993 (CEN 1993c), respectively, are followed. To achieve that, indications about number and dimension of samples, conditioning, drying (in case of MC), weighing, measuring of dimensions and expression of results are respected. For the DDPB the samples are cut from the high-density and low-density areas. The density is also calculated from four full size board samples (one of the width of the board showed in Figure 7 and three of the width of (b) in Fig.7) so to have a representative density of the whole board. For the BoS the samples used for the calculation of both MC and density are cut widthwise so to have a representative section of all the components of the board. The length of the samples is the width of the panels (550 mm) and the width is 35 mm. 3.5 Static bending test For the static bending test, a universal testing machine (MTS 810, MTS System Corporation, USA) is used, set up according to the specification of the European standard EN310:1993. The supports are adjustable so to adapt to different lengths of the sample and the loading speed is adjustable to obtain the fracture in a required time. 12

18 The samples have standard width (50 ± 1mm) and length related to the thickness (20 times the nominal thickness, plus 50 mm). More information can be found in the Table 2. Table.2 Dimensions and number of samples for static bending test Board type Direction Dimension Number of samples PB1 PB2 PB3 PB4 370 x 50 x 16 mm 18 (6 per every climate condition) 370 x 50 x 16 mm 18 (6 per every climate condition) 450 x 50 x 20 mm 18 (6 per every climate condition) 450 x 50 x 20 mm 18 (6 per every climate condition) 430 x 50 x 19 mm 18 (6 per every climate condition) 430 x 50 x 19 mm 18 (6 per every climate condition) 430 x 50 x 19 mm 18 (6 per every climate condition) 430 x 50 x 19 mm 18 (6 per every climate condition) HDF 150 x 50 x 2,5 mm 18 (6 per every climate condition) High 410 x 50 x 18 mm 6 (standard condition) density DDPB Low 410 x 50 x 18 mm 3 (standard condition) density BOS HDF + PB 410 x 50 x 18 mm 6 (standard condition) The supports are positioned to a distance of 20 times the nominal thickness, equally distant from the center loading head. The pace of the loading is regulated to have the fracture of the sample in 60 ± 30 seconds. For HDF, since the thickness is smaller than the range considered by the standard, the size of the sample is arbitrarily decided to be 150x50 mm and 100 mm for the distance of the supports. The time indication is respected and the calculation is done the same way as for the other materials. During the bending test, the deflection and the force of the loading head is recorded until the fracture of the sample occurs. From the load-deflection curve is possible to calculate the modulus of elasticity (MOE) with the following formula (CEN 1993a): ( ) ( ) where: MOE is in MPa; L is the distance between the supports (mm); b is the width of the sample (mm); t is the thickness of the sample (mm); F 2 -F 1 is the increment of the force (in N) between two points (about 10% and 40% of the total force) where the loaddeflection line is straight (elastic area); a 2 -a 1 is the increment of deflection (in mm) corresponding to F 1 and F 2. The modulus of rupture (MOR) can be calculated from the same curve with the formula (CEN 1993a): 13

19 where: MOR is in MPa; L, b and t are the same as for MOE and F max is the maximum force (in N) reached before fracture of the sample. The test is performed on samples cut parallel and perpendicular to the direction of production, as specified in the sampling chapter. Every second sample is flipped so the samples are bended flatwise in both directions. 3.6 Non-destructive tests BING system The resonance method in this study is performed by the BING (Beam Identification by Non-destructive Grading) system, a simple setup of widely available components and a software developed by CIRAD ( for the data acquisition and elaboration. The system is composed of a microphone, an anti-aliasing filter, an acquisition card (PicoScope) and the software BING (Picotech, n.d.).to achieve better results the samples need to fulfill dimensional requirements, as specified by the software developer: the length should be between 10 to 20 times the thickness and the width two times the thickness. More information about dimension and number of samples can be found in the Table 3. The dimensions of the sample, together with the weight and the MC need to be entered in the BING software before the testing and they will be used of the calculation of MOE. Table 3. Dimensions and number of samples for non-destructive tests Board type Direction Dimension Number of samples PB1 PB2 PB3 PB4 240 x 32 x 16 mm 5 (the same per every climate condition) 240 x 32 x 16 mm 5 (the same per every climate condition) 300 x 40 x 20 mm 5 (the same per every climate condition) 300 x 40 x 20 mm 5 (the same per every climate condition) 285 x 38 x 19 mm 5 (the same per every climate condition) 285 x 38 x 19 mm 5 (the same per every climate condition) 285 x 38 x 19 mm 5 (the same per every climate condition) 285 x 38 x 19 mm 5 (the same per every climate condition) *HDF 150 x 50 x 2,5 mm 18 (same samples as for SB test) *DDPB High density 410 x 50 x 18 mm 6 (standard condition) *BOS HDF + PB 410 x 50 x 18 mm 6 (standard condition) *. are not tested with resonance vibration methods. The supports for the sample are two elastic threads positioned on a sliding system to allow the regulation lengthwise (Figure 9): indeed the supports are placed at the nodal vibration points of the sample (0,224 of the length, from the edges) to have free-free support condition (Guan et al. 2015). 14

20 Fig. 9. BING setup. On the left, pipe and steel ball to struck the sample; in the center, sample on elastic; on the right, microphone. With this system it is possible to test the sample for resonance transversal and longitudinal vibration. In case of the transversal vibration test, the sample is struck perpendicularly to plane with a steel ball on one edge and the microphone is placed on the other edge. The transversal vibrations are recorded and the first four vibration modes (Figure 10a) are considered to calculate the dynamic transversal MOE (Hein et al. 2011) according to Timoshenko beam theory. For the transversal vibration test on full size boards, the elastic threads are replaced with triangular shaped rubber supports, placed at the nodal vibration points. To calculate the MOE, the frequency corresponding to the first vibration mode (Figure 10b) is considered, according to Euler-Bernoulli beam theory. More detailed information about the motion equations of longitudinal and transverse vibration can be found in Brancheriau and Bailleres (2002). 15

21 (a) (b) Fig. 10. Frequencies corresponding to the first four bending modes (a) on a sample of PB, and frequency of the first bending mode (b) on a full size test on a light-weight panel. In case of resonance longitudinal vibration, the sample is struck parallel to plane with a steel ball at the end and the microphone is placed on the other end. The first longitudinal vibration is considered to measure the dynamic longitudinal MOE (Hein et al. 2011). For both the testing methods, the test is repeated four times for every sample, two times for each side, and the average is considered as final result. For a reason of thickness and consequent dimensions of the samples, the BING system is not applied on HDF Time-of-flight To measure the time-of-flight in this study two ultrasonic testing machines are used. The functioning principle behind is rather straightforward and involves two transducers (sender and receiver) connected to the testing case with cables. Both transducers are coupled with the sample at the opposite ends of it (transducers facing each other) (Figure 11a) or they are coupled with the sample on the same face of it at a certain distance (Figure 11b). 16

22 (a) (b) Fig. 11. Time-of-flight testing equipment. (a) Sylvatest Trio, (b) Fakopp Ultrasonic Timer. One transducer (sender) sends an ultrasonic impulse that goes through the sample (as a stress wave) until it meets the other transducer (receiver). The time that the impulse takes to get from one transducer to the other one is measured by the machine and it is called time-of-flight. The distance of the transducers and the time-of-flight are used to calculate the stress wave velocity with the formula: where V is the velocity (m/s), D is the distance of the transducers (mm) and T is the time-of-flight (μs). The velocity is then used to calculate the dynamic MOE with the following formula: where ρ is density and V is velocity in longitudinal direction (Ross 2015). Two ultrasonic testing machines are used in this study: Fakopp Ultrasonic Timer (Fakopp Enterprise Bt., Hungary) and Sylvatest Trio (CBS-CBT, France) Fakopp Ultrasonic Timer The Fakopp U.T. setup used in this study is with triangle transducers (Figure 12). 17

23 Fig. 12. Triangle transducers (Fakopp Enterprise Bt., n.d.). This setup allows making measurements on the surface of the sample. The transducers are coupled with the sample and pressed down by hands to achieve a pressure of approximately 10-15kg for each. The testing machine send an ultrasonic impulse every two seconds and the transit time, between one transducer and the other, is displayed on the screen in microseconds. According to the user s manual (Fakopp Enterprise Bt., n.d.), if two or three consequent measurements differ with less than 1 μs, the data are correct. With the use of these transducers, a time correction needs to be applied to remove the transit time of the impulse through the transducers. The distance is calculated as in Figure 13 and 6,1 μs need to be subtracted from the transit time shown on the screen. Fig. 13. Distance calculation with triangle transducers (Fakopp Enterprise Bt., n.d.). To consider any possible variation of the top and bottom surface of the boards, the measurement is done on both surfaces and the average of the two is considered for the calculation of stress wave velocity Sylvatest Trio The transducers of Sylvatest T. are used to measure the transit time from one end-side of the sample to the other. The readout shows the values of the transit time and the velocity. For this study only the time is taken into account. Differently from how it is specified in the user s manual (CBS-CBT, n.d.), the measurements are performed without drilling holes in the samples. The uncertain distance of the transducers, when placed in holes, would influence the results considering the reduced size of the samples of this study. Therefore the measurements are done by pressing the transducers on the surface on the sides of the samples, as showed in Figure 11a. When a measurement is performed, the result displayed is the average of four impulses sent in sequence. To have a more accurate representation of the properties of the sample, three measurements are done on every sample; one for each surface layer and one for the 18

24 core. The average of the three transit times is considered for the calculation of the velocity. 3.7 Limitations Limitations for this study can be listed as it follows: - the range of the materials is limited to four different types of PB, one type of HDF, one type of dual-density PB and one type of light-weight panel (BoS) because of time limitation - Most of the non-destructive tests and the static bending tests are done on two different sets of samples because some of the NDT methods have different dimensional requirements than the static bending tests - tests on dual-density PB and BoS are done only in one climate condition because of the availability of the material and climate chambers - the tests done on full size boards are performed with material acclimatized only in one climate condition because of the availability of time and climate chambers - the mechanical properties tested are only stiffness and strength in bending mode, with the calculation of respectively MOE and MOR because of availability of time and equipment. 4 Results and discussion 4.1 Moisture content and density Table 4 shows the results for the MC determination. It shows the average EMC values in the three climates conditions. Table 4. MC results for the three climate conditions 20 C, 35% 20 C, 65% 20 C, 85% Board type MC % MC % MC % PB1 6,73 a (0,01) 9,91 b (0,04) 14,01 c (0,12) PB2 7,08 a (0,03) 9,70 b (0,02) 13,32 c (0,26) PB3 7,15 a (0,02) 10,20 b (0,05) 13,57 c (0,04) PB4 6,74 a (0,08) 9,95 b (0,05) 13,72 c (0,13) HDF 6,36 a (0,03) 9,57 b (0,03) 13,38 c (0,06) DDPB *6,38 a (0,05) *9,63 b (0,03) *13,63 c (0,03) BOS 6,27 a (0,03) 8,24 b (0,01) 12,59 c (0,05) The results signed with * are relative to the high density area of the DDPB. In parenthesis is displayed the standard deviation. a, b and c shows if the mean difference is statistically significant at 0,05 level. As explained in the caption of the table, the results from DDPB are not expression of the whole board. The MC is also calculated on samples cut from low-density areas with 19

25 MC (%) very similar results but slightly higher (6,54% for 35% RH, 9,69% for 65% RH and 14,24% for 85%). It can be assumed that the values from high-density and low-density samples are the extremes values and the averaged value of the board fall in between the two set of results. Figure 14 shows the general MC variation trend for all the type of material of this study PB1 PB2 PB3 PB4 HDF DDPB BOS RH Fig. 14. MC increase with increasing of RH for all type of WBP. All the WBP show almost a linear correlation between MC and RH, except for BOS; its values are overall lower than the other WBP. This behavior can be explained by the different hydrophilicity of the paper present in the honeycomb core layer or by the additional glue used for gluing the different components of the panel. Table 5 shows the density results at standard condition. Table 5.Density results at 20 C, 65% RH Board type Density (g/cm 3 ) PB1 0,60 (0,03) PB2 0,72 (0,01) PB3 0,66 (0,00) PB4 0,69 (0,01) HDF 0,87 (0,01) DDPB *0,63 (0,01) BOS 0,35 (0,00) The results signed with * are relative to the high density area of the DDPB. In parenthesis is displayed the standard deviation. The high density samples cut form the DDPB, as showed in the table, have an average value of 0,63 g/cm 3 while the result for the low-density areas is 0,52 g/cm 3. The density 20

26 measured on full size dimension samples resulted to be 0,58 g/cm 3, with a standard deviation lower than 0, Static bending test In the three tables below (Table 6-8), the average results of density, MOE and MOR from static bending test, are reported. Every table shows the results from one climate condition. Table 6. MOE and MOR in static bending of samples acclimatized at 35% RH. Board type Direction Density (g/cm 3 ) MOE (MPa) MOR (MPa) PB1 0,60 (0,03) 2891 (225) 11,36 (1,04) PB1 0,59 (0,00) 2381 (167) 10,35 (1,17) PB2 0,72 (0,01) 3943 (110) 19,61 (1,31) PB2 0,72 (0,01) 3788 (41) 19,19 (0,48) PB3 0,65 (0,00) 2657 (43) 12,51 (1,21) PB3 0,65 (0,01) 2452 (101) 12,65 (1,17) PB4 0,69 (0,01) 2954 (112) 15,17 (1,02) PB4 0,68 (0,03) 2685 (54) 14,40 (0,53) HDF 0,87 (0,03) 5360 (371) 57,47 (3,68) Table 7. MOE and MOR in static bending of samples acclimatized at 65% RH Board type Direction Density (g/cm 3 ) MOE (MPa) MOR (MPa) PB1 0,60 (0,03) 2421 (233) 10,91 (1,37) PB1 0,60 (0,00) 2067 (110) 9,61 (0,97) PB2 0,73 (0,01) 3503 (103) 19,25 (0,90) PB2 0,72 (0,01) 3319 (29) 18,29 (0,71) PB3 0,65 (0,00) 2324 (43) 12,04 (0,70) PB3 0,65 (0,01) 2140 (133) 11,71 (0,66) PB4 0,70 (0,01) 2655 (166) 14,56 (0,99) PB4 0,70 (0,01) 2285 (40) 12,98 (0,71) HDF 0,91 (0,02) 4510 (202) 48,00 (1,92) DDPB 0,64 (0,01) 2076 (72) 9,44 (0,39) 21

27 Table 8. MOE and MOR in static bending of samples acclimatized at 85% RH Board type Direction Density (g/cm 3 ) MOE (MPa) MOR (MPa) PB1 0,60 (0,02) 1871 (157) 8,89 (0,84) PB1 0,61 (0,01) 1498 (235) 7,69 (0,95) PB2 0,73 (0,01) 2812 (71) 17,04 (0,82) PB2 0,73 (0,01) 2675 (31) 16,11 (0,36) PB3 0,66 (0,00) 1913 (42) 10,01 (0,61) PB3 0,66 (0,01) 1746 (51) 9,33 (0,85) PB4 0,70 (0,01) 2132 (80) 11,63 (0,57) PB4 0,70 (0,01) 1870 (32) 10,63 (0,98) HDF 0,87 (0,02) 2886 (241) 31,09 (2,51) The percentage reduction of bending properties with the increase of RH in the climate conditions is presented in the Table 9. Table 9. Percentage reduction of MOE and MOR due to change in RH RH from 35% to 65% RH from 65% to 85% RH from 35% to 85% Board type Direction MOE MOR MOE MOR MOE MOR PB1 16%* 4% 23%* 18%* 35%* 22%* PB1 13%* 7% 28%* 20%* 37%* 26%* PB2 11%* 2% 20%* 11%* 29%* 13%* PB2 12%* 5%* 19%* 12%* 29%* 16%* PB3 13%* 4% 18%* 17%* 28%* 20%* PB3 13%* 7% 18%* 20%* 29%* 26%* PB4 10%* 4% 20%* 20%* 28%* 23%* PB4 15%* 10%* 18%* 18%* 30%* 26%* HDF 16%* 16%* 36%* 35%* 46%* 46%* *. the mean difference is statistically significant at 0,05 level. The PB2 (high density PB) resulted to have better bending properties compared to the other UF PB (PB1 standard PB). These better properties come together with a lower reduction of MOE and MOR at increasing RH, confirming the better quality of the material (load bearing purpose) compared to PB1. The PMDI PB show more similar results with higher values of MOE and MOR of PB4 compared to PB3, but also higher reduction of bending properties for the RH rise. Considering the purposes of the two materials, load bearing and damp areas resistance for PB4 and non-load bearing and dry areas application for PB3, the results related to the bending properties were expected but not the results about their reduction. The HDF has higher bending properties compared to PB but also higher reduction of MOE and MOR. This can be related to the thickness swelling; the thickness (mm) is used in the denominator to the third power in the calculation of MOE and to the second 22

28 MOE stat (MPa) MOE stat (MPa) MOE stat (MPa) MOE stat (MPa) MOE stat (MPa) MOE stat (MPa) power in the calculation of MOR. A change in thickness then would badly affect the MOE and MOR results. The content of the Table 9 is visually displayed in the graphs in the Figures 15 and 16. They show MOE and MOR reduction, respectively. PB1 PB MC MC (a) (b) PB2 PB MC MC (c) (d) PB3 PB MC MC (e) (f) 23

29 MOR stat (MPa) MOR stat (MPa) MOE stat (MPa) MOE stat (MPa) MOE stat (MPa) PB4 PB MC MC (g) (h) HDF MC (i) Fig. 15 (a - i). MOE (MPa) in different climate conditions. The differences between different climate conditions are statistically significant for all the graphs (a-i) at 0,05 level. PB1 PB1 30,00 25,00 20,00 15,00 10,00 5,00 0,00 30,00 25,00 20,00 15,00 10,00 5,00 0,00 MC MC (a) (b) 24

30 MOR stat (MPa) MOR stat (MPa) MOR stat (MPa) MOR stat (MPa) MOR stat (MPa) MOR stat (MPa) MOR stat (MPa) PB2 PB2 30,00 25,00 20,00 15,00 10,00 5,00 0,00 30,00 25,00 20,00 15,00 10,00 5,00 0,00 MC MC (c) (d) PB3 PB3 30,00 25,00 20,00 15,00 10,00 5,00 0,00 30,00 25,00 20,00 15,00 10,00 5,00 0,00 MC MC (e) (f) PB4 PB4 30,00 25,00 20,00 15,00 10,00 5,00 0,00 30,00 25,00 20,00 15,00 10,00 5,00 0,00 MC MC (g) (h) HDF 60,00 50,00 40,00 30,00 20,00 10,00 0,00 MC (i) Fig. 16 (a i). MOR (MPa) in different climate conditions. The differences between red columns are not statistically significant at 0,05 level. The other differences are significant at 0,05 level. 25

31 MOE stat (MPa) MOR stat (MPa) For the PB, two set of samples, parallel and perpendicular to the direction of production are considered. The European standard for sampling (EN326-1:1994) clearly specifies the necessity to consider the two directions separately. In this study the results confirmed the importance of considering the two directions separated. For all the types of PB, in every climate condition, the set of perpendicular samples have lower MOE and MOR compared to the parallel samples. The reduction of MOE is 17%, 5%, 8% and 12% for the PB1, PB2, PB3 and PB4, respectively. The reduction of MOR is 11%, 4%, 3% and 8% for the board in the same order just mentioned. The PB2 shows a good uniformity for both the in-plane directions while PB1 is less consistent and the anisotropy is high. The PB4 shows higher anisotropy than PB3 even if the former material is suitable for load bearing and the latter not. This piece of information should be considered in order to choose the right material for the right purpose. If the final application is a load bearing structure, this difference can be of importance. The correlation between bending properties and density is well known for solid wood (Hein et al. 2011, Missanjo and Matsumura 2016) and, to some extent, for WBP as well (Thoemen et al. 2010). In the Figure 17, MOE and MOR of all the PBs of the study, is correlated with density and the results are good, especially for MOR MOE y = 8458,3x ,4 R² = 0,7315 MOR y = 60,278x - 26,941 R² = 0, ,00 20, , , ,00 0 0,00 0,50 0,60 0,70 0,80 Density (g/cm 3 ) Fig. 17. Correlation between MOE stat (MPa) and MOR stat (MPa), and density (g/cm 3 ) for samples of PB1 to PB4 and DDPB (high and low density samples) acclimatized at 65% RH. The samples of HDF resulted to have lower correlation between static MOE and MOR, and density. The density change, over a rise of RH, is the result of the increase in weight and the volume swelling. For PB, in the range of RH considered, the two factors seem to be very balanced in the change and the result is almost a constant density in the three climate conditions. For HDF the trend is different and the highest density is at 65% RH, showing that moisture adsorption and volume swelling have different pace in the RH intervals 35%-65% and 65%-85%. 26