Density estimation by vibration, screw withdrawal resistance and probing in particle and medium density fibre boards

Transcription

1 Density estimation by vibration, screw withdrawal resistance and probing in particle and medium density fibre boards Ignacio Bobadilla Forestry Engineer, PhD. Madrid; Spain. Francisco Arriaga Architect, PhD. Madrid; Spain Miguel Esteban Forestry Engineer, PhD. Madrid; Spain. Guillermo Iñiguez Forestry Engineer, PhD. Madrid; Spain. Isidoro Blázquez Forestry Engineer. Madrid; Spain. Summary The result of probing 228 pieces of 1000 x 50 x 22 mm of particle and medium density fibre boards are presented in order to establish a density estimation model based on the use of non-destructive and portable methods such as the Screw Withdrawal Resistance Meter, Pilodyn, ultrasound and stress wave vibration. For each class of board (particle or fibre board), we tested two types, standard board and humidity resistant board. In general terms, for each board class and type, the real density variation coefficients are much lower than these obtained in the non-destructive tests, so that regression models do not allow us to establish a suitable estimation, presenting very low determination coefficients. If the results are grouped according to whether boards are particle or medium density, for the second type we obtain a normal distribution, making it possible to establish statistical models linking actual board density and the results of non-destructive testing, with good correlation and determination coefficients. The best estimation tool in this case is ultrasonic wave velocity, which has a determination coefficient of almost 80%. 1. Introduction At present various non-destructive methods are widely used and studied to estimate the physical and mechanical properties of wood. Screw withdrawal resistance and the Pilodyn, for example, have been successfully used in estimating the density of solid wood [1]. Some studies also use the same philosophy with other materials, such as wood composites.

2 We can find many references to the use of ultrasound methods in different types of board to evaluate different properties. The elastic properties of particle board have recently been studied [2] as well as variation in density within boards [3]. Similar studies can be found for oriented strand board "OSB" [4]. Stress wave vibrations have been also used by several authors in OSB [5] and other wood-based panels [6]. Fewer references were found to the use of screw withdrawal and the Pilodyn in the estimation of board properties, although the Pilodyn has already been used to try to assess exposure aging in wafer boards [7]. The aim of this study is to examine less-used non-destructive and portable methodologies for the estimation of the density of particle and fibre board as a prelude to the estimation of mechanical properties. 2. Material tested We tested 228 samples from particle and fibre board by a single manufacturer, with dimensions of 1000 x 50 x 22 mm. Two types of each class were tested, one standard and one humidity resistant. The final list of pieces tested is reflected in table 1. Table 1: Board class and type, dimensions and number of pieces tested. Board class and type Test specimen dimensions (mm) Number of pieces tested Standard particle board 1000 x 50 x Humidity resistant particle board 1000 x 50 x Standard medium density fibreboard 1000 x 50 x Humidity resistant medium density fibreboard 1000 x 50 x The determination of dimensions and preparation of the test pieces for the tests were performed according to European standards [8] EN 326-1:1995 and [9] EN 325: Methodology 3.1 Ultrasound The velocity of ultrasonic wave propagation was recorded using Sylvatest Duo equipment, manufactured by Concept Bois Structure, Switzerland. This technique has been widely studied and used for the characterization of different timber products. This equipment emits a wave at an ultrasonic frequency of 22 khz, which is transmitted through material by means of a transmitter-sensor and is received by a receiver-sensor. The time it takes the Figure 1: Ultrasound equipment used. Sylvatest Duo. wave to pass between the two transducers is recorded to compute the propagation velocity. A second set of pulses is evaluated to determine the maximum energy of the received wave. In this work, ultrasound velocity measurement was carried out on the one meter longitudinal direction of each piece from end to end as shown in figure 1.

3 The moisture content of wooden material has some influence on the propagation velocity of ultrasound, so the material was conditioned to 20±2ºC temperature and 65±5 % relative humidity as can be seen in paragraph 3.6. The results of these measurements are shown in table 2. Table 2: Ultrasound wave propagation speed Type of Board Mean wave velocity (m/s) Max wave velocity (m/s) Min wave velocity (m/s) Standard Particle Board , Humidity resistant Particle Board , Standard MDF , Humidity resistant MDF , Stress wave Vibration-based non-destructive tools generally use the well-known and accurate relationship between the natural frequency of oscillation of a simply supported beam and the dynamic elasticity modulus to estimate the mechanical properties of material. The dynamic longitudinal elasticity modulus is calculated from the standard solution of the wave equation for longitudinal vibration of a slender rod in a free-free support condition [10]. In this project we used it to estimate material Figure 2: Stress wave vibration equipment density. Determination of the longitudinal vibration frequency is performed using the equipment shown in figure 2, by carrying out the procedure described below. The test device used is denominated SWVE Stress Wave Vibration Equipment, while the software used is the PC-based Fast Fourier Vibration analyzer. These were developed by FAKOPP Enterprise [11]. In the test procedure the specimens are placed on two supports with soft polyurethane pillows to ensure that test pieces are free to vibrate. Soft supports have been located at 22 centimetres from the extreme of the specimen. The end or centre of a specimen is hit by a hammer and the impact induces a stress wave of longitudinal or bending vibration caught as sound by a microphone set close to the test piece. The fundamental vibration frequency of the sound is analysed by the fast Fourier transform sound analyser. Using the specimen s length (L, in m) and the longitudinal vibration frequency (f, in Hz), the wave velocity (V, in m/s) can be calculated by means of: V = 2 L f (eq. 1) The results of these measurements are shown in table 3. Table3: Longitudinal and bending vibration frequency Type of board Mean longitudinal vibration Max LVF (Hz) Min LVF (Hz) frequency LVF (Hz) Standard Particle Board 928 1, Humidity resistant Particle Board 970 1, Standard MDF , Humidity resistant MDF ,

4 Type of board Mean bending vibration frequency BVF (Hz) Max BVF (Hz) Min BVF (Hz) Standard Particle Board 50,1 2,1 52,5 48,2 Humidity resistant Particle Board 50,1 1,3 52,1 48,9 Standard MDF 50,2 1,4 51,5 48,5 Humidity resistant MDF 50,3 1,6 52,4 48,7 3.3 Screw withdrawal resistance The screw withdrawal test was performed using the portable Screw Withdrawal Resistance Meter (SWRM) designed by Fakopp Enterprise. This has three parts, as shown in figure 3: a device for screw support, a force transducer to record the maximum resistance value and a worm for screw withdrawal. We used Heco Fix Plus 4 x 70 mm yellow Zinc plated screws, with a penetration depth of 22 mm, from side to side of the board specimen test piece. The results of screw withdrawal resistance measurements are shown in table 4. Figure 3: Screw withdrawal resistance meter by Fakopp Enterprise Table 4: Screw withdrawal resistance measurements Type of board Mean screw withdrawal resistance Max SWR (kn) Min SWR (kn) SWR (kn) Standard Particle Board 1,34 6,7 1,53 1,00 Humidity resistant Particle Board 1,63 11,0 1,89 1,02 Standard MDF 1,82 18,0 2,64 1,16 Humidity resistant MDF 2,86 6,6 3,32 2, Penetration depth The Pilodyn used in this project consists of a calibrated spring that drives a steel needle into material. The depth of penetration is used to evaluate the density or in some cases the level of damage, which depends on surface hardness and density [12]. This equipment was designed to estimate the density and hardness of sawn timber and standing trees. It is a non-destructive method because it only makes a small hole 2.5 mm in diameter with a variable depth of 5 to 20 mm, depending on the penetration resistance of the material. This hole causes no significant damage to timber or boards. The equipment used in this work was the Pilodyn 6J Forest commercialised by Proceq. Figure 4: Pilodyn 6J Forest Because of the high variability of this measurement, we decided to perform two penetration tests on each specimen, and use the mean value for statistical discussion. The results of penetration depth measurements are shown in table 5.

5 Table 5: Pilodyn penetration depth measurements Type of board Mean penetration depth (mm) Max Penetration (mm) Min Penetration (mm) Standard Particle Board 11,2 8,8 13,5 8,5 Humidity resistant Particle Board 9,6 11,2 12,0 7,0 Standard MDF 9,8 9,7 12,0 8,0 Humidity resistant MDF 7,6 10,6 9,7 6,2 3.5 Density Each specimen of wood-based board was weigh. The overall density was determined by dividing the total mass of each piece by its volume, according to EN 323:1994 [13]. The results of density measurements are shown in table 6. Table 6: Density measurements Type of board Mean overall density (kg/m 3 ) Max density (kg/m 3 ) Min density (kg/m 3 ) Standard Particle Board 642 1, Humidity resistant Particle Board 699 1, Standard MDF 714 1, Humidity resistant MDF 744 1, Conditioning and humidity content Conditioning was performed before all of the tests in a climate chamber at 20±2ºC temperature and 65±5% relative humidity. The moisture content was measured using electrical resistance equipment and with the oven drying method, according to EN 13183:2002 [14] and EN 322:1994 [15]. 4. Results and discussion 4.1 Density prediction using Screw withdrawal resistance and penetration depth The density values for each class and type of board are very homogeneous, especially when compared to the screw withdrawal resistance or penetration depth test values. They are for example up to 10 times greater with the screw equipment. It is not therefore statistically reliable in general to predict density by means of these mechanical non-destructive tests. Coefficients of determination are very low, at less than 5% in some cases. Grouping types of board (particle board on the one hand and fibre board on the other), in order to increase the variability of density, is not valid in this case for particle board because data distribution loses normality. However, grouping works well with fibreboard, and it is possible to use Pilodyn penetration depth, because here data distribution is normal. In this case we obtain the regression model (eq. 2), with a determination coefficient "R 2 " of 68,6%. Where: δ is the density in kg/m 3 δ = 796,59 14,5088 Zst 6, P (eq. 2)

6 P is the Pilodyn penetration in mm Zst is 1 if it is a standard board and 0 if it is humidity resistant 4.2 Density prediction using vibration methods Vibration methods are somewhat better means of density prediction because they are generally much less variable. However, considering the classes and types of board separately, determination coefficients remain low. Grouping per class once again, we found the same problems that were cited in the preceding paragraph on particle board. With fibreboards we obtained the best estimation model using ultrasound velocity, with a determination coefficient "R 2 " of 79,6 %. Table 7 contains the different estimators analyzed, when their determination coefficients are higher than 5%. Table 7: Estimation models with vibration methods. δ Density (kg/m 3 ) f L Longitudinal stress wave vibration frequency (Hz) f b Bending stress wave vibration frequency (Hz) Vus Ultrasonic wave velocity (m/s) Zst = 1 if standard board; 0 if humidity resistant BOARD CLASS & ESTIMATION EQUATION R 2 TIPE VARIABLE STANDARD P. Ultrasound vibration δ=449,933+0, x Vus 6,7% HUMIDITY R. P. Longitudinal stress wave δ=204,042+0, x f L 37% Ultrasound vibration δ=26,9517+0, x Vus 43,5% STANDARD MDF Longitudinal stress wave δ=248,293+0, x f L 43% Ultrasound vibration δ=160,945+0, x Vus 53,5% HUMIDITY R. MDF Longitudinal stress wave δ=370,166+0, x f L 29,7% Ultrasound vibration δ=232,315+0, x Vus 47,6% ALL KIND MDF Longitudinal stress wave δ=317,504+0, x f L 13,6806 x Zst 73,6% Bending stress wave δ=696,195+0, x f b 29,8266 x Zst 58,9% Ultrasound vibration δ=207,241+0, x Vus 21,1221 x Zst 79,6% 5. Conclusions The very low variation in density within each class and type of board makes estimating density difficult, especially with those estimators with high variation coefficients. Grouping the boards according to class (particle or fibreboard) improves this situation, increasing (and in some cases tripling) the variability of density. But grouping sometimes also leads (in particle board for example) to a loss of data normalcy. In general the best results are obtained with the vibration method, especially ultrasound, which in the case of fibreboards attains determination coefficients close to 80% (Figure 5).

7 Figure 5: Ultrasound density estimation Figure 6: Pilodyn density estimation The most suitable mechanical method is the Pilodyn penetrometer, which in the case of fibreboards attains determination coefficients close to 69% (Figure 6). Finally, for particleboard, we recommend a higher number of test samples, preferably from a variety of sources, in order to obtain a broader range of densities to thereby achieve normal distribution. 6. Acknowledgments AITIM (Technical Research Association for the Wood Industries) AITIM-FUCOVASA Bobadilla project. UTISA, Mediterranean Boards. Rosa García and Jesús Terrádez. Department of Quality Control. 7. References [1] Bobadilla, I.; Íñiguez, G.; Esteban, M.; Arriaga, F.; Casas, L. Density estimation by screw withdrawal resistance and probing in structural sawn coniferous timber, 15 th International Symposium on Non-destructive Testing of Wood, Duluth, Minnesota, USA, [2] Najafi S.K; Bucur V; Ebrahimi G. Elastic constants of particle board with ultrasonic technique Materials Letters 59 (16): July [3] Kruse K; Broker FW; Fruhwald A. Interrelation between internal bond, density distribution and ultrasonic velocity of particle board Holz als roh-und werkstoff 54 (5): October [4] Vun RY; Wu QL; Monlezum CJ. Ultrasonic characterization of horizontal density variations in oriented strandboard Wood and Fibre Science 35 (4): October [5] Ross RJ; Yang VW; Illman BL; Nelson WJ. Relationship between stress wave transmission time and bending strength of deteriorated oriented strandboard Forest Products Journal 53 (3): March [6] Han GP; Wu QL; Wang XP. Stress wave velocity of wood based panels: Effect of moisture, product type, and material direction Forest Products Journal 56 (1): January [7] Schmidt EL; Dietz MG. Pilodyn evaluation of treated waferboard in field exposure Wood and Fibre Science 20 (1):

8 [8] EN Wood based panels. Sampling, test specimen and inspection [9] EN 325. Wood based panels. Dimensional determination of the samples [10] Kollman, F.; Krech, H. Dynamishe messungen der elastischen Holzeingschafter und der Dampfung, Holz als Roh- und Werkstoff, 18, 1960, pp [11] Divos, F. Portable Lumber Grader, 13 th International Symposium on Non-destructive Testing of Wood, Berkeley, California, USA, [12] Hoffmeyer, P. The Pilodyn instrument as a non-destructive tester of the shock resistance of wood, 4 th Non-destructive Testing of Wood Symposium; Vancouver, WA, USA, 1978, pp [13] EN 323. Wood based panels. Density determination [14] EN Moisture content of a piece of sawn timber. Part 2: Estimation by electrical resistance method [15] EN 322. Wood based panels. Humidity content determination