Tensile and compression properties through the thickness of oriented strandboard

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1 Tensile and compression properties through the thickness of oriented strandboard Caryn M. Steidl Siqun Wang Richard M. Bennett Paul M. Winistorfer Abstract It is well known that the density varies through the thickness of oriented strandboard, with the faces being much denser than the core. Density varies through the thickness because of consolidation characteristics of the wood elements during pressing in a hot-press. Hence, the mechanical properties should vary through the thickness of the panel. To determine the variation in strength and stiffness through the thickness of the panel, a commercial oriented strandboard was sawn into 15 layers to obtain thin-layer specimens for tension and compression testing. Specimens were obtained both parallel and perpendicular to the length of the panel. The specimens were tested in tension using straight-sided specimens and unbonded tabs. For specimens parallel to the length of the panel, the face layers had a tensile strength approximately an order of magnitude greater than the core. Greater face tensile strength was due to a combination of strand orientation and density. An apparatus was designed to test the thin specimens in compression. The average compression strength was significantly higher than average tension strength. However, the average compression modulus of elasticity was significantly lower than average tension modulus of elasticity. These layer tension and compression properties were related to the vertical density profile with high r 2 values (> 0.75), thus indicating that a strong linear relationship exists. The layer properties were used to predict the panel properties. Oriented strandboard (OSB) is one of the many engineered wood products that is gaining increased use in both residential and commercial construction. Although OSB has been used commercially for over 20 years, there are still many aspects of the behavior and properties that are not fully understood. For example, it is well known that density varies through the thickness of the panel, with the faces having higher densities than the core. This enhances the flexural behavior because the denser faces have a greater stiffness, creating a product that is analogous to an I-beam. However, it is not known how the stiffness varies through the thickness of the panel, or how the stiffness is related to the density profile. Increased understanding of the behavior of OSB will enhance the further development and efficient use of this engineered product. The objective of this work is to build a relationship that would be predictive of engineering flexural properties from the vertical density profile. This is accomplished by determining the engineering properties of individual layers of the OSB. The authors are, respectively, Former Graduate Student, Dept. of Civil and Environmental Engineering, 221 Perkins Hall, The Univ. of Tennessee, Knoxville, TN (currently Structural Designer, BKV Group, 222 North Second St, Minneapolis, MN 55401); Assistant Professor, Tennessee Forest Products Center, The Univ. of Tennessee, P.O. Box 1071, Knoxville, TN ; Professor, Dept. of Civil and Environmental Engineering, The Univ. of Tennessee; and Former Professor and Director, Tennessee Forest Products Center (currently Professor and Department Head, Dept. of Wood Science and Forest Products, 210 Cheatham Hall, Virginia Tech, Blacksburg, VA ). The authors would like to thank colleagues Chris Helton, William W. Moschler, and Ken Thomas for their helpful assistance. The authors would also like to thank J.M. Huber for providing experimental materials. This paper was received for publication in July Article No Forest Products Society Member. Forest Products Society Forest Prod. J. 53(6): JUNE 2003

2 Previous researchers have examined various aspects of through-the-thickness properties of wood composite panels. Perhaps the most commonly obtained through-the-thickness panel property is the density, or the vertical density profile (VDP) (Strickler 1959). The vertical density profile of wood composites is formed from a combination of actions that occur both during consolidation and also after the press has reached its final position (Wang and Winistorfer 2000). In order to examine the variation of internal bond (IB) through the thickness of a panel, Xu and Winistorfer (1995) sawed an OSB specimen into 9 layers, and measured the IB of each layer. Although there was a positive correlation between IB and density, the degree of correlation was small (r 2 between 0.20 and 0.25). The lowest IB did not always occur in the low density core layer, and the highest IB did not necessarily occur in the high density face layers. The layer sawing technique was also employed to obtain specimens for water absorption (WA) tests (Xu et al. 1996). These tests revealed that WA was positively correlated to layer density and layer thickness swell. Andrews (1998) determined that there was a negative correlation (r = -0.65) between the location of maximum density in the tension face layer and the bending modulus of elasticity (MOE) of the panel. As the maximum density location moved closer to the panel surface, the stiffness of the panel increased. The location of the maximum density influenced the MOE more than the density value itself. The same relationship was true for modulus of rupture (MOR), but the correlation was lower (r = -0.33). Geimer et al. (1975) examined the effects of layer characteristics on 3-layer particleboards. In one series of tests, face and core layers were separated and tested for stiffness in tension parallel to the board surface. They suggested that there was a nonlinear relationship between the MOE of the face layer and the density of the face layer, while there was a linear relationship between MOE of the core and density of the core layer. Laminate theory was used to predict board properties from layer properties. The predicted stiffness averaged 78 percent of the measured stiffness in two-point loading tests and 87 percent of the measured stiffness in single-point loading tests. Using the density gradient in successive 1/32-inch increments along with the developed nonlinear modulus-density relationships resulted in improved predictions, 92 percent for the two-point loading and 102 percent for the single-point loading test. One of the more extensive studies on engineering properties was conducted by Geimer (1979). He measured tension, compression, and bending MOE and failure stress of full-thickness flakeboards made with uniform densities throughout their thickness and different degrees of flake alignment. Logarithmic relationships between stiffness (or strength) and specific gravity and wave speed were developed. Several important behavioral aspects were determined from this work. The failure stress (or MOR) was highly correlated with the stiffness. The stiffness of boards with a density gradient could be predicted to within ±20 percent using the stiffness-density relationship from uniform density boards. The bending MOR was almost double that of the tension failure stress for the same level of stiffness. Carll and Link (1988) studied the layer behavior of 0.5-inch-thick aspen and Douglas-fir flakeboard panels. The 1/8-inch-thick face layers and 1/4-inch-thick core layers were tested in tension and compression. A logarithmic relationship was developed between the tensile or compressive MOE and the specific gravity and wave speed. These relationships were used with the density measured in 6 layers through the thickness (10%, 15%, 25%, 25%, 15%, and 10% of the thickness) to predict the bending MOE. The predicted MOE was consistently 10 to15 percent higher than the measured MOE. Grant (1997) examined the effects of strand alignment on the mechanical properties of OSB. A mathematical model was constructed to describe the relationship between the orientations of the individual surface layer strata and the unidirectional MOR and MOE of OSB panels. The models confirmed the positive influence of strand alignment on MOE, but it was found that there was a marginal return in improvement of the mechanical properties above a certain threshold. As expected, the contribution to strength and stiffness was found to diminish from the outer surface to the core. Xu conducted a series of theoretical studies on the effects of various layer properties on the MOE of the panel. Xu and Suchsland (1998) concluded in a study that assumed a uniform vertical density profile: 1) the panel MOE was not influenced by particle size; 2) the MOE decreases as the average out-of-plane orientation angle of particles increases; 3) the MOE increases linearly with an increase of either board density or compaction ratio (CR); and 4) in-plane orientation improves MOE in the orientation direction but reduces MOE across the orientation direction, with MOE in the across orientation direction leveling off after the percent alignment exceeded approximately 60 percent. Xu (1999) used laminate theory and simulated linear layer MOE-layer density relationships to examine the effect of different VDPs on the panel MOE. The analysis showed that the MOE benefits from the high density surface layer and increases linearly with an increase in peak density, but the maximum MOE does not occur when the peak density is located at the extreme board surface. These previous studies have examined many different aspects of the through-the-thickness behavior of wood composite panels. There has been an attempt to relate many of these properties to the vertical density profile. The purpose of this study was to determine the through-the-thickness tension and compression strength and stiffness of a commercial OSB panel and to relate the layer mechanical properties with the layer density. A much thinner layer than previously used was chosen for this study to better define the variation of the engineering properties through the thickness. The layer properties were used to predict panel properties. Experimental method Specimen preparation All specimens were cut from one 4- by 8-foot, 23/32-inch-thick commercial southern pine OSB panel. The OSB panel was production sanded and bonded with a diphenyle-methylene diisocyanate (MDI) resin. Fourteen sample sets were cut parallel to face strand orientation and 14 sample sets were cut perpendicular to face strand FOREST PRODUCTS JOURNAL Vol. 53, No. 6 73

3 Figure 1. Detail of offset cuts for tension and compression specimens. Figure 2. Cutting diagram for one sample set. orientation. One bending specimen, three tension, and three compression specimens were cut from each sample set, as shown in Figure 1. Bending specimens were 3 by 19-1/4 inches as specified in ASTM D 1037 (1996). The 2- by 2-inch specimens were cut from the bending specimens after testing to measure the vertical density profile. Compression specimens were 1.5 by 4 inches and tension specimens were 1.5 by 8 inches before making thin slices. To better define the engineering properties through the thickness of the panel, a thin layer was chosen. However, as the layer gets thinner, it behaves less like a homogeneous material. We chose to use 15 layers through the thickness of the panel for tension and compression testing, resulting in a specimen thickness of inch. The final tension and compression specimens were 1 inch wide by 8 inches long and 1 inch wide by 4 inches long, respectively. The OSB material was a multi-layered alignment panel. OSB face layer strands were aligned opposite to the core layer; those specimens cut parallel to panel length produced approximately three specimens parallel to face strand alignment, nine specimens perpendicular to face strand alignment, and three specimens parallel to face alignment, respectively, through the panel thickness. The opposite was true for the specimens cut perpendicular to face strand alignment. These layer changes were confirmed by visual inspection after the thin specimens were obtained. Three full-thickness tension and three full-thickness compression pieces were required for each sample set because only five thin specimens could be obtained from each full thickness piece. When the 15 thin specimens are arranged according to their position in the thickness of the board, they reflect the 23/32-inch full-thickness board. A cutterhead consisting of six 7-1/4-inch diameter, inch-thick, 18-tooth carbide-tipped blades with appropriately sized spacers was mounted on an arbor in a milling machine to achieve the desired sample thickness of inch. The full-thickness pieces were initially 1.5 inch wide, so they could be held in a vice attached to the milling machine table. The machine was set to cut to a depth of 1 inch and to feed 74 JUNE 2003

4 Table 1. The effective panel MOE and MOR. Property No. of specimens tested Mean COV MOE (psi) Parallel , Perpendicular , MOR (psi) Parallel 14 5, Perpendicular 14 3, Average density (pcf) Table 2. Layer properties for perpendicular to panel length tension specimens. Strand No. of Density Strength MOE Layer orientation specimens tested Mean SD a Mean COV Mean COV (pcf) (psi) Perpendicular E Perpendicular E Perpendicular E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Perpendicular E Perpendicular E Perpendicular E Average E a SD = standard deviation. the samples into the cutterhead at a rate of 1.75 feet per minute. Only five specimens could be obtained from each full-thickness tension and compression piece. Layers 1, 4, 7, 10, and 13 were obtained from the first piece. Layers 2, 5, 8, 11, and 14 were obtained from the second piece. Layers 3, 6, 9, 12, and 15 were obtained from the third piece. This was accomplished by cutting all of the first pieces, offsetting the blades inch into the sample, and cutting all of the second pieces. The third pieces were cut in a similar fashion, thus obtaining samples representative of a full-thickness board. Clarification of the offset specimens is shown in Figure 2. After all tension and compression pieces were cut through the thickness, a bandsaw was used to trim off most of the 1/2-inch gripping edge. An end mill was used for the final trimming to separate the thin specimens. In order to prevent damage to the specimens during this process, plastic spacers were inserted in the sawkerfs. This process successfully produced thin specimens of the desired quality and thickness. Tension testing The necked-down, or dog-boned-shaped, specimens typically used in tension testing were not used in this research due to the difficulty of making these specimens from the thin OSB slices. Rather, straight-sided specimens with a uniform rectangular cross section were used, which is similar to the way thin polymer matrix composites are tested (ASTM 1995). Self-aligning grips with non-bonded tabs were used to pull the specimen. The tabs were small pieces of wood with sandpaper adhered to them. This set-up proved to be successful. Most samples failed between the grips with only 13 percent of the specimens failing within the grips. The specimens were tested under deflection control at a rate of in./min. Strain was measured using an extensometer consisting of two linear variable displacement transducers (LVDTs), one on either side of the specimen to correct for bending. The gage length was 4 inches and an average deflection was used in data analysis. The MOE of the tension and compression specimens was obtained from the slope of the stress-strain curve. Some stress-strain curves had a small initial vertical portion before the extensometer began recording deformation. A minimum stress was chosen just above the vertical portion of the stress-strain curve. The beginning slope value was calculated for the portion of the stress-strain curve between the minimum stress and the minimum stress plus 30 percent of the failure stress. The minimum stress was incrementally increased by 2 percent of the failure stress and a regression calculated the slope of the next 30 percent of the curve. Once the upper stress reached the maximum stress, the incremental process stopped. The maximum slope calculated during the step-wise regression was taken as the tension MOE. Thus, the reported tension MOE is the least squares fit over the steepest 30 percent of the stress-strain curve. Values of r 2 for the linear regression were typically greater than 0.9. There were 420 tension specimens cut. While handling and loading samples in the testing machine, some samples broke, thereby reducing the number of tests. Except for one, all of the samples that failed in handling were for strands perpendicular to the applied tension. Thus, there may be some small bias in the tension perpendicular to strands results. Tension tests were conducted on a total of 403 specimens: 206 cut perpendicular to face strand orientation and 197 cut parallel. Compression testing Published test methods for wood panels in compression (ASTM 1994) suggest an apparatus that utilizes spring steel to provide the lateral support. Due to the thin specimens of this study and make-up of OSB, this apparatus was not feasible because the steel tines could pierce into the thin specimens or FOREST PRODUCTS JOURNAL Vol. 53, No. 6 75

5 Compression tests were conducted on a total of 403 specimens: 209 cut perpendicular to face strand orientation and 194 cut parallel. One complete parallel sample set was lost due to an error in the cutting process. Only two other specimens were lost in handling. Some of the compression samples became wedged in between the UHMW blocks after failure. However, the maximum load taken by the specimen before wedging was noted, and the compression MOE was calculated using the appropriate data. Figure 3. Compression test apparatus. Table 3. Layer properties for parallel to panel length tension specimens. Strand No. of Density Strength MOE Layer orientation specimens tested Mean SD a Mean COV Mean COV (pcf) (psi) Parallel E Parallel E Parallel E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Parallel E Parallel E Parallel E Average E a SD = standard deviation. through any voids, which are not uncommon in the core specimens. Hence, a compression testing apparatus was designed to provide lateral support, yet preserve the integrity of the specimen (Fig. 3). Full drawings of the device can be found in Steidl s thesis (Steidl 2000). Lateral support is provided by two ultra high molecular weight (UHMW) plastic blocks. UHMW was chosen for its low coefficient of friction with OSB, which was measured as Preliminary tests confirmed that there was minimal load transfer between the specimen and lateral supports. Like tension testing, the compression specimens were tested under deflection control at a rate of in./min. Since an extensometer could not be attached to the specimen, strain was determined from the testing machine crosshead movement. Other testing A commercial densitometer (QMS Density Profile System QDP-01X) was used to measure the vertical density profile, with the density being measured at 0.02-inch increments through the thickness of the specimen. There were 183 density profiles determined. Density points ranging between the start and end points for each layer were averaged to find the average density for each layer. Results The effective panel MOE and MOR are given in Table 1. An average vertical density profile of the panel is shown in Figure 4. The average panel density was 41.2 pcf. Average density for each layer is given in Tables 2 to 5. Tables 2 and 3 list the number of samples tested, average density, average tension MOE, and average tension strength by layer for perpendicular and parallel cut specimens. Figure 5 shows typical failures for perpendicular (L1 and L15) and parallel (L5 and L9) to panel length specimens. Tables 4 and 5 list the number of samples tested, average density, average compression MOE, and average compression strength by layer for perpendicular and parallel cut specimens. Discussion Tension properties For specimens parallel to the length of the panel, the face layers had a tensile strength and MOE approximately an order of magnitude greater than the core (Table 3). This was due to a combination of a denser face and the face strands being oriented parallel to the applied tension. For specimens perpendicular to the length of the panel, the tensile strength and MOE were relatively uniform through the thickness (Table 2). The denser faces, with the strands oriented perpendicular to the applied ten- 76 JUNE 2003

6 Table 4. Layer properties for perpendicular to panel length compression specimens. Strand No. of Density Strength MOE Layer orientation specimens tested Mean SD a Mean COV Mean COV (pcf) (psi) Perpendicular E Perpendicular E Perpendicular E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Parallel E Perpendicular E Perpendicular E Perpendicular E Average E a SD = standard deviation. Figure 4. Average vertical density profile of OSB panel. sion in testing, had approximately the same strength and MOE as the less dense core, where the flakes were aligned with the applied tension. In specimens with perpendicular strand orientation (L1 and L15), the failure is typical of perpendicular to grain failures (Fig. 5). Specimens with parallel strand orientation (L5 and L9) experience failure within the strands. All of the tensile properties had relatively high scatter, as reflected by an average coefficient of variation (COV) of 39 percent. This was expected due to the small specimen size used in this work. Locally, there is a high variation in the properties of OSB; however, there is much less variation in panel properties as reflected by much lower COVs for the bending specimens. Compression properties For specimens parallel to the length of the panel, the face layers had a compressive strength and MOE approximately an order of magnitude greater than the core (Table 5), which was similar to the tension strength and MOE. For perpendicular to panel length samples, the compression strength and MOE were relatively uniform through the thickness (Table 4). Typically, failure occurred in the compression specimens as a result of strands sliding over one another. The average COV in compression properties was 36 percent. The compressive MOE was about half the tensile MOE, averaging 0.46 of the tensile MOE for specimens with the load parallel to the strands and 0.49 of the tensile MOE for specimens with the load perpendicular to the strands. Carll and Link (1988) also found the compressive MOE to be less than the tensile MOE for OSB, although the difference was smaller. They reported the full thickness compressive MOE of aspen and Douglas-fir OSB panels as about 89 percent of the tensile MOE. Geimer (1979) reported compressive MOE of approximately 92 percent of the tensile MOE for Douglas-fir OSB panels. The compression strength was considerably greater than the tensile strength. The compressive strength averaged 1.52 times the tensile strength for specimens with the load parallel to the strands, although the outer two face layers had essentially the same tensile and compressive strengths. The compression strength averaged 1.99 times the tensile strength for specimens with the load perpendicular to the strands. This is opposite of the findings of Geimer (1979) who found the compressive strength of Douglas-fir OSB panels to be approximately 80 percent of the tensile strength. We do not have a good explanation for this, although we note that our compression testing method did provide full lateral support of the specimen. Relating layer properties to vertical density profile In order to relate the vertical density profile to the layer tension/compression properties, layers were grouped by compression and tension, and for perpendicular and parallel strand orientation. The tension/compression MOE and strength values were plotted versus density (Figs. 6 and 7) and a linear regression was obtained with a corresponding r 2 value (Table 6). The r 2 values are high values (reasonably close to 1), thus the data have a positive linear relationship with the high density values producing the higher property values. A log transform model was also investigated since it has been used by others (Geimer 1979, Carll FOREST PRODUCTS JOURNAL Vol. 53, No. 6 77

7 3-inch-long strands. Geimer s results were obtained based on an estimated strand alignment of 35 percent, which results in an estimated sonic velocity ratio of Geimer s strength values for 1.5- and 3-inch-long strands are essentially the same, so only the 3-inch results are plotted. Several observations can be made from Figure 8. Although Geimer (1979) used a log transform model, the relationship between the mechanical properties and density is essentially linear over the range of interest. The slope of the MOE-density relationship and the strength-density relationship from this work is similar to that from Geimer (1979). Geimer s data shows a much greater stiffness and strength than that measured in this work. Figure 5. Failure photo of specimens cut perpendicular to panel length (bottom), specimens L1, L5, L9, and L15 (L1 and L15 perpendicular to strand orientation, L5 and L9 parallel to strand orientation). Table 5. Layer properties for parallel to panel length compression specimens. Strand No. of Density Strength MOE Layer orientation specimens tested Mean SD a Mean COV Mean COV (pcf) (psi) Parallel E Parallel E Parallel E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Perpendicular E Parallel E Parallel E Parallel E Average E a SD = standard deviation. and Link 1988). Although both the linear and log transform fit the data about equally well, the r 2 values for the linear model averaged 3 percent higher than the r 2 values for the log transform model. Thus, the linear model was used. MOE and strength data for tension parallel to the strand orientation is compared to other models in Figure 8. The model from Xu (1999) is for aspen OSB. The results from Geimer (1979) are for Douglas-fir OSB with both 1.5- and Prediction of panel properties Fundamental engineering mechanics relationships can be used to predict both the stiffness and strength of the panel from the layer properties (e.g., Geimer et al. 1975). The predicted panel MOE from the layer properties was 61 percent of the measured panel MOE for parallel to panel length, and 47 percent of the measured panel MOE for perpendicular to panel length. Geimer et al s (1975) predicted panel MOE was also less than measured, averaging 87 percent of the measured panel MOE. Carll and Link (1988) overpredicted panel MOE by 10 to 15 percent. In an attempt to characterize the through-the-thickness mechanical behavior of OSB, much thinner layers were used in this study than in previous studies. This probably affected the results. As thin layers are removed from the full-thickness panel, several of the strands become severed, with a portion going to each adjacent layer, leaving only partial strands to contribute to strength and stiffness. If these partial strands were neglected, only the complete strands would serve as the effective thickness of the layer. The average strand thickness was approximately half the specimen thickness. Thus, an effective thickness could be considered as half the specimen thickness, which would explain the low predictions. The predicted panel strength from the layer properties was 20 percent of the measured strength for parallel to panel length and 25 percent of the measured 78 JUNE 2003

8 Table 6. Regression equations for tension/compression properties vs. layer density. a MOE vs. density r 2 Strength vs. density r 2 Tension Perpendicular MOE = (DEN) STR = (DEN) Parallel MOE = (DEN) STR = (DEN) Compression Perpendicular MOE = 8220 (DEN) STR = (DEN) Parallel MOE = (DEN) STR = (DEN) a Units are psi for modulus of elasticity (MOE) and strength (STR) and pcf for density (DEN). Figure 6. Regression relationship between compression and tension MOE vs. layer density. Figure 7. Regression relationship between compression and tension strength vs. layer density. strength for perpendicular to panel length. The strength prediction was about half as accurate as the stiffness prediction. This is consistent with the findings of Geimer (1979), who found that the bending MOR was almost twice the tension and compression strength. Although we do not have a complete explanation for this, we suggest that part of the reason is nonlinear behavior near ultimate. A shift in the neutral axis towards the compression face near ultimate would cause an underestimation of the strength using linear elastic theory. Conclusion The sawing technique, as described in this study, was successfully used to prepare individual inch-thick layers through the thickness of a commercial OSB panel. These layers provided tension/compression strength and stiffness values. For specimens parallel to the length of the panel, the face layers had tensile and compressive properties approximately an order of magnitude greater than the core. For perpendicular samples, the tensile and compression properties were relatively uniform through the thickness. This behavior was due to a combination of strand orientation and density. The average compression strength was significantly higher than average tension strength. However, the average compression MOE was significantly lower than average tension MOE. These layer tension and compression properties were related to the vertical density profile with high r 2 values (>0.75), thus indicating a strong linear relationship exists. The layer properties were used to predict panel bending properties. This study enhances the understanding of the mechanical behavior of OSB panels. Coupling this understanding of how the vertical density profile affects the through-the-thickness mechanical properties with research on the effects of the VDP on other parameters (such as FOREST PRODUCTS JOURNAL Vol. 53, No. 6 79

9 Figure 8. Comparison of tension parallel to strand MOE and strength to other data. IB and thickness swell), the effects of changes in the VDP can be ascertained. The information can be used to enhance manufacturing and the development of optimal vertical density profiles. Literature cited American Society for Testing and Materials (ASTM) Standard test methods for wood-based structural panels in compression. ASTM D ASTM, West Conshohocken, Pa Standard test method for tensile properties of polymer matrix composite materials. ASTM D 3039M-95a. ASTM, West Conshohocken, Pa Standard test methods for evaluating properties of wood-base fiber and particle panel materials. ASTM D a. ASTM, West Conshohocken, Pa. Andrews, C.K The influence of furnish moisture content and press closure rate on the formation of the vertical density profile in oriented strandboard: Relating the vertical density profile to bending properties, dimensional stability and bond performance. Master s thesis. The Univ. of Tennessee, Knoxville, TN. Carll, C.G. and C.L. Link Tensile and compressive MOE of flakeboard. Forest Prod. J. 38(1):8-14. Geimer, R.L Data basic to the engineering design of reconstituted flakeboard. Proc. 13th Inter. Particleboard/Composite Materials Symposium. Washington State Univ., pp , H.M. Montrey, and W.F. Lehmann Effects of layer characteristics on the properties of three-layer particleboards. Forest Prod. J. 25(3): Grant, D Effects of the through-thickness strand alignment distribution on the unidirectional bending properties of oriented strand board. Master s thesis. Laval Univ., Quebec, Canada. Steidl, C.M Layer properties of oriented strandboard. Master s thesis. The Univ. of Tennessee, Knoxville, TN. Strickler, M Effect of press cycles and moisture content on Douglas-fir flakeboard. Forest Prod. J. 9(7): Wang, S. and P.M. Winistorfer Fundamentals of vertical density profile formation in wood composites. Part 2. Methodology of vertical density formation under dynamic conditions. Wood and Fiber Sci. 32(2): Xu, W Influence of vertical density distribution on bending modulus of elasticity of wood composite panels: A theoretical consideration. Wood and Fiber Sci. 31(3): and O. Suchsland Modulus of elasticity of wood composite panels with a uniform vertical density profile: A model. Wood and Fiber Sci. 30(3): and P.M. Winistorfer Layer thickness swell and layer internal bond of medium density fiberboard and oriented strandboard. Forest Prod. J. 45(10):67-71.,, and W.W. Moschler A procedure to determine water absorption distribution in wood composite panels. Wood and Fiber Sci. 28(3): JUNE 2003