propiedades del osb

8/15/2019 propiedades del osb

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Tensile and compression properties throughthe thickness of oriented strandboard

Caryn M. Steidl

Siqun Wang✳

Richard M. Bennett

Paul M. Winistorfer✳

Oriented strandboard (OSB) is oneof the many engineered wood products

that is gaining increased use in both resi-

dential and commercial construction.

Although OSB has been used commer-

cially for over 20 years, there are still

many aspects of thebehaviorand proper -

ties that are not fully understood. For ex-ample,itiswellknownthatdensityvaries

through the thickness of the panel, with

thefaces havinghigherdensitiesthan the

core.Thisenhances theflexuralbehavior

because the denser faces have a greater

stiffness, creating a product that is analo-

gous 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

ofOSBwillenhance thefurtherdevelop-ment andefficient useof this engineered

product. The objective of this work is to

builda relationshipthatwould be predic-

tive of engineering flexural properties

from the vertical density profile. This isaccomplished by determining the engi-

neering properties of individual layers of

the OSB.

72 JUNE 2003

The authors are, respectively, Former Graduate Student, Dept. of Civil and EnvironmentalEngineering, 221 Perkins Hall, The Univ. of Tennessee, Knoxville, TN 37996-2010 (cur-rently Structural Designer,BKVGroup, 222 North Second St,Minneapolis, MN 55401); As-sistant Professor, TennesseeForest Products Center, TheUniv. of Tennessee, P.O. Box1071,Knoxville, TN 37901-1071; Professor, Dept. of Civil and Environmental Engineering, TheUniv. of Tennessee; and Former Professor and Director, Tennessee Forest Products Center (currently Professor and Department Head, Dept. of Wood Science and Forest Products, 210CheathamHall, VirginiaTech, Blacksburg,VA 24061-0323).The authors wouldlike to thank colleagues Chris Helton, William W. Moschler, and KenThomasfor 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 2001. Article No. 9350.✳Forest Products Society Member.©Forest Products Society 2003.

Forest Prod. J. 53(6):72-80.

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 ahot-press. Hence,the mechanicalpropertiesshouldvarythroughthe thicknessof thepanel.Todeterminethe variation instrengthand

stiffness through thethicknessof thepanel,a commercial orientedstrandboardwas sawninto 15layers toobtainthin-layer specimens

for tension andcompression testing. Specimenswere obtained bothparallel and perpendicular to the lengthof thepanel. Thespeci-

mens were testedin tension using straight-sided specimensandunbondedtabs. For specimensparallel to the lengthof thepanel, thefacelayershada tensile strengthapproximately anorderofmagnitudegreaterthan thecore.Greater facetensile strength was duetoa

combination of strand orientation and density. An apparatus was designed to test the thin specimens in compression. The average

compressionstrengthwassignificantlyhigherthanaveragetensionstrength.However,the averagecompressionmodulusofelasticity

wassignificantlylower thanaveragetension modulus ofelasticity. Theselayertension andcompressionpropertieswere related to thevertical density profile with high r 2 values (> 0.75), thus indicating that a strong linear relationship exists. Thelayerpropertieswere

used to predict the panel properties.


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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 isthe density, or the vertical density pro-file (VDP) (Strickler 1959). Theverticaldensity profile of wood composites isformed from a combination of actions

thatoccur both during consolidation and also after the press has reached its final

position (Wang and Winistorfer 2000).In order to examine the variation of in-ternal bond (IB) throughthethickness of a panel, Xu and Winistorfer (1995)sawed an OSB specimen into 9 layers,and measured the IB of each layer. Al-though there was a positive correlation

between IB and density, the degree of correlation was small (r 2 between 0.20and 0.25). The lowest IB did not alwaysoccur 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 toobtain specimens for water absorption(WA) tests (Xu et al. 1996). These testsrevealed that WA was positively corre-lated to layer density and layer thicknessswell.

Andrews (1998)determined that therewas a negativecorrelation (r = -0.65)be-tween the location of maximum densityin the tension face layer and the bendingmodulus of elasticity (MOE) of the

panel. As the maximum density locationmoved closer to the panel surface, thestiffness of the panel increased. The lo-cation of the maximum density influ-enced the MOE more than the densityvalue itself. The same relationship wastrue for modulus of rupture (MOR), butthe correlation was lower (r = -0.33).

Geimer et al. (1975) examined the ef-fects 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 thatthere was a nonlinear relationship be-tween the MOE of the face layer and thedensity of the face layer, while there wasa 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 per-cent of the measured stiffness intwo-point loading tests and87 percentof the measured stiffness in single-point

loading tests. Using the density gradientin successive 1/32-inch incrementsalong with the developed nonlinear modulus-density relationships resulted in improved predictions, 92 percent for the two-point loading and 102 percentfor the single-point loading test.

One of the more extensive studies on

engineering properties was conducted by Geimer (1979). He measured ten-sion, compression, and bending MOEand failure stress of full-thicknessflakeboards made with uniform densi-ties throughout their thickness and dif-ferent degrees of flake alignment. Loga-rithmic relationships between stiffness(or strength) and specific gravity and wave speed were developed. Several im-

portant behavioral aspects were deter-mined from this work. The failure stress(or MOR) was highly correlated withthe stiffness. The stiffness of boardswith a density gradient could be pre-dicted to within ±20 percent using thestiffness-density relationship from uni-form density boards. The bending MOR was almost double that of the tensionfailure stress for the same level of stiff-ness.

Carll and Link (1988) studied thelayer behavior of 0.5-inch-thick aspenand Douglas-fir flakeboard panels. The1/8-inch- thick face layers and 1/4-inch-thick core layers were tested intension and compression. A logarithmicrelationship was developed between thetensile or compressive MOE and thespecific gravity and wave speed. Theserelationships were used with the densitymeasured in 6 layers through the thick-ness (10%, 15%, 25%, 25%, 15%, and 10% of the thickness) to predict the

bending MOE. The predicted MOE wasconsistently 10 to15 percent higher thanthe measured MOE.

Grant (1997) examined the effects of strand alignment on the mechanical

properties of OSB. A mathematicalmodel was constructed to describe therelationship 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 alignmenton MOE, but it was found that there wasa marginal return in improvement of themechanical properties above a certainthreshold. As expected, the contributionto strength andstiffness was found to di-

minish from the outer surface to thecore.

Xu conducted a series of theoreticalstudies on the effects of various layer

properties on the MOE of the panel. Xuand Suchsland (1998) concluded in astudy that assumed a uniform verticaldensity profile: 1) the panel MOE wasnot influenced by particle size; 2) theMOE decreases as the averageout-of-plane orientation angle of parti-cles increases; 3) theMOEincreases lin-early with an increase of either board density or compaction ratio (CR); and 4)in-plane orientation improves MOE inthe orientation direction but reducesMOE across the orientation direction,with MOE in the across orientation di-rection leveling off after the percentalignment exceeded approximately 60

percent. Xu (1999)used laminate theoryand simulated linear layer MOE-layer

density relationships to examine the ef -fect of different VDPs on the panelMOE. The analysis showed that theMOE benefitsfromthe high density sur-face layer and increases linearly with anincrease in peak density, but the maxi-mum MOE does not occur when the

peak density is located at the extreme board surface.

These previous studies have exam-ined many different aspects of thethrough-the-thickness behavior of wood composite panels. There has been an at-tempt to relate many of these propertiesto the vertical density profile. The pur-

pose of this study was to determine thethrough-the-thickness tension and com-

pression strength and stiffness of a com-mercial OSB panel and to relate thelayer mechanical properties with thelayer density. A much thinner layer than

previously used was chosen for thisstudy to better definethe variation of theengineering properties through thethickness. The layer properties wereused to predict panel properties.

Experimental methodSpecimen preparation

All specimens were cut from one 4- by 8-foot, 23/32-inch-thick commercialsouthern pine OSB panel. The OSB

panel was production sanded and bonded with a diphenyle-methylenediisocyanate (MDI) resin. Fourteensample sets were cut parallel to facestrand orientation and 14 sample setswere cut perpendicular to face strand

FOREST PRODUCTS JOURNAL Vol. 53, No. 6 73


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orientation. One bending specimen,three tension, and three compressionspecimens were cut from each sampleset, as shownin Figure 1. Bendingspec-imens were 3 by 19-1/4 inches as speci-fied in ASTMD 1037 (1996). The 2- by2-inch specimens were cut from the

bending specimens after testing to mea-sure the vertical density profile. Com-

pression specimens were 1.5 by 4 inchesand tension specimens were 1.5 by 8inches before making thin slices.

To betterdefinethe engineering prop-erties throughthe thickness of the panel,a thin layer was chosen. However, as thelayer gets thinner, it behaves less like ahomogeneous material. We chose to use15 layers through the thickness of the

panel for tension and compression test-ing, resulting in a specimen thickness of 0.047 inch. The final tension and com-

pression specimens were 1 inch wide by8 inches long and 1 inch wide by 4inches long, respectively. The OSB ma-terial was a multi-layered alignment

panel. OSB face layer strands werealigned opposite to the core layer; thosespecimens cut parallel to panel length

produced approximately three speci-mens parallel to face strand alignment,nine specimens perpendicular to facestrand alignment, and three specimens

parallel to face alignment, respectively,through the panel thickness. The oppo-site was true for the specimens cut per-

pendicular to face strand alignment.These layer changes were confirmed byvisual inspection after the thin speci-mens were obtained.

Three full-thickness tension and threefull-thickness compression pieces wererequired for each sample set becauseonly five thin specimens could be ob-tained from each full thickness piece.When the 15 thin specimens are ar -ranged according to their position in thethickness of the board, they reflect the23/32-inch full-thickness board.

A cutterhead consisting of six7-1/4-inch diameter, 0.094-inch-thick,18-tooth carbide-tipped blades with ap-

propriately sized spacers was mounted on an arbor in a milling machine toachieve the desired sample thickness of 0.047 inch. The full-thickness pieceswere initially 1.5 inch wide, so theycould be held in a vice attached to themilling machine table.Themachine wasset to cut to a depth of 1 inch and to feed

74 JUNE 2003

Figure 2. — Cutting diagram for one sample set.

Figure 1. — Detail of offset cuts for tension and compression specimens.


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the samples into the cutterhead at a rateof 1.75 feet per minute.

Only five specimens could be ob-tained from each full-thickness tensionand compression piece. Layers 1, 4, 7,10, and 13 were obtained from the first

piece.Layers 2, 5, 8, 11, and 14 were ob-tained from the second piece. Layers 3,6, 9, 12, and 15 were obtained from thethird piece. This was accomplished bycutting all of the first pieces, offsettingthe blades 0.047 inch into the sample,

and cutting all of the second pieces. Thethird pieces were cut in a similar fash-ion, thus obtaining samples representa-tive of a full-thickness board. Clarifica-tion of the offset specimens is shown inFigure 2.

After all tension and compression pieces were cut through the thickness, a bandsaw wasused to trimoffmost of the1/2-inchgripping edge. An end mill wasused for the final trimming to separate

the thin specimens. In order to preventdamage to the specimens during this

process, plastic spacers were inserted inthe 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 test-ing were not used in this research due tothedifficulty of makingthese specimensfrom the thin OSB slices. Rather,straight-sided specimens with a uniformrectangular cross section were used,which is similar to the way thin polymer matrix composites are tested (ASTM1995). Self-al igning grips withnon-bonded tabs were used to pull thespecimen. 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 specimensfailing within the grips.

The specimens were tested under de-flection control at a rate of 0.020in./min. Strain was measured using anextensometer consisting of two linear variable displacement transducers(LVDTs), one on either side of the speci-men to correct for bending. The gagelength was 4 inches and an average de-flection was used in data analysis.

The MOE of the tension andcompres-sion specimens was obtained from theslope of the stress-strain curve. Somestress-strain curves had a small initialvertical portion before the extensometer

began recording deformation. A mini-mum stress was chosen just above thevertical portion of the stress-straincurve. The beginning slope value wascalculated for the portion of thestress-strain curve between the mini-

mum stress andthe minimum stress plus30 percent of the failure stress. The min-imum stress was incrementally in-creased by 2 percent of the failure stressand a regression calculated the slope of the next 30 percent of the curve. Oncethe upper stress reached the maximumstress, the incremental process stopped.The maximum slope calculated duringthestep-wiseregression was taken as thetension MOE. Thus, the reported ten-sionMOE is the least squares fit over thesteepest 30 percent of the stress-strain

curve. Values of r

2

for the linear regres-sion were typically greater than 0.9.

There were 420 tension specimenscut. While handling and loading sam-

ples in the testing machine, some samples broke, thereby reducing the number of tests. Except for one, all of the sam-

ples that failed in handling were for strands perpendicular to the applied ten-sion. Thus, there may be some small

bias in the tension perpendicular tostrands results. Tension tests were con-ducted on a total of 403 specimens: 206cut perpendicular to face strand orienta-tion and 197 cut parallel.

Compression testing

Published test methods for wood pan-els in compression (ASTM 1994) sug-gest an apparatus that utilizes springsteel to provide the lateral support. Dueto the thin specimens of this study and make-up of OSB, this apparatus was notfeasible because the steel tines could

pierce into the thin specimens or


Table 1. — The effective panel MOE and MOR.

Property No. of specimens tested Mean COV

MOE (psi)

Parallel 14 934,000 0.08

Perpendicular 14 505,000 0.07

MOR (psi)

Parallel 14 5,230 0.18

Perpendicular 14 3,520 0.10

Average density (pcf) 183 41.2 0.03

Table 2. — Layer properties for perpendicular to panel length tension specimens.

Density Strength MOE

Layer Strand

orientation No. of

specimens tested Mean SDa Mean COV Mean COV

- - - - (pcf) - - - - - - - - - - - - - (psi) - - - - - - - - - -

1 Perpendicular 14 52.5 2.50 616 0.37 3.79E + 05 0.36

2 Perpendicular 13 49.1 1.71 522 0.47 2.85E + 05 0.34

3 Perpendicular 13 44.7 1.63 384 0.41 2.38E + 05 0.33

4 Parallel 14 40.3 1.28 616 0.61 5.09E + 05 0.62

5 Parallel 13 37.4 0.69 699 0.41 5.32E + 05 0.366 Parallel 14 36.1 0.37 576 0.33 4.50E + 05 0.31

7 Parallel 14 35.2 0.32 495 0.34 3.82E + 05 0.32

8 Parallel 14 34.7 0.33 446 0.27 3.77E + 05 0.36

9 Parallel 14 34.8 0.29 594 0.42 4.23E + 05 0.29

10 Parallel 14 35.5 0.46 546 0.22 5.33E + 05 0.34

11 Parallel 14 36.9 0.64 672 0.33 4.65E + 05 0.37

12 Parallel 14 39.1 0.85 615 0.59 4.64E + 05 0.43

13 Perpendicular 13 42.5 1.45 484 0.39 2.50E + 05 0.31

14 Perpendicular 14 46.7 1.58 466 0.26 2.49E + 05 0.29

15 Perpendicular 14 50.2 1.16 634 0.41 3.33E + 05 0.25

Average 13.7 41.0 1.02 558 0.39 3.91E + 05 0.35a SD = standard deviation.


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through any voids, which are not un-common 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

highmolecular weight (UHMW)plastic

blocks. UHMW was chosen for its low

coefficient of friction with OSB, which

was measured as 0.25. Preliminary testsconfirmed that there was minimal load transfer between the specimen and lat-eral supports.

Like tension testing, the compressionspecimens were tested under deflectioncontrol at a rate of 0.020 in./min. Sincean extensometercould notbe attached tothe specimen, strain was determined from the testing machine crosshead movement.

Compression tests were conducted ona total of 403 specimens: 209 cut per-

pendicular to face strandorientation and 194 cut parallel. One complete parallelsample set was lost due to an error in thecutting process. Only two other speci-mens were lost in handling. Some of thecompression samples became wedged in betweenthe UHMW blocks after fail-

ure. However, the maximum load taken by the specimen before wedging wasnoted, and the compression MOE wascalculated using the appropriate data.

Other testing

A commercial densitometer (QMSDensity Profile System QDP-01X) wasused to measure the verticaldensity pro-file, with the density being measured at0.02-inch increments through the thick-ness of the specimen. There were 183density profiles determined. Density

points rangingbetween the start and end points for each layer were averaged tofind the average density for each layer.

Results

The effective panel MOE and MOR are given in Table 1. An average verticaldensity profile of the panel is shown inFigure 4. The averagepanel densitywas41.2 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 ten-sion MOE, and average tension strength

by layer for perpendicular and parallelcut specimens. Figure 5 shows typicalfailures for perpendicular (L1 and L15)and parallel (L5 and L9) to panel lengthspecimens. Tables 4 and 5 list the num-

ber of samples tested, average density,average compression MOE, andaveragecompression strength by layer for per-

pendicular and parallel cut specimens.

Discussion

Tension properties

For specimensparallel to the lengthof

the panel, the face layers had a tensilestrength and MOE approximately an or-der of magnitude greater than the core(Table 3). This was due to a combina-tion ofa denser face and the face strands

being oriented parallel to the applied tension. For specimens perpendicular tothe length of the panel, the tensilestrength and MOE were relatively uni-form through the thickness (Table 2).The denser faces, with the strands ori-ented perpendicular to the applied ten-

76 JUNE 2003

Figure 3. — Compression test apparatus.

Table 3. — Layer properties for parallel to panel length tension specimens.


Layer Strand

orientation No. of


- - - - (pcf) - - - - - - - - - - - - - (psi) - - - - - - - - - -

1 Parallel 14 53.6 3.14 2185 0.36 1.08E + 06 0.26

2 Parallel 14 49.5 2.04 1640 0.33 7.87E + 05 0.19

3 Parallel 14 44.3 1.73 730 0.61 4.29E + 05 0.44

4 Perpendicular 14 39.9 1.17 288 0.37 1.42E + 05 0.53

5 Perpendicular 12 37.4 0.58 227 0.22 1.10E + 05 0.45

6 Perpendicular 12 36.3 0.38 166 0.25 9.56E + 04 0.257 Perpendicular 14 35.6 0.30 163 0.32 1.27E + 05 0.46

8 Perpendicular 11 35.1 0.28 187 0.27 1.04E + 05 0.38

9 Perpendicular 14 35.1 0.31 147 0.24 1.16E + 05 0.64

10 Perpendicular 11 35.6 0.43 164 0.35 1.56E + 05 1.22

11 Perpendicular 11 36.7 0.54 259 0.49 1.76E + 05 0.37

12 Perpendicular 14 39.1 0.98 347 0.49 2.86E + 05 0.45

13 Parallel 14 42.7 1.42 807 0.45 5.77E + 05 0.46

14 Parallel 14 47.0 2.01 1568 0.42 8.43E + 05 0.18

15 Parallel 14 51.6 1.69 1992 0.54 9.17E + 05 0.30

Average 13.1 41.3 1.13 725 0.38 3.96E + 05 0.44a SD = standard deviation.


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sion in testing, had approximately the

same strength and MOE as the lessdense core, where the flakes were

aligned with the applied tension. Inspecimens 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 fail-

ure within the strands.

All of the tensile properties had rela-

tively high scatter, as reflected by an av-

erage 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 propertiesas reflected bymuch lower COVs for the

bending specimens.

Compression properties

For specimens parallel to the length of the panel, the face layers hada compres-sive strength and MOE approximatelyan order of magnitude greater than thecore (Table 5), which was similar to thetension strength and MOE. For perpen-dicular to panel length samples, the

compression strength and MOE wererelatively uniform through the thickness(Table 4). Typically, failure occurred inthe compression specimens as a result of strands sliding over one another. The av-erage COV in compression propertieswas 36 percent.

Thecompressive MOEwas about half the tensile MOE, averaging 0.46 of thetensile MOE for specimens with theload parallel to the strands and 0.49 of the tensile MOE for specimens with theload perpendicular to the strands. Carlland Link (1988) also found the com-

pressive MOE to be less than the tensileMOE for OSB, although the differencewas smaller. They reported the fullthickness compressive MOE of aspenand 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 tensilestrength. The compressive strength aver-aged 1.52 times the tensile strength for specimens with the load parallel to thestrands, although the outer two face lay-ers had essentially the same tensile and compressive strengths. The compres-sion strength averaged 1.99 times thetensile strength for specimens with theload perpendicular to thestrands. This isopposite of the findings of Geimer (1979) who found the compressivestrength of Douglas-fir OSB panels to

be approximately 80 percent of the ten-sile strength. We do not have a good ex-

planation for this, although we note thatourcompression testing methoddidpro-vide full lateral supportof thespecimen.

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 perpendic-

ular and parallel strand orientation. Thetension/compression MOE and strengthvalues were plotted versusdensity (Figs.6 and 7) and a linear regression was ob-tained with a corresponding r 2 value(Table 6). The r 2 values are high values(reasonably close to 1), thus the datahave a positive linear relationship withthe high density values producing thehigher property values. A log transformmodel was also investigated since it has

been used by others(Geimer 1979, Carll


Figure 4. — Average vertical density profile of OSB panel.

Table 4. — Layer properties for perpendicular to panel length compression specimens.


Layer Strand

orientation No. of


- - - - (pcf) - - - - - - - - - - - - - (psi) - - - - - - - - - -

1 Perpendicular 14 52.4 2.25 860 0.46 1.98E + 05 0.37

2 Perpendicular 14 49.0 1.61 1013 0.32 1.53E + 05 0.22

3 Perpendicular 14 44.7 1.57 893 0.36 1.44E + 05 0.384 Parallel 14 40.2 1.17 1129 0.43 2.29E + 05 0.43

5 Parallel 14 37.4 0.66 1106 0.33 2.27E + 05 0.24

6 Parallel 14 36.0 0.36 895 0.38 2.08E + 05 0.34

7 Parallel 14 35.2 0.32 849 0.36 1.83E + 05 0.30

8 Parallel 14 34.7 0.33 764 0.36 1.93E + 05 0.30

9 Parallel 14 34.8 0.29 906 0.40 2.08E + 05 0.26

10 Parallel 14 35.4 0.47 1127 0.41 2.48E + 05 0.41

11 Parallel 14 36.9 0.62 983 0.32 2.20E + 05 0.27

12 Parallel 14 39.1 0.84 837 0.40 1.65E + 05 0.38

13 Perpendicular 13 42.5 1.49 923 0.50 1.40E + 05 0.38

14 Perpendicular 14 46.7 1.52 922 0.29 1.53E + 05 0.24

15 Perpendicular 14 50.3 1.09 1024 0.50 1.64E + 05 0.39

Average 13.9 41.0 0.97 949 0.39 1.89E + 05 0.33

a SD = standard deviation.


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and Link 1988). Although both the lin-

ear 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, thelinearmodel was used.

MOE and strength data for tension

parallel to the strand orientation is com-

pared to other models in Figure 8. The

model fromXu(1999) is for aspen OSB.

The results from Geimer (1979) are for

Douglas-fir OSB with both 1.5- and

3-inch-long strands. Geimer’s resultswere obtained based on an estimated strand alignment of 35 percent, whichresults in an estimated sonic velocity ra-tio of 1.59. Geimer’s strength values for 1.5- and 3-inch-long strands are essen-tially the same, so only the 3-inchresultsare plotted.

Several observations can be madefrom Figure 8. Although Geimer(1979)used a log transformmodel, the relation-ship between the mechanical propertiesand density is essentially linear over therange of interest. The slope of theMOE-density relationship and thestrength-density relationship from thiswork is similar to that from Geimer (1979). Geimer’s data shows a muchgreater stiffness and strength than thatmeasured in this work.

Prediction of panel properties

Fundamental engineering mechanicsrelationships can be used to predict boththe stiffness and strength of the panelfrom the layer properties (e.g., Geimer et al. 1975). The predicted panel MOEfrom the layer properties was 61 percentof the measured panel MOE for parallelto panel length, and 47 percent of themeasured panel MOE for perpendicular to panel length. Geimer et al’s (1975)

predicted panel MOE was also less thanmeasured, averaging 87 percent of themeasured panel MOE. Carll and Link

(1988) overpredicted panel MOE by 10to 15 percent.

In an attempt to characterize thethrough-the-thickness mechanical be-havior of OSB, much thinner layers wereused in this study than in previous stud-ies. This probably affected the results.As thin layers are removed from thefull-thickness panel, several of thestrands become severed, with a portiongoing to each adjacent layer, leavingonly partial strands to contribute tostrength and stiffness. If these partialstrands were neglected, only the com-

pletestrands would serveas the effectivethickness of the layer. The averagestrand thickness was approximately half the specimen thickness. Thus, an effec-tive thickness could be considered ashalf the specimen thickness, whichwould explain the low predictions.

The predicted panel strength from thelayer properties was 20 percent of themeasured strength for parallel to panellength and 25 percent of the measured

78 JUNE 2003

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.


Layer Strand

orientation No. of


- - - - (pcf) - - - - - - - - - - - - - (psi) - - - - - - - - - -

1 Parallel 13 53.8 1.89 2606 0.33 5.12E + 05 0.25

2 Parallel 13 49.3 1.92 1835 0.30 3.79E + 05 0.31

3 Parallel 13 44.2 1.61 1128 0.38 2.05E + 05 0.36

4 Perpendicular 13 39.7 1.09 486 0.35 7.29E + 04 0.38

5 Perpendicular 13 37.4 0.56 429 0.39 5.58E + 04 0.28

6 Perpendicular 13 36.3 0.36 345 0.35 4.90E + 04 0.19

7 Perpendicular 12 35.6 0.30 365 0.38 5.13E + 04 0.27

8 Perpendicular 13 35.0 0.28 334 0.66 4.84E + 04 0.38

9 Perpendicular 13 35.0 0.30 367 0.37 5.13E + 04 0.31

10 Perpendicular 13 35.6 0.42 331 0.26 4.34E + 04 0.19

11 Perpendicular 13 36.7 0.54 589 0.58 8.37E + 04 0.54

12 Perpendicular 13 39.0 0.96 753 0.48 1.32E + 05 0.55

13 Parallel 13 42.6 1.37 1609 0.26 2.99E + 05 0.30

14 Parallel 13 46.9 1.93 1877 0.31 3.69E + 05 0.22

15 Parallel 13 51.5 1.42 1814 0.48 3.63E + 05 0.40

Average 12.9 41.2 1.00 991 0.39 1.81E + 05 0.33

a SD = standard deviation.


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strength for perpendicular to panellength. The strength prediction wasabout half as accurate as the stiffness

prediction. This is consistent with thefindings of Geimer (1979), who found that the bending MOR was almost twicethe tension and compression strength.Although we do not have a complete ex-

planation for this, wesuggest that part of the reason is nonlinear behavior near ul-timate. A shift in the neutral axis to-

wards the compression face near ulti-mate would cause an underestimation of the strength using linear elastic theory.

Conclusion

The sawing technique, as described inthis study, was successfully used to pre-

pare individual 0.047-inch-thick layersthrough the thickness of a commercialOSB panel. These layers provided ten-sion/compression strength and stiffnessvalues. For specimens parallel to thelength of the panel, the face layers had tensile and compressive properties ap-

proximately an order of magnitudegreater than the core. For perpendicular samples, the tensile and compression

properties were relatively uniformthrough the thickness. This behavior was due to a combination of strand ori-entation and density. The average com-

pression strength was signif icantlyhigher than average tension strength.However, the average compressionMOE was significantly lower than aver-age tension MOE. These layer tensionandcompression properties were related to the vertical density profile with highr

2 values (>0.75), thus indicating astrong linear relationship exists. Thelayer properties were used to predict

panel bending properties.

This study enhances the understand-ing of the mechanical behavior of OSB

panels. Coupling this understanding of how the vertical density profile affectsthe through-the-thickness mechanical

properties with research on the effects of the VDP on other parameters (such as


Figure 6. — Regression relationship between compression and tension MOE vs.

layer density.

Figure 7.— Regressionrelationship between compressionand tension strength vs.

layer density.

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 = 13500 (DEN) - 351600 0.804 STR = 27.14 (DEN) - 778 0.929

Parallel MOE = 30220 (DEN) - 662200 0.824 STR = 84.84 (DEN) - 2554 0.890

Compression

Perpendicular MOE = 8220 (DEN) - 234900 0.880 STR = 40.39 (DEN) - 1016 0.797

Parallel MOE = 13230 (DEN) - 278200 0.777 STR = 75.12 (DEN) - 1798 0.845aUnits are psi for modulus of elasticity (MOE) and strength (STR) and pcf for density (DEN).


9/9

IB and thickness swell), the effects of changes in the VDP can be ascertained.The information can be used to enhancemanufacturing and the development of optimal vertical density profiles.

Literature cited

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__________1995.Standardtestmethod for ten-

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Geimer, R.L. 1979. Data basic to the engineer-ing design of reconstituted flakeboard. Proc.13th Inter. Particleboard/Composite Mate-rials Symposium. Washington State Univ.,

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___________, H.M. Montrey, and W.F.Lehmann. 1975. Effects of layer characteris-tics on the properties of three-layer particle-

boards. Forest Prod. J. 25(3):19-29.

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strand alignment distribution on the unidirec-tional bending properties of oriented strand board. Master’s thesis. Laval Univ., Quebec,Canada.

Steidl, C.M. 2000. Layer properties of oriented strandboard. Master’s thesis. The Univ. of Tennessee, Knoxville, TN.

Strickler, M. 1959. Effect of press cycles and moisture content on Douglas-fir flakeboard.Forest Prod. J. 9(7):203-215.

Wang, S. and P.M. Winistorfer. 2000. Funda-mentals of vertical density profile formationin wood composites. Part 2. Methodology of vertical density formation under dynamicconditions. Wood and Fiber Sci. 32(2):220-238.

Xu, W. 1999. Influence of vertical density dis-tribution on bending modulus of elasticity of wood compositepanels: A theoretical consid-eration. Wood and Fiber Sci. 31(3):277-282.

__________ and O. Suchsland. 1998. Modulusof elasticity of wood composite panels with auniform vertical density profile: A model.Wood and Fiber Sci. 30(3):293-300.

__________and P.M. Winistorfer. 1995. Layer thicknessswell and layer internal bond of me-dium density fiberboard and oriented strand-

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80 JUNE 2003

Figure 8. — Comparison of tension parallel to strand MOE and strength to other

data.

propiedades del osb

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