How Variability in OSB Mechanical Properties Affects Biological Durability Testing

Transcription

1 8 S.F. Curling et l.: Biologicl Durbility of OSB Holzforschung 57 (2003) 8 12 How Vribility in OSB Mechnicl Properties Affects Biologicl Durbility Testing By Simon F. Curling, Jerrold E. Winndy, Chrles Crll, Jessie A. Micles nd Anton Tenwolde USDA Forest Service, Forest Products Lbortory, Mdison, Wisconsin, USA Keywords Oriented strndbord Decy Modulus of rupture Strength Specimen size Summry Loss in bending strength of wood hs been shown to be more sensitive mesure of decy thn is weight loss. Using modulus of rupture s the decy criterion is problemtic for oriented strndbord (OSB) becuse of vrition in mechnicl properties due to prticle orienttion nd size. Moreover, the smll specimen size required for such tests increses the vrince in mechnicl properties. This study compred the vrince in bending strength of ASTM D1037 stndrd-sized specimens nd smll specimens from two smples of commercil OSB. The smll specimens were found to hve significntly higher level of vrince in bending strength thn the stndrd-sized specimens. A simple method of sorting the specimens bsed on strnd orienttion on the tensile surfce significntly reduced the level of vrince mesured. The effects of differing levels of vrince on the size, design nd limittions of the experimentl study re presented. Introduction The durbility of wood nd wood-bsed products hs trditionlly been evluted using loss in dry weight s the decy criterion. Mny stndrd methods use this criterion, such s ASTM 2017 (ASTM 1998), EN113 (British Stndrds Institution 1981) nd ENV (British Stndrds Institution 1998). However, reserch hs shown tht loss in the bending strength or modulus of rupture (MOR) of wood is more sensitive mesure of decy, prticulrly erly decy, thn is weight loss (Wilcox 1978; Winndy nd Morrell 1993; Kim et l. 1996; Curling et l. 2000). A method hs been developed for evluting the fungl durbility of wood specimens using MOR s the decy criterion (Curling et l. 2000). However, the testing of composite bord mterils with this method hs rised the question of how the vribility of MOR shown by smll test specimens ffects test results. The MOR vlues of composite mterils vry s result of prticle composition, orienttion nd size. These fctors re criticl in strength-bsed decy tests but not in weight-bsed tests. Vrince in bending properties of oriented strndbord (OSB) test specimens is known to be correlted with the size of the wood elements (e.g., strnds) tht constitute the mteril nd to be inversely relted to test specimen size (McNtt nd Superfesky 1984; Geimer et l. 1999). Vribility in OSB strength vlues cn be estimted for ASTM D1037 (ASTM 1996) stndrd-sized mm specimens (McNtt nd Superfesky 1984; McNtt 1984, 1986), but vribility in strength for the smller mm specimens used for decy tests is not known. These specimens my show considerbly greter vrition in strength thn do stndrd-sized specimens, which is likely to be relted to the positioning nd orienttion of strnds in tht portion of the test specimen in which mximum bending moment is developed (Lufenberg 1984; McNtt 1984). When nlyzing chnges in bending strength, vrince in the dt must be tken into ccount. Higher levels of vrince decrese the sensitivity of the experiment by incresing the minimum difference in bending vlues required for the difference to be sttisticlly significnt. The methodology developed for evluting the decy resistnce of solid wood specimens involves the use of mtched wood specimens (Curling et l. 2000). However, for tests of composite bord mterils, there re no recognized mens for mtching test specimens. The number, orienttion nd size of surfce prticles re redily observble nd re fctors known to influence the strength of wood composite bords. We nticipted tht sorting test specimens on the bsis of surfce strnd composition, size nd orienttion might llow for some degree of mtching, thus reducing vrince, lthough it ws unknown to wht extent this would prove vible. Methods Smples of commercil southern pine OSB pnels were obtined from two different suppliers in the Piedmont re of Georgi nd the Mississippi Delt, USA (designted s smples A nd B). The pnels consisted entirely of southern pine strnds. Ech smple consisted of fifteen m pnels. Test specimens were tken from five pnels selected t rndom from ech smple. To remove edge effects, strips (250 mm) were removed from ech (longitudinl nd horizontl) edge. Copyright 2003 Wlter de Gruyter Berlin New York

2 S.F. Curling et l.: Biologicl Durbility of OSB 9 Fourteen strips, lternting between 25 mm (the desired test size) nd 75 mm (the size specified by ASTM D 1037) in width were cut from ech bord prllel to the 2.4 m direction. The strips were cut into 320 mm long specimens. One hundred specimens 25 mm wide nd fifty specimens 75 mm wide from ech smple were then tested using four-point bending test (thirdpoint loding using lod hed spn of 270 mm nd loding rte of 1.25 mm/min) to determine MOR (Winndy nd Morrell 1993; Curling et l. 2000). During testing, specimens were discrded if the filure occurred outside the middle third of the test spn or if the filure ws other thn tensile filure. The 25 mm specimens were visully sorted into six ctegories bsed on strnd distribution nd orienttion on the tension surfce in the middle third of the spn, which corresponds to the re of gretest bending moment. A number of visul chrcteristics were evluted so tht one specimen could be plced in more thn one ctegory. The 75 mm specimens were not ctegorised. Sorting ctegories were bsed on the following criteri: 1) Unligned The mjority of strnds showed no generl or uniform lignment in ny direction. 2) Aligned The mjority of strnds showed generl or uniform lignment in one direction. This group ws further divided into two subctegories: 2) Angled lignment The mjority of strnds were ligned in direction not prllel (i.e., 30 off the long xis) to the length of the specimen. 2b) Prllel lignment The mjority of strnds were ligned prllel (i.e., < 30 off the long xis) to the length of the specimen. 3) Single strnd A single strnd ccounted for the mjority ( 80%) of the re of mximum bending moment. 4) Multiple strnd The re of mximum bending moment contined multiple strnds. Exmples of these ctegories re shown in Figure 1. Anlysis The men bending strength, vrince nd stndrd devition were determined for ech ctegory. To compre test dt with previously published dt, the coefficient of vrition for the specimens ws lso clculted s s Cv = 100 (1) x where Cv = coefficient of vrition, S = stndrd devition of bending strength nd x = men of bending strength. Results Differences between smples The men, stndrd devition nd vrince of MOR for ech smple (A nd B) nd for both 25 nd 75 mm wide specimens re given in Tble 1. The differences in the vrince between the 25 nd 75 mm specimens were tested using n F-test nd found to be sttisticlly significnt t p 0.10 (smple A) nd p 0.05 (smple B). For both smples, the MOR of the 75 mm specimens ws significntly higher (t-test, p 0.05) thn tht of the 25 mm specimens. Effect of sorting Tble 2 shows the distribution of the specimens in the visul ssessment ctegories. The men, stndrd devition, men vrince, nd coefficient of vrition of MOR of the specimens in ech ctegory re shown in Tbles 3 nd 4. For smple A, sttisticl nlysis (F-test) showed tht the vrince in bending strength of the 25 mm speci- Tble 1. Modulus of rupture of OSB specimens Group n Men MOR Stndrd Vrince COV A 25 mm mm B 25 mm mm COV is coefficient of vrition. Tble 2. Distribution of 25 mm wide specimens in visul ssessment ctegories Ctegory Distribution (%) Smple A Smple B Fig. 1. Exmples of sorting ctegories for 25 mm specimens: () unligned, (b) ngled lignment, (c) prllel lignment, (d) single strnd. Specimen () would lso fit into the multiple strnd ctegory. Discrd 16 8 Unligned Aligned Angled Prllel Single strnd Multiple strnd Specimens were discrded when filure during the bending test occurred outside the re of mximum bending moment. Dt from these specimens were not used in the nlysis.

3 10 S.F. Curling et l.: Biologicl Durbility of OSB mens in the multiple strnd ctegory ws significntly (p 0.05) lower thn tht of specimens in ll other ctegories with the exception of the prllel-ligned ctego- Tble 3. Modulus of rupture of 25 mm specimens in smple A Ctegory Men MOR Stndrd Vrince COV Unsorted Unligned Aligned Angled Prllel Single strnd Multiple strnd Tble 4. Modulus of rupture of 25 mm specimens in smple B Ctegory Men MOR Stndrd Vrince COV Unsorted Unligned Aligned Angled Prllel Single strnd Multiple strnd Tble 5. Level of significnce of differences in vrince between sorted 25 mm nd unsorted 75 mm specimens Ctegory Significnce of difference b Smple A Smple B Unsorted Unligned NS 0.05 Aligned Angled Prllel NS 0.05 Single strnd Multiple strnd NS 0.05 F-test t p b Levels of significnce were p 0.10 nd p NS desig ntes not significnt. ry. Bending strength of the prllel-ligned specimens ws significntly lower thn tht of specimens in the unsorted nd ngled-ligned ctegories. For smple B, sttisticl nlysis (F-test) showed no sttisticlly significnt (p 0.1) differences in the vrince of the bending strengths of the sorted 25 mm specimens. Comprison of 75 mm nd 25 mm specimens Multiple F-test nlyses were performed to determine the significnce of differences in the vrince of bending strengths of the 25 nd 75 mm specimens; the results re shown in Tble 5. The vrince in bending strength of the unsorted 25 mm specimens ws significntly higher thn tht of the 75 mm specimens. Therefore, ny result of no significnt difference between ny 25 mm ctegory nd the 75 mm specimens indictes tht sorting into tht ctegory reduced the vrince in bending strength of those specimens in vlue equl to tht of the 75 mm specimens. Comprison of test results with published dt To determine whether the vrince in bending strength resulting from our nlysis is comprble with tht mesured in other studies, the coefficients of vrition of the dt were compred to those of previously published dt (Tble 6). Discussion Effect of sorting The MOR of the 75 mm wide specimens ws significntly (p 0.05) higher thn tht of the 25 mm wide specimens. This my be due to the lrger number of strnds in the wider specimens, which cused lod shring effect within the re of gretest bending moment. The dt lso show significnt (p 0.05) difference in vrince in MOR between the smples. For both specimen widths, greter vrince occurred in smple A compred with smple B. It is lso pprent tht using unsorted 25 mm specimens incresed the level of vrince. The differences in vrince between the 25 nd Tble 6. Comprison of coefficients of vrition of test dt nd previously published dt Source Mteril Test Coefficient of vrition for type two sizes of specimens 75 mm 25 mm Current study Commercil OSB, A 4 point Current study Commercil OSB, B 4 point McNtt (1984) Commercil OSB 3 point McNtt et l. (1990) Commercil OSB 3 point McNtt et l. (1990) Commercil OSB 3 point McNtt et l. (1992) Lb OSB 3 point McNtt et l. (1992) Lb OSB 3 point 18.60

4 S.F. Curling et l.: Biologicl Durbility of OSB mm specimens were significnt t p 0.05 for smple B but only t p 0.1 for smple A. In this study, the coefficients of vrition (COVs) for the 75 mm specimens from smple A fell within the rnge shown in previously published dt (Tble 6). However, the COVs of smple B were considerbly lower thn tht previously reported. In comprison, the COVs of the 25 mm specimens in smple A were t the higher end of the previously published rnge nd those of smple B were in the middle of the rnge. Sorting hd vrying effects on specimen vrince. For smple A, sorting specimens into the prllel-ligned nd multiple strnd ctegories significntly reduced vrince compred to tht of unsorted specimens. In ddition, sorting specimens into these ctegories reduced the vrince such tht the difference between these specimens nd the 75 mm specimens ws no longer significnt. When compring COVs, it is pprent tht sorting smple A into the prllel-ligned nd multiple strnd ctegories lowered the clculted vrince into the middle of the rnge of vrince vlues determined from previous dt. For smple B, where the initil vrince ws significntly lower thn tht of smple A (nd tht in other published studies), sorting lowered the vrince but not to ny significnt level. In ddition, for ll ctegories of sorted specimens in smple B, the vrince ws still significntly (p 0.05) higher thn tht clculted for the unsorted 75 mm specimens in this smple. Therefore, where the mteril hd higher levels of vrince (e.g., smple A), sorting reduced tht vrince to cceptble levels, llowing the use of the smll (25 mm wide) specimens. This sorting method would be pproprite for studies evluting the comprtive effect of some tretment or process on mechnicl properties. However, such method using smll specimens would not be pproprite in studies intended to determine the bsic mechnicl properties of pnels. Influence on experimentl design The level of vrince in experimentlly derived dt cn ffect the size nd complexity of the study. One importnt fctor is the number of replictes required; enough replictes must be used to mke the results sttisticlly vlid, but the study must lso be kept prcticl. The number of smples required to sttisticlly differentite between results cn be obtined on the bsis of the stndrd devition of the men MOR of the specimens. Eqution (2) (Snedecor 1961) is the formul for clculting the number of smples required to get the Tble 7. Number of replictes required to differentite loss in MOR in smple A Specimen Men SD Replictes required to differentite stted MOR loss 10% 20% 25% 30% 75 mm Unsorted 25 mm Sorted 25 mm Unligned Aligned Angled Prllel Single strnd Multiple strnd Tble 8. Number of replictes required to differentite loss in MOR in smple B Specimen Men SD Replictes required to differentite stted MOR loss 10% 20% 25% 30% 75 mm Unsorted 25 mm Sorted 25 mm Unligned Aligned Angled Prllel Single strnd Multiple strnd

5 12 S.F. Curling et l.: Biologicl Durbility of OSB desired level of differentition t confidence level of 95%: 4σ 2 n = (2) L 2 where n = number of replictes, σ = stndrd devition nd L = llowble error. For exmple, to detect difference of 10% between control nd test specimens, the mximum llowble error is 5% vrition bout the men for both the control nd test specimens. Combined together, this gives n error of 10%, below which no sttisticl significnce cn be ttched. As the minimum level of chnge to be detected increses, the number of replictes increses. Tbles 7 nd 8 show the number of replictes (both control nd test) required to sttisticlly detect the given chnge in MOR using the levels of vrince previously described. For both smples, the number of replictes required for detecting 10% chnge in MOR is too lrge to be prcticl. The number of replictes needed to detect 20% nd 30% chnges in MOR is more prcticble. Tble 7 further shows the benefit of sorting the smples into the prllel-ligned or multiple strnd ctegories becuse of the reduction in number of replictes nd/or increse in sttisticl detection level. Agin, however, sorting hd no beneficil effect on smple B (Tble 8) s result of the very low vrince of the 75 mm specimens. Conclusions The use of 25 mm wide specimens with the four-point bending test resulted in greter vrince in modulus of rupture (MOR) thn did the use of stndrd 75 mm wide specimens. Where the vrince ws high (smple A), sorting the 25 mm specimens into the multiple strnd or prllel-ligned ctegory lowered their vrince to tht of the stndrd specimens, n cceptble level of vrince. Although this ws not the cse for smple B, the levels of vrince for the 25 mm specimens were within the rnge of vlues previously reported for 75 mm specimens nd could therefore be considered cceptble. The dt lso show tht sorting to reduce vrince in the bending strengths of specimens llows detection of smller chnges in bending strength. As the level of vrince of bord cnnot be judged without testing, sorting is recommended becuse it reduces vrince when it is likely to be high enough to be confounding fctor. Acknowledgments This project ws prtilly funded through co-opertive greement with the Prtnership for Advncing Technology in Housing (PATH) nd the U.S. Deprtment of Housing nd Urbn Development. References ASTM ASTM D 1037 (96). Stndrd test methods for evluting properties of wood-bsed fibre nd prticlebord mterils. Annul book of ASTM stndrds, vol , Americn Society for Testing nd Mterils, West Conshohocken, PA. ASTM ASTM D Stndrd method of ccelerted lbortory test of nturl decy resistnce of woods. Annul book of ASTM stndrds, vol , Americn Society for Testing nd Mterils, West Conshohocken, PA. British Stndrds Institution Stndrd EN113. Determintion of toxic vlues of wood preservtives ginst wood destroying bsidiomycetes cultured on gr medium. British Stndrds Institution, London, Gret Britin. British Stndrds Institution Drft Europen Stndrd ENV (1998) Pnel products: Method of test for determining the resistnce of pnel products mde of or contining wood, ginst wood destroying bsidiomycetes. British Stndrds Institution, London, Gret Britin. Curling, S.F., J.E. Winndy nd C.A. Clusen Experimentl method to simulte incipient decy of wood by bsidiomycete fungi. Int. Res. Group on Wood Pres., Doc. No. IRG/WP/ Geimer, R.L., J.W. Evns nd D. Setibudi Flke furnish chrcteristion: Modelling bord properties with geometric descriptors. U.S. Deprtment of Agriculture, Forest Service, Forest Products Lbortory, Mdison, WI, Res. Pp. FPL-RP 577. Kim, G., W. Jee nd J. R Reduction in mechnicl properties of rdit pine wood ssocited with incipient brownrot decy. Mokche Konghk 24, Lufenberg, T.L Flkebord frcture surfce observtions nd correltion with orthotropic filure criteri. J. Inst. Wood Sci.10, McNtt, J.D Test methods for bsic properties of woodbsed pnels: Pst experience nd tody s needs. Proceedings, 18 th Int. Prticlebord/Composite Mterils Symp., Mrch 27 29, Wshington Stte University, Pullmn, WA, USA. pp McNtt, J.D Tension, compression, nd sher properties of wferbord from smll-specimen tests. Forest Prod. J. 36, McNtt, J.D. nd M.J. Superfesky How some test vribles ffect bending tension nd compression vlues for prticle pnel products. U.S. Deprtment of Agriculture, Forest Service, Forest Products Lbortory, Mdison, WI, Res. Pp. FPL-RP 446. Snedecor, G.W Sttisticl Methods. Iow Stte University Press, Ames, IA, 593 pp. Wilcox, W.W Review of literture on the effects of erly stges of decy on wood strength.wood nd Fiber 9, Winndy, J.E. nd J.J. Morrell Reltionship between incipient decy, strength nd chemicl composition of Dougls-fir hertwood. Wood nd Fiber Sci. 25, Received June 18 th 2001 Simon F. Curling Jerrold E. Winndy Chrles Crll Jessie A. Micles Anton Tenwolde USDA Forest Service Forest Products Lbortory One Gifford Pinchot Drive Mdison, WI USA