An alternative method to quantitatively determine the wood failure percentage on shear test specimens was developed, experimentally tested and successfully applied to specimens with typical combinations of adhesive types and wood species used by the glulam industry in Switzerland. The method consists of astaining technique for abetter differentiation of wood fibers and adhesive, and an image processing procedure for astandardized estimation of the fractured surface ratio covered by wood fibers and adhesive, respectively. The semi-automatic method allows for amore objective determination of wood failure percentage and is suitable for quality control in the glulam industr

Künniger, Tina

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DOI 10.1007/s00107-008-0236-6K U R Z O R I G I N A L I A · B R I E F O R I G I N A L SHolz Roh Werkst (2008) 66: 229–232A semi-automatic method to determine the wood failure percentageon shear test specimensTina Ku¨nnigerPublished online: 12 April 2008© Springer-Verlag 2008Subject An alternative method to quantitatively determinethe wood failure percentage on shear test specimens was de-veloped, experimentally tested and successfully applied tospecimens with typical combinations of adhesive types andwood species used by the glulam industry in Switzerland.The method consists of a staining technique for a betterdifferentiation of wood fibers and adhesive, and an imageprocessing procedure for a standardized estimation of thefractured surface ratio covered by wood fibers and adhesive,respectively. The semi-automatic method allows for a moreobjective determination of wood failure percentage and issuitable for quality control in the glulam industry.1 IntroductionTo provide an adequate bonding quality of their products,glulam manufacturers have to perform shear tests on speci-mens which are cut out of beams of their running produc-tion. Shear strength is measured and wood fiber percentage(graded as 0% to 100% wood failure) is estimated to confirmthe bonding quality according to EN 386. A major problemto estimate the wood fiber percentage is the difficult per-ceptibility of adhesive on the fractured surface especially iftransparent adhesives like 1P-PUR or MUF are used. Crafts-men report that a deviant estimation of more than 20% woodfailure is not uncommon. Glulam industry requires a stan-dardized method for estimating the wood failure percentagewhich is inexpensive to install and simple to apply.T. Ku¨nniger ()Wood Laboratory, Empa, Swiss Federal Laboratories forMaterials Testing and Research,Ueberlandstrasse 129,8600 Duebendorf, Switzerlande-mail: tina.kuenniger@empa.ch2 Materials and methodsBased on a literature review on contrast staining forwood adhesive identification (Dibuz and Shelton 1967,Gibson and Krahmer 1980, Ginzel and Stegmann 1970,Paulitsch 1986, Plath 1970, Schriever 1981, Zeppenfeld andGrunwald 2005) several staining techniques were selectedand tested on over 450 shear test specimens which were pro-duced with different wood-adhesive combinations: Norwayspruce (Picea abies), European larch (Larix decidua) andash (Fraxinus excelsior); 1P-PUR, MUF, UF, MF, 2P-EPIand PRF.For each combination, two 25 mm thick, planed laminawere spread with adhesive, sandwiched together and pressedunder laboratory conditions. All commercial adhesives areclassified as type II adhesives according to EN 301 for usein structural applications. The applied amount of adhesiveand all processing parameters were in accordance with man-ufacturer recommendations. The bonded specimens werestored for at least 7 days at standard conditions (20 ◦C/65%RH) prior to further processing to individual shear testspecimens.The shear strength of glue lines was determined accord-ing to EN 392. Additionally, some previously tested speci-mens were supplied by the Swiss glulam industry for con-trast staining and further assessment.The staining solutions were applied on fracture surfacesof the specimens by brushing or spraying. Stained spe-cimens were stored for approx. 30 minutes at 20 ◦C and50% RH to allow for a reaction of the dye with eitherthe wood or the adhesive matrix to generate a sufficientcolor contrast. The most promising staining technique forevery wood-adhesive combination was selected and thestained specimens were analyzed with the image processingsoftware.13230 Holz Roh Werkst (2008) 66: 229–232DatInf ® Scientific Color (2007) is a commercial imageanalysis software for sizing areas in digital images whichare distinguishable in terms of color. The software runs onstandard-PCs and supports different kinds of image formatse.g. BMP, JPG, and TIF.For comparison of results of the visual rating currentlyapplied by the glulam industry and the method presentedin this paper, both methods were applied on each shear testspecimen. First, the fractured surfaces were scanned andthe images printed. Then, the percentage wood failure wasestimated on the fractured surfaces of the shear test spe-cimens by visual assessment by two people. These resultswere marked on the paper prints. Afterwards, the specimenswere treated with a proper solution for contrast staining.The stained fracture surfaces were scanned again and fi-nally semi-automatically assessed by the image processingsoftware.Statistical analysis was performed to compare the resultsand describe the differences between these two methods. Forevery specimen the difference in results (wood failure per-centage WFP) between visual estimation (old method) andsemi-automatic method (new method) was calculated as:∆WFP = WFPold method −WFPnew method [%] .Statistical analysis was performed for various specimengroups of wood-adhesive combinations.3 Results and discussionFor the tested wood-adhesive combinations the acid-baseindicator (Dibuz and Shelton 1967) and/or the detectionreagent DACA (Paulitsch 1986) induced sufficient colorcontrast between wood and adhesive to allow for the semi-Fig. 1 Program window in DatInf ® Scientific Color; left the two stained parts of a shear test specimen; right the analysed surface with whiteareas indicating wood failure of 55%Abb. 1 Programmfenster in DatInf ® Scientific Color; links die zwei gefa¨rbten Ha¨lften einer Scherprobe; rechts die analysierte Fla¨che, wobeiweiße Bereiche Holzfaserbruch in Ho¨he von 55% abbildenautomatic measurement of the wood failure percentage withthe DatInf ® Scientific Color software (Fig. 1). In combi-nation with 1P-PUR, MUF, MF and 2P-EPI wood specieswith pH < 5.8 (e.g. spruce and larch wood) showed a sat-isfactory contrast in color when stained with the acid-baseindicator (adhesive green; wood orange to dark red), whilewood species with pH > 5.8 (e.g. ash) had to be stainedwith DACA to produce a satisfactory color contrast betweenwood fiber (light green to blue) and adhesive (pink). For UFresin the acid-base indicator was suitable for wood specieswith pH > 5.8 and DACA for wood species with pH < 5.8.The dark brown color of PRF causes a good color contrast ingeneral, which can be further improved by staining the woodwith malachite green.In Fig. 2 an assessment procedure is proposed.On the basis of general experience, the wide range ofshade and pattern on the colored fracture surfaces makesa fully-automatic assessment impossible, thus a plausibilityappraisal of the results is still required.Statistical analysis of all result data showed that the dif-ference between the visual and the semi-automatic methodis dependent on the wood species and the adhesive typeand to a lesser extend on the experience of the evaluatingperson. On average over all test specimens, the wood fail-ure percentage determined with the semi-automatic methodwas slightly lower compared to the results of visual rat-ing (∆WFPold−new +8%). This is explained mainly by thefact that the new method interprets every failure closeto the interface as adhesion failure. The highest differ-ences in results were identified on Norway spruce speci-mens bonded with UF (∆WFPold−new +30.2%; SD±17.3)or MUF (∆WFPold−new + 22.2%; SD ± 19.0) adhesive. Inthose cases the conventional method resulted in much higherpercentages of wood failure, simply because the adhesion13Holz Roh Werkst (2008) 66: 229–232 231Fig. 2 Procedure to determinethe wood failure percentage onshear test specimensAbb. 2 Auswertungsroutine fu¨rdie Bestimmung desFaserbruchanteils an ScherprobenFig. 3 Differences in woodfailure percentage (∆WFP) ofvarious wood-adhesivecombinations (S spruce, L larch;Boxplot: Box Perc. 25/50/75,Whisker Perc. 5/95, × Perc.1/99, – min/max,  mean)Abb. 3 Differenz desHolzfaserbruchanteils (∆WFP)von verschiedenenHolz-Klebstoff-Kombinationen(S Fichte, L La¨rche; Boxplot:Box Perc. 25/50/75, WhiskerPerc. 5/95, × Perc. 1/99, –min/max,  Mittelwert)failure was hardly visible without staining. In contrast, larchspecimens bonded with MUF showed nearly identical re-sults with both methods just as spruce specimens glued withPRF. On specimens bonded with PUR slightly higher woodfailure percentages (∆WFPold−new − 4.5%; SD ± 11.8 forlarch wood; ∆WFPold−new − 7.2%; SD ± 21.3 for spruce)were determined with the new method compared to theresults of conventional visual rating. These results are visu-alized in Fig. 3.In addition to the determined wood failure percentage, inmost cases contrast staining allows a definition of the fail-ure mode, because adhesion failure and cohesion failure areproducing different types of color patterns (mirror-invertedor an irregular color pattern).13232 Holz Roh Werkst (2008) 66: 229–2324 ConclusionThe proposed semi-automatic method can be used alterna-tively for determining wood failure percentage on shear testspecimens.The procedure requires additional effort in regard to ma-nipulation of the specimens and instrumentation includingsoftware, but delivers more objective results than the visualmethod. Additionally, the new method enables a definitionof the failure mode based on the pattern of the coloredfracture surfaces. This could be advantageous for identify-ing sources of existing bonding problems. Finally an elec-tronic documentation of analyzed surfaces and results ob-tained is possible, which is of great convenience for qualitycontrol.In conclusion, the semi-automatic method described herecan be recommended for both the quality control in the glu-lam industry and for research purposes.More detailed information about the new method is givenin Ku¨nniger (2007).Acknowledgement Financial support of ‘Fonds zur Fo¨rderungder Wald- und Holzforschung’, FFWH Nr. 2006.05 is gratefullyacknowledged.ReferencesEN 301 (2001) Adhesives, phenolic and aminoplastics, for load-bearing timber structures – Classification and performance re-quirementsEN 386 (2001) Glued laminated timber – Performance requirementsand minimum production requirementsEN 392 (1995) Glued laminated timber – Shear test of glue linesDibuz JJ, Shelton FJ (1967) Glueline Identification. For Prod J17(10):20–22DatInf ® Scientific Color (2007) www.datinf.deGibson MD, Krahmer RL (1980) Staining to Make Urea-Formalde-hyde Resin Visible on Glued Wood Surfaces. For Prod J30(1):46–48Ginzel W, Stegmann G (1970) Nachtra¨gliche Anfa¨rbung von Harn-stoff-Formaldehydharz-Bindemitteln auf beleimten Holzspa¨nenzur visuellen Beurteilung der Leimverteilung. Holz Roh- Werkst28(8):289–292Ku¨nniger T (2007) Automatische Bestimmung des prozentualenFaserbruchanteils bei der industriellen Klebfestigkeitspru¨fung.FFWH Abschlussbericht. Empa Wood Laboratory, DuebendorfPaulitsch M (1986) Methoden der Spanplattenuntersuchung. Springer-Verlag, Berlin Heidelberg New York TokyoPlath L (1970) Mikroskopische Untersuchung von Holzverleimungen.Adha¨sion 14(9):318–323, 336Schriever E (1981) Einfacher qualitativer Nachweis von Harnstof-formaldehydharzen und Diisocyanaten in Holzwerkstoffen. HolzRoh- Werkst 39:227–229Zeppenfeld G, Grunwald D (2005) Klebstoffe in der Holz- undMo¨belindustrie. 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Asemi-automatic method to determine the wood failure percentage on shear test specimens

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Asemi-automatic method to determine the wood failure percentage on shear test specimens

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