A TECHNIQUE TO MEASURE STRAIN DISTRIBUTIONSIN SINGLE WOOD PULP FIBERS

Laurence M ottPost-doctoral Resean:h Associate

and

Stephen M. Shaler

and

Leslie H. GroomResearch Te<:hDololistUSDA Forest Service

Southern Research StationPineville. lA 71360

(Received December 1995)

AMTRAcr

Environmental _-!mL'1£ electron microscopy (ESEM) and digital imase correlation (DIC) ~ usedto measure micros1r8in distributions on the surface ofwood pulp fibers. A loadina stage incorporatinga fiber lriPPinl system wu desianed and built by the authon. Fitted to the tensile substaae of anESEM or a Polymer Laboratories MINlMATtester, it provided a reliable fiber straining mechanism.Black spruce lateWood fibers (Picea nwriana (Mill) B.S.P.) of a near-zero microfibril anile displayeda characteristically linear load elonption fonn. ESEM wu able to provide real-time, hi&h mapificationimages of strainina fibers, crack growth, and complex sinale fiber failure mechanisms. Digital imagesof sinale fiben ~ also captured and used for subsequent DIC-based strain analysis. Surface dis-placement and strain maps revealed nonunifonn strain distributions in seemingly defect-free fiberregions. Applied teDsile displacements resulted in a strain band phenomenon. Peak strain (concen-tration) values within the bands ranIed from O.9CMI to 8.8%. It is hypothesized that this commonpattern is due to a combination offacton including the action ofmicrocompressive defects and straininaof amorphous cell-wall polymeric components. Strain concentrations also corresponded well to lo-cations of obvious strain risers such as visible cell-wall defects. Results suaest that the ESEM-buedDIC system is a useful and accurate method to assess and, for the fint time, measure fiber micro-mechanical properties.

Keywords: Fibers, micromechanics, strain, digital imase correlation, tensile testing, environmentalV~!11'Jne electron microscopy.

INTR.ODUcnON controls the mechanical properties of individ-ual wood pulp fibers (Page et al. 1977). By thesame technique, fiber defects have also beenshown to influence fiber properties includingmaximum strain potential and fiber failure

Single fiber tensile tests conducted under p0-larized light confirmed that the microfibril an-gle of the S2 secondary cell-wall layer largelyWood IIIId Fiber~. 28(4), 1996, pp. 429-437C 1 996 by die Society ol W God Scj-. T ecb8oI-.J

Associate Professor

Department of Forest ManagementUniversity of Maine

Orono, ME 04469-5755

WOOD AND FIBEa K3ENa, ~ 1996, v. 28(4)430

mechanisms. However, prosress with regardto characterizinl mechanical properties of re-cycled fibers (Wuu et aI. 1991) and other fibercharacteristics, such as fracture criteria, is sloW;and no empirical data exist that can quantifysuch basic relationships as (collapsed) fiber cell-wall Poisson ratios. Evidence of individual de-fect impact on the mechanical response of thecell wall is lacking, and at present statisticaldescriptions of fiber defect/length relation-ships represent the extent of fiber microme-cbanics understandina (Page and El-Hosseiny1976). The objective of this study was to de-velop a sinale fiber tensile testina techniquecapable of extracting micro mechanical prop-erty data from reliable single fiber tensile tests.Ofparticular interest was the ability to observefracture sequences in sinale fibers and measuremicrostrain distributions surrounding naturaland processing-induced defects.

FIO. I. The experimental let-Up for ESEM-baIed dia-ital imap: ~latiOD (DIC) evaluatiOD of IiDIle fiber mi-crostraiDI aDd failure. It 00DIiIts of. miaoeXteDlOmetrylocated in an ESEM. Sinale fibers are secured by . free-fiber-aljpmeDt b8Jl and lOCket uambly. Dilitally cap-tured i~ of teDsi1e teIU ~ poned to . workstatiODfor analysis by DIC via. PC-baled fi'amearabber.

MATERIALS AND METHODOlOGY

Sinale fibers were observed under load usingenvironmental scanning electron microscopy(ESEM). The ESEM is similar to conventionalscanning electron microscopes (SEM) in thathigh m.pification and ~ depth offield im-ages can provide insight into material char-acteristics. However, the unique design of theESEM column and secondary electron detectorpennit the specimen chamber to operate underincreased pressW'eS of S- 20 Torr (Cameron andDonald 1994). Water vapor is often used toprovide the chamber atmosphere, in which caseit is possible to imaac biological specimenswithout subjectina them to potentially hannfuldehydration processes or conductive coatin&-A chamber water vapor pressure of approxi-mately 6 Torr was used in this study. Undersuch conditions single wood pulp fibers wereexamined in the ESEM in their near naturalstate. Additional information on this instru-meat is available in the literature (Cameronand Donald 1994; Sheehan and Scriven 1991).

ESEM observation of single fiber testing

In an effort to establish the reliability of me-chanical property data obtained from the de-

veloped free-fiber-alianment gripping mecha-nism, various single fiber types were tested us-ing the loading stage in combination with theESEM and a Polymer Laboratories MINI-MAT tensile tester. The fiber griPPina systemwas based upon a ball and socket mechanism(FJI. I). Epoxy droplets applied close to thefiber ends prior to testing provided the balljoints. Although this assembly has been prov-en to reduce fiber failure at the ariPS by up to40% (Mott 1995), it does not completely re-move the influence that gripping mechanismshave upon stress and strain distributions with-in the fiber. For the purposes of this study,however, s~ strain was most frequentlymeasured close to the fiber midspan, and acommon fiber length to width ratio was ap-proximately 30: 1. For such geometries, evenin bjply anisotropic plates where the averagefibril angle approaches 15°, the effect of grip-ping on strain component distribution at mid-span is negligible (WU and Thomas 1968). Theeffect is further redu(:Cd in near-zero microfi-bril angle fibers because the collapsed fiber isnot a single orthotropic plate. Instead. the op-posite walls of the fiber restrict the shear de-formation that results from gripping, which

431Mort et aJ.-MEASURING STRAIN IN SINGLE WOOD PULP FIBERS

frequently leads to normal stress-strain non-uniformity .

More details on this technique and prelim-inary findings of this study, which includeESEM failure and crack initiation sequencesand a critical assessment of fiber macrome-chanical property data, are reported elsewhere(Mott 1995; Mott et al. 1995; Groom et al.1995). This paper describes a micromechani-cal investigation of fiber cell-wall material us-ing digital image correlation.

Single fibers were introduced into the ESEMspecimen chamber under an ambient equilib-rium moisture content. A carefully controlled(vacuum) pump-down schedule for the ESEMchamber was used to maintain fibers in theirnatural hydrated state (Cameron and Donald1994). The specimen chamber pump-down se-quence was characterized by an 8-stage watervapor flooding cycle while simultaneously p0-sitioning the fiber out of the beam path to re-duce the potential for radiation damage. Thefinal ESEM operating parameters for observ-ing single fibers were a chamber pressure of 5to 6.3 Torr and a substage temperature ofap-proximately 21 OC. Under these carefully mon-itored environmental conditions, a single fiberequilibrium moisture content of approximate-ly 8.5% was calculated using the Carrier equa-tion (Siau 1984).

The wet bulb depression temperature (a re-quired variable in the Carrier equation) underestablished ESEM test conditions was deter-mined by first obtaining the corresponding dewpoint temperature using a Peltier cooling sub-stage. Corresponding dry bulb and the dewpoint temperatures were then used to find theequivalent wet bulb depression temperature(120C) with the aid of a psychrometric chart(Siau 1984). A condenser lens strength of 56-60% (instrument setting) and an acceleratingvoltage of 15 ke V were used when examiningthe fibers.

kraft pulping the 55th growth ring of a maturetree to a 47% yield (kappa number 56). Duringpreparation, single fibers were also flattenedand dried into ribbon shapes prior to tensiletesting. More information concerned with fi-ber drying, moisture content equilibration, andadhesive application is reported in Mott (1995)and Mott et al. (1995). Fiber drying and flat-tening facilitate easy handling and permittreatment of the fiber as an orthotropic bilam-inate material (page et al. 1977). Flattened fi-bers also simulated collapsed single fiber ge-ometries in paper sheets and other biocom-posites.

Digitally captured (640 x 480 pixel) imagesof fibers under a uniaxial tensile displacementwere used to assess surface microstrains. Theexperimental setup is presented in Fig. 1. Ob-taining suitably high signal to noise ratio im-ages of straining fibers required that a slowelectron beam scan rate of 4.3 s/frame be em-ployed. This precluded any further need forimage averaging, which is often necessary whenemploying DIC analysis (James et al. 1989).Following completion of the pump-down andflooding procedure, single fibers were assumedto be under the calculated equilibrium con-ditions (this process can take up to 2 min). Thetensile substage was then moved into the beampath. A low magnification (x 300) and an in-creased scan rate (2.3 frames/s) were selectedtogether with an increased working distance ofapproximately 10 rom. This procedure wasadopted to reduce the potentially damagingeffects of the electron beam. Despite visibleimage noise, fiber surface features such as pit-fields were plainly evident, and a fiber gaugelength was accurately measured using residentESEM software (:t 1 Ilm).

A suitable area over which strain analysiswas to be performed was quickly identified,and the necessary ESEM parameters were ad-justed to obtain optimum image quality. Afinal magnification of approximately 2,500 wasusually selected prior to commencing tensiledisplacement at a rate of 1 Ilm/S (console con-trolled). Optimum brightness, contrast levels,beam and condenser strengths were achieved

Digitally captured ESEM images for DIC

For surface-microstrain assessment purpos-es, single black spruce (Picea mariana (Mill)B.S.P.) latewood fibers were obtained by batch

WOOD AND FIBER ~CE. 0eID'-1996. V. 28(4)432

0 '93 \43 \86

lime (-)FIG. 2. Cbarac;teristic shape of a sinale fiber load elon-

gation curve. Strainins of the fiber is suspended prior tocapturins a dilital imaae to obtain distortion-free i~Elapsed time between stoppiq and resuming strainina ofthe fiber is approximately 4.S s or I frame.

FIG. 3. A sinI1e black spruce latewood fiber (low fibrilanile) strained to failure. The brash fracture surface ischaracteristic of fibers with a slow 52 layer microfibril

anile.

Following image acquisition, each fiber wasalso strained to failure in the ESEM. This re-quired subjecting each fiber to approximately15-20 s more beam time. Fracture surfaceswere then used to assess whether beam damagehad significantly affected mechanical proper-ties. The black spruce fibers used in this studydemonstrated a brash fracture surface, as iscommon with mature fibers of low microfibrilangle (Fig. 3). The fibers were also prone tocracking and failure at locations other thanregions examined by ESEM. These facts pro-vided an initial confinnation that beam-relat-ed radiation damage was not significantly af-fecting the mechanical response of the fibercell-wall material. Average strain to failure for218 black spruce latewood fibers as deter-mined by the MINIMA T testing facility wasapproximately 4.25%.

manually and then held constant throughoutthe testing procedure.

Measurement of the microstrain field dis-tributions using DIC required obtaining tWosuccessive digital ESEM images over an ap-proximate 5-s displacement (or loading) inter-val. These images were designated an imagepair (Fig. 2). Prior to obtaining any images forDIC, each fiber was loaded so as to achieve alinear load elongation trace. Extension was thensuspended and an initial image capfilred to aremote PC resident framegrabber (approxi-mately 1/30 s capture time). Suspending thetensile test is essential when employing slowscan rates in dynamic microscopy. Failure todo so results in time-delay distorted imageswhere pixels in the lower portion of an imageare temporally distorted due to the fact thatthey are representative of a specimen that hasbeen subject to a proportionally greater strain.

After capturing the initial image, displace-ment was resumed employing a cross-head ex-tension rate of I lI.m/s for approximately 5 s.Prior to capturing a second image, extensionwas again suspended. The acquired image pairwas then used to perform a DIC analysis offiber surface displacements resulting from boththe 5-lI.m fiber extension and the small irreg-ular (purely) translational movements of thetensile substage.

Principles of DIC

Surface displacements of fiben were deter-mined using a digital image correlation (DIC)technique. Digital image pairs were analyzedusing a DIC program written to utilize a SiliconGraphics workstation (Mott 1995). Bi-cubicsplines were used to generate continuous sur-face intensity fields from the discrete digital

433Molt et Q/.-MEASURING STRAIN IN SINGLE WOOD PULP fIBERS

FIG. 4. A diaitally captured image of a single woodfiber. The fiber was slowly dried between glass slides toexploit the phenomenon of anisotropic shrinkaae. The re-sultina shrinkaae patterns created sufficient image contrastto perlnit subsequent DIC assessment of surface displace-

ments.

function tolerance values. A total of 36 cor-relation analyses were performed, each con-sisting of the same 441 subimage locations.Generated response surface models indicatedan image subregion area of approximately 292pixels, provided a near minimum correlationerror with reasonable computation time. Inaddition, the area exhibited a unique surfacepattern but was small enough to encompassapproximate uniform strains in the x,y plane.an assumption of the DIC technique.

DIC accuracy and calibration

To obtain the necessary sub-pixel accuracy,a continuous intensity function bi-cubic inter-polation routine was applied to respectiveESEM image pairs. This made it feasible tomeasure intensity values between discrete pix-el locations and to measure respective dis-placements with a sub-pixel precision of:t 0.1pixel. At a magnification of2,SOO this was equalto approximately :t 0.008 I!m. This value wasdetermined by a rigid body translation tech-nique, conducted digitally and then mechan-ically inside the ESEM chamber.

itnages. The intensity field of the first imageof a pair was warped using an affine mappingfunction. The difference in intensity values be-tween the warped first image and the unwarpedsecond image were then calculated, squared,and summed over a defined subregion (Chu etat. 1985). This error was minimized throughan iterative process of choosing the six affinewarping function coefficients using a Powellalgorithm (Press et al. 1990). The warpingfunction coefficients resulting in minimum er-ror (maximum correlation) then define themovement of a central (control) point of thedefined subregion. This process is repeated fora range of control points. The resulting whole-field displacement maps are then used to cal-culate strains using large-strain definitions ofstrain displacement equations (Fung 1965).

The scale of a random surface body pattern,either artificially applied or natural, dictatesthe density of chosen displacement measure-ment (control) points and the required size ofextracted subregions. To measure microstraindistributions on a single fiber surface frequent-ly required choosing control points that weresubstantially less than IlJ.m apart (single fiberswere approximately 15--30 IJ.m wide when flat-tened into the familiar ribbon shape). An ar-tificially applied surface pattern would there-fore have to consist of extremely small parti-cles. Such a pattern could not be produced atthe time of this study. Instead, single fiberswere dried in a manner that utilized the effectsof anisotropic fiber shrinkage. This createdsuitable surface patterns in the form of finesurface creases and wrinkles (Koran 1974). Theshrinkage creases (Fig. 4) were randomly dis-tributed, produced acceptable ESEM contrastlevels to promote accurate correlation, and wereofa scale fine enough to permit the use of smallimage subregions and the close positioning of

chosen correlation (control) points.DIC was performed using extracted image

subregions. The optimum size was determinedby performing multiple correlations on singleimage pairs. A range of subregion sizes from(7 x 7) 72 to 372 pixels were chosen over arange of intensity difference cross-correlation

WOOD AND FIBER SCIENCE, 0I:tcItIw 1996. V. 21(4)434

~ "at~..t 1~~I4~J

9 14 .~,t

1 1 3 4 5 . 7~ 8

t..~

-

a)-I . 648 pixels

Flo. S. Approximate position of the 19(14 x 14pixel)data sets to determine mean displacement tolerance val-

ues.

b)

FIG. 6. A 3-0 view of the u-and v-microextensomeuydisplacement components. a) shows the u-component vec-tor of (transverse) fiber displacement; b) displays the lon-litudinal) displacement component coincident to the di.rection of applied strain (fiber longitudinal axis). Kraftpulped black SpnlCe 4711& yield. The arrows represent thedirection of applied displacement. Displacement tolerance- 0.1 pixel.

as 3-D surface maps. Examples of u and vdisplacement component vectors for a blackspruce late-wood fiber are presented in Fig. 6.The u-component of displacement representspassive deformation in the fiber transverse axisresulting from an applied tensile displacementin the fiber longitudinal axis (v). Displacementvector v represents the longitudinal responseof the cell wall to same applied tensile dis-

placement.

Digital warping of a single duplicated imagewas performed by means of a Silicon Graphicslibrary routine. This provided evidence of theDIC program's ability to determine displace-ments in a rapid and efficient fashion. Appliedstrains in the x,y plane were recovered with noreported error. This was to be expected in suchnoise-free images. True noise-influenced (dis-placement) measurement precision was estab-lished using mechanically translated ESEMimages. Translation was conducted by movingthe tensile substage small amounts in the x,yplane. Mean tolerances in .the x and y axes werethen determined by running 19 sets of corre-lation arrays per image pair (4 image pairs).Each set consisted of a regular 14 x 14 pixelarray (Fig. 5). Analyses of variances (ANOV A)and subsequent multiple comparison of m~stests (Tukey's multiple range test) indicatedthat no statistically significant differences ex-isted between the reported mean values for uand v displacement between any of the chosen19 data sets (P = 0.00). It was concluded, there-fore, that the system was not affected by spa-tially nonrandom noise. This was confirmedin subsequent tests using the same 19 data setlocations applied to translations of varyingmagnitude. Mean u and v standard deviationwere reported as tolerance value of 0.08 pixelsand 0.1 pixels, respectively.

To visualize single fiber surface displace-ment due to microextensometry-induced de-formation, the u and v (horizontal and verti-cal) components of displacement were plotted

Determination of strains

Surface strain values were detennined byfirst calculating the change in distance (in pixelunits) between respective neighboring subre-

435AI«t" IIl.-MEASURINO STRAIN IN SINGLE WOOD PULP FIBERS

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0

of

]t:'-'£a

1>- 150~

t r..:.

~t ..CJ.

> 0.000>0.013> 0.026> 0.048

/(~

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(~

lion locations resulting from tensile displace-ment. The Lagrangian strain tensor definitionwas then used to obtain normal strains in thex and y directions. The system dependent sub-pixel precision level (:f: 0.1 pixel) required thatthe control points of subregions have a 20pixel spacing in order to obtain a strain mea-surement precision of :f: 0.5% or better. Toobtain a strain precision of :f: I % required asubimage spacing of only 10 pixels (0.77 Jlmat a 2,500 magnification).

Figure 7 displays rendered strain maps forsingle black spruce latewood fiber. Accompa-nying displacement surfaces are displayed inFig. 6. The fiber contained a number of mi-crocompressive kinks or creases resulting froma modified preparation technique whereby twofibers were dried, one bridging the other. Straincomponents ~ and En indicated that a creasedfiber cell wall results in large strain variations.A rational explanation of strain nonuniformityis offered.

rr210 3S0

X coordinate(fiber transverse axis)

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,-'"

o~~

of

.8t:---

fu>-

.(].< o. ()()()< - 0.013< - 0.023

Fiber strain distributions

The ESEM-based DIC system was success-fully used for calculating the strain distribu-tions of six defect-free black spruce latewoodfibers under small applied strains of 0.3% to0.5%. Component Eyy indicated that strainin&in the direction of loadine is distinctly non-uniform. Banded regions of high strain con-centrations (0.9% to 8.8%), similar in characterto Luders' bands which form in some metals(Illston et al. 1987), developed in the fiber wallunder uniaxial tensile displacements. Eu wasmore uniform although strain concentrationsof a lesser magnitude were evident. Eu was alsolargely negative, providing confirmatory evi-dence of a Poisson effect. These patterns werecommon to all defect-free fibers of black sprucelatewood with a low microfibril angle tested inthis preliminary study, the results of which arediscussed in more detail in other sources (Mott1995; Mott et al. 1995; Groom et al. 1995).

A ~nable exp~tion for strain non-uniformity was thought to be the manifesta-tion of microcompressive defect "pull-out."

X coordinate(fiber transverse ax.is)

FIG. 7. Quantitative characterization of a) Iyy and b)Iu in a micro-cJased relion of a wood fiber ceO waD. The~~~ linea ~ut tbe approximate locations of theceO-wall defect crases. Applied strain - O.S3~. Initialsubimaae spacina - 13 pixels. Strain tolerance - O. 7~. Adistinct Poiuon effect wu noticeable in aU sinate fiberstrain mapa.

Microcompressive pull-out occurs as smallcrimps, creases, and kinks are removed fromthe cell wall under increasing tensile strain.Evidence of the hi&h strain potential in mi-crocompressively dama!~ regions is also

436 WOOD AND FIBD. s::IENa, 0cI0bw 1996, V. 21(4)

confirmed premature yielding of this typewould have serious implications with regardto damage accumulation in single fibers subjectto low repetitive or cyclic loading. The prob-able and potentially catastrophic prematureplastic deformation of the cell wall in this studyoccun"ed in sinale fibers that were subject toapplied strains of I % or less.

found when examining a fiber load elongationcurve, which is frequently irregular and shal-low in initial stales as the integrated effects ofmicrocompressive pull-out take effect (p.and Seth 1980). A further potential cause ofthe strain band effect in the yy axis that cannotbe discounted is the periodic and natural oc-currence of amorphous cellulose regions andthe presence of interstitial lignin or hemicel-lulose polymer phases of the cell-wall com-posite structure.

An initial assumption of DIC theory is thatstrain within any chosen subregion is uniform(Chu et al. 1985). This assumption was deemedvalid when testing defect-free and minimallydefected fiben. However, highly localizedplastic deformations, such as those which oc-cur around the tip of a growing crack in ductilematerials, are problematic and would be dif-ficult to measure unless small enough subre-gion areas were attainable. ESEM video mi-croextensometry indicated that single fibersfrequently display what can be considered aductile crackina mechanism, characterized bya blunt crack tip. These cracks are commonlyseen to extend from pit apertures and pit bor-ders under increasing tensile displacemenLStrain map pairs were rendered for several fi-ben that included a visible bordered piL Highstrain concentrations were found to corre-spond with common displacement or load-in-duced crack locations at the pit aperture andat the pit border where the greatest S2 layermicrofibrillar deviation is known to occur.

The strain values close to the pit aperturewere extremely large, however, and althoughrepresentative of regions where high strainconcentrations are likely to occur (they cor-respond to actual crack locations), the valuesare likely in error. The probable source of thiserror is a violation of the DIC assumption thatstrains within subregions be uniform (Chu etal. 1985). The failure of DIC to predict strainvalues in the locality of pits supported the hy-pothesis that IaJ'Ie strain gradients exist in smallregions « l#l.m2) immediately adjacent to theaperture edge and that hi&hly localized plasticdeformation of this region was probable. A

OONa.USIONS

ESEM-based DIC was used to determinesin&1e fiber microstrain distributions. Usingpresent ESEM and DIC capabilities and care-fully controlled fiber preparation techniques,it has been possible for the first time to mea-sure single wood fiber Poisson effecu, to quan-tify natural and induced cell-wall defecu interms of strain potential, and to explore andquantify single fiber failure mechanisms. Ex-pected strain nonuniformity has been mea-sured, and evidence for a quasi-ductile fracturemechanism and of a premature plastic failureregion in the locality of natural defects nowexists. For such analyses, a :to. I pixel dis-placement measurement error and an opti-mfm DIC subimage area of 292 pixels wasadequate.

Smaller DIC measurement error, in the or-der of 0.0 I pixel, now appears feasible usinlimproved DIC algorithms (Lif et al. 1995).Improved computing facilities used in com-bination with such aJaorithms will promotefaster DIC analyses and more p~se data toconfinn the initial findinp of this study. Forthe present, however, ESEM-based DIC pro-vides a powerful new tool to investigate themicromechanical process of fiber deformationand fracture, which together provide the bridgebetween fiber composite structure and mor-pholoay and mechanical properties. Investi-gation of this process is fundamental to theimprovement of biocomposite materials.

AaNOWLEDOMENTS

The authors would like to acknowledge thesuppon of USDA NRI grant 94-37500-1199and Mcln~ Stennis grant MEO9607. In ad-

dition, the authors would like to thank An-nukka Liukkonen and The University of MainePaper Surface Science Program and Christo-pher Paduan for technical support. This isMAFES document No. 2004.

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