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E N G I N E E R I N G P A P E R T U B E S T O I M P R O V E W I N D I N G P E R F O R M A N C E

O F V A R I O U S M A T E R I A L S

T e rr y G e r h ar d t, S e n i or Re se ar c h Fe l l ow, S on oc o Pr od u c ts Com p an y , M ad i son , W I

Yan p i n g Q i u , S e n i or Pr o fe s s i on a l S c i e n t i s t , S on oc o Pr od u c ts Com p an y , M ad i son , WI

Ch u c k Joh n son , Re se ar c h Assoc i a te , S on oc o Pr od u c ts Com p an y , M ad i son , W I

Dav i d Rh od e s , T e c h n i c a l M an age r , S on oc o Pr od u c ts Com p an y , H ar tsv i l l e , S C

A B S T R A C T

O ve r th e p as t 10 ye ar s , S on oc o h as c on d u c te d fu n d am e n ta l , so l i d m e c h an i c s r e se ar c h

c on c e r n i n g s tr uc tu r al b e h av i o r o f sp i r a l ly wou n d p ap e r tu b e s. T h e s c op e o f thi s p r ogr am h as

inc luded exper imental , numer ical , and analyt ical mechanics approaches as documented in re ferences

1 -7 ) . For r e c e n t n on - l in e ar f in i t e el e m e n t r e se ar ch , we h a ve u se d AB A Q U S an d d e ve l op e d u se r -

def ined mater ia l subroutines . Th ese subroutines feature a propr ietary 3D const i tut ive mod e l for

paperboard. Th e m ode l uses non -l inear stress-s train propert ies of So no co paperboard mea sured in 3

pr inc ip le d irec t ions . A n im po r t a n t r e sea r ch ob je c t i ve i s t o deve l op i n n ova t i ve tube des i g n s tha t

e n a b l e ou r c u s tome r s t o imp r o v e th e i r w i n d i n g o p er a t i o n s

T o ac h i e ve th i s ob je c t i ve , we h ave

d e ve l op e d se ve r a l p a te n te d te s t d e v i c e s th a t m e asu r e tu b e p r op e r t i e s fu n d am e n ta l to wi n d i n g

appl icat ions . Tests to measure core radial s t i f fness on the ins ide and outs ide E c ) with respect to an

external pressure and radial s trength have been deve loped. This paper descr ibes the tes t methods and

p r e se n ts d a ta to ve r i fy m e c h an i c s r e se ar c h f in d i n gs b y wa y o f two c or e ap p l ic a t ion s . T h e se ar e

e xam p l e s o f wh e r e c or e s we r e e n g i n e e r e d u s i n g m e c h an i c s t e c h n o l ogy to i m p r ove wi n d i n g

c ap ab il ity : 1 ) d e ve l op m e n t o f an ex tr e m e l y h i gh E ~ c or e for wi n d i n g l ow fr ic t ion , c oa te d a l u m i n u m ,

an d 2 ) c or e s for wi n d i n g te x t i l e yam s an d p o l ye th y l e n e s tr e tc h f il m b ase d on r ad ia l s t if fn e s s o f

ins ide d iameter .

N T R O D U C T I O N

Son oco p roduces paper tubes for industr ia l pack aging us in g a sp iral w indin g process f igure 1) .

Because paper i s an anisotropic mater ia l , the sp iral winding process y ie lds a general ly anisotropic

structure . To com pl icate m atters , paper tubes are frequently loaded into the non-l inear s tress-s train

region dur ing use . Over the past 10 years , we have deve loped several exper imental , numer ical , and

an a l y ti c a l too l s to a i d e i n th e d e s i gn o f tu b e s to m e e t a wi d e var i e ty o f c u s tom e r r e q u i re m e n ts .

E xp e r i m e n ta l m e c h a n i c s wor k a t S on oc o h as two c on c e n tra t i on s. T h e f ir st is to ge n e ra te

m ate r ia l b e h av i or d a ta to c om p l i m e n t th e d e ve l op m e n t o f c on s t itu t i ve m od e l s . T h e s e c on d i s to

m im ic loading con di t ions tubes are subjec ted to in the fie ld . The overal l goal i s to dev e lop a

capabi l i ty to des ign tubes to meet spec i f ic requirements and to tes t for compl iance wi th those

requirements .

A n u m e r i ca l m e c h an i c s c ap ab i li ty h as b e e n d e v e l op e d for s itu a t ion s w h e r e t e s t in g an d an a l y t ic a l

m o d e l s c an n ot p rov i d e a r eason ab l e f r am e wor k for p r od u c t d e s ign or u n d er s tan d i n g . Pr ob l e m s w i th

c om p l e x b u c k l i n g an d c on tac t h ave b e e n su c c e s s fu l l y so l ve d . Pr opr ie tar y c on s t i tu t i ve m od e l s h ave

b e e n i n c or p orate d in to A B A Q U S to an a l yze th e se p r ob l e m s .

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Severa l ana ly t i ca l mo del s have been deve loped over the year s . Both l inear and non- l inear

mod el s have been produced , inc lud ing a genera l 3D non- l inear paper model . W i th these model s and

ABAQUS, many product conf igura t ions and load ing have been succes s fu l ly modeled .

The ba lance o f th i s paper w i l l d is cus s the app l i ca t ion o f these t echnolog ies and capab i l it i es

th rough two des ign cases .

C O R E S F O R W I N D I N G C O A T E D A L U M I N U M P R O D U C T S

Many researcher s have computed in te rna l s t r es ses in web mater i a l s wound around paper o r

meta l cores. For exam ple Pfe i f f e r (8 ), Yagod a (9 ), Hakie l (10), and Wi l l e t t and Poesch (11) have

cons idered ro ll s cons t ruc ted by cen te r wind ing . These a re typ ica l ly non l inear , one-d imen s iona l

fo rmula t ions in the rad ia l d i r ec t ion o f the ro l l. Roi sum (12) p resen t s an ex ce l l en t r ev iew.

In the pub l i shed wind ing m odel s , core o u t s ide d iam eter s t i ffnes s (E~) i s in t roduced th rough a

boundary condi t ion . Gerhard t and Qiu (4 ) pub l i shed Ee fo r paper tubes . Data i s f rom measurements

wi th s t r a in gauges on th ree- inch d iameter tubes . Analy t i ca l , numer ica l , and exper imenta l r esu l t s in

th i sp ape r show tha t core s t i f fnes s va lues pub l i shed in the l i t e r atu re were incor rec t .

Bern ie Bec ker o f AL CO A presen ted a pape r a t the four th 1WEB (13) descr ib ing h i s e f fo r ts to

so lve wind ing prob lems as soc ia ted wi th two f a i lu re mode s , V-bu ck le and s ag co l l apse (13).

F igures 2a and 2b i l lus tr a t e these f a i lu re modes . Becker , us ing W IN D ER l , es t ab l i shed an

excep t iona l ly h igh Ec per formance r equ i r ement needed to so lve th i s p rob lem. F igures 3 and 4 show

the e f f ec t o f increas ing tube E~ on r ad ia l and t angen t ia l s t res s in the woun d on a luminum . As Be cker

i l lus t rates in f igure 9 (13) , theoret ical ly , the higher tube s t i f fness widens the process window for web

tens ion control .

For thi s wind ing app l i ca t ion , we deve loped and m anufac tured an ex t r em ely s t i f f paper tube to

the des i r ed E l eve l. Tube des ign was accom pl i shed us ing a p ropr ie t a ry mul t i -g rade l inear -e last i c

equa t ion to es t imate tube deformat ions und er ex te rna l p res sure . W i th th is model , Ec va lues o f

var ious cons t ruc tions can be ca lcu la ted . Requi red inpu t s to the model inc lude e l as t i c mod ulus va lues

for the pap er g rades und er co ns idera tion , Qiu , e t a l . (7 ) . Becker descr ibes the r esu l ts o f us ing the new

core in (13).

Large d iameter tubes , 406 .4 mm (16 inch) , a r e used fo r wind ing in the me ta l s indus try . An

issue is that these are too large to tes t us ing our patente d radial crush tes ters (14) . Thus to ver i fy this

des ign methodology , we deve lope d a t es t dev ice to d i r ec tly measure Ec .

E ~ T e s t D e v i c e

The l a rge- sca le t es t dev ice i s p ic tu red in F igure 5 . A tube i s in i t ia l ly mo unted as shown. The

ent i re sect ion then s l ides into an extremely large pressure vessel . Dur ing the tes t , a i r pressure is

increased in increment s to about 125 ps i . Dur ing pres sur i za t ion , the two arms shown ro ta te and

sensor s mo unted on the ends measure ou t s ide d iameter deformat ion . Av erage ou t s ide d iam eter i s

me asured a t s evera l d i f fe ren t p res sure l eve l s .

1WIN DER V4.2, A proprietary winding analysis software package developed by the Web Handling Research Center,

Oklahom a State University.

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T yp ic a l d a t a c o l l e c t e d i s sh o w n in F igu r e 6 . B y d e f in i t i on ( 12 ) , t h e s lop e o f t h is l in e

m u l t ip l i e d b y t u b e ou t s id e d iam e t e r i s t h e E ~ va lu e . N o t e t h e l i n e a r i t y o f h e p r e ss u r e - OD c h a n g e

response. For t h i s t e s t an d t h e ID s t i f f n e s s t e s t t h a t f o l l ow s , w e an a lyz e ave r age d iam e t e r c h an ge s .

D at a f r om b o t h te s t s sh ow a lin e ar r e sp on s e a t l ow p r e s su r e s .

For O D p r e s su r e l oad in g o f t u b e s , th e f ir s t b u c k l in g m od e i s an ova l sh ap e . A s d i sc u s se d in

t h e c om p an ion p ap e r , t h e r ad ia l c r u sh t e s t e r p r oh ib i t s t h i s b u c k l in g . M o r e o ver w h e n u s ed i n t h e

f i e l d t h e w o u n d ma t e r i a l a l s o r e st r a i n s t h i s b u c k l i n g m o d e i n t h e t u be. T h u s , t h e l in e ar r e sp on se

in F igu r e 6 i s c on s i s t e n t w i t h f i e ld l oad in g o f t h e se l ar ge tu b e s . A t t e m p t s t o m e asu r e E f r om s t r a in

gage s a t s e ve r a l loc a t ion s ar ou n d t h e t u b e c i r c u m f e r e n c e w o u ld b e u n su c c e s s f u l . T u b e b u c k l in g

w o u ld b e in ad ve r t e n t ly in t e r p re t e d a s a n on - l in e ar p r e s su r e - O D c h an ge r e sp o n se .

C O R E S F O R W I N D I N G T E X T I L E S

M an y p ap e r t u b e s ar e u se d t o w in d syn t h e t i c t e x t i l e yar n s su c h as p o lye s t e r , n y lon an d lyc r a TM

w h i c h a r e p r o d u c e d a t h i g h - s p e e d s ( u p t o 7 0 0 0 m / m i n ) . T h e s e y a m s a re w o u n d u n d e r t e n s i o n a n d

m o s t e x h i b i t s o m e r e c o v e r y a f t er w i n d i n g . I t i s n o t u n u s u a l f o r a 3 0 - p o u n d p a c k a g e o f p o l y e st e r t o

w in d f or 16 h ou r s . T h e w in d e r s w i l l ge n e r a l ly h ave a s in g l e s t a t ion th a t c an ac c om m od a t e t h re e or

f ou r p ac k age s . A so l id s t e e l w in d in g m an d r e l w i t h e x t e n s ib l e e la s t om e r gr ip p e r t in gs su p p or t an d

h o ld s t h e c or e d u r in g w in d in g b y th e in s id e d iam e t e r . T h e in s id e d iam e t e r ( ID ) an d t h e w in d in g

m an d r e l d iam e t e r ge n e r a l ly d i ff e r b y on e t o t w o m i l l im e t e r s . T h i s s e t o f p r oc e s s c on d i t ion s d r ive s

t w o p r im ar y c or e r e q u ir e m e n t s . F i r s tly , a c or e m u s t h ave e n o u gh r ad ia l s t r en gt h t o w i t h s t an d th e

p r e s su r e o f w o u n d yam ove r t im e . S e c on d ly , t h e c or e ID s t i f fn e s s m u s t b e su f f i c i e n t t o m a in t a in an

ID lar ge r th an t h e w in d in g m an d r e l .

D e f o r m a t i o n i n a P a p e r T u b e

U n d e r e x t e m al p r e s su r e l oad in g , d e f or m at ion s in p ap e r t u b e s ar e q u i t e d i f f e r e n t t h an t h ose f ou n d

in i so t r op ic or e ve n t r an sve r se ly or t h o t r op ic m at e r ia l s . G e r h ar d t ( 2 ) d i s c u sse s t h e se d i f f e r e n c e s an d

p r ov id e s a t h e or e t i c a l f r am e w or k t o e s t im at e d e f or m at ion s in p ap e r t u b e s . In t h i s p ap e r , d a t a f r om

S tr a in gau ge m e asu r e m e n t s c o n f i r m s t h e e la s t i c it y so lu t ion f or l oad l e ve l s l e s s t h an ab ou t 50% o f

u l t im at e s t r e n gt h . A t yp ic a l d e f or m at ion p a t t e r n i s i l lu s t r a t e d b y t h e d o t t e d l in e s in F igu r e 7 . U n l ik e

an i so t r op ic t u b e , r ad ia l d e f or m at ion on t h e ou t s id e d iam e t e r gr e a t ly e xc e e d s r ad ia l d e f or m at ion on

t h e in s id e d iam e t e r. A s d e sc r ib e d in t h e p ap e r , t h i s is c au se d b y an e x t r e m e ly l ow r a t io o f m od u lu s in

t h e r an d q d i r e c t ion s , r e sp e c t ive ly .

I D S t i ff n e s s T e s t D e v i c e

i

T o m e a su r e ID s t i f fn e s s , S on oc o d e v e lop e d an ac c e s sor y f or t h e r ad ia l c r u sh t e st e r. T h i s sy s t e m

is a l a se r - b ase d d i sp lac e m e n t s e n so r an d as soc ia t e d h ar d w ar e an d so ft w ar e . F igu r e 8 i s a s c h e m at i c

o f t h e b as i c c on f igu r a t ion o f th e ID m e asu r e m e n t sy s t e m . T h e la se r i s r o t a te d a s d i sp lac e m e n t d a t a i s

ac q u ir ed . T yp ic a l d a ta i s sh ow n in F igu r e 9 . Pr e s su r e i s p lo t t e d on th e Y a x i s an d in s id e d iam e t e r

c h an ge , or ID c om e d ow n , i s p lo t t e d on t h e X ax i s . D at a f r om 5 r e p l i c a t ion s i s sh ow n .

R ad ia l c r u sh s t r e n gt h o f t h i s p ar t i c u lar t u b e w as ab ou t 800 p s i . N ot e t h a t u p u n t i l ab ou t h a l f

the fa i lure load , pressure-deformat ion response i s fa ir ly l inear . The s traight l ine represents the s lope

o f t h i s l in e ar p or t ion o f th e c u r ve . W e d e f in e t h e ID s t i f fn e s s o f t h i s tu b e a s t h e s lop e o f th i s l in e , or

a p p r o x i m a t e l y 8 5 , 0 0 0 ( p s i/ i n ).

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Mo dulus v alues have b een m easured for recycled paperboa rd in al l di rect ions (Qiu, et al. , 7) .

Using these values , tes ts have conf i rmed that the elas t icity solution (2) provides rel iable ID s t i ffness

es t imates . W e mea sured tube diameter changes at several text i le ma nufactur ing operat ions . By

combining des ign technology and f ield measurements , special tubes were des igned for specif ic yam

appl icat ions . The issue of creep/mois ture content is again impor tant as, for some yams, w inding

t imes can be as long as 18 hours .

W e der ived a propr ietary, elas t icity solut ion for mul t i -grade tubes . Based on this mo del and

exper iments , an opt imized, mul t i -grade s tructure was discovered. As descr ibed in Patent 5,505,395

(15), the const ruct ion features a w eaker grade located in the center of the tube wal l .

I D S t if fn e s s E f f e c t o f M o i s t u r e C o n t e n t

The relat ionship between tube ins ide diameter s t i f fness and mois ture content i s shown in Figure

10. In this study, four different tube types were tested. ID stiffness measurements were conducted at

four di f ferent mois ture content levels . In analys is of the data, a regress ion curve was determ ined for

each tu be type. Each data set was then norm al ized to yield an ID s t i f fness of 1.0 at a mo is ture content

of 8.5 . Note the dramat ic imp act of mois ture content on this perform ance at t ribute.

S U M M A R Y

Over the pas t 10 years , researchers at Sonoco have conducted fundamental , sol id mechanics

research conceming s t ructural behavior of spi ral ly wound, paper tubes . The scope of this program

has included exper imental , numerical , and analyt ical mec hanics approaches . As descr ibed in this

paper , a por t ion of this research has been publ ished whi le many detai ls have remained propr ietary.

This paper descr ibes several tes t devices and des ign methods that measure tube proper ty data

fundam ental to winding appl icat ions . Such data ver i f ies me chan ics-based des ign methods developed

for var ious f ield appl ications . W e also describe f ield examples w here pape r tubes were successful ly

engineered us ing mechanics technology to improve winding capabi l i ty. These include a high

s t i f fness tube des igned for winding low f r ict ion, coated aluminum and tubes des igned for winding

text i le yam s based on s t if fness of ins ide diameter .

A C K N O W L E D G E M E N T S

The author s would l ike to thank Sonoco ' s s enior management for cont inuous funding ofa

mec hanics research program over the pas t 13 years . These individuals include Char l ie Coker , Peter

Brown ing, Tom Coxe, Harr is DeLoach , Trent Hi l l , Russel l King, and Ear l Norm an. Other

individuals that have contr ibuted great ly to the success of this program include Lind a Hi l l, W im v an

de Camp , Wil l iam Blakeney, Bi l l Mil ler , Mike Schock, Way ne Honeycut t , and Ray Hayes . Final ly,

Kath y Tutcher capa bly typed this manuscr ipt .

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R E F E R E N C E S

1. Bank, L. C. , T ~ D. G erhardt , and J . H. Gordis Dynamic Mech anical Propert ies o f Spiral ly

Wou n d Pap e r T u b e s .

A SM E ou r na l o f V ib r a ti on S t r ess an d Rel i ab i l i t y in D esign

Vol .

111, p . 489-490, 1989.

2 . G erhar dt , T . D. Ex ter na l Pressure Load ing of Spiral Paper Tubes: Theory and

Experiment.

3 . Bank, L. C. , E . Cofie , T . D. G erhardt A N ew Test M ethod for the Determ ination of the

Fl e xu r a l M od u l u s o f S p i r a l l y Wou n d Pap e r T u b e s .

ASM E J o u r n a l o f E n g in eer i n g

M ater i a ls and T echno logy

Vo l . 114, p . 84-8 9, 1 992.

4 . G e r h ar d t , T . D . , Y . P . Q i u Pap e r T u b e De for m at i on s Du r i n g W i n d i n g Pr oc e sse s . AS M E

Proceedings: M echan ics of Ce l lu los ic Mater ia ls , An ahe im , CA, p . 1-6 , N ov . 8-13 , 199 2.

5 . Gerh ardt , T . D. , J . F. Staples , R. G. Lucas Vibrat ional Characteri s tics o f W oun d Paper

Rol l s : Exper iment and Theory. Tapp i ou r na l Vol . 76 , p . 121 -128 , 199 3 .0

6 . G e r h ar d t , T . D . , R . W. K e am s Rad i a l Cr u sh T ak e s th e M e asu r e o f a Cor e .

Conver t ing

Vo l . 12 , No . 2 , p . 50-52, 1994.

7 . Qiu , Y. P. , M . Mi l lan , C. H. Lin , T . D. G erhardt Nonl inear Properties of High Strength

Pap e rb oard s . AS M E Pr oc e e d in gs : M e c h an i c s o f Ce l l u l os ic M ate ri al s - 1997 , AM D-V ol .

221, M D -V ol . 77 , Northwestern Univers i ty , p . 1-18, June 29 - Ju ly 2 , 1997.

8 . Pfe i f fer, J .D. , Ham ad, W .Y. , Ho w Core St i f fness and Poisso n Ratio A ffec t Energy Balan ce

Ro l l Structu re Formulas . , Presented at the F irst International Co nference on W eb

Handl ing, May 17-22, 1991 Oklahoma State Univers i ty , St i l lwater , Oklahoma.

9 . Yagod a , H .P . , Re so l u t i on o f a Cor e Pr ob l e m in W ou n d Ro l l s .

ASM E Jou r n al o f

A pp l i ed M echan ics

Vol . 47 , p 847 , 1980 .

10. Hak ie l , Z . , Non l inear M ode l for W oun d Rol l Stress , TA PP I o u r n a l Vol 70 (5 ) , p . 113 ,

1987.

11 . W i ll e t t, M .S . an d Poe sc h , De te r m i n i n g th e S tr es s Di s tr ib u t i on s i n W ou n d Ro l l s o f

M agn e t i c T ap e Us i n g a Non l i n e ar F i n ite Di f fe r e n c e Ap p r oac h .

ASM E J ou r n al o f

A pp l i ed M echan ics Vol 55 , p 365 , 1988 .

12 . Ro i su m , D .R. , T h e M e asu r e m e n t o f We b S tr e s se s Du r i n g Ro l l W i n d in g . Ph .D. T h e s is ,

Oklahoma State Univers i ty , St i l lwater , Oklahoma, 351 p

13 . B e c k e r , B . J. A S ys te m s Ap p r oac h to Re d u c i n g W i n d i n g De fe c t s a t Al c oa -W ar r i ck

Operations . Proceed ings of the Fourth International Con ference on W eb Handl ing,

Ok lahom a State Un ivers i ty , p . 10 2-114 , June 1 - Jun 4 1997.

14. Ro wland s , R .E. , Sal ik l is , E .P. , W ise , J .T., Gerhardt , T .D . , App aratus and M ethod for

Test ing Tubular Products. , Patent num ber 5 ,33 9,69 3, 1994.

15. Qiu , Y.P. , Gerhard t , T .D . , Mult igrade Paperboard W inding Cores for Y am s and Fi lms

Having Enhanced Res is tance to Ins ide Diameter Reduction . , Patent number 5 ,505,395,

1996.

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V Buckle

Collapse

Sag

Collapse

Figure 1 . Schem atic of paper tube wind ing

Figure 2a & b. Sag Collapse and V-buckle .

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- 2 0 0 0 . 0 0

- 3 0 0 0 . 0 0

- 4 0 0 0 . 0 0

- 5 0 0 0 . 0 0

- 6 0 0 0 . 0 0

7 0 0 0 . 0 0

8 0 0 0 . 0 0

t I I t t I I I I I I I I I I I 1 I I

I t ) t O t O t o t O t o t o t o t o

CO CO OO OO O OO OO OO QO O

- . e - - T a n g e n t i a l P r e s s

E c = 4 5 , 2 0 0 p s i

. o - - T a n g e n t i a l P r e s s

E c = 7 1 , 0 0 0 p s i

C o i l r a d i u s

F i g u r e 4 T a n g e n t i a l s t r e s s at t h e a t t h e i n s i d e d i a m e t e r

f o r c o r e s w i t h d i f f e r e n t s t i f f n e s s v a l u e s [ 1 3 ]

o5 a5

7 9


8/10

60

50

40

3

o

20

10

F i g ure 5 Ec m ea s u rem en t dev i ce

0 0 002 0 00 4 0 00 6 0 008 0 01 0 012

O D C h a n g e i n )

Figure 6 Typ ical E~ test results

_ m

f v

I .

0 014

0 016

8 0


9/10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . ' .

J

. . ' '.

..

F i g u r e 7 R a d i a l P r e s s u r e L o a d i n g a n d A s s o c i a t e d D e f o r m a t i o n

Stepper

Motor

x , ,

Laser

Displacement

Sensor

F i g u r e 8 I D M e a s u r e m e n t S y s t e m

8 1


10/10

1000 t

900

I D = 9 4 r a m ( 3 . 7 " )

C} .

:3

t / )

==

0 .

800

700

600

500

400

300

200

100

t = 0 . 3 0 0 "

~ ~ ~ d ~ l. r u st

L _ K , I D S t i f f n e s s K = 8 5 2 9 0 ( p s i /i n . )

0 -

' 1 '

0 . 0 0 0 0 . 0 0 5 0 . 0 1 0 0 . 0 1 5 0 . 0 2 0 0 . 0 2 5

I D C o m e d o w n ( in . )

F i g u r e 9 A p p l i e d R a d i a l P r e s s u r e v s I D

1.6

A 1 .4 -

t J

S=

.~. 1 . 2 -

(~ .

0 1 . 0 -

, 0 .8 -

, = =

, i . I

0 . 6 -

0 . 4

4

V

I I I I I

6 8 10 12 14 18

M C ( )

F i g u r e 10 I D S t i f fn e s s v s C o r e M o i s t u r e C o n t e n t

8