a comparison of static bending, compression and tension

8/11/2019 A Comparison of Static Bending, Compression and Tension

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Wood Science and Technology Vo]. 7 (1973) p. 241--250

9 by Springer-Verlag 1973

A C o m p a r i so n o f S t a ti c B e n d i n g , C o m p r e s si o n a n d T e n s i o n

P a r a l l e l to G r a i n a n d T o u g h n e s s P r o p e r t ie s o f C o m p r e s si o n W o o d

a n d N o r m a l W o o d o f a G i a n t S e q u o i a

B y R . A . C 0 C ~ : r ~ E L L a n d R M . K ~ U D S 0 ? ~

University of CMifornia, Forest Products Laboratory, Richmond, Cal.

Summary

Compression wood (CW) of the giant sequoia studied had higher values than normal wood

(NW) in crushing strength and ultimate stress in tension parallel to grain, in toughness, in

modulus of rupture, and in work to maximum load and total work in static bending. In the

green condition CW had higher values than NW in stress at the proportional limit and work

to the proportionM limit, and about the same modulus of elasticity in static bending. In the

dry condition CW was about equivalent to NW in work to the proportional limit, but was

sIfghtly weaker fn s~ress at propor~fon~,l limi~ and modulus of ela.stfef~y ~ s~atie bending.

The compression wood of this giant sequoia, even though formed when the tree was suppressed

and having relatively narrow rings, can therefore be said to be essentially equivalent to

normal wood so far as the mechanical properties tested in this study are concerned.

ntroduct ion

In the course of obtaining wood test material for a mechanical properties stud y

[Cockrell et a l 1971], a second-growth giant sequoia tree Sequoiagigantea (Lindl.)

Decn.) was found leaning against another tree as a result of stream-bank under-

cutting. Because youn g giant sequoia trees usually are straight-growing and have

well-developed buttressed bases, they are seldom found in the leaning condition.

Therefore, this was considered an unusually good opportunity to obtain com-

pression wood for the species. Upon cutting the sequoia, a well-developed semi-

circle of compression wood (CW) 33 rings wide was found on the low side (Fig. l ) ;

this provided test material for comparing properties of compression wood with

those of normal wood (N-W).

The tree was 15.1 inches in diameter at breast height and 81 feet high, with a

spike (dead) top lodged against the crown of a large ponderosa pine; the main

trunk was straight and was inclined at an angle of 26 degrees with the vertical.

The first live branches were 48 feet above the ground. There were 86 rings at the

stump, which was abo ut 1 foot above ground level. This suggests th at the sequoia

was about 55 years old when it became lodged in the pine, and that it had been

suppressed for the 33-year period, during which time the compression wood was

produced.

Although compression wood is a common growth defect of coniferous trees,

test results have been reported for only a few species, and therefore there are

relatively limited test data to indicate the extent and exact nature of how such

*) Given at FPRS meeting in Dallas, Texas, June 1972


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242 R. A . Coekrell and R. M. Knu dson

Fig. 1. Cro ss section showing zones of compression wood (CW), norm al wood (NW), and

tension-side wood (TSW)

wood d if f er s i n m e c ha n i c a l p r ope r t i e s f r om no r m a l wood . S t ud i es r e po r t e d by

M a r kwa r d t a nd W i l s on [ 1935 ] a nd m or e c om pl e t e s t ud i e s by P i l l ow a nd L uxf o r d

[1937] ind ica te lower s t i f fness and , a l lowing for i t s g r ea te r dens i ty , a genera l

de f i c ie nc y i n m os t o t he r p r o pe r t i e s ; t h i s i s m or e p r ono unc e d i n t he d r y t h a n i n

t he g r e e n cond i t ion . On a ka [ 1949 ], s um m a r i z i ng t he l i t e r a t u r e a nd i nc l ud ing h i s

o w n s t u d y o f Pinus densi/lora wood , s t a t e s t ha t . . . the inc rease ( in s t r ength)

wh i c h a c c om pa n i e s t he de c r e a s e i n m o i s t u r e c on t e n t i s s m a l l e r i n c om pr e s s i on

woo d . P e r e m [ 1960 ] c onc lude s t h a t whe n s t r e ng t h va l ue s a r e c om pa r e d d i r e c t l y

(neglec ting we ight d i f f e r ences ) , compress ion woo d gene ra l ly appe ar s to be s t rong er

t ha n n o r m a l wood . T he s e i nve s t i ga t o r s r e po r t e d t e s t d a t a on s om e or a ll o f c e r t a i n

aspec t s o f bending , o f comp ress ion pa ra l le l to gr a in , o f t ens io n pa ra l le l to gr a in ,

a nd o f t oughne s s . T he p r e s e n t s t ud y wa s de s i gned t o c om pa r e i n s om e w ha t m or e

de t a i l t he s a m e p r ope r t i e s i n the c om pr e s s i on wood a nd no r m a l wood o f t h i s

pa r t i c u l a r g i a n t s e quo i a . F i b r i l o r i e n t a t i on a nd s h r i nka ge c ha r a c t e r is t i c s w il l be

de a l t w i t h i l l a s ubs e que n t s t udy .

T wo 4 - f oo t bo l t s we r e c u t f r om t h e t r e e , w i t h t he l a r ge e nds 8 a nd 9,0 f e e t a bo ve

t he s t um p . T he l ar ge - a nd s m M l - e nd d i a m e t e r s i n s ide ba r k we r e r e s pe c t i ve l y

11 . 7 - - 11 .2 a nd 10 . 5 - -10 . 2 i nc hes . C om pr e s s i on wood , no r m a l wood , a nd t e ns ion -


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Comparison of properMes of normal and compression wood of a giant sequoia 243

side wood TSW) test blanks ap prox ima tely 11 1 } inches in cross section were

cut from the different zones of the cross section as depicted in Fig. 1. Bending

tests were performed on 1 1 16-inch specimens and compression parallel to

grain tests were mad e on 1 1 4-inch specimens, all in accordance with t he

ASTM 143-52 secon dary method. Tension parallel to grain specimens had an

effective cross sect ion of 0.375 0.1875 3/s 8/1G) inches; toughness specimens

were 0.79 0.79 11 inches in leng th; both these test s followed the ASTM 143-52

pri mar y method . Specific gra vit y values were based on green volume and oven-

dry weight of 1 1 4-inch specimens.

Fibril angle measurements were made by means of a microscope having a

rotating stage and an ocular fitted with cross hairs; 8 blocks of CW, 7 of NW,

and 5 of TSW were sectioned with 20 measurements made on each section.

Compression wood fibril orien tatio n of the S 2 layer was clearly revealed by the

striations and elongate pit apertures in the traehei d walls of stained radial sections.

NormM and tension-side wood fibril orientation was observed on unstained radial

sections in which iodine crystals were formed by treating with chlorine water and

soaking in IKI solution and mounting on a microslide in 40 per cent nitric acid

[CockrelI 1946].

e s u l t s a n d d i s c u s s i o n

Table 1 presents dat a on rings per inch and fibril angle. Table 2 gives dat a on

moisture content, specific gravity, crushing strength and tension parallel to grain,

and toughness. Table 3 contains data on static bending tests. Table 4 shows

specific strength indices.

Table 1. Rings per inch and latewood fibril angle

Rings per inch Fibril angle

of latewood

standard

number* deviation degrees

Compression 8)

wood 24 3 21

Normal 4)

wood 8 -- 2--4

Tension side 9)

wood 36 9 2

* Numbers in parentheses refer to number

o

specimens used for

each value.

Growth r ings

The pattern of growth-ring width can be seen in Fig. 1, and

average num ber of rings per inch is indicated in Table 1. Normal-wood test

specimens were taken from the outer part of the 46- and 37-ring central cores.

The compression wood and corresponding rings on the tension side were quite

narrow, probably as a result of the suppressed growth condition and loss of the

top. Because the zone of narrow-r inged wood on the tension side was only 0.625

inches wide at t he ce nter Fig. 1) and 0.81 inches wide at its wi des t- -ne ar th e

17a WoodScience and Technology,Vol. 7


4/10

244 R A Cockrell and R M Knudson

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O 5"q ~O "~ gq

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C o m p a r i so n o f p r o p e r t i es o f n o r m a l a n d c o m p r e s s i o n w o o d o f a g i a n t s e q u o i a 4 5

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9 ~ ,

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6/10

2 4 6 1 . A . C o c k r e l l ~ n d 1 ~, M :. K n u d s 0 n

~9

~ 2 cq cv

O O

~

[-..

9 9 9 O


7/10

Comparison of propert ies of norm al and compression wood of a gian~ sequoia 247

p o i n t o f t r a n s i ti o n f r o m c o m p r e ss io n w o o d - - o n l y t e s t s p e c i m e n s fo r t e n si o n

p a r a l l e l t o g r a i n h a d w o o d a l l o f t h i s c o n d i t i o n . A c c o r d i n g l y , m o s t o f t h e s e s p e c -

i m e n s c o n s i s te d h a l f o f n o r m a l w o o d a n d h a l f o f n a r r o w - r i n g e d w o o d .

Moisture content

G r e e n m o i s t u r e c o n t e n t ( T a b l e 2 ) v a r i e d i n v e r s e l y a s th e

s p e c if ic g r a v i t y ; t h i s i s s im i l a r t o d a t a o b t a i n e d b y P i l lo w a n d L u x f o r d [1 93 7].

E q u i l i b r i u m m o i s t u r e c o n t e n t o f c o m p r e s s i o n w o o d w a s s l i g h t l y h i g h e r t h a n

n o r m a l w o o d , a n d t h i s a l so c o n f o r m s t o t h e i r f in d i n g s.

Strength comparison C o m p a r i so n o f v a l u e s ( T a b l e 2 ) f o r m a x i m u m c r u s h i n g

s t r e n g t h i n c o m p r e s s io n p a r a l le l t o g r a i n r e v e a ls c o m p r e s si o n w o o d s t r o n g e s t a n d

t e n s io n - s i d e w o o d s p e c im e n s w e a k e s t in b o t h g r e e n a n d d r y c o n d i t io n s . E x a m -

i n a t i o n o f s p e c i fi c s t r e n g t h i n d i c e s ( T a b l e 4 ) sh o w s t h a t i n t h e g r e e n c o n d i t i o n

a ll t h r e e a r e e s s e n t ia l l y e q u i v a l e n t , w h e r e a s i n t h e d r y c o n d i t i o n t h e y d i ff e r, w i t h

t h e o r d e r b e i n g i n i n v e r s e r e l a t i o n t o t h e i r s p e c i fi c g r a v i t y .

I n m a x i m u m s t r e s s i n t e n s i o n p a r a l l e l t o g r a i n , C W i s s t r o n g e s t a n d T S W i s

w e a k e s t i n t h e g r e e n c o n d i t i o n , w i t h s p e c i f i c s t r e n g t h i n d i c e s v a r y i n g i n i n v e r s e

o r d er . I n t h e d r y c o n d i ti o n , t h e C W is m u c h s t r o n g e r t h a n t h e o t h e r tw o a n d i ts

s p e ci fi c s t r e n g t h i n d e x is a p p r e c i a b l y h i g h e r t h a n n o r m a l w o o d . T h e s e re s u l ts d o

n o t a g r e e w i t h P i l lo w a n d L u x f o r d [1 9 37 ], w h o r e p o r t e d l o w e r t e n s i o n v a l u e s f o r

C W i n b o t h g r e e n a n d d r y D o u g l a s- fi r a n d o l d -g r o w t h r e d w o o d a n d i n g r e e n

p o n d e r o s a p i n e . P e r e m [ 1 96 0 ] d i d n o t r e p o r t o n t e n s i o n va l u e s. O n a k a [ 1 94 9 ]

s t u d i e d t h e r a t i o b e t w e e n t e n s i l e s t r e n g t h a n d m a x i m u m c r u s h i n g s t r e n g t h a n d

g a v e a v e r a g e v a l u e s f o r C W a s 1 . 9 6 : 1 a n d f o r N W a s 4 .1 9 : 1 i n t h e g r e e n c o n d i -

t i o n ; f o r t h e d r y c o n d i t i o n h e r e p o r t e d 1 .4 2 : 1 f o r C W a n d 1 .5 3 : 1 f o r R T W . O u r

d a t a f o r g r e e n g i v e a v e r a g e v a l u e s f o r C W a s 2 . 3 1 : 1 , a n d f o r N W , 2 . 9 2 : 1 ; f o r

d r y , v a l ue s a r e CW , 2 .45 : 1 and R- W, 1 .76 : 1 .

N e i t h e r o f t h e s e t w o s t u d i e s r e p o r t e d o n s tr e s s a t t h e p r o p o r t i o n a l l i m i t in

t e n s i o n p a r a l le l to g r a i n ( w h ic h i s i n c l u d e d in T a b l e 2 ). E x a m i n a t i o n o f t h e s e

d a t a s h ow s C W m u c h l o w e r i n b o t h g r ee n a n d d r y t es t s th a n N W o r T S W , w i t h

c o r r e s p o n d i n g l y l o w e r sp e c if ic s t r e n g t h i n d ic e s f o r C W . T h u s i t a p p e a r s t h a t i ts

h i g h e r b r e a k i n g s t r e n g t h g i v e s C W m u c h g r e a t e r c a p a c i t y f o r p l a s t i c d e f o r m a t i o n

t h a n N W o r T S W h a v e , a n d t h e g r e a t e r st r e n g th o f C W i n t e r m s o f t o u g h n e ss

s u p p o r t s t h i s o b s e r v a t i o n . U n d o u b t e d l y , t h e p r o n o u n c e d d if f e r e n c e i n f i b ri l

o r i e n t a t i o n i n t h e s e c o n d a r y w a ll ( i n d ic a t e d i n T a b l e 1 ) as w e l l as t h e m a n y f i n e

f i s s u r e s i n t h i s w a l l i n c o n t r a s t t o t h e c o m p a c t w a l l s t r u c t u r e o f n o r m a l w o o d

h a v e m u c h t o d o w i t h t h e g r e a t e r s t r e tc h a b i l i ty o f t h e C W .

F i g . 2 i l l u s t r a t e s t y p i c a l f a i l u r e s o f s t a t i c b e n d i n g s p e c i m e n s o f t h e t h r e e t y p e s

o f w o o d . T h e n o r m a l w o o d ( S 7 a n d 6 ) s h o w s m o r e i r r e g u l a r p u l l in g a p a r t o f t h e

f i b e r s t h a n t h e c o m p r e s s i o n w o o d (R T 6) w h i c h h a s a b r i t t l e fa i l u r e . T h e s p e c i m e n s

f r o m t h e t e n s i o n s i d e o f t h e t r e e ( S 3 a n d S 1) a ll h a d t h e n a r r o w - r i n g e d w o o d o n t h e

t e n s i o n s i d e o f t h e b e a m , a n d t h i s l o w - d e n s i t y w o o d u s u a l l y f a il e d i n a b r a s h

m a n n e r . T h e f i r s t v is i b le e v i d e n c e of f a i lu r e i n C W b e a m s w a s a l w a y s i n t en s i o n ,

w h e r e a s i n I ~ W b e a m s i t w a s a l w a y s i n c o m p r e s si o n .

G r e e n C W i s s t ro n g e r t h a n g r e e n ~ a n d T S W i n s tr es s a t t h e p r o p o r t i o n a l

l i m i t a n d m o d u l u s o f r u p t u r e i n s t a t i c b e n d in g . S p e c if ic s t r e n g t h i n d e x v a lu e s ,

h o w e v e r , h a v e a d e f i n i t e i n v e r s e t r e n d w i t h s p e c i fi c g r a v i t y , C W b e i n g l o w e s t

a n d T S W b e in g hi g he s t. F o r t h e d r y c o n d i t io n , t h e m o d u l u s o f r u p t u r e h a s t h e

s a m e r e l a t i v e o r d e r a s i n t h e g r e e n c o n d i t i o n a m o n g t h e t h r e e t y p e s o f w o o d , b u t


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248 R.A. Cockrell and E. M. Knu dso n

Fig. 2. Typical failures of green static bend ing specimens showing side and tension-sideviews:

4B 6 and 4B S T--no rmal wood (NW); 4B S1 and 4B S3--tension-side wood (TSW);

4A N6 and 4B N6--eompression wood (CW)

700

tb

lreenl

600

500

O0

300

200

oo

o

o o,1

r D r y

/ P.L.=Lood

ol

proport;ona{ limit

/

/

/

?

fP.L. P.L

t/.,

0,2 0,3 0,4 0,5 0,6 0,7 02 O,g 1 ,0in.1,1 0 0,1 02 0,3 O,& 0,5

Deflection Eleftection

q

/ /

/ I

EL.=

Load

at

proportionaL I

l im]t 1

I

I

I

I

I

I

I

I

I

\

~

I

0,6 0,7 in. 0,8

Fig. 3. Comparison of static bending stress-gtrain curves of I~TWand CW tested a) green and

b) dry


9/10

C o m p a r i s o n o f p r o p e r t ie s o f n o r m a l a n d c o m p r e s s i o n w o o d o f a g ia n t~ s e q u o i a 4 9

0

0

9

0

0

o

r

0

0

o,1

0

oo

0

0


10/10

250 1%. A. Cookrell and R. M. Kn uds on

t h e s t r e s s a t t h e p r o p o r t i o n a l l i m i t i s l o w e s t f o r C W a n d h i g h e s t f o r N W . ( F ig . 3

s h o w s th e s e r e s u l ts . ) C o n s i s t e n t w i t h t h i s, t h e w o r k t o p r o p o r t i o n a l l i m i t o f d r y

C W i s s l i g h tl y lo w e r t h a n t h a t o f N W .

A b s o l u t e v a l u e s f o r m o d u l u s o f e l a s t ic i t y in s t a t i c b e n d i n g o f N W a r e s l i g h tl y

h i g h e r t h a n C W i n b o t h g r e e n a n d d r y c o n d it io n s , w i t h s p e c if ic s t r e n g t h s h o w i n g

a g r e a t d i s p a r i t y b e c a u s e i t v a r i e s w i t h t h e s q u a r e o f s p e c i fi c g r a v i t y . T h i s lo w e r

r i g i d i t y o f C W i s c o n s i s te n t w i t h t h e l o w e r v a l u e e x h i b i t e d b y C W i n s t re s s a t

p r o p o r t i o n a l l i m i t i n t e n s i o n p a r a l l e l t o g r a i n .

T h e g r e a t e r f l e x i b i li t y a n d c o m p r e s s i v e a n d t e n si le s t r e n g t h o f C W i s r e f le c t e d

i n i ts m u c h h i g h e r v a lu e s f o r w o r k to m a x i m u m l o a d a n d t o t a l w o r k ; t h i s a ls o

a g r e e s w i t h i t s h ig h e r t o u g h n e s s v a l u e s .

T a b l e 5 g iv e s r a t i o s o f s t r e n g t h o f C W a n d N W i n g r e e n a n d i n a i r d r y c o n-

d i t i o n s f o r t h e s p e c i e s t e s t e d b y P i l lo w a n d L u x f o r d [ 19 3 7] , a n d f o r th e g i a n t

s e q u o ia . T h e h i g h e r t h e r a t io , t h e s t r o n g e r C W i s i n c o m p a r i s o n ~ d th n o r m a l

w o o d . T h e s e r a t i o s a re q u i t e v a r i a b l e a m o n g s p e c ie s a n d b e t w e e n g r e e n a n d d r y

c o n d i t i o n s , b u t t h e r a t i o s f o r g i a n t s e q u o i a c r u s h i n g - s t r e n g t h p a r a l l e l to g r a i n ,

m o d u l u s o f r u p t u r e , a n d m o d u l u s o f e l a s t i c it y r e s e m b l e m o s t c l o se ly t h o s e f o r

s e c o n d - g r o w t h r e d w o o d a n d a r e h ig h e r t h a n r a t i o s f o r t h e o t h e r s p e c i es m e n t i o n e d .

A l t h o u g h g i a n t s e q u o i a r a ti o s fo r t o u g h n e s s, a n d f o r d e fl e c ti o n to m a x i m u m l o a d

i n b e n d i n g , d o n o t p a r t i c u l a r l y r e s e m b l e t h o s e f o r s e c o n d - g r o w t h r e d w o o d , t h e y

a r e d i s t i n c t l y h i g h e r t h a n t h e r a t i o s o f m o s t o f t h e s e o t h e r s p e c ie s .

e f e r e n c e s

Am erican Society for Test ing an d Materials. 1952. Stand ard m etheds for test ing small clear

specimens of t im ber. D 143-52. Philade lphia, P ennsy lvania.

Cockrel l , R. A. 1946. Influence of f ibri l angle on longitudinal shrinkage of ponderosa pine

wood. Jo urn al of Forestry, 44 (11): 876--878.

Cockrell , R. A. , K nuds on, R. M., Stangen berger , A. G. 1 971 . Mechanical prop ert ies of

southe rn sierra old- and second-growth gian t sequoia. California Agricultural Ex perim ent

Stat ion, Bullet in 854.

Markwardt , L. J . , Wilson, T. R. C. , 1935. Strength and related propert ies of woods grown

in the United States. U.S. Dept . of Agr. Tech. Bul . 479.

Onaka, F. 1949. Studies on compression and tension wood. Woed Research Inst i tute , Kyoto

Un iv. B ul . l~ o. 1. (Translat ion l~ o. 93. Canad a D ept . of Northern Affairs an d Na tional

l~esources, Forestry Branch, Ottawa 1956).

Perem , E. 1960. The effect of compression wood on the mechanical propert ies of white spruce

and red p ine . For . Prod . L abora tor ies of Canada Tech . ~o te 13.

Pillow, M. Y., Lu xfor d, 1%. 17. 193 7. Stru cture , occurrence, and pro per ties o f com pression

wood. U.S. Dept. of Agr. Tech. Bul. 546.

(Received May 15, 1972)

D r. 1%. A. Cockrel], Professor of Fo res try (Wood Science),

1%. M. K nu dso n, W ood Technologist,

School of Fo restry and Conversat ion,

Fores t Products Labora tory ,

U nive rsi ty of California,

1301 South Street, l~ichmond, Cal.

a comparison of static bending, compression and tension

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