IrLATIONS IRP WOOID
TlHIS l IEIOT IS ()NL Ct A SEIIIS ISSUE
TO AIlu TiH NATION'S uIEfNSE EFOrlT
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
Digitized by the Internet Archive
S-7CITIC GRAVITY-S:-_'"I -: R3L.ATI0:,S 70R L 00D-
Studies at the 7aor. st Products Labor-tor niv -.now;n tn.-nt t
specific gravity of ,oo substance is nearly thr sae tfor" all necie -,
and has a value of about 1.5. Since the lullk specific ,ravit- of 'oi
is less t-lin uniti for most species, it is evie(nt tht ra to ,id rri
portion of the volume of a piece o' -"oo3. is occuied 'by7 to v-)riou- c-,1
cavities and pores. For these reasons, the specific rnrvit,- of oir n-.r,-
-ood is an excellent i'Aex of the amount of 7wood substance r. ,-
hence of the strength properties.
The relation of specific -ravit,, to the mecnnnic'l rroner'i" of
wood 7,va.- be considered from the st;,ndpoint of (1) differences bet-r,.en
species, and (P) differences between pieces of the same qpeciei. Con:,il-
ering, different species, the General relation of specific .:ravitv to
strength is illustrate.d by to -idely different -oods, nstic, n 1n
Florida species, and balsa, a very lirht Centr'l American qnecip5. r-
wise compression tests on ?reen mnteri'-l gC-am the re :-,lts of table 1,
which show t iat rmstic had nine ti.es tne average sjrcific rr-vit- of'
balsa, and '?ws also nine times as hi< in crusnin;f str'u tn aion<21 tne
grain. eightt for weight the end-ise crushir'n str-en i, of tnes lie r ;e
species are substantially equal.
Table l.--A_ comparison of the specific ravities nd t:" strength v ]ui
of t'-o a 'idel' difere nt ':oo(s in ti, re .r.. on:lition
Species of w.ood :Specific F-r-vity: Crushin:.: Srfcit'ic
:based on 7weiht strength : str:n-"t
:and volume of parallel
:-oo3 -"hen oven : to grain
1 2 7)
d r '7"
square inch: (Col. 2 t Col. 1)
l,' stic..................... : i.O: 5, 0 :, ,71O
Balsa ..................... : .1 C44 -<
LThis mimeograph is one of a series of nro -reos report is.uer b:/ t. c
Forest Products Laboratoryr to idi. te Iation-' C ... 'for ,rit
here reported are preliminary, and ... u revis d as -dd1tion'l I'.
become available .
!:imco o. 1303
Some properties increase directly with increase in specific
gravity while others incr-Fse more rapidly. Crushing strength parallel
to grain and shrinkage are examples of rropcrties that vary directly
with the sPrcific gravity. Modulus of rupture, on the other hand, varies
from one species to another as the 1-1/4 power of the specific gravity.
Other properties are related to specific gravity by equations of still
higher powers; for instance, the exponent of specific gravity for the
variation in hardness is 2-1/4. It is evident, therefore, that small
differences in specific gravity may result in large differences in cer-
tPin strength properties.
Specific gravity affords an index of strength also for dif-
ferent pieces of the same species. In fact, the relationship is closer
than that between the averages of different species. Furthermore, the
curve representing the relationship of pieces within a species is
usually of a slightly hi>hr-r power than that representing the average
values for different species. (See accompanying figures).
Some species of wood contain relatively large amounts of resins,
gums, and extractives, which, of course, add to the weii'ht but do not
contribute to the strength as would a like amount of wood substance.
Furthermore, the different species of "wood vary somewhat in the struc-
tural arrangement of the fibers. For these reasons it is aprarmnt that
t'7o species which may be identical in specific gravity may, exhibit dif-
ferent average strength characteristics. This fact is illustrated by
the scattering of points in the accompanying figure showing the relation
betweem modulus of rupture and specific gravity. Hence the specific
gra"-ity relationship should be taken as a general trend rather than a
perfectly uniform law. A departure from the general curve that a)Qlies
to most species usually indicates some exceptional characteristic of a
species, which may make it particularly desirable> for certain use re-
quirements. (The term extractives is used for the compounds that can
be removed from the nood of some species by passing cold or hot 'ater,
alcohol, or other solvent through it when it is in the form of sawdust.
Extractives may be referred to in terms of the solvent used, such as hot-
w"ater extractives, for example.)
Minimum Specific Gravity Requirement
The minimum strength values that mT:. be expected from random
stock of arty species may be materially rai- i by eliminating a rela-
tively small portion of the material. This is accomplish d by fixing a
minii:.um specific gravity r(quirement (table ?) as one of the srcifica-
tions for aircraft wood, thus rej ctlin, li'ht-wi&ht stock. TI-. insrec-
tion can '.iually he made satisfactorily bh,' visu-1il -ex.ninnt ion, but in
c rtain cia5es it r- ber desirable or even necessary to re-crt to actual
specific gr;ivity d'termninations. Such determinations made from tine to
time ar, 'of valu; to aircraft inspectors in familiarizin,- thenm with th'
r ltion b, tee n nTr.arancr and specific gravity.
I o /
( o yo /
10 6 < /o
o^- o/ o o
1 o o o 0 0
2 _o- -- -- -- ---_^ -^ / _____- ^ __ -4 ___
a __ __ __ ^ A __-- --
2oib __ ___reh
i <85 oa % ... .. .
/ :*y *; *
--------~ ~ ~ ~ ~ -A^ -9--p- -^ } ^ ^ ^
- ........ . .. ...... .. .....__ ., .- .... ..... .. ....... .....
C _______ _______ __________ __-___ 1, _..
0 / 0 2 03 04 0.5 06 07 C8 0
The relation bhtweell bending strength and specific gravits for 115 hardwoods and 48 softwoods. 1 1nh puint repr,, unt? the iveragi of a imIter o ts,
up to several hundred for the mtore important species, on Si nll, cletr, st mright-graincd epeci enlis. The, pecitic grnivil vi l>s f ie leutiite fr the wood wheii in o dry and its volnu e ;in test
Z M 27311 F'
-1-'^---- ------------- -- --
2 __000 ------.- -------
0 00 06
6000 -- 0 1
0 -- -
0 OiJ 0.2 0.3 0.4 0.5 0.6 0.7
Ti'h T(lt ion of iiod!lh iii f r fpi'rI lion, ti' gr nvit v \, 1, ,ir> I h:, ihr xvight of the wood whlion oven dry and its VoltnM ,, WhiCn green
Z M E7312 F
While no maximum specific gravity limitations are necessarIr, it
is worthy of note that greater uniformity of reiiht and streno-th can be
obtained by removing the exceptionally dens' stock.
Table 2.--Iinimum acceptable specific gravity values for aircraft ooJlp
:Specific gravityy (bnsp
: on volume and weic-ht
: when oven-dry)
Ash, black (Fraxinus nigra)............
Ash, commercial v;hite (Fraxinus sr.). .
Basswood, American (Tilia glabra)......
Beech, American (FauIs grandifolia) ....
Birch (Betula sp.) .....................
Cherry, blrck (Piunus serotina) ........
Cottonwood, enst-rn (Populus deltoides)
1.,. rock (Ul..us thomasii ............
Hickory (true hickories) (Hicorip sp.).
Khaya (iBha.' sp.) ("African mahogany")
Mahogany, true (Srietenia sp.).........
Maple, sugar (Actor saccharum)..........
Oak, commercial :.-hite and red (quercus
Sweetgum (L' .uidambar styraciflua).....
Walnut, black (juglans nigrra) ..........
Yellowpoplar (Liriodendron tulic-ifera).
Baldcypress (Taxolium distichum)............
Douglas-fir (:-s .Jot ?1-r tr.:ifolia .........
Incense-cedar, California (Libocedrus
Pine, red (Pinus resinosa) ..................
Pine, sugar (Pinus lambertiana) .............
Pine, eastern white (Pinus strobus)... ......
Pine, western white (Pinus monticola) .......
7edcedar, western (Thuja plicata) ...........
Spruce (Picea sn.)................... ........
White-cedar, Port Orford (Chamaecy7rrisf
White-c it-r, i,orthern (Thuja occidentalis).
UNIVERSITY OF FLORIDA
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