(\l3.9'y w CJo-^v_i^X- SPECiriC GRAVITY-STRENGTH RELATIONS ECR WOOD October 1941 rt is m 01 \ is one cr a secies issued TO AID THE NATION'S DEFENSE EEFORT No. 13€3 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin Digitized by the Internet Archive in 2013 http://archive.org/details/specisOOfore SPECIFIC GRAVITY- STRATH RELATIONS FOR TCOOP- Studies at the Forest Products Laboratory have shown that tl specific gravity of wood substance is nearly the same for all species, and has a value of about 1.5. Since the "bulk specific gravity of woo 1 is less than unity for most species, it is evident that a considerable oortion of the volume of a piece of wood is occupied by the various cell cavities and pores. For these reasons, the specific gravity of oven-dry " r ood is an excellent index of the amount of rood substance present, and hence of the strength properties. The relation of specific gravity to the mechanical properties of wood may be considered from the standpoint of (1) differences between species, and (?) differences between pieces of the same species. Gonri 1 - ering different species, the general relation of specific gravity to strength is illustrated by two widely different ^oods, mastic, a dense Florida species, and balsa, a very light Central American species. End- wise compression tests on green materi.nl gave the results of table 1, which show that mastic had nine tir.es the average specific gravity of balsa, and was also nine times as high in crushing strength along the grain. Weight for weight the endwise crushing strengths of tnese diverse species are substantially equal. Table 1. — A comprrison of the specific gravities and t rie strength values of two widely different ^oo^s in the green condition Species of wood Specific gravity based on weight and volume of wood when oven dry Crushing strength narallel to =rrain Specific strength Mastic. Balsa. . 1 . 01 .11 Pounds r>er square in ch 5,ggo (Col. 2 1 Col. 1) 5,710 5.8RO —This mimeograph is one of a series of progress reports issued by the Forest Products Laboratory to aid the ligation's d f :e effort. Results hero reported are preliminary and may be revised as additional Latj become av? a 1 ab 1 e . Kimeo No. 1303 Some properties increase directly with increase in specific gravity while others increase more rar>idly. Crushing strength parallel to groin and shrinkage are examples of properties that vary directly with the specific 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- tain 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 higher 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 weight but do not contribute to the strength as would a like amount of wood substance. Furthermore, the different species of ^ood vary somewhat in the struc- tural arrangement of the fibers. For these reasons it is apparent that two 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 mod\ilus of rupture and specific gravity. Hence the specific gravity relationship should be taken as a general trend rather than a perfectly uniform law. A departure from the general curve that applies 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 wood 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- water extractives, for example.) Minimum Specific Gravity Requirement The minimum strength values that may be expected from random stock of any species may be materially raised .by eliminating a rela- tively small portion of the material. This is accomplished by fixing a minimum specific gravity requirement (table 2) .as one .of the specifica- tions for aircraft wood, thus rejecting light-weight stock. The inspec- tion can usually be made satisfactorily by visual examination, but in certain cases it may be desirable or even necessary to resort to actual specific gravity determinations. Such determinations made from time to time are 'of value to aircraft inspectors in familiarizing them with the relation between appearance and specific gravity. -?- 1303 o o o o o o o / o° *- ) o o 6 (3D / ° o o o o J u c 1 o c ) o / / • o • o D 5/ y o c 1 o < » 6> > c ° \ • O y • • • O o o 8 cf> c 6> D O o '5 o • • o • c 1 o oq GB5D /" f 1 5 O 6> .♦ •• • y*~ >/ • • • < / • r o oo ° I K (**•• • • • o c o / • • o • {$ • • • • • < o • • & • */ ^ • > V /<£ ' • • • J\ w # 0.1 2 0.3 0.1 0.6 03 1.0 04 0.5 0.6 SPECIFIC GRAVITY -The relation between bending strength and specific gravity for 115 hardwoods and 48 softwoods. Each point represents the average of a number of tests, up to several hundred for the more important species, on small, clear, straight-grained specimens. The specific gravity values are calculated from the weight of the wood when oven dry and its volume at test 2 M 27311 F 16000 14000 3; ^ 1 2000 kj <\ -• u. 10000 8000 6000 4000 Z000 fl o LO tO-l u O/ o n <¥/ ~ & c 1 ' o D R U 1 t C P 1 % C n o P V $ i o u ( *>J$ ?o n D d 0$& °- , 8 ° c c % D t ' 8 J cy >° 9 tf> O n j y n -\ /^ * 4o o° 3 , v V 0.1 0.2 0.3 0.4 SPECIFIC GRAVITY 0.5 0.6 0.7 The relation of modulus of rupture tn specific gravity for small, clear specimens of white nsh tested in a green condi- tion; the specific gravity values are hosed on the weight of the wood when oven dry and its volume when green Z M E7312 7 While no maximum specific gravity limitations are necessai--, it is worthy of note that greater uniformity of weight and strength can be obtained by removing the exceptionally dense stock. Table 2.— Minimum acceptable specific gravity values for aircraft ■■/pods : Specific gravity (based : on volume and weight : when oven- dry) HARD100DS Ash, black ( Fraxinus nigra ) : 0.1+8 Ash , commercial white (Fraxinus sp . ) : .56 Basswood, American ( Tilia glabra ) : .Jib Beech , American ( Fagus grandi folia ) : . 60 Birch (B e tula sp. ) . : . Rg Cherry, black (Prunus serotina) : ,H? Cottonwood, eastern ( Populus deltoides ) : .39 Elm , rock ( Ulmus thomasii ) : . 60 Hickory (true hickories) ( Hicoria sp. ) : . 71 Khaya (Khaya sp. ) ("African mahogany") : .1+2 Mahogany, true ( Srietenia sp. ) : .1+6 Maple , sugar ( Ac^r saccharum ) : . 60 Oak, commercial white and red ( Que reus sp.)...: .62 Sweetgum (Li guidambar styraciflua ) : .1+8 Walnut , black ( Juglans nigra ) ■ -TT : .52 Yellowpoplar ( Liriodendron tulipifera ) : .38 CONIFERS Baldcypress ( Taxodium distichum ) : .1+3 Douglas-fir ( Pseudotsuga taxifolia ) : .1+5 Incense-cedar, California ( Libocedrus : decurrens ) : .32 Pine, red ( Pinus resinos a) : .1+6 Pine , sugar~~( pinus lambertiana ) : . 3U Pine , eastern white ( Pinus strobus ) : . 3U Pine, western white ( Pinus monticola ) : .38 Hedcedar, western ( Thuja rlicata ) : .31 Spruce (Picea sp. ) : .36 White-cedar, Port Orford ( Chamaecyparis : lawsoniana ) : .1+0 White-cedar, northern (Thuja occidentalis) . . . . : .29 -3- 1303 UNIVERSITY OF FLORIDA 3 1262 08866 5129 i