METliCDS FCR DETERMINING THE SPECIFIC GRAVITY OF WOOD FOR AIRPLANE USE December 1941 THIS REPORT IS CM CI A SERIES ISSUED TO AID THE NATION'S DEFENSE EEEORT Nc. 1314 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin METHODS FOR DETERMINING THE SPECIFIC GRAVITY OF WOOD FOR AIRPLANE USE 1 The specific gravity or density of wood, exclusive of the mois- ture it contains, affords a good indication of the strength properties. Hence, in the absence of actual strength tests to establish the quality of individual pieces of wood for highly-stressed parts, such as spars, a mini- mum specific gravity requirement has been established for each species. There are several methods of expressing the specific gravity of wood, but that used in aircraft inspection is the spec ific gravity based on the we ig ht and t he_ volume when oven dry , Either of the two methods described below is suggested for de- termining the specific gravity based on weight and volume when oven dry. The second method should be used only when the apparatus essential to the first method is not available. First Method (Immersion Method). Selection and Pre o araticn of Test Specime n At least one specimen should be cut from each piece of stock to be examined, A convenient form of specimen is a section 1 to 2 inches in length (direction of the grain) taken from a board, plank, or part. The vol- ume of the specimen should preferably not be less than 5 cubic inches- nor more than 25 cubic inches. The object of limiting the length of the test specimen in the direction of the grain is to reduce the time required for drying. For the determination of specific gravity by this first method, the specimen need not be regular in shape. All loose splinters should be removed. Determination of Spec.i fie Gravity After selecting a representative test specimen, proceed as follows: l) Put the specimen in an oven at 212° F. (100° C. ) and dry until the contained moisture is evaporated and the weight becomes constant. This will require from 1 to 2 days. If a number of specimens are placed in the oven for drying at the same time, they should be open-piled to allow free access of air to all surfaces of each piece. —This mimeograph is one of a series of progress reports issued by the Forest Prociucts Laboratory to aid the Nation's defense effort. ^•Satisfactory specific gravity determinations can be made on specimens less than 5 cubic inches in volume by using a suitably sensitive balance such as that shown in figure 2. Greater precision in technique is required with very small ..than, with large specimens to insure a given standard of accuracy. Mimeo, No, 1314 P) ffeigh the oven-dry specimen whil< arm. It is rost eonvmi to record the weight in metric units (f-rair. i ala thereof)* 7 ) Determine the volume by the : on i *ho ; ' after bk>1 8 turn- proofing the specimen, as follows: (a) After the oven- dry weight has been ostein . specimen, preferably while still warm, in hot paraffin. Tit" ~ood tech- nique no surplus paraffin should adhere to the simple, but in I vent that it does, any surplus should be scraped off. (b) Find the volume of the specimen by determinir : of water it displaces when immersed. A container holding sufficient ~ater for the conpl I rsion of the specimen is placed on one pan of a balance. The container an I r are then balanced with weights added to the other pan. - ana o^ a b pointed rod or other r^rip, the specimen is held compli I - Bubmer ad not touching the container while the scales are again c- 1. (fig. 1). additional -eight reauired to restore balj ' .of water dis- placed by the specimen, and, if in rrars , is rically equal to the oven- dry volume of the specimen in cubic centimeters. It is important that the determination of the volume by ^ej be made as quickly as possible after the immersion of the specimen, since any absorption of water by the specimen directly influences the accuracy of the result, ty estimating the volume of the specimen end niacin- on t approximately the required ^eights before the specimen is immersed, the time necessary for balancing may be reduced to a minimum. The rod or grip by means of which the specimen is neld in position should be as small as practicable. Care should be taken not to lo~- j r the specimen into the -ater to a much greater depth than re: to sub: r it completely; otherwise, the weight of the water r'isrlac ^ -- the ro^i or *trip may increase appreciably the weight required to balance, and may seriously impair the accuracy o^ the result. h) Compute the specific gravity from the equation, specific ~rav- T) xt y =» , i D is the oven-dry ^eirht of the specimen in "is the oven-dry volume of the specimen in cubic centimeters. !fhen t 1 as obtained under paragraph 2, and the volume as obtained under paragra 3 are in metric units rs suggested, the values may be sul ted directly in the formula above. (See example). 131U 1 co U © O •H 1 -P x> 3 •H Xi © © >>X P. Pi X> -P 'H •H 3 xi W) o^xf © Pi © © X-H ^ w w X CO •H hO-p t i-H -H X ft © hO ft is -H X O © o-o ^ o rt Pi aS aS (D 2 X! a CO •> -p •H •H ft •H •> © CO P, CO ft • H hO g co Xi S H h ^ © ID bOX X -p • -p a d O -H Cm CD p, © •H O Pi © o aj p CD XI 3 ft O CO r— 1 CO ^ -P p •H l> xi xj a O CD 3 © P -p xi £ fl © -p •H X! aj O -P ft ■p Cm a 3 i x • a 1 3 W X) © rH CD O aS o >» > a i-H Cm i-H rH $ O Di aS X co O 40 CO .H •H a xi P, w> a fli Pi CD Pi fi •H g © 5 Pi -P Pi •h >» a x> -s co Pi •M CD Pi Cm ■p 5 © CD O XS -H © PI O O (A • Pi CD Pi O ft O i-H W aS P. Cm CO «H Xi -P CO © -p © 32 3 X! Pi Pi 6 ■P-P a) O •H a >s-p -P P, t»0 O 10 c as Pi 3 © V ft-H ^) Pi ft < 1 1 • 1— 1 © Pi 3 So Figure 2. — Satisfactory specific gravity determinations can be made on speci- mens less than 5 cubic inches in volume provided a suitably sensitive balance, such as shown here, is employed. Second Method (Measurement Method) The second method differs from the first in that the volume is de- termined by measuring the specimen, rather than by immersing it in water. Selection and Preparation of T est Specimen The general instructions for the selection of test specimens as described for the first method should be followed with the additional require- ment that the specimens be regular in shape. The cross section should hence be square or rectangular, and the ends should be parallel. After selecting a representative test specimen, proceed a.s follows: 1) Put the soecimen in an oven at 212° F. (100 ° 0.) and dry un- til the contained moisture is evaporated and the weight becomes constant. 2) 7'eigh each standard oven-dried specimen while warm* It is most convenient to record the weight in metric units. If the specimen becomes out of shape in drying to a degree that the voluue cannot be determined accurately by measurements, it should again be cut to regular shape before weighing. 3) Carefully measure the width, height, and length of each oven- dried specimen, taking average measurements when this appears desirable. Prom these compute the volume of the specimen. If the volume is determined in English units, it may be converted to metric units by the conversion fac- tors below. U) Compute the specific gravity from the equation specific gr^v- rhere D is the oven- dry the volume in cubic centimeters. ity - ;=•, where D is the oven-dry weight of the specimen in grqms, and V is General Information Conversion Factors In computing specific gravity by the above formula, all measure- ments must be expressed in metric units. Then any of the measurements are expressed in English units, the conversion factors given below may be used: 1 inch = 2.^k centimeters 1 cubic inch = 16.U cubic centimeters 1 ounce = 28.4 grams 1 pound = U53.6 grams 1.311+ -3- Accuracy In order to Insure frood results, the weights and volumes Lther • od should be obtained to an accuracy of at lenst one-half of L I : at. Lteisture Content The moisture content of the spo> _s usually o" r tain ■■• -Lorn wil the specific gravity* The only additional Bten is to o v ti I of specimen immediately after saline;. It is important that 1 ht be immediately after b awing, since wood is subject to moist' -s on osure to the air. The degree and rapidity of chanre are dei I on the moisture content of the piece and the condition of the air to it is exposed. The percentage moisture content of the rood frorr: which n t«st speci- men is cat is expressed by the formula W — U I'oisture content, in percent = — « — x 100 where 7f = original weight of the specinv n before dr. D - oven-dry weight of the specim n. Sxanrole 1 Calculate the specific gravity, '^ased on weight ar ion oven dry, of b sample of Sitka spruce on which the following data been obtained by the first method discussed. Weight Then oven dry (par. 2) = 57*^ grams Volume (weight in grams re- v Lr d *o v p1"-.co after im- -ir.en) (par. t) = lMO«7 eu. cm. Specific gravity -y = ifefy = 0,l+1 Bxample i C-'lculate the specific gravity, based on - ie t and ".. -u ov n dry, of a samr.lj of Sitka spruce op whicJ! thi folio- been obtained by the aecond method describ Weight when oven dry (par. 2) = 57 « 4 grams Measurements when removed from oven (par. 3) ! Ave. length = 1.08 inches Ave. width - 2.U7 " Ave. thickness = 3*27 " Volume in cu. cm. = 1.08 x 2.47 x 3.27 x 16. 4 = 1^3.0 cu. cm, D S7.4 Specific gravity = y = 1I47 q = 0.40 Example 3 Calculate the srecific gravity, based on weight and volume when oven dry, of a sample of Sitka spruce of less thnn 5 cubic inches on which the following data have been obtained by the f irst method described. Weight when oven dry (par. 2) = 5« ^9 grams Volume (weight in grams required to balance after immersing specimen ) (r»ar. 3) = 13.29 cu. cm. Specific gravity =..g = -l^H = ° ' ^ v i3-<-9 Example 4 Calculate the moisture content of the sample of Sitka snruce used in Example 1. The weight of the sample, immediately after sawing, was found to be 63.9 grams. Weight when oven dry = 57 .^ grams W — D h7 Q _ f;7 lj Moisture content, in percent = — — - x 100 = ' ^ x 100 = 11. 3 D 57 . - 1314 _ 5 -