UFDC Home  Federal Depository Libraries of Florida & the Caribbean  UF Government Documents Collection  Internet Archive   Help 
Material Information
Record Information

Full Text 
SIATE &PRIATE FUNKSh
MAtTIA) 0C CAICIATINC Tilt1 STIP1tNC;TE ANI) MOIVIUS 0Cf ELASTICITY 011 P4ywcul) IN CCAIPEISSICN lIevised December 19443 S r ALN J~ A This reportt Is On. of a Series Issued In Cooperation with the AlRM VNAVYCl VI t COMAITIM on AIICVAf I)LMIN Cg?#TtA Under the Supervision of fhm AltIMAUTICAL 90AASM No. 1315 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin in Coopwamio with the University of Wi~tonfin IMT 0D$ P? CALCTMIATINTG THE 'STRY,'ITGT .AIM 11ODTUU7 OF ELAIT!C!TY OF PLYWOOD IY COMPRPWE3lON1 By J. A. LIGKA, Engineer This report presents a method developed at the Forest Products Laboratory for calculating the compressive strength and moduli of elasticity of plywood. This method is based on tests which to date have been limited to (1) plywood in which the grain of the alternate plies is at right angles, (2) loading condi tions in which the compressive stress is either parallel or perpendicular to the direction of the grain of the face plies, and (3) plywood in which all plies are of the same species. The tests are being continued to amplify the data and to cover aditional plywomod constructions. The methods indicated are, hence, subject to modification as additional data become available. The for mulas and discussion relate to members or specimens of such size and supporting conditions that the compressive stress is the controlling factor; accordingly, the data here presented are n 'ot applicable to ply'o od colums or plates where buckling is the controlling factor. The following formulas, Which mathematically are only approximate, have been found by test to be satisfactory for computing the compressive strength and modulus of elasticity of plywood, when the stress is parallel or perpendicular to the direction of the face grain and the grain of alternate plies is at right angles : 'S a E L 1 + Z/ AL M L A I + r Tk (2) Ak E ,,i where: A, is the total area. AL, is the area of the plies whose grain is parallel to the direction of the applied forces. 1This is one of a series of progress reports prepared by the Forest Products Laboratory relating to the use of wood in aircraft. Results herereported ~are preliminary and may be revised as additional data become available. Report No. 1315 I AT, is the area of the plies whose rain is perpendicular to the direction of the applied forces (AT = A AL). k A AL AT AL A, 3, is the averae unit stress over the total area (either the unit maxi mum crushing stress or the unit fiber stress at proportional limit, Depending on whether SL is the maximum crushing stress or propor tional limit value). SL, is the unit compressive stress parallel to the grain (longitudinal). Ea, is the modulus of elasticity of plywood in compression. qT, is the modulus of elasticity of wood in compression perpendicular to grain (tangential). EL, is the modulus of elasticity of wood in compression parallel to grain (longitudinal). EL Em ER The value of the ratios = r and = r, where R is the modulus of elasticity of wood in compression perpendicular to the grain (radial), are available for a few species, although the data are incomplete in that little is known about the variation of these ratios with changes in speci fic gravity and moisture content. These data are reported in "Elastic Properties of Wood," Forest Products Laboratory Report 1529 and supplements thereto. For species on which no data are available, the use of average values of rT = 0.05 and rR = 0.10 is succested, w',ich will Vive a rood approximation of results. It Tay be noted that the strength and moduli of elasticity of plywood are largely dependent on the relative :rea of the. pywood whose grain is parallel to the direction of test. Since rm is small, the value of the term, r Tk, is small compared to unity so that a good approximation of the property desired can be obtained even if this term is omitted. The formulas presented herein have been found satisfactory for plywood made from rotarycut veneer. No tests on plywood from quarter sliced veneer hnve thus far been made. Presumably the formulas will prove applicable to this latter case if rR i substituted for rT in formulas (1) and (2) and k = E. report No. 1315 2 One of the common plywood constructions met in practice is that in which all plies are not of the same species. The strength and molull of elasticity of this type of plywood cannot be calculated by means of equations 1 and 2, although the basic formulas are in general believed to be applicable. While subsequent tests need to be made to check this theory, it in believed that the following formulas can be used for calculating the compressive strength and moduli of elasticity of multispecies plwyood: 12 AL2 L2 A Ll AL1 1 ASL2 A2 r 2 A (3) A E AL L T2A L E 1 +2EL1 L + L2 E 12 +2 Ea = A L1 A +1 r AT 1 ) + j 2 \ AL2 + rT2 AT2 *** (4) A where the terms have the meanings previously indicated and the subscripts 1, 2, etc. refer to the different species, The unit stress and moduli of elasticity values to be used in the formu las can be obtained from previously published data. To give a more correct result, these values should be adjusted to the specific gravity of the plywood and to the expected moisture content when in service, U. S. Department of Agriculture Technical Bulletin Nvo. 479, "Strength and Related Properties of Woods Grown in the United States," will probably furnish most of the desired values for obtaining maximum stresses. While no values for the moduli of elasticity in compression will be found. therein, they may be obtained by increas ing the bending moduli values by 10 percent to correct for shear deformation. Basic stress values and methods for determining working or design stresses fcr structural timbers may be obtained from U. S. department of Agriculture Miscellaneous Publication No. 185, "Guide to the Grading of Strlctural Timbers and the Determination of "Corking Stresses." ANC Bulletin 18, "Tesign of Iood Aircraft Structures," provides information for a number of aircraft woods. The appropriate values to be substituted in the formulas will depend upon the infor mation desired or upon the manner in which the plywood is to, be used. Report No. 1315 3 LIT".1RAT'r~r CI=f (1) .IY4VYCIVIL COrI!MITT7 0CT AIRCRAFT IrESIGN CRIT I.A 19~4T. sign of wcod aircraft structures, A C Bulletin 19, 247 PP., il lus. (2) 'CTp. v., TrFPvr, J. T., and McHURfNEY, R. S. 19~5.Elastic properties of wood, Forest Products Laboratory Report NO. 1529 and supplements. (3) MART ;kFDT, L. J. and '1LSI7N, T. R. C. 19Y35. Strength and Related Properties of Woods Grown in the United States. U. S. Pept. Agr. Tech. Bul. No. 479, 90/ pp., illus, (4) WILSONT, T. It. C. 1934. Guiide to the grading of structural timbers and the determination of working stresses. U. S. Dept. A, r. Misc. Pub. 19F, (16 pp., illus. 4 (rort No. 1315 WIVERhiry OF FLORIDA 3 1262 08866 6036 