EFFECT OF MOISTURE ON TUE COMPRESSIVE % STRENGTU OF PEyWCCD AND LAMINATED WOOD February 1943 <4fc R ( 1 SSI ICTED Of ■i*i < This document contains the National Defense of meaning of the Espionage transmission or rlu- revelal manner to an Unauthorized per Information so classified mav t> sons in the military and n Males, appropriate civilian offio the Federal Governrneni therein, and to United, - and discretion w\ thereof. , ecting Jnited n the USC .50:31 and 32. Its i of. its contents in any law. per- s of nPPOSriunt • > This Rupert is One cf a Scries Issued in Cooperation with the ARMy-NAVy-CIVIL COMMITTEE on AIRCRAFT DESIGN CRITERIA Under the Supervision of the AERONAUTICAL UCARU i M No. 13C6 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/moistureOOfore EFFECT OF MOISTURE ON THE C PISCES SIEVE STRENGTH OF PLYWOOD AND LAMINATED WOOD- By JOHN T. DROW, Associate Engineer and J. A. LISKA, Assistant Engineer The strength properties of wood are known to be influenced by its moisture content. Frequently it is not feasible to condition specimens or structures to the same or to a standard moisture content at the time of test. Consequently, a means of adjusting test results for the ©ffect of varying moisture contents on strength properties is necessary to attain comparability among the tests of a series or to other similar data. A method of adjusting test data on the strength properties of wood at different moisture contents has been developed and published by the Forest Products Laboratory .-- There has, however, been no specific information deal- ing with moisture content- strength relations for such wood assemblies as ply- wood or laminated construction. Adjustments of results from tests of these forms have been possible only by assuming that the adjustments for solid wood members apply. To determine the validity of this assumption, compres- sion tests were made at the Forest Products Laboratory on plywood and lami- nated wood specimens of Douglas-fir and Sitka spruce. Material Rotary cut veneer of several thicknesses was produced from four specially selected logs, two of Douglas-fir (coast type) and two of Sitka spruce, at Tacoma, Wash., dried and shipped to Madison, Wis., where it was glued up at the Laboratory with casein glue into plywood and laminated wood of various numbers of plies. Veneer sheets placed in corresponding locations in the plywood and laminated wood assemblies were taken from adjacent —This mimeograph is one of a series of progress reports issued by the Forest Products Laboratory to aid the Nation's war program. Results here reported are preliminary and may be revised as additional data become available. The work here reported was done in cooperation with the Aeronautical Board. p -U. S. Dept. Agr. Tech. Bui. 282; U. S. Dcpt. Agr . Tech. Bui. ^79; U. S. Dept Agr. Unno. Pub., "Wood Handbook;" Forest Products Lab. Mimeo. 1313 J and ANC Handbook on the Design of Wood Aircraft Structures (Restricted). Mimeo. No. 1306 -1- positions in larger sheets of veneer in order to make, the .two kinds of assemblies as nearly alike in material as possible. The specimens for these tests were cut from plywood and laminated beams made up from the material described and used in a previous series of tests. The specimens were side or end matched to provide, insofar as possible, similar specimens for each moisture condition. In each laminated specimen the grain of all plies was either parallel or perpendicular to the long axis. In each plywood specimen, the grain of the face plies was either parallel or perpendicular to the long axis. Three types of plywood specimens were used: 9~ply> of l/l 6-inch veneer, with a nominal thickness of c j/l6 inch; 5~ply» °^ 3/i^-inch veneer, with a nominal thickness of 15/16 inch; and 7~ply» °^ 3A 6-inch veneer, with a nominal thickness/l-5/16 inches. The 5- and 7~pl.Y specimens were H inches long, the 9~ply specimens 3-l/2 inches long. Conditioning The veneer had been dried to an unknovvn moisture content it bhe producing plant, when manufactured, and was reconditioned to approximately 10 percent moisture content at the Laboratory before being glued up. The gluing operation added moisture so that, after gluing, it was necessary to condition the plywood and laminated wood beams to approximately 10 percent moisture content. After these beams were tested they were stored for about k- years in a room exposed to fluctuating atmospheric humidity and, occasion- ally, heated artificially. When the beams were cut into specimens for the tests herein reported, their moisture content averaged about 6 percent. The 9-ply specimens were subjected to either 16° F. and 65 percent relative humidity or S0° F. and 90 percent relative humidity. The 5~piy an( * 7~ply specimens v;ero subjected to one o r the above conditions or to either S0° F. and ~}0 percent relative humidity or &0° F. and 80 percent relative humidity. Conditioning was continued until the specimens appeared to have reached constant weight. The equilibrium moisture content attained by the specimens appeared to be somewhat lower, in general, than vould he expected in the case of solid wood subjected to the same conditioning processes (table l). _ 2_ Ivameo. No. 1306 Table I. -- Average moisture content of test specimens after st orage to constant weight in various conditions of tempe rature and re 1 a t i ve humidity Moisture content after conditioning at -- Material 80° F. and 76° F. and:gO° F. and 80° F. and 30 percent. 65 percent : SO percent 90 percent relative relative relative relative humidity humidity humidity humidity . Percent Percent Percent Percent Solid wood (average equilibrium moisture 6 12 17 22 Douglas-fir : 5- and 7~plyj 3/l6-inch veneer. : 5.« g.6 i 12.7 ■ 16.6 t •' — — 19.0 Sitka spruce 5- and 7~ply> 3/l 6- inch veneer. 9 ply, l/l6-inch veneer........ 6.2 10.0 1>I.2 IS. '4 — 10.3 — 20. g Testing Procedure A total of 6*+0 specimens, four for each variable were tested. Half were of laminated and half of plywood construction. All were tested in com- pression, with the direction of the applied load parallel to the long dimen- sion of the specimen as indicated in figure 1. Properties Investigated Data were obtained for determinations of moisture content, specific gravity, fiber stress at proportional limit, maximum crushing strength, and modulus of elasticity. No correction was applied to strength values for differences in specific gravity, as specimens from each log varied but slightly in this respect. Formulas used in computing these properties were: Moisture content (percent) M c Wei&ht a t time of test - oven-dry weight x 100 oven- dry weight Specific gravity: ".'eight in grams at time of test - G = estimated weight of glue /, percent moisture \ , . , . , . , . _ , [i + £ Yc)n ' x v °l ume in cubic centimeters at time of test Mimeo. No. I306 -y Fiber stress at proportional limit: o _ Load at pr oport ional limit in pounds PL Cross-sectional area of specimen in square inches Maximum crushing strength: S„ = Maximum load in pounds Cross-sectional area of specimen in square inches Modulus of elasticity: p _ Load in pounds x gage length in inches c Cross-sectional area in square inches x deformation in inches between gage points corresponding to above load Analysis of Results to represent the — p The formula derived in previous Forest Products Laboratory research— / relation between strength values and moisture content for solid wood is S - S x 10-™ in which S is the strength value at moisture content M, S is the strength value that would obtain at zero moisture content if the equation were valid to that point, and K is a constant or parameter to adapt the equation to a particular set of data. Values computed from this formula are on a straight line when strength values are plotted to a logarithmic scale as ordinates and the corresponding values of moisture content to an arithmetic scale as abscissas. The slope of this line with respect to the axis of abscissas is -K. To investigate the applicability to plywood and laminated wood of the methods of adjustment that have been derived for solid wood, it was neces- sary to determine: (l) whether the data from tests of plywood and laminated wood conform to a straight-line relationship when plotted as described above, and (2), whether the slopes of such lines are the same as those of curves found for solid wood. In making these determinations, individual graphs were drawn for specimens from each log, for each of the two types of construction, and for each of the three strength properties under consideration. Figure 2 gives examples of these graphs. Conformity to Linear Relationship As previously indicated, tests of the ( >-ply constructions were made on specimens at but two values of moisture content. Hence, they afford no data for study of conformity to a linear relationship. No. 1306 ^_ The graphs representing the ^-ply and 7~ply specimens, on which tests were made at ^our differing moisture content values, indicated a linear relationship between moisture content and strength. In many of the graphs, the four points were almost perfectly on a straight line. In other cases, three points were practically on a straight line, with the fourth point falling to one side or the other. In a number of instances, however, a straight-line relationship was not apparent. The degree of agreement found between plotted points as to their straight-line relationship was considered as satisfactory as could be expected from the limited number of tests considered. It was, therefore, concluded that the strength-moisture relationship for plywood and laminated wood can be represented by the formula derived for solid wood. Comparison of Value of K for Flywoo d and Laminated \Yoou w ith Correspo nd- ing Values for Solid V.'ood The general equation of a straight line on the graphs representing the data of this test is which is equivalent to Log S = log S - KM -KM S = S x 10 ^ o the equation previously presented. The parameter K, as previously indicated, is numerically equal to the slope but of opposite algebraic sign. Thus, K is positive and the slope negative for those properties that increase with decrease in moisture content, which includes all properties investigated in this series of te$ts. The parameter K may also be expressed in terms of the percentage change in strength properties for a 1 percent change in moisture content. The figures given in table 2-2 of the ANC Handbook on the Design of Wood Aircraft Structures (19^+2) are such average percentage correction factors for solid wood. As shown in the Appendix, these percentage corrections and the values of K are readily convertible. Table 2 presents the average values of percentage change as found from the tests of plywood and laminated wood, and for comparison certain values from U. 3. Dent. Agr . Tech. Bui. 282 and from Table 2-2 of the ANC Handbook. Lines with the slopes represented by these comparison values are also shown in figure 2. It is to be expected that the effects of moisture content on maximum crushing strength and on fiber stress at proportional limit in compression parallel to the grain will be the same for laminated as for solid wood. Furthermore, since resistance to compression is, in plywood, contributed principally by the plies that are parallel to the direction of load, it is o. l.o. 1306 _[-_ to be expected that moisture effects on plywood will be the same as for solid wood whether the loading is parallel or perpendicular to the face plies. These expectations with respect to maximum crushing strength are in general confirmed by the comparisons in columns 3 bo 9 °^ table 2, although it may be noted that the values for plywood and laminated construction tend to be slightly higher than those for solid wood. The data of table 2 (columns 7 bo 12) indicate also that the fiber stress at proportional limit and maximum crushing strength differ but little with respect to the effects of moisture content. This is in agreement with data for softwood species given in table l6 of U. S. Dept. Agr. Tech. Bui. ^-79 where the ratios of strength at 12 percent moisture to strength in the green condition are given as 1.66 for fiber stress at proportional limit and 1.97 f° r maximum crushing strength. For perpendicular-to-grain tests on luminated wood, the values in columns 10 to 12 of table 2 are not directly comparable to those in columns k to 6 because of the difference in areas loaded in test (see foot- note h, table 2). Values in columns 7 to 9 refer furthermore to maximum crushing strength, whereas those in columns k to 6 relate to proportional limit values. It may be noted that values in columns 10 to 12 are, in general, slightly higher than those in columns 7 bo 9> which suggests that for this kind of test fiber stress at proportional limit may be influenced more by moisture content than is maximum crushing strength. Values in columns lb to IS for plywood and for laminated wood loaded parallel to the grain when compared to values in columns 13 to 15 indicate that the effect of moisture on modulus of elasticity in compression parallel to grain is slightly greater than on modulus of elasticity in bending. On the ot hur hand, the values for laminated wood tested perpen- dicular to the grain show that the effect of moisture is much greater on modulus of elasticity in compression perpendicular to grain than on modulus of elasticity in bending. Othe r R el at. ions h ips Agreement to a straight-line relationship between moisture content and strength is, in general, somewhat better, for plywood than for laminated wood. The variation of points for the laminated construction is greater for specimens tested perpendicular to the grain than for those tested parallel to the grain. There seems also to be a greater variation for plywood tested with outer plies perpendicular to the direction of load than for the opposite construction. The various points repres snting fiber stress at proportional limit are more erratic than those for maximum crushing strength, and the points for modulus of elasticity are still mor >rratio. The slop... • : 1 ars to bu independent of the number of plies in the laminated wood, and no consistent variation is apparent in the plywood. : :•• o. Mo. 1306 _6_ Conclusions The conclusions frori this study, based on compression tests of Douglas-fir and Sitka spruce, aro: 1. The type of moisture adjustment formula that has "been found suit- able for v. r ood is applicable to plywood and laminated construction. 2. The parameter that adapts the type formula to a specific property and species is variable for either solid wood or the other forms, but the values found in this series of tests are for the most part within the same range as that previously found for solid wood. **** APPENDIX Deriv ation of Correction Factors of Table 2- 2 in the ATJC Handbook on the D-si-n of VJ ood A ircraft Structures (1942 ) The fundamental equation representing the relationship between mois- ture content and strength of wood is -KM S - S x 10 o 1 b Si = strength value at moisture content Mi §2 = strength value corrected to moisture content Mp Then S-, = S x lO" 10 ' 1 (l) i o and w S 2 - S Q x 10" A ^ (2) If the moisture content changes from Mi to Mp_, there will be a change in the strength property. The ratio of the new strength value Sp_ to the original value Sj_ is found by dividing equation (2) by equation (1) !§. = S o X 10 "^ 2 „ 10 " PJJg . 10 K(Ml " M 2 ) s x io • ICf^ 1 ( L -- M 2 ) or S 2 = 3]_ x 10 or S 2 = S 1 (10 K ) JVI 1 " M 2 Mimeo. No. 1306 , F, the change in strength, expressed as a ratio to the original strength, is then: 3 - S S Mj - M 2 P = -l_I--£- 1 = (10 K ) -1 S l o- n Mi - *'? = 1, this becomes Sg ' s i - 10 K - 1 - P , where F_i is the percentage change (expressed decimally) in strength for a decrease of 1 pur cent in moisture content. whence 10 = 1 + P_i ('0 Combining equations (3) and (k) - 2 = h (i + F^y 1 ' h2 (5) Values of P , are given in table 2-2 of the ANC Handbook on the Design of Vibod Aircraft Structures, that is, the values of table 2-2 are the percentage changes in strength value that accompany a decrease of 1 percent in moisture Content. (The parentheses in the title of table 2-2 are erroneous . ) In the use of equation (5) it is necessary to observe the algebraic ■ Lgn of P_]_ as given in table 2-2. It may noted that when correction is to be made to a lower moisture content, M]_ - M? is positive and the original strength is multiplied by a power of (l + P_]_)j and ./hen correction is to a higher moisture, Mi - Mo is negative and the original strength is divided by a power of (l + P , ). Mimeo. No. I'jOC Table 2. — Percentage increa se in str ength corresponding to a decrease of 1 percent in moisture content . Values for plywood and laminated wood compared to values for solid wood. Direc- tion of face and alter- nate plies with respect to di- rection of load (1) Parallel Perpen- dicular Parallel Perpen- dicular Species (2) Douglas- fir Sitka spruce Douglas- fir Sitka spruce Douglas- fir Sitka snruce !>ou las- fir Sitka spruce Log ber- (3) Maximum crushing strength Values from -- Tech. Bui. - No . 2S2 Table 2k CO (5) ANci Table 2-2 (6) Solid wood Min. :Max 3-2 3-2 k. 5 L 3-2 3-2 k.5 4-5 3-2 3-2 4-5 6.7 6.7 6.1 6.1 ^9-2 ±9.2 6.7 6.7 6.1 6.1 6.7 6.7 6.1 6.1 5-5 5-5 5-3 5-3 \.3 5-5 5-5 5-3 5-5 5-5 5-3 5-3 Values from present tests 5-ply:7-ply: J-ply (7) 5-5 5-2 6.5 6.5 5 A 5.0 9-5 6.R 5-5 5-7 5-5 6.0 5-5 5-5 7-0 6.5 (S) (9) Fiber stress at proportional limit Values from present tests 5-ply:7-ply:9-ply (10) (11) Laminated wood ': 6.5 : 6.0 6.0 6.0 k-1 5.0 : 5-5 I . 3-5 = : 6.9 : 6.7 ' 6.3 6.6 6.0 7.0 7-0 :' S.O : : 6.0 : 5-0 6.5 ■ • 5-0 7-0 ': S.O : : 7-0 : 6.S s.o 7-5 £.0 9.0 ': S.O : Plywood : 5-5 : 6.5 5.0 5-5 3-5 k.O ■ 5-3 : k.O : :' 6.5 : 6.5 5-5 6.0 6.0 5.0 7-5 I S.O : : 5-5 6.5 6.2 4.5 k.o 5.S i k.2 : i 6.5 : 6.5 6.5 6.5 6.9 7.0 7-1 : g.O : (12) 6.0 6.5 9.0 7-5 9.1 9.0 6.5 9-0 5.0 5-5 7.0 6.5 5-5 0.0 7-5 6.5 Modulus of elasticity In bending values from -- Tech. Bui. - No . 2S2 Table 2k (13) [Ik) AN&2- Table 2-2 :i5) Solid wood Min. :Max 1.0 1.0 1.2 1.2 1.0 1.0 1.2 1.2 2.6 2.6 3.0 3.0 2.6 2.6 1.3 l.S 1-7 1-7 1.0 2.6 ': 1.0 2.6 : 1.2 3-0 ': 1.2 3-0 : 1.0 2.6 ': 1.0 2.6 : 1.2 3.0 ': 1.2 3-0 : 1.7 1.7 1.8 l.S 1-7 1-7 l.S l.S 1.7 1.7 In compression values from present tests 5-ply:7-ply (16) (17) 9- ply (is) Laminated wood 3-5 1.3 5.0 6.0 7.0 3.0 l.S 2-5 1-7 3-9 5-2 7.0 S.7 Plywood 2-7 l.S 1-7 1.7 S.O 7.5 9.5 6.0 2.5 l.S 3.0 1.7 3.0 2.1+ 1.6 1-5 : 1 j : 1 c . : 1 •7 = 7 . : 3 ,5 : : 2 S : : 1 3 i : 2 : 1.3 1.5 1.0 2.0 2-5 2.5 2.1 2.0 —These numbers do not apply to columns k, 5, 6, 13, lk, or 15. 2 -U. S. Dei.t. Agr. Tech. Bui. 2S2, "Strength-moisture Relations for Wood," 1932 'ANC Handbook on the Design of Wood Aircraft Structures (Restricted), 19^2. k These values apply to fiber stress at proportional limit in the standard compression-perpendicular-to- grain test, in which load is applied to only part of the area of the specimen. Other values on the same lines, except in columns 13 to 15, are for laminated specimens loaded perpendicular to the grain over the full area (see figure 1). Mimeo. No. I3O6 Z U 4-5199 F Wt^ WOOD GRAIN OF ALL LAMINATED GRAIN OF ALL PLIES PARALLEL PLIES PERPEN- TO DIRECTION DICULAR TO 01- OF LOAD RECTION OF LOAD 5-PLY 1-PLY 9-PLY PLYWOOD GRAIN OF FACE AND ALTERNATE PLIES PARALLEL TO DIRECTION OF LOAD. GRAIN OF FACE AND ALTERNATE PLIES PERPEN- DICULAR TO DI- RECTION OF LOAD LEGEND t = ^-" L--4 1 16 ' L * S" 1 6 > L /A i L 3f FIG. 1 CONSTRUCT/ON OF SPECIMENS USED TO TEST MOISTURE CONTENT-STRENGTH RELATIONS IN PLYWOOD AND LAMINATED WOOD IN COMPRESSION 2 M 4-5500 F DOUGLAS-FIR L 6 z vf/r/c/j < 'PRUCE- -LOG Z MOi 1 p "•** • 3 ^ "x ^ 3 - H-, *. ■ k ^{ *^ ^,J» -- . ^ », ■-^^ ^v. 5>- — - ' i % ^ ! V/„ " - A? /A **-.. - -» 1 ik >* ^ ■K - s ~-x s •- tc : >J* V ~i MAXIMUM CRUSHING STRENGTH \ y S Iv i i M d> '/; W :r US 'H TV G S T ^ ^ rv N , * \ ^ < \ "IN -2r > v . 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