26 



Hardwood Record — Veneer & Panel Section 



July 10, 1919 



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its strength properties and the variation of these proper- 

 ties with the number of plies and the ply thickness. 

 TESTS FOR WAR AND NAVY DEPARTMENTS 

 Prior to our entrance into the war, over 150,000 tests 

 had been made at the Forest Products Laboratory at material to splitting is represented as the "splitting 

 Madison, Wisconsin, on about I 30 species of native wood, energy." 



to drop upon the center of the 31/4 by 3 '/4 -inch test piece 

 from a height of one-half inch. This drop held the test 

 piece upon the spear, which was then dropped from in- 

 creasing heights with an increment of one-half inch until 

 failure due to splitting occurred. The resistance of the 



which furnished data for selecting substitute species suit- 

 able for aircraft use. No such data were available, how- 

 ever, on the mechanical properties of plywood. 



Early in 1 9 1 8 an investigation was inaugurated to 

 furnish data of this kind for the War and Navy Depart- 

 ments which were vitally interested in the results and 

 which furnished the funds for the project. 



The term "splitting energy" is applied to the total work 

 done to produce failure and is computed by adding to- 

 gether the distances through which the spear fell and 

 multiplying by the weight of the spear. 

 THE TABLES 



Table 1 gives the results of the column-bending, ten- 

 sion and splitting tests on three-ply panels, in which 



In the tests all plies of any given panel were of the same all plies are of the same species and the grain of the face 

 species and thickness, and the grain of successive plies plies is at right angles to that of the core. Since the 

 was at right angles. This series comprises a total of about column-bending measure varies with the ratio of core to 

 30,000 tests on 32 species of wood. Some of the other total pane] thickness as well as with the number of plies, 

 plywood investigations undertaken at the Forest Products ^nd different combinations of species, the figures are only 

 Laboratory involved a study of the effect on strength of strictly applicable to similar three-ply construction. 



The tensile strength of plywood, as tabulated in Table 

 1 , is based upon the total cross-sectional area, which in- 

 cludes the area of the cross-banding or the veneer hav- 

 ing grain running at right angles to the direction of the 

 force. Inasmuch as the tensile strength of wood is very 



th. 



thick- 



changing the number of plies, of varying ttie core 

 ness, and of using different combinations of species. 



To eliminate variable factors which influence the 

 strength of plywood, all the veneer was cut by the rotary 

 process, and was glued with one kind of glue. The 

 veneer was either glued by the veneer manufacturer or a much higher parallel to the grain than it is perpendicular 

 commercial panel manufacturer. In most cases three ship- to the grain, the veneer stressed across the grain contri- 

 ments of veneer for each species were received from dif- butes very little to the strength of the plywood. In gen- 

 ferent manufacturers, and in many cases from different eral, the tensile strength of a plywood member may be 



parts of the country. A shipment of three-ply wood in- 

 cluded five panels cf each of eight thicknesses ranging 

 from one-tenth to one-half inch in thickness. 

 DESCRIPTION OF TESTS 



Each of the panels received at the laboratory was trim- 

 med to a twenty-inch square, and cut into eight pieces 

 of suitable size and shape for the experiment intended. 

 The following tests were carried out: Column-bending, 

 tension, splitting, and warping. 



Tests were made to determine the strength of plywood 

 in bending. The column-bending test was used in prefer- 

 ence to the usual cross-bending tests on account of the 

 small loads involved and the large bends obtained in the 



considered equal to that of the combined tensile strength 

 of the veneer having the grain parallel to the force. Table 

 2, column e, tabulates the tensile strength of single-ply 

 veneer parallel to the grain, in pounds per square inch as 

 computed from the results of tests on three-ply construc- 

 tion of Table 1, on the assumption that the veneer which 

 is stressed across the grain carries no load. From this 

 column the approximate tensile strength of any combina- 

 tion of the species listed in any veneer thickness may be 

 computed. 



The splitting energy as such has no numerical applica- 

 tion in design. It is useful, however, for comparing dif- 

 ferent panels in their resistance to rupture when stressed 



thinner plywood, which makes difficult the support of at fastenings by suddenly applied loads. 



plywood in cross-bending tests. Table 1 shows the splitting resistances of various con- 

 While the actual strength of wood in direct tension is structions of plywood. The splitting resistance depends 



seldom reached in any structural member, there is a to a large extent upon the holding power of the glue, but 



greater possibility of obtaining higher tensile stresses in it increases quite appreciably with an increase in the num- 



plywood than in ordinary wood on account of the superior ber of plies. 



strength of fastening offered. In airplane construction, in particular, warping of ply- 



The tension tests were made on 3 by 1 2 -inch specimens, wood due to adverse atmospheric conditions must be a 



and the center portion was trimmed to approximately 1 

 inch in width. Specimens were held in ordinary flat grips 

 and tested in direct tension to rupture. 



The factors of splitting resistance obtained from the 

 splitting test have relative value only. They give an in- 

 dication of the relative strength of fastenings, when sub- 

 jected to shock, and facilitate the comparison of different 

 species and constructions. 



The cone spear used in the splitting test, with the rod 

 attached, weighs 1 1.22 pounds. The spear was allowed 



minimum. Data showing the relative merits of the various 

 species with reference to their ability to retain a smooth, 

 undistorted surface are therefore of considerable import- 

 ance. Results obtained at the Laboratory indicate that 

 panels of low-density species tend to remain flatter than 

 panels of high-density species when subjected to adverse 

 atmospheric conditions. Warping increases in general 

 with an increase in density. 



In three-ply construction warping may be reduced by in- 

 creasing the ratio of core to total panel thickness. On the 



