This report is one of a series issued in cooperation with the Army-Navy-Civil Committee on Aircraft Design Criteria under the supervision of the Aeronautical Board. Information reviewed and reaffirmed March 1956 and 1962. Original report issued April 1945

Forest Products Laboratory (U.S.)

Sanborn, W. A.

Undetermined

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ERECT OE THICICNESSr01 PLYWCCDREINEORCIN6 PLATE SApril 194This Report is Cne of a SeriesIssued In Cooperation with th eARMY-NAVY-CIVIL COMMITTE EonAIRCRAFT DESIGN CRITERIAUnder the Supervision of th eAERONAUTICAL BOARDNo.=152 7UNITED STATES DEPARTMENT OF AGRICULTUREFOREST SERVIC EFOREST PRODUCTS LABORATOR YMadison, Wisconsi nIn Ceoperrtion with the University of WisconsinON TH.E BEHAVIOR CF S0LID, WOO ID AIRCR4E':SPARSEFFECT OF THICKNESS OF PLYWOOD R EINFORCING PLATES ON THEBEHAVIOR	 OF SOLID WOODAIRCRAFT -SPARS UNDER CHANGE SIN MOISTURE CONTENT1ByW. A. SAN BORN, Enginee rSummaryThis report presents the results of an investigation made to deter -mine the effect of the thickness of plywood reinforcing plates on the split-ting of solid wood spars in aircraft when the moisture content is change dand to compare the effectiveness of square and pointed plate ends in pre -venting splits from extending into the uncovered parts .Twenty.-four full scale specimens were used for this study. In these ,combinations of yellow birch plywood plates of six thicknesses varying fro mone-tenth to three-fifths of the thickness of the spars were glued t oquarter-sawed Sitka spruce spars of 4, and 6-inch depths with a cold-settin gurea-formaldehyde glue . One specimen of each combination of spar depth an dplywood thickness was glued with material at about 6 percent moisture con-tent and another at about 12 percent .Extreme moisture conditions that might be encountered by aircraft i nservice were simulated in rooms kept at constant temperature and relativ ehumidity. No splitting was observed in any of these specimens . For thi sreason, neither the effect of the thickness of the plywood reinfor=cin gplates nor the effect of gluing spars with material at 6 instead of 12 per-cent moisture content on the tendency of the spars to split could be defi-nitely determined . The results indicate that the splitting of such spar sin service has been due to other contributing factors . Since no splittingoccurred in these tests,it was also impossible to compare the effectivenes sof square or pointed ends of the plywood plates in preventing splits fro mextending into the uncovered portions of the spars .Some minor damage to specimens resulted from abrupt reductions i nrelative humidity . Such reduction caused checking of the edges of the spar s1This report is one of a series of progress reports prepared by the Fores tProducts Laboratory to further the Nation's war effort . The results herereported are preliminary and may be revised as additional data becomeavailable .Report No. 1527	-1-in some instances and, in specimens with plates 0 .225 'inch thick (three-fifths the spar thickness), caused cracking of the glue lines between th eedges bf the spars and the plates .The extent to which the spars were restrained from changing depth b ythe plywood plates increased slightly with increase in the plate thickness .Since the restraint was great, however, even for thin plywood plates, and .the increase in restraint obtained by increasing the plate thickness wa ssmall, the use of a thin plywood plate in preference to a thicker one coul dbe expected to provide only a small advantage in th e, prevention of checkingor splitting of solid spars .Introductio nWhen wood aircraft constructed with solid spars reinforced with ply -wood plates have been used in arid regions, checking or splitting of th espars has been found to . occur at the -ends of, the plates . It has been re-ported2 .that' this does not - occur when the : total thickness of the plywoo dplates does ,not exceed; one-sixth of the ,thickness of the spar . ,, 'The purpose. of this investigation was' to determine the effect of th ethickness of plywood reinforcing plates on the splitting of solid wood spar sunder, changes in moisture content and to compare the effectiveness of squar eand pointed plate ends in preventing splits from extending into the uncov-ered parts.Test SpecimensFor test specimens, yellow birch plywood plates having one end square 'and dne pointed were glued to each side of quarter-sawed Sitka spruce spar s , .of 4- and 6-inch-depths as shown in figure 1 . Plywood thicknesses of 0 .035,0 .070, 0 .100, 0.125, 0 .185,-and, 0 .225 inch were used, approximating one-tenth, one-sixth, one-fourth, one-third, one-half, and three-fifths of the -_3/4-inch thickness of the spars .?CAA Safety Regulation Release No . 154 dated November 12, 1943: 'tMany air-planes of various models having wood spars and - being operated in ari d-regions have developed longitudinal spar cracks in the vicinity of th eplywood reinforcing plates . These cracks result from the tendency of th espar to shrink when drying takes place. The cross-grain portion of theplywood resists this tendency and causes a cross-grain tensile failure in.the basic spar. Crack's start under the plywood plates ; usually, but notnecessarily, at .a bolt - hole or cixt-out and spread in each direction until ,in most cases, they extend a .short distance beyond the ends of the plate swhere the resistance to spar"shrinkage disappears 	 When plywood plate s'are on both faces of . the spar and their total thickness is t less than ap-proximately one-sixth of the total thickness , at that point ,  cracks .havenot been encountered . On the other hand, a thick plywood plate on only . ,one side of the spar apparently causes no cracks to occur . "Report No. 1527	-2--Two specimens were made of each	►binati on of spar depth and plywoodthickness, one of which was glued with i terial at about 6 percent moistur econtent and the other at about 12 percen't.MaterialsThe Sitka spruce spars were cut from clear, straight--grained portion sof five kiln-dried 'quarter-sawed planks that were cut consecutively from th esame growth rings of one log. Their average specific gravity, based o nweight when oven dry'and volume at 12 percent moisture content was 0 .37 ,with individual pieces varying not more than + 0 .02 from the average .Yellow birch plywood-s complying with ANC Specification AN- P-51lbwere manufactured in the Laboratory . Veneers selected for aircraft gradeplywood and conditioned to about'12 percent moisture content were glued i na hot press. with a phenol-f ermaldehyde film glue under a pressure of 20 0pounds per square inch at a temperature of 320° F .The average thickness and the specific gravity of each plywood base don .weight when oven dry and volume at 12 percent moisture content were a sfollows :Average thicknes sThree-ply0.038inch '0.069 inch0,100 inch0.118 inch0 .185 inchFive-ply0,220 inch	:64 .Preparation of Specimen sA side-matched pair of plywood plates was glued to each spar with aurea-formaldehyde cold-setting glue . Parts for six specimens at a time weretaken from the conditioning rooms, and glue, mixed in the proportions of 6 5grams of water to 100 grams of dry g1 le, was applied with a glue spreader a ta coverage rate of 25-grams per square foot . After an assembly time not ex -ceeding 15 minutes, the assemblie s ' were pressed at a-pressure of 125 pound sper square incth for a perio d; of 6 hours or more . Upon removal from th epress, the specimens were returned to the humidity rooms in which the com-ponent parts had been stored and were reconditioned for 14 days . The edge sof the specimens were then dressed flat on a joiner .Specific gravi y0.58.60.59.6 3,66Report No . 1527Test Procedur eHigh and low moisture contents of spars that might be attained i naircraft in service were produced by the use of conditioning rooms in whic hthe temperatures and relative humidities were automatically controlled a sfollows :Room A. 70° F., 64 percentRoom B . 80° F., 30 percentRoom C . 80° F., 90 percentRoom D . 140° F., 25 percentrelative humidityrelative humidityrelative humidityrelative humidityThe 4 percent moisture content produced in room D was selected a sthe probable minimum that could be expected in the spars of aircraft oper -ated in arid regions .Rooms A and B were used for the conditioning of parts prior to as -sembly and fbr reconditioning the specimens after gluing . After recondi -tioning, the specimens were subjected to changes in moisture content i nfour successive stages as follows :Stage 1. (From 12 percent to 6 percent .) The specimens glued at 12 percen tmoisture content were conditioned in room B for•29 days .Stage 2 . (From 6 percent to 17-1/2 percent .) All specimens were condi -tioned in room C for 30 days .Stage 3 . (From 17.-1/2 percent to 7 percent .) All specimens were again con -ditioned in room B for 29 days .Stage 4. (From 7 percent to 4 percent .) All specimens were conditioned i nroom D for 10 days.The specimens were examined for splitting or other possible damage a tintervals during each exposure . At the conclusion of each stage of condi -tioning, each specimen was examined and weighed, and the width was measure dwith a vernier caliper at seven stations as shown in figures 2 and 3 . Thewidths of both the plywood plates and of the spar were measured at station s3, 4, and 5 .After completion of the four stages of conditioning, all of th especimens=were stored in room A for 203 days after which they were agai nweighed-and measured .-Forest Products Laboratory Report No . 1597, "Study of Temperature an dMoisture Content in Wood Aircraft Wings in Different Climates . "(February 1944 )Report No . 1527	-4-Explanation of Chart s'IsThe dimensional change data are shown , graphicallyfor 6-inch spars .in figure 2 and for 4-inch spars in figure 3 . . The changes in width at eachstation that accompanied the greatest changes in moisture content of th especimens above and below that at which the plates were glued are shown o nindividual graphs in each of these figures for each thickness of plywoo dplate .The relationship . between the change indepth for each percent chang ein moisture content of quarter-sawed Sitka spruce spars reinforced wit hyellow birch plywood plates and the thickness of the plates is shown i nfigure 4. The changes in depth shown were measured at mid-length of each,plywood plate (point 4, figs . 2 and 3) . The average dimensional change o fthe Sitka spruce spars measured at station 1 was assumed to correspond t o, that of a spar without plates .	_Discussion of Result sMoisture Content of SpecimensThe moisture changes attained in each humidity condition by th especimens of a given depth and moisture content at gluing were found to b eapproximately the same . The moisture contents of specimens shown in table ;1 were computed from_ the changes in weights of the spars and the know nequilibrium moisture contents of-wood in each humidity condition .Table 1.---Moisture contents of Sitka spruce spars wit hyellow birch plywood plates after each_stage of conditioningMoisture conten t-------------------------------------------- -Conditioning	:6-inch spars glued at :4-inch spars glued a tstage 	 :	: 12.0	,	6.4 12,0'	: 5 . 5percent :	percentPercent :	Percent	:percent :Percent :percentPercentStage 1 7.4	 6.4 . . . . . . . . . ..Stage 2 17.7	17 .3 17 .5 . 17 . 2Stage 3 8.0	8 .1 7 .0 7 . 0Stage 4	- 4.8	4 .8 4,0 3.9'Final storage 10.7	10.7 10.0 10 .0Report No. 1527	-5-Effect of Moisture Change sNo visible splitting of the quarter-sawed faces of the Sitka spruc espars beyond the plywood plates was found 'in any of the specimens afte rcompletion of the conditioning program . Seventeen specimens showed no' .damage of any kind, while minor defects developed by the moisture change swere found' in .Seven specimens .	*After from 3 to 7 days' exposure to a humidity that lowered thei rmoisture content, checking of the flat-sawed edges of the Sitka spruce wa sfound in the following specimens : 'Specimens glued at 6 percent moisture content4-inch. Spar ; 0.-125--inch plate s6-inch'spar, 0.100-inch :plate s' 6-inch spar, 0 .125-inch plate sSpecimens glued at 12 percent moisture content6-inch spar, 0.070-inch plate sThe checks in the first three, which were from 1-1/2 to 3 inche slong and occurred in stage 3, disappeared upon continued exposure to . lowhumidity. After 3 days' exposure. at 80° F. and 30 percent relative hu-midity, the fourth specimen had developed a check 9 inches long, but this ;closed partially during subsegizent conditioning .Light cracks appeared in the'glue lines between the edges of th espars and the plywood plates in three of the four spars with 0 .225-inchplates, after from 3 to 7 days' exposure to a humidity that lowered thei rmoisture content . ' The specimens and the stage in which the cracks de-veloped were as follows :Specimens Stage4-inch spar glued at	6 percent moisture content 36-inch spar glued at 12 percent moisture content 16-inch spar glued, at	6 percent moisture oontent 3In the. first two specimens the cracks were small and disappeare dupon continued exposure . Cracks were more extensive in the third specime nand about 1/4 inch in depth . They closed partially with continued. exposure .Dimensional Changes_ of Spar sIn each specimen, the ratio of the change in width of the plywoo dplates to the change in moisture content was con tant ' for all of the change sin moisture content . Although this dimensional-change coefficient in indi-vidual specimens varied from the average value for each thickness of plywood ,Report No. 1527	-6-no consistent difference was found between specimens differing in depth o rin moisture content at time of gluing (fig. 4) . The ratio of change in widt hto the change in moisture content of free quarter-sawed Sitka spruce was als oapproximately constant within the range of moisture contents employed .The extent to which the plywood plates restrained the solid spar sfrom changing in depth with changes in moisture content increased slightl ywith an increase in the thickness of the plywood plates . The plates of al lthicknesses, however, restrained the solid spars to a considerable degree .The dimensional change of the Sitka spruce adjacent to the joint was reducedabout 75 percent by plywood plates only 0 .035 inch in thickness, The reduc-tion was about 80 percent with 0 .07-inch plates (one-sixth the thickness ofthe spar) and increased to 88 percent with 0 .225-inch plates . Since thesedifferences are small, restriction of the thickness of the 4einforcin gplates to not more than one-sixth the thickness of the spar- as a means o fpreventing checking or splitting of the spars does not appear to be justi-fied .ConclusionsTest specimens of quarter-sawed Sitka spruce spars reinforced wit hyellow birch plywood plates did not split when exposed to atmospheric con-ditions as extreme as those likely to be encountered by aircraft in service .The splitting of spars of similar size and construction in aircraft i stherefore apparently due to other contributing factors . The use of bolte dfittings or the combination of working loads with the stresses induced b ymoisture changes may be expected to have an influence on the checking o rsplitting of spars. A more extensive investigation than was here contem-plated would be required for their evaluation .Since splitting of the solid spars in the test specimens was not ac-complished, a comparison could not be made of the effectiveness of squar eor pointed ends on plywood reinforcing plates in preventing such split sfrom extending beyond the plates .The effectiveness of gluing spars with the material at 6 instead o f12 percent moisture content in preventing the occurrence of splitting wa snot demonstrated .When abrupt reductions in humidity conditions caused the moistur econtent of spars with plywood plates to be unequally distributed, stresse swere induced that tended to cause checking of the edges of the spars .Under these conditions, the spars with 0 .225-inch plywood plates (totalin gthree-fifths of the spar thickness) showed a tendency to fail at the glu elines between the plates and spars . Although these failures were slight ,they might be expected to become progressive with fluctuations in moistur econtent .Report No . 1527	-7-A comparison .of thechan§es in dimension with changes in_ m.oi.stureconte.x t of. spars_ with plywood plates of varying thickness afforded an 'esti -mate 6f the relative stresses. involved. The extent-,to which the spa-re . wererestrained from changing width-by the plywood plates . increased slightlywith an increase in the thickness of the plywood . The restraint wa's great ,however, even for thin plywood plates, and the increase in restraint ob-tained by increasing the plate, thickness was small. The use of a thin- plywood plate inp'eference to a,thicker one, therefore, can be expected• .t oprovide only, a small advantage in the prevention of checking or splittin gof solid spars .	-Report No, 1527	-•8 -A0S_•a	 ==•1MM=~ =EM■■■H■I1■N■L ■■i■■■ !■I■►1■11INOME111M MEIN ► ■It, 111 U I1I l~ g l99 ~ ,111`I:}f11 .;RN 1 1N-- ~	I	-i- I I	Iono	on oooo o~o ?(S300N/1 H1d30 NI 39NVH70Sti ~S/ SPRUCEI "SPAR9 rPLYWOOD PLATE	000,~0 0	0 0SIDE V/EW OF SPEC/MENOIO IWITH 0.035-INCH PLYWOOD PLATES005fr005 _WITH 0070-INCH PLYWOOD PLATES005005WITH 0./00-INCH PLYWOOD PLATES0.050 ---------- -005 ._WITH 0./25-INCH PLYWOOD PLATES0.050.05WITH 0/85-INCH PLYWOOD PLATES005~~ -------005do,WITH G.225-INCH PLYWOOD PLATESI	I77	L6-	/6"	4SITKASPRUCE	PLYWOOD PLATESPAR	-AAR0	f)SIDE VIEW OF SPECIMEN0.151110.~0. .	r005 WITH 0035-INCH PLYWOOD PLATES0.10005005	WITH 0070-INCH PLYWOOD PLATES0./00050D.0.100-/NCH PLYWOOD PLATES01 00.05O005WITH W25-INCH PLYWOOD PLATES010O005	,005WITH 0./85-IA/CH PLYWOOD PLATESO.IO205 -1_nn ■	WITH 0.225-INCH PLYWOOD PLATESOrrrft4.f4.tr4.SPARS GLUED AT /2 PERCENT MOISTURE CONTENT	SPARS GLUED AT 5 i, PERCENT MOISTURE CONTENTLEGENDPERCENTMOISTURE CONTEN T/74.	4CENTER OF S/THA SPRUCE	---PLYWOOD PLATESFigure 3 .--Changes in depth of 3/4- by 4-inch quarter-sawed Sitkaspruce spars with yellow birch plywood reinforcing plates due t ochanges in moisture content from that at which they were glued .Z M 80418 F0■D •O"tom	tu tu A7d-9 H̀ON/-92O ■Q WW a tu ELIZ a~	o 1C7	Q. 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Effect of thickness of plywood reinforcing plates on the behavior of solid wood aircraft spars under changes in moisture content

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Effect of thickness of plywood reinforcing plates on the behavior of solid wood aircraft spars under changes in moisture content

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