widths, and lengths.

The purpose of this investigation was to study the strength of such

stock, and the effects of cross grain, scarf joints, and orientation of

laminations,

The veneer was fabricated into 2-inch thick stock, most of which was

straight-grained and made with continuous laminations. Some was purposely

so fabricated as to make planks with cross grain in which the grain slopes

were crossed in adjacent plies. Other planks were made up from straightgrained veneer end with scarf joints in alternate plies.

A hot-setting phenolic resin glue was used in making the scarf joints.

The individual sheets of veneer were glued together with a urea-resin glue

(Plaskon 250-2).

The glue, together with possible slight densification from the pressure applied in gluing, added about 7 percent to the unit weight of the laminated material. Lesser additions would result if glue was used in the film

rather than the liquid form.

The tests included static and impact bending, compression parallel to

grain, shear parallel to the grain, and toughness.

In these tests the straight-grained laminated Sitka spruce gave average

values essentially similar to averages from previous tests on solid Sitka

spruce, except that in work values in static bending, a measure of shock

resistance, the laminated stock was definitely lower than the solid. This

is not, however, confirmed by impact bending and toughness tests, which are

also measures of shock-resistance.

This mimeograph is one of a series of progress reports prepared by the

Forest Products Laboratory to further the Nation's war effort. Results

here reported are preliminary and may be revised as additional data become available.

Mimeo, No. 1512 -1-
g y py 

only with center loading.

Strength values in laminated material with grain slopes of 1 in 10 are

not sufficient to justify use of such material in parts that are highly

stressed in compression, bending, or tension parallel to the grain,

A popular concept has been that by interlacing the grain (reversing

grain slopes in adjacent laminations) in stock built up from thin laminations

the deleterious effects of cross grain on such properties as bending and tension could be overcome and wood with steep grain slopes made equal to

straight-grained material. This is not confirmed by the present tests. It

is, however, definitely indicated that under flexure (as in static or impact

bending or toughness tests) grain sloping oppositely in adjacent laminations

is superior to grain of the same slope oriented alike throughout.

Beams with glued scarf joints at a slope of 1 in 15 in alternate laminations and at the same point in the length were deficient as compared to

beams with all laminations continuous. The reduction was particularly large

in those properties indicative of shock resistance. Tension tests of the

veneers disclosed that the scarf joints were not as strong as can be produced by thoroughly good technique. Scarf joints are being successfully used

in aircraft parts formed of sawed laminations. Rotary-cut veneers afford

opportunity, because of their lesser thickness, for better joint distribution

than thicker material. Also, because of the lesser thickness and better distribution, the effect of a jointed lamination at the tension face of a member

stressed in bending would be less.

The general conclusion is that on the basis of strength values Sitka,

spruce laminated from rotary-cut veneer is satisfactory as an alternate to

solid members or members laminated from sawn stock, provided the same requirements for straightness of grain, limitation of defects, and specific

gravity of the wood (exclusive of glue) are observed.

1512 -2-
Material

The veneer was obtained from No. 1 Sitka spruce logs. Some of the

logs were taken from the pond of the Harbor Plywood Company and some were

obtained from the Polson Logging Company, both of Hoquiam, Washington. The

cutting of the veneer, clipping of the defects, drying of the veneer, edge

gluing of the sheets, and scarf jointing of some of the sheets was done under

the supervision of Laboratory representatives at the plants of the Harbor

Plywood Company, Hoquiam, Washington, Portland Plylock plant of the M. and M.

Woodworking Company, and the B. P. John Furniture Company at Portland, Oregon.

Immediately after the rotary cutting of the veneer it was run through

a commercial drier set to a temperature of 250° to 300° F. The drying period

was 30 minutes, and the resulting moisture content of the veneer was 2 percent or less. During the inspection of the veneer it picked up considerable

moisture, particularly near the edges, resulting in some warping. It was,

therefore, necessary to redry the veneer before further processing, such as

edge gluing and scarf jointing. During the second drying the temperature

was 300° to 350° F. and the drying period 25 minutes. This again brought the

veneer to a moisture content of 2 percent or less. The veneer was edge-glued

or scarf-jointed or both edge-glued and scarf-jointed and was shipped to

the Laboratory.

Fabrication of Material into 2-Inch Stock

The following procedure was used in preparing end gluing the veneer

into 2-inch thick planks for test purposes.

Forest Products Laboratory unpublished report, "Investigations of RotaryCut Aircraft Veneer--Sitka spruce," by E. M. Davis and W. Y. Pillow.

1512 -3-
Specimens with slopes of grain of 1 in 15 and 1 in 10, together with

corresponding controls were made. In one-half of those specimens with a

slope of grain, the slope in all plies was parallel, whereas in the other

half of the specimens, the slope of grain in adjacent plies crossed. The

specimens with slope of grain of all plies parallel were cut from straightgrained material after gluing as shown in figure 1. Those with slope of

grain crossed in alternate plies had each ply cut to desired slope of grain

before gluing and then assembled to give desired arrangement of slope of

grain in alternate plies as also shown in figure 1. To obtain this crossing

of grain, it was necessary to turn each alternate ply over from its original

position as it came from the lathe, Consequently, open. side of veneer was

glued to open side and closed side to closed side. Material with straight

grain and with slope of all plies parallel was also glued with veneers open

side to open side and closed side to closed side to eliminate this factor in

later comparisons.

The veneers were glued with urea-resin glue (Plaskon 250-2). The

single spread was 18-3/4 grams per square foot of glued surface, the pressure 100 pounds per square inch, the assembly time 30 minutes, the room temperature 70° to 75° F., while the time in pross varied from 5 to 40 hours.

Most planks were in the press 20 hours. Preliminary tests indicated that

this gluing technique and glue spread would give good glue joints.

To obtain information on the dry weight of the glue used, the weight

of the veneer just prior and immediately after the spreading of the glue

was recorded. The glue mixture consisted of 100 parts dry glue to 65 parts

water, by weight.

After gluing, the planks were placed in the 62 percent 70° F. humidity

room and left there until they were of constant weight and approximately

uniform moisture content,

1512 -4-
previously mentioned.

After gluing and conditioning, the planks were planed to a 2-inch

thickness. All planing was done on one side and this side of the specimen

was placed in compression; no surfacing was done on the tension side of the

specimen and this outside lamination was thus of original thickness. The

method of marking and matching is shown in figure 1 where

S = static bending; and straight

I = impact bending

V = vertical

H = horizontal

P = parallel

C = crossed

Beams with Scarf Joints in Veneers

These specimens were cut from boards made of 1/7-inch veneer, alternate plies of which had a scarf joint with a slope of 1 in 15. Those scarf

joints were placed one above the other and the specimens so cut that the

scarf joints were at the middle of the length of the specimen. The specimens

were so fabricated that at least one outside ply of full thickness contained

a scarf joint. All veneer composing the 2-inch planks from which these

specimens were cut were straight-grained. The scarf cutting was done at a

plant in the west coast region with a traveling head scarfing machine. A

phenol glue was used. After a single spread on each of the two pieces to be

joined the glue was allowed to dry and was then set in a hot press. The

setting in the press required about 2 minutes.

1512 -5-
The 2- by 2- by 30-inch static and impact bending specimens (center

loading), and also the shear and compression-parallel-to-grain specimens,

were tested in accordance with standard procedure as outlined in American

Society for Testing Materials Standards entitled, "Standard Methods for

Testing Small Clear Specimens of Timber," serial designation D143-27, except

that all static bending tests were continued until the load had dropped to

200 pounds. The work done to to this point is considered "total work." A

few of the static bending specimens were tested with third-point loading,

the deflection being measured at the center of the span, (In computing

values of work, the deflection was multiplied by 0.87 to compensate for

measuring deflection at center of span rather than under load points).

The toughness specimens were 5/8 by 5/8 by 10 inches and were tested

over an 8-inch span with center loading. Of the four toughness specimens

cut from a single impact or static bending specimen, two were tested with

the load applied on the tangential face and two with the load applied on the

radial face, the two for each load direction being obtained from diagonally

opposite corners of the beam.

The schedule of tests is given in the following tabulation:

1512 -6-
1512 -7-
Explanation of Tables

Table 1 is a summary of results of tests of laminated Sitka spruce

beams with laminations vertical and those with laminations horizontal. For

both these placements of laminations, average values are shown for (1)

specimens with straight grain, (2) specimens with sloping grain in which the

grain in all plies is parallel, end (3) specimens with grain slopes in

adjacent plies crossed. Ratios given at the end of the table compare specimens with laminations horizontal to specimens with laminations vertical.

Table 2 is a summery of the results of tests of laminated and solid

specimens, all of which were straight-grained.

are from U.S.D.A. Technical Bulletin No. 479,

The values for solid specimens

"Strength and Related Properties

of Woods Grown in the United States," except for toughness values, which were

taken from an unpublished report entitled "Toughness Tests of Airplane Woods

(Sitka Spruce and White Ash)."

Specific gravity values are given for laminated beams based on the

oven-dry weight of the wood including the glue, and also when the glue is

excluded. The pressure applied during gluing caused a slight compression of

the veneers and amounted to from 1 to 2 percent The specific gravity

values enumerated herein are based on volume at test.

The laminated specimens were tested at about 10 percent moisture content, and most strength values for solid wood were, for comparison, adjusted

to 10 percent. Sheer was adjusted to 10.7 percent, and no adjustment was

made for toughness. No reliable means of adjusting the toughness values for

moisture is available. The toughness values for solid specimens were adjusted

Unpublished report, "Preliminary Tests on Laminating 1/7-inch Sitka Spruce

Veneer for Airplane Spar Stock," by M. Leonard Selbo, dated July 19, 1943.

1512 -8-
nated beams without scarf joints. Ratios comparing laminated beams with scarf

joints and laminated beams without scarf joints are also included.

Discussion of Results

The principal factors investigated included:

(1) Laminations vertical or horizontal,

(2) Strength of 1aminated beams and solid beams,

(3) Slope of grain in laminated beams, and

(4) Scarf joints in laminated beams.

Beams with Laminations Vertical or Horizontal.

The laminated beams were made up of l/?-inch veneer glued together

with urea-resin glue (Plaskon 250-2). The glue added some 7 or 8 percent to

the oven-dry weight of the wood.

Information on the strength of beams, grouped according to whether

laminations are vertical or horizontal and according to whether the material

is straight-grained, has a sloping grain with the grain in all plies parallel, or a sloping grain in which the grain in adjacent plies is crossed,

is given in table 1.

The ratios, as given at the bottom of the table, for modulus of rupture and modulus of elasticity are, in practically all instances, near 100

percent, indicating that these properties are not affected by the lamination

placement. This is true whether the material is straight-grained or has a

1512 -9-
sidered significant. The modulus of elasticity is appreciably higher for

laminated then for solid beams, and it seems likely that the glue has added

somewhat to the stiffness. The laminated beams are also slightly higher in

maximum crushing strength parallel to the grain. In work values the laminated

beams are considerably lower. This apparent deficiency is not, however,

substantiated by results of the impact bending tests in which laminated end

solid beams have the same average values. Furthermore, in toughness, another

measure of shock resistance, the laminated and solid materials had practically

the same average values when the load was applied on the radial surface; when

the load was applied on the tangential surface, the laminated beams averaged

considerably higher in toughness than the solid. Considering all the values

measuring shock resistance, it appears that laminated stock is not deficient

in this property as compared to solid.

From a consideration of all strength properties, the data indicate

that with wood of the same specific gravity laminated and solid stock are, in

general, comparable in strength properties.

Laminated Beams with Sloping Grain

The relation of strength values of laminated beams to slope of grain

is shown by table 3. Since, as has been previously pointed out, the orientation of laminations does not materially affect the strength of beams, values

for beams with laminations horizontal and those for beams with laminations

vertical were averaged, and these averages used in computing the ratios shown

in table 3,

Some beams with laminations vertical were tested with the load applied

at the center and some with the loads at the third points. Since a previous

1512 -10-
with a slope of grain of 1 in 15 crossed in adjacent plies average slightly

higher than those for grain of this slope in the same direction in all plies.

The comparisons for material with a slope of grain of 1 in 10 are

similar to those for the slope of 1 in 15 , hut the deficiencies as compared

to straight-grained material are larger. The reduction in work values in

static bending with load applied at the third points, is very large. An

appreciable reduction is also shown in work values in center loading tests in

static bending, drop in impact bending, and toughness values. As for a slope

of 1 in 15, the strength properties for material with a slope of grain of 1

in 10 crossed in adjacent plies is somewhat higher in practically all strength

properties than when the slope of grain is parallel in all plies.

The suggestion is frequently advanced that in laminating members from

comparatively thin material interlacing of fiber direction will compensate

for the effect of grain deviations and that consequently restrictions on

slope of grain may be made less stringent than for single piece members. It

has been further suggested that by such interlacing material superior to that

having thoroughly straight grain will result. (Superiority in such respects

as resistance to shear, splitting , or cleavage along planes at an angle to

glued surfaces is obvious.)

The present tests indicate much the same effects from noninterlaced

grain as previously found for the same slopes in single piece beams Onthe

other hand, some improvement is, in general, indicated for systematic crossing

or interlacing. No reason for the indicated superiority in stress at proportional limit in flexure is apparent. The betterment shown in modulus Of

elasticity is perhaps due partially to reduction in the contribution of shearing distortion to deflections. With grain slopes crossed in adjacent

Unpublished report, "The Influence of Spiral and Diagonal Grain on the

Mechanical Properties of Sitka Spruce and Douglas-Fir," by T. R. C. Wilson

and R. F. Luxford, dated August 27, 1919.

1512 -11-
the laminations is not thought to be a factor. Usually the difference in

shear with plane of failure "radial" or "tangential" is not large, and

previous tests on solid Sitka spruce have shown a slight advantage when the

shearing plane was tangential. As shown in table 2, the average value for

shear of laminated stock is practically the same as for solid stock previously

tested.

Scarf Joints in Laminations of Laminated Beams

Information on the effect of scarf joints in veneer in laminated

glued beams is given in table 4. Only alternate plies contained scarf joints,

but one outside ply of full 1/7-inch thickness contained a scarf joint, and,

in the bending tests with laminations horizontal, the beam was placed with

this lamination at the tension face where it would have the largest effect.

Laminated beams with scarf joints in alternate plies were inferior in

all strength properties studied as compared to matched beams without scarf

joints. As usual, the greater percentage reductions were in those properties

measuring shock resistance (work values in static bending and height of drop

in impact bending). The specimens with horizontal laminations were lower in

most strength properties as compared to unjointed matched specimens than

were those with vertical laminations. Since only alternate plies had scarf

joints, it follows that in beams in which the laminations were vertical,

scarf joints affected only half the width of the tension face, whereas in

beams with laminations horizontal, the scarf joints extended entirely across

this face. Since the tensile strength is ordinarily lowered by a scarf joint,

the lower efficiency of beams with the scarf joint extending entirely across

the tension face is to be expected.

Preliminary to the tests on laminated glued beams with scarf joints,

tension-parallel-to-grain tests were made on specimens cut from several sheets

of veneer with scarf joints and on end-matched controls. (The specimens were

1512 -12-
(2) Laminated beams and solid beams of the same weight, not considering the weight of the glue, have, in general, equal strength properties.

(3) Vertically-laminated and horizontally-laminated beams. are comparable in most strength properties. When the laminations are horizontal

the beams are somewhat more resistant to shock than when the laminations are

vertical.

(4) A slope of grain of 1 in 15 lowers most strength properties only

slightly, but decreases shock resistance very considerably, The deficiency

is somewhat less when the slope of grain in adjacent plies is crossed.

A slope of grain of 1 in 10 appreciably reduces most strength properties. The reduction in shock resistance is especially pronounced. Similar

to the finding for a slope of 1 in 15 the reductions in strength for a slope

of 1 in 10 are somewhat less when the grain in adjacent plies is crossed.

(5) Scarf joints with a slope of 1 in 15 in laminated beams when at

the point of high stress caused a reduction in all strength properties

studied. The reduction was especially large in tnose properties measuring

shock resistance. Improvements that can undoubtedly be made in the technique

of preparing scarf joints would lessen these deficiencies.

1512 -13-