DURABILITY OF GLUED JOINTS BETWEEN 

ALUMINUM AND END-GRAIN BALSA 

September 1947 

This Report Is One of a Series 

Issued In Cooperation with the 

ARMY-NAVY-CIVIL COMMIlTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1566 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
DURABILITY OF GLUED JOINTS BETWEEN ALUMINUM AND END-GRAIN BALSA1 

By HERBERT W. EICKNER, Engineer 

Summary 

Tension-type specimens of aluminum cubes bonded to end-grain balsa 

with each of three typical two-stage bonding processes were subjected to 

different exposure conditions and tested after periods of 32 to 52 weeks. 

Joints in specimens bonded by using one metal primer and a 

resorcinol-resin glue decreased in strength after immersion in isopropyl 

alcohol. Joints in specimens bonded by using another metal primer and a 

resorcinol-resin glue decreased in strength after continuous exposure to 

high humidity, alternate exposure to high and low humidity, and immersion 

in gasoline and ethylene glycol. Specimens glued by using a metal primer 

and an intermediate-temperature-setting phenol-resin glue, showed a 

partial LOSS of strength after immersion in water for 2 weeks but little 

or no further change thereafter. 

Introduction 

Aluminum alloy sheets glued to a core of end-grain balsa comprise 

a form of sandwich construction that has been investigated for use in aircraft. Before this type of sandwich construction can be widely used in 

aircraft, however, the durability of the glued joints when exposed for 

prolonged periods to high and low humidities, to high and low temperatures, 

and to soaking in water and aircraft fluids must be known. A previous 

study2 was made by the Forest Products Laboratory on the durability, under 

these types of exposure, of joints between metal and wood made with a 

number of gluing processes; but in that study tests were made only on lapjoint shear specimens of aluminum and steel glued to flat-grained yellow 

birch, fabricated, for the most part, by direct-bonding processes. It was 

the purpose of this study, therefore, to make similar tests on three 

1

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

Products Laboratory relating to the use of wood in aircraft. Results 

here reported are preliminary and may be revised as additional data 

become available. 

2"Durability of Glued Wood- to-Metal Joints," Forest Products Laboratory 

Report No. 1570. 

Report no. 1566 -1-
two-step gluing processes (designated as gluing processes 1, 2, and 3) 

with tensile-type specimens made of aluminum facings glued to end-grain 

balsa cores to determine the durability of the glue joints in this type 

of sandwich construction. 

Preparation of Specimens 

The type of specimen (fig. 1) used throughout this study consisted of two 1-inch cubes of 17 ST aluminum alloy bonded to a 1/2-inch 

thick section of end-grain balsa. Holes, 1/4 inch in diameter, in each 

aluminum cube were used for attaching a universal-joint loading 

apparatus to the specimen. 

Preparation of Aluminum Cubes 

Aluminum bar stock, 1 inch square, was cut into 1-inch lengths on 

a metal-cutting bandsaw, and the holes were drilled and reamed. After 

the face of the cube was lightly milled, it was cleaned and etched. 

Cubes to be glued by gluing process 1 were cleaned by washing 

them with acetone, and then immersing them for about 20 minutes at 140° 

to 150° F. in an etching solution composed of the following ingredients 

(by weight): 

Sodium dichromate 1 part 

Sulphuric acid (conc.) 10 parts 

Water 30 parts 

The cubes were washed in cold and warm water to remove the etching solution and then air dried. 

Cubes to be glued by gluing process 2 were cleaned in a similar 

manner, except that before the acid etching the grease was removed from 

them by washing them with acetone and then immersing them for 3 to 5 

minutes at 160° to 185° F. in a solution formulated as follows: 

Sodium metasilicate, anhydrous 3.6 ounces 

Alkyl aryl sodium sulphonate 0.4 ounces 

Water 1 gallon 

The cubes were thoroughly washed in cold and warm water to remove the 

metasilicate solution before they were placed in the sodium dichromatesulphuric acid etching solution. 

Cubes to be glued by gluing process 3 were cleaned first with 

acetone and then with metasilicate and washed in the sanae manner as those 

glued with gluing process 2. Etching of the cubes was then done by 

Report No. 1566 -2-
immersing them for 1 to 3 minutes at 140° to 150° F. in a solution of the 

following composition (by weight): 

Chromic acid 5 parts 

Water 95 parts 

The cubes were washed in cold and warm water to remove the etching 

solution and then air dried. 

Preparation of Balsa Sections 

The balsa planks used in the preparation of the specimens ranged 

in density from 6 to 9 pounds per cubic foot at 12 percent moisture content. Sections, 1/2-inch thick, were cut from the ends of the balsa 

planks on a table saw. Selection was made from among these pieces so as 

to obtain balsa free from rot, pith, and checks, and having a density of 

about 7 pounds per cubic foot, but no selection was made to eliminate 

the inclusion of balsa having short fibers. 

Balsa for use with gluing processes 1 and 2 was selected at the 

same time to insure uniformity in general quality. Balsa sections for 

use with gluing process 3 were selected at a different time, and there 

is a possibility that this balsa wight have been of slightly different 

quality than was used with the other two gluing processes. 

The balsa pieces were bonded together with urea-resin glue into 

6- by 6-inch sections, 1/2-inch thick. The square balsa sections were 

conditioned for at least 1 week at 80° F. and 30 percent relative 

humidity before they were glued into specimens. 

Fabrication of Specimens 

Four different commercial glues (designated as glues A, B, P, and 

R) and one sizing agent (H) were used in the preparation of the specimens. 

Specimens were prepared in groups of 25 by using the alining jig 

as shown in figure 2. Twenty-five aluminum cubes to which the primer had 

previously been applied were placed in the bottom section of the jig. 

The secondary adhesive was applied to the primed surface of these cubes 

and also to the 25 cubes for the top of the assembly. With two of the 

processes, secondary adhesive was also applied to both sides of the core. 

After suitable assembly period, the core was placed on top of the cubes 

in the alining box, additional alining spacers were placed on top of the 

core, and the upper set of cubes was placed between the spacers on top of 

the core. The assembly was pressed at 100 pounds per square inch in a 

cold or hot press depending upon the type of secondary glue used. A soft 

caul was used to equalize the pressure. Following the curing of the 

secondary glue, the jig was disassembled, and the core with cubes bonded 

to it was cut into the individual test specimens. 

Report No. 1566 -3-
Eight hundred individual tension specimens were prepared with each 

of gluing processes 1 and 2 and 450 with gluing process 3. The gluing 

conditions used with each process were as follows: 

Gluing Process 1. Metal-priming adhesive A was thinned with two 

volumes of a solvent distributed by the glue manufacturer to one volume 

of adhesive, and four successive coats were sprayed on the cleaned and 

etched faces of the aluminum cubes. After 16 hours of air drying, the 

glue film was cured for 30 minutes at 310° F. The glue-film thickness 

was about 0.003 inch, The cured film was lightly sanded with No. 1/2 

emery cloth. A medium spread of resorcinol-resin glue R was applied by 

brushing to both the balsa core and to the sanded priming coat on the 

metal cubes. An open assembly period of approximately 10 minutes and a 

closed assembly period of 5 minutes elapsed before the assembly was 

pressed for 16 hours at 100 pounds per square inch in a room at 75° to 80° 

F. 

Gluing Process 2. Six spray coats (two passes each) of metalpriming adhesive B, thinned with 0.8 volumes of the manufacturer's solvent 

to one volume of glue, were applied to the cleaned aluminum cube surface. 

A five-minute flash-off period was allowed between coats and 16 hours of 

air drying after the final coat. The primer film was cured for 20 

minutes at 320° F. The film thickness was about 0,003 inch. The film was 

lightly sanded with No. 1/2 emery cloth, A medium spread of resorcinolresin adhesive R was applied by brushing to both balsa and primed metal, 

and then an open assembly of 10 minutes and a closed assembly of 5 minutes 

elapsed before the assembly was placed under a pressure of 100 pounds per 

square inch for 16 hours at 75° to 80° F. 

Gluing Process 3. Metal-priming adhesive B was applied and cured 

on the face of the cube in the same manner as used in gluing process 2. 

The balsa was sized, by spraying with sizing agent H at a rate of approximately 1 gram per square foot of surface. The sizing coat was then 

allowed to air dry at 75° to 80° F. for at least 24 hours before 

secondary gluing to the primed aluminum cubes. A medium spread of an 

acid-catalyzed, intermediate-temperature-setting phenol-resin glue P was 

applied by brushing only on the primed aluminum surface. After an open 

assembly period of 3 to 5 hours, the cubes and core were assembled and 

pressed for 1 hour at 220° to 230° F. and 100 pounds per square inch, 

Test Procedure 

Following a conditioning period of about 1 week at 80° F. and 30 

percent relative humidity, the specimens were sorted into groups for 

exposure and testing, Thirty specimens made by each gluing process were 

selected for test as control specimens, The remaining specimens (770 

with gluing processes 1 and 2 and 420 with gluing process 3) were divided 

into groups of 70 specimens, The selection of these specimens and also 

of the specimens for control tests was made so that each group consisted, 

Report No. 1566 -4-
as far as possible, of the same number of specimens from all jig loads 

made with a given gluing process. The 11 groups of specimens made with 

gluing processes 1 and 2 were subjected to all of the following 11 

exposure conditions, and the 6 groups of specimens made with gluing 

process 3 to the first 6 of the following exposures: 

1. Continuous exposure to air at 80° F., and 30 percent relative 

humidity . 

2. Continuous exposure to air at 80° F. and 97 percent relative 

humidity. 

3. A repeating cycle consisting of 2 weeks at 80° F. and 97 

percent relative humidity, followed by 2 weeks at 80° F. and 30 percent 

relative humidity. 

4. Continuous exposure to air at 158° F. and 20 percent relative 

humidity. The specimens were teated following a conditioning period of 

1 week at 80° F, and 30 percent relative humidity. 

5. A repeating cycle consisting of 1 day at 158° F. and 20 percent relative humidity followed by 1 day at -67° F. in a cabinet cooled 

with solid carbon dioxide. The specimens were tested following a conditioning period of 1 week at 80° F. and 30 percent relative humidity, 

6. Continuous soaking at room temperatures in tap water. 

7. Continuous soaking at outdoor temperatures in 100-octane 

gasoline. 

8. Continuous soaking at outdoor temperatures in 73-octane 

gasoline. 

9. Continuous soaking at outdoor temperatures in ethylene glycol. 

10. Continuous soaking at outdoor temperatures in isopropyl 

alcohol de-icer fluid. 

11. Continuous soaking at outdoor temperatures in aircraft 

lubricating oil, 

The specimens for exposures 7 through 11 were immersed in sealed 

containers of the fluids indicated and allowed to stand outside the 

Laboratory with no protection against temperature changes. 

Ten specimens from conditions 1, 2, 3, 4, and 5 were removed for 

testing after 4, 8, 16, 24, 32, and 52 weeks of exposure; from condition 

6 after 2, 4, 6, 7, 8, 16, and 24 weeks of exposure; and from conditions 

7, 8, 9, 10, and 11 after 2, 4, 6, 8, 12, 24, and 52 weeks of exposure. 

Specimens were also to be removed from conditions 1, 2, 3, 4, and 5 after 

104 weeks of exposure. The selection of the specimens from the various 

Report No. 1566 -5-
exposure conditions to be tested at any one test period was always made SO 

that a representative sampling was obtained including approximately equal 

numbers of specimens from each jig load. 

All specimens were tested to failure in tension by the use of the 

testing fixture (fig. 1) and universal testing machine loading with uniform head movement of 0.05 inch per minute. A record was made of the 

failing load and estimates as to the percentages of balsa failure, of 

secondary glue to wood, of primary glue to metal, and failures in the 

primary glue or between the primary glue and the secondary glue. 

Results 

Tables 1 and 2 present the average results of these tension tests 

on the groups of specimens following the several exposures. 

The control tests on the specimens bonded with gluing processes 1 

and 2 averaged 1,183 and 1,143 pounds per square inch, respectively, with 

average failures in the balsa of 50 and 65 percent, respectively. The 

control tests on the Joints bonded with gluing process 3 averaged somewhat 

lower, with a tensile strength of 1,027 pounds per square inch, but the 

failure was 53 percent in the balsa, These values may be considered to be 

equal to or somewhat higher than those usually obtained in unpublished 

work at this Laboratory with a number of adhesives in this type of construction, and consequently may be regarded as satisfactory joints, 

In some instances, the test results following exposure showed that 

the exposure affected decidedly the quality of the glued joint and balsa 

core, 

Results of Gluing Process 1 

With gluing process 1, a decided decrease in the quality Of the 

glued joint or of the balsa core was observed in only 2 of the 11 exposure 

conditions during 52 weeks of exposure. These two conditions were continuous exposure to 97 percent relative humidity at 80° F, and continuous 

immersion in isopropyl alcohol. In exposure to the high humidity, 8-weeks 

test results showed a decrease in the average tensile strength of about 

20 percent from the dry control strength, and there was a continual 

decrease with exposure until, when the specimens were tested after 52 

weeks of exposure, the strength was less than 50 percent of the dry control strength, Since a high percentage of failures occurred in the balsa, 

this loss in strength might be attributed in part to the deterioration of 

the balsa in this exposure condition. When the specimens were soaked 

Report No. 1566 -6-
continuously in isopropyl alcohol, a decided decrease 3 in Joint quality 

was shown in the tests made after 24 weeks, and evidence of decreasing

strength appeared in the tests made after 4 weeks. After 52 weeks of 

exposure there was a further decrease in joint quality, as the average 

tensile strength was only 33 percent of the dry control strength with 

failures of only 20 percent in the balsa, This loss in joint strength 

could be attributed to the effect on the glue itself, 

Results of Gluing Process 2 

With gluing process 2, 7 of the 11 exposure conditions caused a 

significant decrease in the quality of the glued joint or of the balsa 

core during 52 weeks of exposure. Continuous exposure to 97 percent 

relative humidity at 80° F. and exposure alternately to 97 and to 30 

percent relative humidity at 80° F., were the two tests conditions that 

had the greatest effect on joint specimens glued by this process, In 

the continuous high humidity exposure, tests made after 4 weeks of 

exposure showed an average strength of only 64 percent of the dry-control 

strength with 39 percent failure in the balsa. After 52 weeks of exposure the joint strengths had continued to decrease until the average 

strength was only 27 percent of the dry-control strength and the failure 

was only 17 percent in the balsa. In the alternating high and low 

humidity exposure, test results after 16 to 52 weeks of exposure showed 

strengths that continued to decrease from 70 down to 25 percent Of the 

dry-control strength, and the average failures decreased from 40 to 19 

percent in the wood. With both exposure conditions, the specimens 

tested after the longer exposure periods had a large percentage of 

adhesion failure of the primer coat from the metal. Continuous exposure 

to 1580 F. at 20 percent relative humidity and exposure alternately to 

158° F. at 20 percent relative humidity and to -67° F., materially reduced the strength of joint specimens glued by gluing process 2 in 32 and 

52 weeks of exposure. These losses of strength under continuous exposure 

to 158° F. could be attributed in part to deterioration of the balsa, 

since the observed failures were mainly in the balsa, For joints glued 

with gluing process 2, there was a deterioration of the joints soaked in 

73-octane gasoline, 100-octane gasoline, and ethylene glycol. In the 

tests made following 2 to 12 weeks of soaking in gasoline, the Joint 

quality remained approximately constant, but the level of strength was 

generally slightly below that of the dry control specimens. The joint 

quality progressively decreased after 24 weeks of soaking, and after 52 

weeks the average strengths were about 50 percent of the dry-control 

strength and the failure was less than 20 percent in the balsa. No 

decided decrease in the quality of the joints soaked in ethylene glycol 

3This decrease in joint quality is somewhat contradictory to the results 

obtained in the durability study of lap-type specimens of aluminum to 

wood (footnote 2). It was observed that the priming coat on these 

tensile specimens seemed to be somewhat more porous than it was on the 

lap specimens, which might have affected the results, although application of the primer was done in the same way. 

Report No. 1566 
was noted until the tests made after 62 weeks. As this last test furnished the only evidence of deterioration in quality of joints when soaked 

in ethylene glycol, no positive conclusions can be drawn until additional. 

data are available. 

Results of Gluing Process 3 

While the average strengths of the bonds obtained by the use of 

gluing process 3, were slightly lower than those obtained by the other two 

gluing processes, a decided change in joint quality occurred in only one 

of the six exposure conditions used. Two weeks of soaking in water 

decreased the joint quality by reducing the strength of the joint and the 

percentage Of failure in the balsa. Thereafter, however, the joints 

maintained an average of about 70 percent of the dry-control strength, 

with about 30 percent failure in the balsa, through the 24th week. 

Joints bonded by the other two processes also showed some decrease in 

strength when tested after soaking In water, but the major portion of 

these decreases can be attributed to normal decrease in wood strength

with higher moisture content. 

Report No. 1566 -8-