INVESTIGATlON OF METHODS OF 

INSPECTING BONDS BETWEEN CORES 

AND FACES OF SANDWICH PANELS OF 

THE AIRCRAFT TYPE 

September 1947 

This Report is One of a Series 

Issued In Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAIT DESIGN CRlTERlA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1569 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
INVESTIGATION OF METHODS OF INSPECTING BONDS BETWEEN CORES 

AND FACES OF SANDWICH PANELS OF THE AIRCRAFT TYPE

1 

By BRUCE G. HEEBINK, Engineer 

and 

ALVIN A. MOHAUPT, Technologist 

Introduction 

Sandwich construction is finding limited use at present in the aircraft industry, and plans are being made for its more extensive application 

to structural parts. Manufacturing processes involving recently developed 

(and often unproven) adhesives and assembly techniques are presently used 

for bonding various core and face materials. It is therefore highly 

desirable to have, as a working tool, a dependable quality-inspection 

method or nondestructive test to insure the detection and subsequent 

rejection of inferior bonds between the cores and faces, 

The ANC Subcommittee on Wood Aircraft Structures requested that 

along with other research on sandwich construction, the Forest Products 

Laboratory evaluate the various inspection methods that have been proposed 

and also attempt to develop and evaluate other inspection and nondestructive test procedures. It is the purpose of this report to present 

the results of this work covering the period from late in 1945 to May 1947. 

Summary and Conclusions 

The following 10 methods, which appeared to offer promise of determining the quality of joints between cores and faces of sandwich panels 

without relying upon destructive tests, were investigated: 

1. Visual inspection 

2. Special lighting 

3. Tapping 

4. Supersonic inspection 

5. Exposure to vacuum 

1This 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. 

Report No. 1569 -1-
6. Vacuum-cup test 

7. Internal-pressure test 

8. Heating complete panel 

9. Local heating 

10. Button-tension test 

All of the methods had some merit and under certain limited conditions were capable of detecting actual voids or unglued areas between the 

face and core. The tapping test, performed by a specially trained person, 

appeared to be the most practicable and dependable. 

A careful visual inspection immediately after the panel was removed 

from the hot press or bag often revealed blistered areas that disappeared 

after the panel cooled. On special combinations of materials, such as 

translucent glass-cloth faces on an expanded rubber core, these blisters 

could be made visible again by special lighting, but on combinations 

involving opaque faces, such as aluminum, special lighting effects had no 

merit. 

Scanning by supersonic vibrations revealed voids, but was sensitive 

to varying density. Weak bonds between the face and a balsa core could not 

be isolated, as their attenuation of supersonic vibrations was found to be 

essentially the same as that of strong bonds. Because panels having honercomb cores contain essentially complete voids between the faces, it is 

probable that an unbonded area between tho face and core could not be 

detected. 

Vacuum-induced air pressure exerted a slight tensile force upon the 

bond between the face and the core that extended an existing blister when 

the face was impervious and core material slightly pervious to air. The 

same force could be applied locally by means of a vacuum cup, but both 

methods were found to be ineffective and unreliable. 

The magnitude of the forces tending to separate the faces from the 

core would be greatly increased by quickly releasing an externally applied 

air pressure after several hours of exposure. This test was capable of 

extending unbonded areas as small as 1/2 inch in diameter so that they 

could be easily detected. The range of this test could be further increased 

by conducting it at an elevated temperature so that the glue bonds were 

stressed while hot. Tests revealed, however, that poorly bonded area8 

could withstand this test. 

Heating, applied either to the complete panel or locally, proved to 

be ineffective and unreliable as an inspection method. 

Tension exerted through a button glued to the face of the panel was 

found to be the only method used that would detect poorly glued areas. 

Since this test was time-consuming and involved, however, it was considered 

impractical for use over a complete panel, but it appeared to have 

application for "spot" tests. 

Report No. 1569 -2-
None of the tests investigated presented practical and dependable 

means of inspecting sandwich panels for quality of joints, and it appeared 

that combinations of the test methods would offer little promise of 

improvements. For the present, therefore, it is considered that carefully 

controlled process specifications, substantiated by sufficient destructive 

tests and supplemented by rigid inspection, must be relied upon to Insure 

uniformly high-quality joints in sandwich panels. 

Test Procedures and Discussion of Results 

Visual Inspection 

Visual examinations were made immediately after the sandwich panels 

were removed from the press or bag. In each of these processes, the 

panels were cured with heat and pressure, and upon removal of the pressure, 

the face of an unbonded area often rose to form a blister. These blisters 

were extended for a short interval only or until the drop in panel temperature reduced the internal pressure of the panel. During this short 

interval the blisters were outlined with a wax crayon line for future 

identification. Such blisters were visible on aluminum- and glass-clothfaced panels, but could not be seen so readily in plywood-faced panels. On 

glass-cloth-faced panels, a blister could sometimes be seen by the variation in color or transparency of the face, but this was not always true, 

became many times such a variation appeared to be perfectly sound when the 

tapping method was used. 

The visual inspection method appeared to have only limited possibilities. If a panel has blisters when removed from the press or bag, the 

presence of defective area is demonstrated and the panel can be rejected 

immediately. If no blisters are visible, however, the absence of defective 

areas is not proved and the panel must be subjected to further tests by 

more dependable methods. 

Special Lighting 

The use of lights was tried for determining faulty areas or blisters 

in various types of sandwich construction, but mainly in panels with glasscloth faces. Various light arrangements were used in an attempt to bring 

out the defects, such as lighting from the edges of the panel and passing 

light through the panel. By these arrangements and other variations of 

light angles, some blistered areas could be detected, but not with any 

degree of reliability. Poorly bonded areas could not be located. The 

possibilities of successful detection of faults and blisters by this method 

appeared to be less promising than those of other methods, and therefore 

efforts to use lighting were discontinued. 

Report No. 1569 -3-
Tapping 

Tapping is one of the simplest methods that has been used for testing 

for voids in the glue bond between the faces and core material of sandwich 

construction, The only equipment necessary for this test was a small piece 

of metal, such as a coin or a small light hammer. A special hammer was used 

for most of the tapping tests herein reported. It consisted of an aluminum 

ball about 3/4 inch in diameter attached to a 6-inch handle. The panels 

(usually 1/2 by 36 by 36 inches) were supported near two opposite edges to 

intensify the tone produced by tapping and thus to make it easier to determine the faulty areas. A well-bonded area produced a clear tone, while an 

unglued area usually produced a lower tone or dull thud. 

This method proved reasonably satisfactory for detecting areas where 

the faces of the sandwich were not firmly attached to the core. It was 

found that if there was Intimate contact between the face and core, however, 

no difference in tone quality could be detected. Poorly bonded areas, 

therefore, could not be differentiated from well-bonded areas. It was also 

evident that very light tapping was more selective than heavy blows. Considerable experience was required consistently to locate defective areas 

because panels of different construction gave off different tones and the 

tones on a single panel varied with the position of the spot tapped on the 

panel. The variation in tone was especially noticeable within a few inches 

of the edges of a panel. 

Supersonic Inspection 

Metal products, such as steel castings, forgings, and tin plate, are 

sometimes inspected by the use of supersonic vibrations. Hidden flaws, 

voids, and other defects are located by their attenuating effect upon these 

high-frequency vibrations. It was thought that by proper adjustment for 

density these devices might be adapted for use on sandwich constructions. 

Several panels, containing known defective areas, as shown in figure 1, 

were subjected to supersonic inspection under laboratory conditions. In 

these tests the transmitter and receiver units were entirely submerged in 

water and mounted so that they could be made to contact the panel at 

directly opposite points on the faces. Each unit covered an area of about 

1 square inch. The panel was passed between these two units while the 

transmitter was operating at a frequency of 440 kilocycles. Observations 

during these tests revealed considerable variation in attenuation as the 

aluminum-balsa panels were passed between the heads of the instrument. This 

variation was attributed to the variations in density of thebalsa. It was 

found from repeated trials that poorly bonded areas could not be located, 

but that areas having actual voids between the face and the core were 

consistently detected. 

While it is possible that tests made at a higher frequency might be 

more selective, it is improbable that a differentiation could be made 

between poorly glued and well-bonded areas by supersonic methods alone. 

Report No. 1569 -4-
Although no tests were made, this method would appear to be quite 

ineffective when used on sandwich panels with honeycomb cores. The voids 

in the honeycomb cores are continuous from face to face, so that the 

presence of voids in the joints between the faces and coresundoubtedly

could not be detected, 

Exposure to Vacuum 

The force exerted by vacuum-induced air pressure may be used to 

apply a moderate load to completed sandwich panels provided the face and 

core materials are relatively impervious to air. The magnitude of this 

load is dependent entirely upon the rate of air flow through the face and 

core of the panel, but under ideal conditions cannot exceed atmospheric 

pressure, or about 14 pounds per square inch. 

Tests made by this method were conducted upon small aluminum-faced 

panels in an autoclave having a window in its door through which the 

immediate effect of quickly drawing a high vacuum upon the completed panels 

could be observed. It was found that areas having poor bonds could not be 

detected, and areas having no bond whatsoever were difficult to locate 

unless the area was large and the face very thin. Defects of this type 

could be located more easily and with greater accuracy by tapping, The 

vacuum-test method, therefore, appeared to have little value. 

Vacuum-cup Test 

The vacuum-cup method of inspection was similar in principle to 

exposure of the entire panel to a vacuum. A portion of the face was 

covered by an inverted container in which a vacuum was drawn. Vacuuminduced air pressure within the portion of the panel under the container 

exerted a force tending to push the face off, provided that the face was 

impervious to air. In this type of test the most severe load was applied 

when the face was airtight and the core was not. Under these conditions 

the ultimate load was equal to about atmospheric pressure. 

Original tests at the Laboratory were made by using a glass funnel 

for the vacuum cup. Its open end, which was in contact with the panel, 

was coated with rubber to provide an airtight joint. If a blister 

appeared under the glass funnel when a high vacuum was drawn, it could be 

observed by reflected light. 

The results resembled those obtained when the complete panel was 

tested under vacuum. Areas containing weak bonds could not be located, 

but large areas containing open joints could sometimes be detected. 

Later, an instrument operating upon the same principle but incorporating a dial indicator for actual measurement of the face movement 

was tried. This device is shown in figure 2. For accurate deflection 

Report No. 1569 -5-
readings the rubber gasket, as used on the funnel, had to be omitted, and 

therefore, considerable leakage developed under the aluminum cup, especially 

on panels that were not perfectly plane and smooth. 

The results obtained with this Instrument were similar to those with 

the inverted glass funnel except that, because of the reduced degree of 

vacuum, areas having no bond would often be undetected. 

Internal-pressure Test 

After exposure to air pressure for several hours, a sandwich panel 

having airtight faces and a core that is slightly pervious to air tends to 

develop an air pressure within the core (and in any voids between the face 

and the core) equal to the external pressure. If this external pressure is 

suddenly released the air entrapped within the voids exerts a temporary 

force on the face that may cause a blister. The void, or unbonded area, 

can then be easily located by visual inspection. 

Tests by this method of inspection were conducted on small (12-inch) 

and large (36-inch) square sandwich panels of various constructions. 

Tests on small panels.--The small panels had aluminum faces 0,012, 

0.020, or 0.032 inch thick glued to 1/4-inch end-grain balsa cores. The 

gluing process involved a primary cement cured on-the aluminum and a 

secondary phenolic glue between the primed aluminum and the balsa core. 

These panels were carefully prepared to include unbonded areas (no glue) 

and areas of poor bond (secondary glue only) at certain points that could be 

located by means of a template. Tension tests on similar areas of poor bond 

yielded erratic tensile values ranging from 0 to 220 pounds per square inch 

as compared to a normal tensile strength of 800 to 1,000 pounds per square 

inch for well-glued areas. The location and size of these defective areas 

are shown in figure 1. 

These panels were first thoroughly inspected by tapping, and a record 

was made of the defective areas thus located. This inspection was followed 

by a test consisting of exposure for 16 hours to air pressure of about 75 

pounds per square inch, succeeded by a quick release of pressure. The defects 

detected by visual inspection after this test were again recorded. Later an 

additional test for 24 hours at a pressure of 80 pounds per square inch, 

followed by quick release of pressure, was performed, and the results were 

again recorded. It was thought that a pressure test at elevated temperature 

(not exceeding the fabrication temperature) might bo more effective in 

locating poorly bonded areas, The panels, therefore, were exposed to an air 

pressure of 75 pounds per square inch at a temperature of 230° F. far 

periods of 3 and 20 hours. The results of inspections following each of 

these tests are presented in table 1. 

None of the poorly bonded areas could be Located by any of the tests, 

and only the larger unbonded areas were detected in most cases, It appeared 

also that the detection of unbonded areas became more difficult with an 

increase in face thickness, 

Report No. 1569 -6-
Air-transmission tests.--To determine the relative air-transmission 

rates of two types of cores, 3/4-inch buttons were glued to the outer face 

of two panels in various positions as shown in figure 3. A 1/8-inch hole 

was drilled through the button and face of the panel and a 1/8-inch copper 

tube soldered in place. The joints between the button, panel, and copper 

tube were tested for leakage by submerging the panel in water and applying 

a low air pressure to the loose end of the copper tubes. The panels were 

then placed one at a time in an autoclave and exposed to an air pressure of 

75 pounds per square inch. The copper tubes were threaded through a sealed 

fitting in the autoclave walls and mounted so that the end of each was 

directly under an inverted water-filled container as shown in figure 4. 

The air passing through the core to the various points at which the 

copper tubes were attached, was forced out and collected in the inverted 

containers. Two panels, 1/2 by 12 by 12 inches in size, one with aluminum 

faces glued to an end-grain balsa core and one with aluminum faces glued to 

a paper honeycomb core, were subjected to this test. The results, recorded 

in table 2, show that the paper honeycomb core is far more pervious to air 

than the balsa core. There also appears to be no relation between rate of 

flow and distance to the nearest edge. The variations within one panel are 

probably due to nonuniformities in the core materials, or to open-edge glue 

joints between individual pieces of core. 

Some core materials, such as cellular hard rubber and cellular 

cellulose acetate, have "closed bubble" constructions, which are undoubtedly 

almost impervious to air. Such core materials would require long exposure 

to air pressure before the air would penetrate any existing voids for an 

adequate test. Other core materials, such as glass cloth and cotton honeycomb, have open or "screenlike" weaves that offer little resistants to the 

flow of air and consequently produce little force on the core-to-face bond 

when the external pressure is quickly released. 

Tests on large panels.--The tests on large panels by this inspection 

method were made in a steel autoclave 40 inches in diameter. The panels 

were first thoroughly tested by tapping on both faces to determine any areas 

having voids before they were pressure-tested. Such areas were outlined with 

a wax-crayon line, The panels were then piled on a small car, with stickers 

placed between the panels to allow for air circulation and free expansion of 

the faces, and wheeled into the autoclave. Air was supplied from a line 

that maintained a pressure of between 60 and 75 pounds per square inch. For 

the stronger core materials, such as paper honeycomb and glass-cloth honeycomb, the pressure was applied as fast as the line would allow, but for the 

low-strength cores, such as cotton-cloth honeycomb, the pressure was brought 

up to 75 pounds per square inch in three equal steps at about 20-minute 

intervals. Pressure was maintained in the autoclave for 15 hours, which was 

assumed to be sufficient time for the pressure in the core (and possible 

voids) to become approximately equal to the line pressure. The exhaust 

valves were then opened wide, and the autoclave pressure was reduced to 

atmospheric pressure within 2 minutes. 

Report No. 1569 -7-
Of the 57 panels pressure-tested in this manner, 6 were found to be 

defective. One of these 6 had a hard-rubber core that crushed in several 

spots, presumably because the application of air pressure was too rapid. In 

the further testing of panels with hard-rubber cores, therefore, the threestep procedure of applying pressure was used, All the others that failed 

were panels with aluminum faces bonded to paper honeycomb cores with various 

adhesives. The failure in all cases was primarily in the glue line rather 

than in the core. This inspection method apparently applied higher stresses 

on panels having paper honeycomb cores than those having glass-cloth honeycomb or cotton-cloth honeycomb cores that were made with the same adhesives 

and methods of fabrication. "he glass-cloth and cotton-cloth honeycomb are 

more pervious to air and therefore allow the entrapped air to escape very 

rapidly without exerting appreciable pressure on the face. Figure 5 shows 

one of the panels having paper honeycomb core and aluminum faces that 

exploded in the pressure test. 

One panel, made with 0.020-inch aluminum faces bonded to paper honeycomb core with an adhesive composed of a thermosetting liquid and a thermoplastic powder, had an unbonded area in the middle, which was located by 

tapping (fig. 6). The panel was later subjected to the pressure test 

previously discussed, and a large blister (also outlined in fig. 6) was 

formed. From the shape of the blister it can be seen that the air pressure 

tore the faces from the core to form a channel to the edge of the panel, 

which allowed tho air to escape. 

The air-pressure method has an advantage over the tapping method 

because of the possibility that a very poorly glued area may fail when subjected to the stresses developed by the air pressure. No determinations 

were made as to the exact pressure that existed on the inside of the panels, 

but it undoubtedly varied according to the various types of construction. 

The loss of air under pressure in panels having aluminum faces is entirely 

through the core material, while in those having glass-cloth faces impregnated with a laminating resin and likely to be porous the loss occurs through 

both the core and the faces. From tho air-transmission tests it was found 

that the rata of air transmission through different core materials varied 

considerably (table 2). The resultant pressure when a panel is pressuretested will vary accordingly, and panels made of different materials will be 

subjected to different internal pressures, although the autoclave pressure 

is the same. 

Heating Complete Panel 

Heating a complete panel involved the use of heat to produce air 

pressure within a panel by the expansion of the air in the cells of the 

core and in any possible voids. Six glass-cloth-faced panels in a 

stickered pile were heated for 1 hour in a steam-heated oven equipped with 

a temperature recorder and a fan for circulating the air. Since the temperature for this inspection method (250° F.) was the same as the curing 

temperature used in fabricating the panels, the temperature alone should 

have had no damaging effect upon the panels. 

Report No. 1569 -8-
The six panels tested consisted of eight-ply glass-cloth faces 

impregnated with a low-viscosity laminating resin bonded to paper honeycomb, 

glass-cloth honeycomb, or cotton-cloth honeycomb cores, Two panels of each 

combination were included in the test. No effect was noticed upon the 

glass-cloth honeycomb and cotton-cloth honeycomb panels, but one paper honeycomb panel developed a few blisters. The other paper honeycomb panel 

developed some smaller light-colored areas that appeared the sane as 

blisters, but which could not be detected by tapping. 

Very little work was done with this method because the bond was being 

tested at a temperature higher than that expected in service conditions and 

the degree of pressure developed in the panels was questionable, The 

probability that the pressure on the inside of the faces varied with the 

materials making up the sandwich panel was indicated by the varying airtransmission rates. 

Local Heating 

The only equipment used for the local-heating inspection method was an 

electrically heated flatiron. The panels had aluminum faces glued to balsa 

or paper honeycomb cores and were previously tapped on both faces to determine any questionable areas. Both questionable and well-bonded areas were 

heated for a short period with the iron and then removed and tapped to determine the condition of the bond. In many cases blisters developed that could 

be seen by visual examination. 

This method was also considered unsatisfactory, however, because the 

bond was stressed at a high temperature and many of the adhesives were of a 

thermoplastic nature. Further, no simple way was found to determine the 

stresses produced on the bond by the pressure developed inside the panel, or 

by the thermal expansion of the heated portion of the face. 

Button-tension Test 

A mechanical pulling test for inspecting sandwich panels was developed 

at the Laboratory. It consisted of gluing an aluminum button to the face of 

a panel and applying tension to the button by means of a mechanical device 

operated by air pressure. The panels inspected by this means consisted of 

various combinations, including aluminum, plywood, and glass-cloth faces and 

balsa, cellular hard-rubber, cellulose acetate, and paper honeycomb cores. 

The panels were first inspected by tapping before the buttons were glued in 

place. 

Preparatory to gluing the buttons to the panels, the chosen spots on 

the panel and the face of each button were sanded with fine emery paper, 

cleaned with acetone, and coated with two or three applications of a hightemperature-setting thermoplastic resin adhesive. A drying period of about 

one--half hour elapsed between each coat and 1 to about 3 hours after the 

final coat. The buttons were glued in position with the aid of a vacum 

Report No. 1569 -9-
blanket containing a special adapter (fig. 7). A thermocouple to determine 

the temperature during the gluing operation was held with a balsa plug 

against the bottom of the hole in the button. The vacuum blanket was taped 

to the panel and a vacuum hose attached, The heating element consisted of 

a metal core surrounded by a coil, which, in turn, was connected in series 

with a lamp bank for varying the resistance of the circuit and thereby 

changing the heating capacity. The protruding end of the metal core of the 

heating element was fitted into a recess in tho special adapter of the 

vacuum blanket directly over the button. Although the metal core was 

closely fitted in the recess, the thin air film acted as an insulator; but 

by displacing this air with oil, efficiency was greatly increased. The 

complete arrangement used for gluing the buttons onto the panels is shown 

in figure 8. The buttons were glued on the panels at a temperature of 

275° F. for 20 minutes. 

A pneumatic pulling device was used to apply tension to the buttons 

as shown in figure 9. It consisted of a cylinder andpiston, with a rod 

leading from the piston for attachment to the buttons. The attachment was 

made by means of a loose-fitting pin. Air flow was regulated by a valve, 

and the air pressure in the cylinder was registered on a gage. Frictional 

resistance was held to a minimum by allowing air to leak past tho piston and 

piston rod. Constant pressure in the cylinder was maintained by adjusting 

the inlet valve to obtain steady-flow conditions. The area of the button was 

one-half square inch and the net area of tho piston was such that by multiplying the gage pressure by 19.1, the tension under the button could be 

determined in pounds per square inch. 

The ultimate tension values that were obtained at random from various 

exploratory panels are recorded in table 3 and, in most cases, are in the 

relative order of tensile strengths of these combinations as determined by 

normal tension-testing methods, except that the tension values obtained from 

the aluminum-balsa panel were abnormally low. More comparable values were 

obtained from buttons used as controls on the panel referred to in table 4. 

In addition, two specially constructed aluminum-faced panels, one 

with a balsa, core and the other with a paper honeycomb core, were fabricated 

with nine defective areas (fig. 10) to which buttons were glued and pulled 

with the mechanical pulling device, The ultimate tension results obtained 

on these panels are recorded in tables 4 and 5. In these tests, lower 

values were obtained for unbonded or poorly bonded areas than for well-bonded 

areas, 

The two types of core yielded different ultimate tensile values, 

which agreed fairly well with their respective tensile strengths. The maximum tensile stress that can be exerted by the button depends on the bond 

between the button and the face, but this bond appeared capable of consistently developing stresses in excess of 300 pounds per square inch. 

Report No. 1569 -10-
Insufficient tests were made to determine the safe load that might 

be applied in a nondestructive tension test of this type. Once this value 

is established, however, for any combination of face and core material, 

the button-tension test could be used as an acceptance test on flat or 

nearly flat surfaces. The buttons could later be removed by softening the 

thermoplastic glue bond with heat or a solvent. Because of the amount Of 

time and equipment required, this test is not practicable for a complete 

inspection method, but is rather for randon tests or for tests on highly 

stressed areas. The buttons are glued at 275° F., which may be higher 

than the curing temperatures of certain resins or adhesives and, therefore, 

on such panels the test is unsafe. 

Report No. 1569 -11-
Table 1.--Results of inspection of-small defective aluminum-to-balsa panels by- tapping, internal 

pressure, and a combination of internal pressure and heat 

1 Each 12- by 12-inch panel had four defective areas as shown in figure 1. 
Table 2.-Results of air-transmission tests on sandwich panels with 

aluminum faces on balsa and paper honeycomb cores 

1 Leak in joint; not tested 

Report No. 1569 
Table 3.-Preliminary results of button-tension test-on sandwich 

panels 

Report No. 1569 
Table 4,--Summary of results of button-tension tests on aluminum-balsa 

sandwich panel 

(continued) 

Report No. 1569 
Table 4.--Summary of results of button-tension tests on aluminum-balsa 

sandwich panel (continued) 

1 "Position" here refers to a defective area of definite size on the panel 

as shown in figure 10. 

2 First trial reached 898 p.s.i. without fracture, 

3First pull reached 1,012 p.s.i. without fracture. 

Report No. 1569 
Table 5.--Summary of results of button-tension tests on aluminun-paper 

honeycomb sandwich panel 

(continued) 

Report No. 1569 
Table of results of tests on aluminum-paper

honeycomb sandwich panel. 

1Position here refers to a defective area of definite size on the panel 

as shown on diagram (fig. 10). 

2Aluminum tore from core at 38 p.s.i. 

Report No. 1569 
Figure 1.-Locationand diameter of defective areas in small test 

panels. 




Figure 6.--Sandwich panel of aluminum-to-paper honeycomb with an 

outlined defective aea determined by tapping, and the blister 

formed after subjecting the panel to the internal-pressure test. 

(The boack bands are stripes of a metal-to metal glue applied to 

the aluminum before the panel was fabricated.) 



Figure 10.-Locationand diameter of defective areas in large test 

panels.