Supplement to 

REPAIR OF AIRCRAFT SANDWICH 

CONSTRUCTIONS 

May 1950 

This Report is One of a Series 

Issued In Cooperation with the 

AIR FORCE-NAVY-ClVIL SUBCOMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AIRCRAFT COMMITTEE 

of the 

MUNITIONS BOARD 

No. 1584-A 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5, Wisconsin 

In Cooperation with the University of Wisconsin 
Supplement to 

REPAIR OF AIRCRAFT SANDWICH - CONSTRUCTIONS1,2 

By 

A. A. MOHAUPT, Technologist 

and B. G. HEEBINK, Engineer 

Forest Products Laboratory,3 Forest Service 

U. S. Department of Agriculture 

Summary 

This work was undertaken in an attempt to improve the appearance and 

tensile strengths of repairs to sandwich-type constructions having glasscloth reinforced facings similar to those presented in Forest Products 

Laboratory Report No. 1584. Methods employing room- temperature-setting 

resins and contact pressure resulted in repairs considered to have satisfactory tensile strength and containing no entrapped air pockets. 

The repair methods involving vacuum blankets on sandwich panels with 

glass-cloth facings, although showing improvement on previous results, were 

still relatively low in strength and often contained air or gas pockets between the plies of the repair facings. On an experimental scale, this 

difficulty was overcome by placing the entire panel to be repaired in a 

rubber bag and applying vacuum pressure, but this method might be impracticable on large structures. 

Repair strengths were improved by increasing the number of plies in 

the repair-patch facings above that of the six-ply panel facings or by increasing the width of the scarfed area. Repair efficiencies of more than 90 

percent were obtained with repairs having seven-ply facings on a 2-1/2-inch 

scarf, or six-ply facings on a 3-inch scarf. 

A core replacement without facings was acceptable when one facing of 

the sandwich panel was repaired at a time. A core replacement with facings 

was desirable if only contact pressure was being used in a one-step process 

in which both facings were applied and cured at the same time. 

1This progress report is one of a series prepared and distributed by the 

Forest Products Laboratory, under U. S. Navy, Bureau of Aeronautics 

(No. NBA-PO-NAer 00619) Amend. No. 1, and U. S. Air Force (No. 

USAF-PO-(33-036)48-41E). Results here reported are preliminary and 

may be revised as additional data become available. 

2This report presents information supplemental to Forest Products Laboratory 

Report No. 1584 of the same title, 

3Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 

Report No. 5384-A Agriculture- Madison 
A method of repairing curved, solid, glass-cloth laminates was devised 

for use with room-temperature-setting resins. The repairs appeared to be 

satisfactory, but no strength or rain-erosion tests were conducted. 

Introduction 

The purpose of this study was to devise improved methods of making 

repairs in sandwich-type constructions having glass-cloth-reinforced plastic 

facings, and to evaluate, by inspection and tensile tests, the quality of 

the repairs developed. The work was performed mainly on flat sandwich panels 

with various core materials, and some supplementary work was done on curved 

specimens of solid glass-cloth-reinforced laminates. One type of construction used for radomes consists of glass-cloth-reinforced. facings on glasscloth honeycomb core, and a considerable portion of the work was done on this 

combination. The methods of repair discussed in this report are based on the 

assumption that both sides of the sandwich panel are accessible. 

Sandwich Constructions 

The three combinations of aircraft sandwich construction used for 

evaluating repair techniques were: glass-cloth facings on balsa core, glasscloth facings on paper honeycomb core, and glass-cloth facings on glass-cloth 

honeycomb core. The panels had cores 1/4 inch thick and six-ply facings of 

112-114 cloth, cross-laminated and impregnated with resin A, a hightemperature-setting, high-viscosity, contact-pressure laminating resin of the 

polyester type. 4 The glass-cloth facings were impregnated 

with resin approximating 45 percent of the total weight, of resin and glass 

cloth. The panels were made by wet laminating the facings, assembling the 

panels on a flat aluminum mold, and bag molding them under vacuum pressure 

at a temperature of 225° F. for one hour. 

Repair Techniques 

The several methods that were devised to make repairs in glass-clothfaced sandwich panels can be classified as (1) high-temperature or (2) roomtemperature methods. In either case, the assumption was made that both sides 

of the panels were accessible. 

Preliminary Preparation of Sandwich Panels 

The size of the repair area, which was oval in shape, was 2-1/2 by 

8-1/2 inches and was the same for all panels (fig. 1). Preliminary preparation of the repair area consisted of cutting holes at the end of the oval 

-Appendix, note 1. 

Report No. 1384-A -2-
repair by means of a centerless hole saw in a hand brace, of sawing out the 

damaged area with a reciprocating-saw attachment for an electric drill, and 

of scarfing the area around the hole with a flexible-disk sander. The 

fiber sanding disks used were coated with abrasive grit No. 80. 

High-temperature Methods 

High-temperature procedures, summarized in table 1, required heat 

normally used to cure the resins in accordance with the manufacturer's 

instructions. In this series resin A was used end cured 

at a temperature of 225° F , for about 1 hour. 

Method 1.--Repairs by method 1 were made on sandwich panels with 

glass-cloth facings on balsa and on paper honeycomb cores. After Preliminary preparation of the repair panels, core replacements or plugs were cut 

to the correct size and shape and fitted carefully into place. These core 

plugs consisted of sandwich panels with two-ply glass-cloth facings on balsa 

and on paper honeycomb cores. The facings were sanded with coarse sandpaper 

or emery cloth to provide a clean roughened surface. Patches for the facings 

were made with six plies, the same number as used in the original facings. 

Glass cloth was impregnated with 55 to 60 percent of resin A 

and stretched out smoothly on a sheet of colored cellophane. A second sheet 

of cellophane was placed on top of the glass cloth to facilitate handling. 

The outlines of the various plies of the patch were scribed on the cellophane 

and cut out with a scissors. The first ply was large enough to cover the 

core plug and to lap the scarfed facing about 1/2 inch around the entire 

repair area. Each successive ply was progressively larger, with the final 

ply covering the 2-inch scarf, 

With the core plug in place, resin A was brushed on 

the core plug and on the scarfed area of the panel. After removing the cellophane from one side of the smallest ply of the patch it was centered on the 

repair area. The cellophane was then removed from the other side of the 

first ply and the wrinkles and air bubbles were worked out. The rosin that 

adhered to the cellophane reduced the resin content of the impregnated glass 

cloth approximately 10 percent. The second and successive plies were laid 

in the same manner until the patch was completed. The entire repair area 

was covered with cellophane, which was trimmed about 1 to 2 inches beyond the 

repair and over which an electrically heated, cloth-surfaced, vacuum blanket 

was taped. The same procedure was followed to repair tho other facing. A 

vacuum was drawn on the two blankets, and the temperature of each was raised 

gradually to 225° F. and maintained there constantly for 1 hour. 

Method 2.--Method 2 was also carried out on sandwich panels of glasscloth facings on balsa and on paper honeycomb cores. It differed from method 

1 in that seven-ply repair facings were used instead of six-ply facings. All 

other details were the same. 

Method 3.--Method 3 followed the same general procedure as method 1, 

but the number of plies in the facing repair was increased from six plies to 

eight plies and the scarf was increased from 2 inches to 2-1/2 inches wide. 

Report No. 1584-A -3-
Method 4.--The sandwich construction used for method 4 consisted of 

glass-cloth facings on balsa core. The details of this method were similar 

to those of method 1, but the method incorporated a 2-1/2-inch scarf with a 

seven-ply repair patch. The entire panel was placed in a rubber bag, as in 

normal bag-molding procedures, and the repair was cured under vacuum pressure in an air-circulating oven at a temperature of 225° F. for 1 hour. 

Room-temperature Methods 

Repairs made with vacuum blankets at high temperatures were not reliable because expanded air or gas often was trapped between the plies of the 

repair patch. In an attempt to eliminate this, room-temperature-setting 

resins were tried under positive pressure or under contact pressure only. 

There are several promoters or accelerators available that can be used in 

conjunction with various catalysts to produce cure of a polyester resin at 

room temperature or with the application of very little heat. In order to 

obtain a longer pot life of these resins the amount of accelerator recommended by the manufacturer was reduced. Because of the uncertainty of the 

degree of cure with the reduced amount of accelerator, the repairs were given 

a final cure in an oven or under a bank of' heat; lamps. The details of repair 

methods employing room-temperature cure are described in the following 

paragraphs and are summarized in table 1. 

Method 5.--The damaged areas of two sandwich panels with six-ply 

glass-cloth facings on 1/4-inch balsa or paper honeycomb cores were prepared 

in method 5 as referred to under Preliminary Preparation of Panels. Oval 

core plugs 2-1/2 by 8-1/2 inches consisting of two-ply glass-cloth facings 

on a balsa or a paper honeycomb core were fitted into place in their respective panels and the facings were roughened with coarse sandpaper or emery 

cloth. The repair patches for the facings were composed of seven plies 

impregnated with the low-viscosity, contact-pressure polyester resin B5 

mixed with 1.6 percent benzoyl peroxide catalyst and 3.0 percent promoter. 

The oval-shaped glass-cloth plies were prepared in the same manner as discussed under method 1, except that the room-temperature-setting resin B was 

used in place of resin A. Resin was applied to the core plug and the scarfed 

area of the repair panel. The first ply covered the core plug and 1/2 inch 

of the scarfed area on all sides. Each successive ply was progressively 

larger, and the final ply covered a 2-1/2-inch scarf. After the final ply

was laid down and the wrinkles were removed, a piece of cellophane was placed 

over the whole patch and some of the air in the repair patch was worked out 

with a hardwood squeegee. Both facings were repaired in this manner, and the 

sandwich panel was placed in a rubber bag and cured under positive air pressure (14 pounds per square inch) overnight at a temperature of 100° F. To 

insure a complete cure, the repair panel was given an additional cure of 

1 hour at 225° F. in a circulating oven. 

Method 6.--The sandwich construction for method 6 consisted of six-ply 

glass-cloth facings on 1/4-inch glass-cloth honeycomb core. As in method 5, 

5Appendix, note 2. 

Report No. 1584-A -4-
a seven-ply repair patch impregnated with resin B mixed with a roomtemperature promoter was used on a 2-1/2-inch scarf. After the repair patch 

was laid up and covered with cellophane, it was cured at room temperature 

without pressure for a minimum of 16 hours. This initial cure was followed 

by a final cure for 2 hours under a bank of heat lamps producing a temperature of about 175° F. on the exposed surface of the panel. 

Method 7.--With use of glass-cloth facings on glass-cloth honeycomb 

sandwich construction, the same procedure was followed in method 7 as was 

described under method 5, except that a core plug without facings was employed and the intial cure of the repair patch was made at room temperature 

without pressure. The core plug without facings was employed in an attempt 

to minimize the variation in thickness of the facings. 

Method 8.--Method 8 differed from method 5 in several respects. The 

procedure involved a two-step instead of a one-step repair on a sandwich 

panel of glass-cloth facings on glass-cloth honeycomb core. The two-step 

repair consisted of fabricating and curing one facing (with the core plug in 

place) before starting the second facing. The core plug was composed of only 

the core material. The resin used for this repair was a low-viscosity, 

contact-pressure laminating resin C,6 formulated for curing at room temperature with 22 parts of styrene, 1.2 parts of 1-hydroxycyclohexyl hydroperoxide-1, and 3 parts of an accelerator. The initial cure was at room 

temperature with contact pressure overnight, and was followed by a final cure 

at a temperature of 225° F. for 1 hour in an oven, 

Method 9.--In method 9 the damaged area of a sandwich panel of glasscloth facings on glass-cloth honeycomb core was sanded to produce a 3-inch 

scarf. A core plug consisting of a sandwich panel with single-ply facings 

was fitted in place. The repair patches for the facings were prepared and 

fabricated as discussed under method 5, except that resin C, mixed with 22 

parts styrene, 1.2 parts 1-hydroxycyclohexyl hydroperoxide-1, and 1.5 parts 

of a room-temperature-setting accelerator, was used, The number of plies in 

the repair was also reduced to six. The resin had an initial set within 16 

hours at room temperature and was given a final cure in an oven at a temperature of 225° F. for 1 hour. 

Minor Repairs to Solid Laminates 

Erosion of the plastic parts in frontal areas of high-speed aircraft 

by rain has caused serious trouble with glass-cloth-reinforced laminates. 

The erosion of these parts varies from shallow pits of very small diameter 

to large eroded areas that penetrate several plies of the laminate. Methods 

of repair for these parts, which would be equally applicable to similar 

defects of glass-cloth-laminate facings of sandwich parts, were explored. 

These eroded areas of typical-sample test pieces were sanded with a small 

6Appendix, note 3. 

Report No. 1384-A -5-
emery disk or a small emery stone to provide a clean surface having beveled 

edges. The resin used for repair was resin B compounded 

with 3 percent of a room-temperature-setting promoter. 

For the small holes, some of the room-temperature-setting resin was 

mixed with 15 to 20 percent of chopped glass fibers and the mixture was 

worked in the holes carefully to eliminate as many small air bubbles as 

possible, and it was then covered with cellophane to prevent contact of the 

resin with the atmosphere, After an overnight cure at room temperature, the 

cellophane was removed and the surface was sanded smooth. Usually an excess 

of the mixture of resin and glass fibers was used to allow for shrinkage and 

sanding . 

The larger beveled repair areas were also repaired with the roomtemperature-setting resin. Patches of 112-114 glass cloth were cut to the 

shape of the repair area. The number of plies required depended upon the 

depth of the repair. As in the repairs to sandwich facings, the first ply 

was the smallest arid the size of each successive ply increased in accordance 

with the slope of the beveled surface. The repair area was coated with the 

room-temperature-setting resin, and the first ply of dry cloth was laid in 

place and brushed with additional. resin. The successive plies were treated 

in the same manner. In irregular places where the glass cloth did not fill 

in properly, resin mixed with chopped glass fibers was added. After the 

repair was built up to the proper level, it was covered with cellophane and 

the air was worked out with a hardwood squeegee. Curing and finishing were 

done as described for the small pits. 

Test Method 

Three longitudinal-tension specimens were taken across the repair 

area, and three control tension specimens were cut from outside the repair 

area, as shown in figure 1. The tension specimens were 1-1/2 by 16 inches 

with the center portion tapered to 1 inch wide to reduce concentration of 

stresses near the grips. In order to prevent crushing of the ends held by 

the grips of the testing machine, the core was sawed out and a hard-maple 

block was inserted between the facings. 

No tests were made to determine the efficiency of the repairs made on 

the solid glass -cloth laminates. 

Results and Discussion 

The results of the longitudinal-tension tests for the various repair 

methods discussed are summarized in table 2. Methods 1 to 3, inclusive, involving the heated vacuum blankets for obtaining curing temperatures and 

pressure, produced results that were erratic, partially because of the varying amounts of air that were being drawn from the core through the facings 

during the cure. An attempt was made to cut the test specimens from the 

Report No. 1584-A -6-
unblistered portions of the repairs. The methods of repair involving the 

placement of the entire panel in a rubber bag (method 4) produced a better 

appearance because there were fewer air or gas pockets in the facings. Pressure was not always necessary in making a satisfactory repair as is shown by

methods 6 to 9, which utilized room-temperature-setting resins. A number of 

repairs were made under contact pressure only, but if heat was applied before 

the resin was initially set, the contact pressure was insufficient to prevent 

gas pockets from forming between the plies of cloth. After the initial set, 

heat only was applied without any visible harmful effect. 

The amount of promoter or accelerator and the working temperature had 

considerable effect on the working life of the resin. In one case the 

amount of accelerator recommended by the manufacturer reduced the pot life 

of the resin at room temperature to such an extent that it was impractical 

for repair use. With a little experience it would be possible, however, to 

determine the amount of accelerator required for a certain temperature range. 

From previous tests, reported in Forest Products Laboratory Report 

No. 1584, "Repair of Aircraft Sandwich Constructions," the maximum repair 

efficiency that was obtained from sandwich panels with glass-cloth facings 

was 69.6 percent. The average efficiency was about 61 percent. These repairs were composed of six plies (in panel facings of six plies) on a 2-inch 

scarf, with the first ply being the same size as the repair plug. Method 1, 

which also consisted of a six-ply repair on a 2-inch scarf, but with the 

first ply being 1/2 inch larger than the core plug on all sides, gave an 

average repair efficiency of approximately 76 percent. In method 2 the 

number of plies in the repair was increased to seven and the repair efficiency was increased to about 85 percent. It was observed that the major 

portion of failures in these two methods was in shear along the scarf. To 

overcome this difficulty the scarfed area was increased to 2-1/2 inches. 

Method 3 utilized the 2-1/2-inch scarf and an eight-ply repair. This combination resulted in an average repair efficiency of about 100 percent, and 

also produced a majority of tension failures in the facing material. In 

other repairs (methods 4 to 8, inclusive,), a seven-ply repair was used on a 

2-1/2-inch scarf, and repair efficiencies of 91.8 percent and higher were obtained. The repair efficiency of method 9, in which the number of plies in 

the repair was reduced to six and the scarfed area was increased to a 3-inch 

width, was 95.8 percent. This last method is more desirable for radome construction, since it is desirable that the facings and core thicknesses remain 

unchanged. 

Core plugs of two constructions were used in the repairs. One type 

consisted of the core material that was used originally in the sandwich panel, 

and the other was similar but with facings of one or two plies of glass cloth. 

A core plug without facings was satisfactory for tho two-step process where 

one facing was fabricated and cured at a time (method 8). It might also be 

applicable when positive pressure is used, as in method 5. When, however, a 

one-step process with room-temperature-setting resins was used on a core plug 

without facings (method 7), the air included in the core when the second 

facing was started, made it difficult to keep tho opposite facing from 

separating from the core. This difficulty was eliminated when a core plug 

with facings was used. 

Report No, 1584-A -7-
No strength tests were conducted on the repairs made on the solid 

glass-cloth laminates. Visually, the repairs appeared satisfactory. One 

difficulty experienced in this type of repair was the elimination of small 

air bubbles from the mixture of resin and chopped glass fiber. The fibers 

made the mixture so viscous that the air bubbles would not rise to the surface on standing, and, therefore, the mixture had to be applied gradually 

so that the air could be worked out. 

Appendix 

Note 1. Resin A - A high-temperature-setting, high-viscosity, contactpressure laminating resin of the polyester (diallyl phthalatealkyd) type. 

Note 2. Resin B - A high-temperature-setting, low-viscosity, contactpressure laminating resin of the polyester (styrene-alkyd) type. 

Note 3. Resin C - A high-temperature-setting, low-viscosity, contactpressure laminating resin of the polyester (styrene-alkyd) type. 

Report No. 1584-A -8-
Table 1.--Summary of details of repair methods 
Table 2.--Tensile strength of repairs in sandwich panels having glasscloth facings1 

Report No, 1584-A 
Figure 1.--Cutting diagram for test panels with glasscloth facings, showing location and shape of repair 

area and tension specimens. 

(ZM 84227 F) 

Report No. 1584-A