kGRICULTUR E ROOM
teNG-11IPM CASE STUDY CIF SANDWICH
PANEL CONSTRUCTION 1114
FIL EXPERIMENTAL UNIT
October 1959
No. 211G5
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UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
FOREST SERVICE
MADISON 5, WISCONSIN
In Cooperation with the University of Wisconsin
LONG-TERM CASE STUDY OF SANDWICH PANEL
CONSTRUCTION IN FPL EXPERIMENTAL UNIT!
By
L. J. MARKWARDT, Assistant Director
and
LYMAN W. WOOD, Engineer
2 Forest Service
Forest Products Laboratory,—
U. S. Department of Agriculture
Introduction
Although structural sandwich construction has attained its well-deserved
recognition only in recent years, its concept and a vision of its possibilities are not new. An efficient sandwich composed of metal facings
and a plywood core was produced commercially some four decades ago,
and no doubt there were other even much earlier applications. World
War II witnessed one of the most spectacular modern structural sandwich applications in the design of the mosquito bomber by DeHaviland,
employing birch plywood facings with a lightweight balsa wood core.
The possibilities through structural design of utilizing materials more
efficiently and of achieving lightweight construction constitute the impelling challenge of structural sandwich construction. The relatively
recent significant advances in synthetic resins and adhesives that
fostered glued-laminated construction, the development of fabricating
techniques, and the post-war production and availability of a great variety of facing and core materials, have ushered in a new era of structural sandwich constructions unlimited.
1
—Presented at Building Research Institute Conference on Sandwich
Panel Design Criteria, Washington, D. C, November 17-19, 1959.
—Maintained at Madison, Wis. , in cooperation with the University of
Wisconsin.
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Background
Some of the early developments in structural panel design with special
application to prefabricated houses employed the stressed-skin principle of construction. This is an efficient form of construction in
which the siding or facing serves to carry stress, in contrast to its
function as a parasite covering in most types of conventional construction. The stressed-skin principle, which in one form or another has
been extensively employed in prefabricated buildings, involves the
bonding of the skins to longitudinal stringers, simulating box beam
design. Stressed-skin construction should not be confused with sandwich construction, which involves the concept of a distributed and continuous core. One definition of sandwich construction that has hada
great deal of study by a great many engineers in the aircraft, materials, research, and construction fields is as follows:
"A construction comprising a combination of alternating dissimilar
simple or composite materials assembled and intimately fixed in relation to each other so as to use the properties of each to specific
structural advantages for the whole assembly."
The shortage of housing in the immediate postwar period and the potential of increasing production through prefabrication led to early
consideration of the possibilities of structural sandwich design. Among
the first prefabricated houses that were developed employing sandwich
panels were the homes produced by Lincoln Industries, Inc. , Marion,
Virginia. The panels were constructed of aluminum facings bonded to
paper honeycomb core of the expanded or Christmas-bell type. A
report on these homes is presented in a companion paper.
Research Problems
The application of a relatively new and untried construction to housing
naturally raises a great many questions--how can effective bonding be
achieved in the attachment of the facings to the core; what type of adhesives should be used to insure the long serviceability required for
house construction; what type of paper should be employed in the paper
honeycomb and what pH should be required; how long should paper be
expected to maintain its integrity and function structurally as a honeycomb core; what provision should be made to develop resistance to
high humidities or possible wetting of the core; could improved serviceability of the core be achieved through resin impregnation and
what degree of impregnation would be required; what is the effect on
the honeycomb core of installing pipes for radiant heating within
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the panel; what are the strength and structural characteristics of sandwich panels of different construction; what principles of design can be
employed; and what accelerated aging tests, if any, could be used to
aid in evaluating performance and long-range serviceability?
The need for information on these and related questions led to the
development of an extensive research program extending -over .a number of years, and one which is still going on. 'Reports of research
results relating to many of these specific problems are now available
and serve as a basis for greatly improved technical developments in
structural sandwich panel design.
It was recognized that even with the extensive research seeking an
answer to the many questions and with the encouraging results of accelerated aging tests, there was still a question as to how far accelerated aging tests could be depended on to give an indication of longtime serviceability. It is obvious that the most effective and convincing answer to the question of serviceability and durability is through
a record of actual performance over a long period of time.
Experimental Unit Employing Sandwich Construction
To obtain such necessary long-time exposure and service test data on
sandwich constructions, it was decided to erect an experimental test
unit to simulate house conditions. Such a unit was accordingly built
on the grounds of the Forest Products Laboratory in 1947 and equipped
with cold weather temperature and humidity control in order to create
conditions of exposure similar to those existing in actual dwellings
for these long-time exposure tests.
Some of the design details necessarily differed from those that would
be employed in an actual house. For example, provision needed to be
made for the possible removal and replacement of individual panels
without disturbing the rest of the construction. Also the desirability
of installing the different sandwich panels so that each could react
independently was apparent. The joints between roof and wall panels
were accordingly designed for easy removal and to allow unrestrained
bending or bowing of each panel individually in response to thermal
effects or moisture changes.
Because of the research nature of this project, detailed technical information was obtained on the strength and stiffness of each of the variety of panels that have entered into the construction, and accurate
records have been kept of the performance since erection. The FPL
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experimental unit comprises one of the most extensively documented
•constructions employing structural sandwich construction in existence,
both from the standpoint of technical details of the strength and stiffness of the structural units, but also with respect to continual measurements of such features as moisture content of the wood components,
temperature and temperature effects, and bow of the panels. Measurements of bow of each of the wall and roof panels have been made
regularly over a period of years.
General Features
The overall size of the experimental unit is 12 feet 6 inches by 38 feet
6 inches. The interior is divided into two 12-foot by 15-foot rooms
and one central 8-foot by 12-foot utility room. The two side walls
are constructed of 20 sandwich panels, installed north and south in
matched pairs. Ten sandwich panels were used for the roof. Four.
of the wall panels and two of the roof panels had aluminum facings;
all the others had plywood or other wood-base facings. Three panels
were used for each end of the building. The three panels on the east
end were of sandwich construction, but the three on the west end were
of stressed-skin construction. One of the panel units in each end contained a window. An exterior door is on the north side and opens into the utility room; the opposite panel on the south contains a window.
Another special feature of the construction was the use of sandwich
panels over a crawl space for the floor of the east room, with copper
heating pipes installed in the panels during fabrication to determine
the effect of radiant heating with hot water on the long -range performance. The west room has a concrete subfloor with radiant heating to
permit study of wood finish floors under such heating conditions.
The complete test unit is shown in figure 1.
Details of Panel Design
•
•
Cores. -- The cores of all sandwich panels were paper honeycomb, but
two different kinds were used. These may be designated the expanded,
or Christmas bell type, and the corrugated paper type. The expanded,
or Christmas bell type, represents a long established fabrication
principle in which sheets of paper assembled flatwise are bonded along
continuous narrow bands at regular intervals across the sheet, the
bonds being staggered in adjacent sheets (fig. 2). Such material can
be readily fabricated with as many sheets or in such thickness as required, and with any desired size of honeycomb cell. Segments of
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the bonded sheets are cut off in accordance with the required thickness
of the sandwich. The segments are then expanded to develop the honeycomb pattern, and bonded to the facings of the sandwich panel with
the axis of the honeycomb perpendicular to the facing. These cores
of the expanded type were used in the aluminum-faced panels only.
In the corrugated paper type of core, the corrugations or flutes are
formed by running the flat sheet through a corrugating machine such
as is used in the fabrication of container fiberboard. The corrugated
sheets are then assembled and bonded at the node to form a block of
honeycomb material. Segments of the block are sawn off to provide
core of the desired thickness. Alternatively, the corrugated sheets
may be assembled to provide a number of different honeycomb patterns.
Three such patterns used in the sandwich panels made at the Laboratory are illustrated in figures 3, 4, and 5.
Paper used inthe corrugated-paper core is a typical kraft paper weighing about 53 pounds per ream (24 by 36 inches), impregnated with
about 15 percent of a water-soluble phenolic resin. Bonding of the
sheets composing the core was with an acid-catalyzed phenolic resin.
Bonding of the core to the facings was with an acid-catalyzed intermediate -temperature -setting phenolic resin.
The expanded core was commercially manufactured of kraft paper impregnated with a thermosetting phenolic resin. It was bonded to the
aluminum facings with a phenol -vinyl adhesive.
Cores designated PN are made of resin-treated corrugated paper as-.
sembled and glued at the nodes with the direction of all of the flutes
parallel. Segments the thickness of the panel are then sawn from
the block and assembled in the panel with the axis of the honeycomb
cells perpendicular to the facings.
The core designated XN consists of resin-treated corrugated kraft
paper sheets glued together so that the corrugations of adjacent sheets
are at right angles, the assembly being sawed into panel thicknesses
and laid on edge between the facings. In such assembly the axis of the
flutes in alternate layers are perpendicular and parallel, respectively,
to the plane of the facings.
Cores designated XF are similar except that as placed in the panel the
flutes are all parallel to the plane of the facings, but with the flutes
of alternate layers parallel and perpendicular to the length of the
panel, respectively. This kind of placement is obviously weak in
flatwise compression, but has better thermal insulation properties.
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Walls. --Wall panels under test include both 4- by 8-foot sandwich
panels, 3 inches thick, with plywood or veneer facings, which were
made in the Laboratory, and 3- by 8-foot panels, 2 inches thick,
with aluminum facings which were commerciall-y fabricated. Wall
panels were designed for a wind load of 20 pounds per square foot.
Some of the Laboratory- made panels have resin-treated paper overlays. All of the panels in the north, south, and east walls are of
sandwich construction; the same combination of core and facing types
are used in matched pairs in both the north and south walls. For
comparative purposes, the west wall was built of insulated 4- by
8-foot stressed-skin panels, consisting of plywood facings glued to
a wood framework.
Five types of facings were used in the Laboratory-made panels:
(1) 1/4-inch, three-ply Douglas-fir of exterior type, Sound-2-Sides
grade; (2) 1/4-inch, three-ply Douglas fir, exterior type, Sound2-Sides grade, with a resin-treated paper overlay on one face; (3)
two-ply Douglas-fir of 1/10-inch veneers with the grain of the veneers at right angles and a resin-treated paper overlay on one
side; (4) single ply 1/8-inch Douglas-fir veneer with resin-treated.
paper overlays on both sides; and (5) 3/8-inch five-ply Douglas-fir,
exterior type, Sound-2-Sides grade (for floor panels).
Ten panels representing all types described were installed in both
the north and the south wall S of the test unit. The east wall consists
of three panels with cores of type XN and 1/4-inch, three-ply Douglasfir facings. As already mentioned, the west wall consists of stressedskin panels. One interior partition consists of uninsulated stressedcover panels, and the other is made of three sandwich panels containing type XN cores and 1/8-inch veneer facings overlaid on both
sides with resin-treated paper.
Panels in the north and south walls are fastened together with continuous 3/4-inch top and 1-5/8-inch bottom plates seated in grooves
formed by projecting the facings beyond the cores at top and bottom of
the panels. These grooves also receive cleats glued to the roof panels
over the top plate. Fastenings were made with roundhead screws
with cut washers. Because the panels were actually 1/2 inch less
than the nominal 3- or 4-foot width, a 1/2-inch space remained between each pair, which was filled with felt strip insulation. Vertical
joints were taped inside and out with flexible waterproof tape.
Roof. --Ten sandwich panels were used for the roof of the experimental
unit. These are 14 feet long and span the width of the structure with
9-inch overhangs. Panels were designed for a load of 25 pounds per
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square foot. The eight laboratory-made roof panels are 4 feet wide
and 4-1/2 inches thick, with facings of 1/4-inch, three-ply Douglas fir; and all three of the corrugated paper type cores used in the
wall panels are represented. Three of the eight panels thus made
have ventilating flues 2 by 3 inches in cross section and 6 inches
apart which run lengthwise through the panel. The facings of one
of the ventilated panels have paper overlays, and one was given two
coats of aluminum paint with an additional coat of inside white paint
on the interior face.
The other two roof panels were factory made. They are 3 feet wide
and 3 inches thick, with aluminum facings on expanded paper honeycomb cores.
The roof panels are attached to the walls by means of glued cleats
I inch thick that seat over the 3/4-inch plates in the grooves in the
top edge of the wall panels. No fastening is provided between roof
panels but a felt gasket is installed between each pair. Those directly over partitions are seated in them with cleats. A metal roof
cover-seam is first covered with a water-resistant pressure tape
and then a metal cap strip. This permits easy removal of any roof
panel when required.
The assembly method is such that it permits roof panels and exterior wall panels to bow inward or outward. Cleats of the wall and
roof panels that fit into the partition walls prevent crosswise movement.
Floors. --The east room, underneath which is the crawl space, is
floored with sandwich panels 12 feet long by 3 feet 8-1/2 inches wide
by 6 inches deep, with type XN cores and 3/8-inch, five-ply Douglasfir facings. The panels were designed for a load of 40 pounds per
square foot. Copper hot water heating pipes were installed in these
panels during manufacture to determine the effect of radiant heating
upon them, (fig. 6). Panels are connected together with an insulated
spline similar to that used in the east and west walls. Supplementary
provision is made for hot-water baseboard heating along the exterior
walls in this room.
The west room has a concrete subfloor in which radiant heating pipes
are placed 1 inch below the upper surface. The concrete was laid
on a 6-inch gravel fill poured over roll roofing. Asphalt-impregnated
and untreated wallboard insulation was placed along the outside walls
and a 6-inch reinforcing mesh was laid over the heating pipes. Wood
sleepers are spaced 16 inches on center and anchored to the concrete
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subfloor. Strip flooring (25/32- by 2-1/4-inch oak) was laid over the
sleepers; sections of this flooring were installed at 3, 6, 9, and 12
percent moisture content to study the optimum moisture content of
floors subjected to the effect of radiant heating.
Doors and Windows. --Three flush doors were made with cores of type
XN. The exterior door had a birch frame and two-ply cross-banded
birch facings made of 1/ 16-inch veneer. The interior doors had
Douglas-fir frames, and one was covered with the paper-overlaid
1/8-inch Douglas-fir veneer and the other with the overlaid twoply Douglas-fir. A glue of the same general type used in the panels
but made by a different manufacturer was used in these doors. Wall
panels with door or window openings were made up with rough frames
for the openings glued in place when the panels were pressed.
Details of Fabrication and Assembly
All but the six aluminum-faced panels used in the FPL experimental
house were fabricated at the Forest Products Laboratory. The aluminum-faced panels were obtained from a commercial source, and complete information is not available on the method of assembly and the
bonding. The aluminum facings were 0.02 inches in thickness and
smooth faced.
In fabricating the sandwich panels with veneer and plywood facings at
the FPL, a considerable amount of hand labor was required because
of the variety of facings and cores employed, and because of the need
of exploratory tests to determine suitable methods and materials.
Fabrication of Cores
A typical kraft pulp made in large quantities for fiberboard boxes and
other purposes was selected as the base material for the honeycomb
cores, primarily because it is extensively produced, and also because
its durability compares favorably with that of most other types of
paper. Strength tests indicated that a ream weight of 45 pounds would
be adequate. Although a lighter paper probably would have sufficient
strength for certain types of panels, the 45-pound paper would minimize corrugating difficulties and be easier to handle during core fabrication than a lighter paper.
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The paper was treated with a water-soluble phenolic resin to provide
the necessary strength under wet conditions and to make it resistant
to attack of decay fungi. Exploratory tests showed that, while a
lower resin content would probably give adequate improvement in
wet strength, about 15 percent of resin based on the weight of the
treated paper would be necessary to impart adequate decay resistance. At this resin content, cores made from this paper retained
about 60 percent of their dry compressive strength when soaked in
water. Compared with dry, untreated paper, the resin-treated
paper had about 90 percent as much tensile strength in the water
soaked condition.
Unsized kraft paper 4 feet wide, obtained from a commercial source,
was impregnated with resin on a commercial machine. The web of
paper was passed through a resin bath, then through squeeze rolls
to remove excess resin, and dried in a chamber having two heating
zones, one at 275 0 F. and the other at 290 0 F. The resin was diluted
to about 40 percent concentration by weight in a mixture half water
and half alcohol. Use of alcohol in the diluent was found helpful in
preventing breaking of the paper web as it passed through the drying
tower.
The finished core blocks were band sawed to the required size within
a tolerance of 0.015 inch considered necessary for satisfactory gluing to the panel facings. Cores for wall panels were sawed to thickness of 2-1/2 to 2-11/16 inches according to the type of facing later
to be glued to them; those for roof panels were sawed 4 inches thick,
and those for floor panels 5-1/4 inches thick. Tolerance was checked
with a jig. After being sawed, the core strips were cleaned of sawdust with an air jet and cut to desired lengths with a circular cutoff
saw.
Fabrication of Facings
Douglas-fir plywood used for the facings and the 1/8-inch veneer were
obtained commercially. The two-ply cross-banded facings were made
from 1 / 10-inch Douglas-fir veneer cut at the Laboratory and glued up
unsanded.
Facings longer than 8 feet were made by scarfing two 8-foot panels
together at a slope of I in 12 with a melamine glue cured in a hot press.
The overlay paper used on some of the facings was a commercial product reported to contain about 25 percent of phenolic resin in addition
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to a dried phenolic-resin glue line on one side. This paper was applied to the plywood and veneer in a commercial hot press at a pressure of 175 to 200 pounds per square inch for 6 minutes at 285° F.
Several small blisters or unbonded areas 1 to 3 inches in diameter
were observed in some of these overlay faces, particularly near
the scarf joints of the longer facings; they were attributed to nonuniform thickness of the base plywood in the region of the scarf joint.
The overlay paper was applied so that the machine (grain) direction
of the paper was parallel to that of the adjacent ply of both the threeand two-ply plywood, but at right angles to the grain of the single
veneer facings.
Assembly of Panels
Exploratory Work. - -The fabrication of the full-size panels was preceded by exploratory tests in which small test panels about 1 inch
thick were made to try out different gluing and fabricating methods.
The glue used to bond facings to cores was selected to afford desired
durability and strength as well as to meet the pressing and assembly
time requirements for fabrication. While exceedingly durable bonds
can be obtained with phenolic f melamine, or resorcinol glues, the
short pressing cycle and long assembly time necessary for fabricating the panels favored an intermediate-temperature-setting phenolic
glue. It was found that the same glue used to fabricate the core
would meet these requirements, and hence was used for this purpose
also. In tension tests, this glue gave an average tensile strength of
about 70 pounds per square inch with an average core failure of 80
percent when used to bond 1/4-inch three-ply plywood facings to cores
of type XN.
Compression tests of type XN paper honeycomb cores showed a crushing strength of about 50 pounds per square inch at a temperature of
200° F. The small test panels were therefore glued at pressures
ranging from 15 to 45 pounds per square inch and inspected for adequacy
of contact between core and facing and for possible crushing of the
cores. At the lower pressures poor contact was evident, while the
higher pressures caused occasional crushing. Compression measurements showed that collapse of the core occurred when compression
exceeded 0.030 to 0.035 inch.
Assembly Details. -- The most satisfactory gluing pressure, as indicated by these tests, was 20 to 25 pounds per square inch, with a compression of about 0.015 to 0.020 inch. Later experience with fabrication of full-size panels, however, resulted in lowering the pressure
Report No. 2165
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to 15 pounds per square inch for cores of type XN when some crushing
persisted at the higher pressures, although 20 pounds was found low
enough to avoid crushing in gluing facings to cores of type PN. Such
low pressures were needed because the cores for the large panels
varied in thickness as much as one thirty-second inch. Panels with
cores of type XF were pressed with stops in the press, so that the
actual pressure exerted upon them was unknown. The stops permitted 2 to 3 percent compression of the panel assembly.
Glue was applied to all cores and facings with hand rollers at the rate
of Z2 grams a square foot, one-half to the core and one-half to the
facing. After glue was applied, the cores and facings were allowed
to stand for 3 to 20 hours before being assembled, so that solvent
could evaporate. They were then assembled and put into a hot press
for cure of the glue at 230° F.
All panels, regardless of the type of core, were step-pressed, because of their size, in a single-opening 50- by 50-inch hot press
at a platen temperature of 230 0 F. The usual procedure was to insert the first 4 feet of panel and cure it for 40 minutes. The press
was then opened and the panel advanced 2 feet for a period of 25 minutes. This process of advancing 2 feet and curing for 25 minutes was
continued until the final step, which was cured for 40 minutes. Thus
every point on the panel was curedfor 40 minutes or more.
Sizing and Routing of Panels. -- Wall panels were trimmed, after as
sembly, to a standard size of 3 feet 1 1 - 1 / 2 inches by 7 feet 1 1 1 2
inches. This was done with a metal-cutting saw on the factory-made
aluminum-faced panels. The FPL wall panels were jointed on one
side with a machine jointer and then sized to width on a shaper with
a vertical cutting head. The ends were rough trimmed to length on
a band saw and finished on the shaper.
Grooves in the tops of the wall panels were made by routing out
the cores to a depth of 1-3/4 inches for the continuous head plate
and roof cleats, those in the bottom of the panels to a depth of 1-3/4
inches for the sole plates. A shaper was used for routing, with cutters
mounted on the vertical spindle. Sides of the panels used in the east
and center walls were routed to a depth of 1-1/8 inches to receive
the insulated splines that connect the panels of these walls.
Panels of the north and south walls that would be adjacent to partition
panels were designed with cleats glued in place to fit into the grooves
of the adjoining partition panels.
Screw holes were predrilled in the facing edges where wall panels
would be screwed to sole and head plates and roof cleats.
Roof panels made at the Laboratory were trimmed to final size of
3 feet 11-1/2 inches by 14 feet and routed at the ends to receive filler
or nailing cleats glued in place to provide a nail base for facia boards
and cornice. Lengthwise cleats were glued with a resorcinol glue
to the edges of roof panels that would be joined to interior partitions,
and crOss cleats were glued to the panel ends for later attachment
to the tops of the wall panels.
The aluminum roof panels were sized with a metal-cutting saw and
fitted with cleats and end fillers which were glued in place with a
commercial wood-to-metal glue at 300° F. and a pressure of 15
pounds per square inch.
Floor panels made at the Laboratory were trimmed to a final size of
3 feet 8-1/2 inches by 11 feet 11-1/2 inches. Panel ends and outside
edges of the two panels at either end of the floor were trimmed flush.
Edges where panels joined were routed to a depth of 1-1/8 inches to
receive the insulated floor splines used to fasten them together. Slots
were cut into the bottom facings of the panel ends to permit bending
of the radiant heating pipes for connection to the water supply and return lines.
Window and Door Frames. --Because of the 3-inch thickness of the
wall panels, window frames were specially designed for the checkrail windows used to provide a means of weatherproofing the joints
between panels and frames. Window frames were installed so that
they projected on the exterior side. Window frames were made from
nominal 2- by 6-inch stock. A shallow saw cut was made into the
outside plywood panel cover above the head jamb to receive metal
flashing, and was filled with caulking compound to insure a positive
seal. The inside casing acts as an inside window stop.
The outside cloox frame was also made from 2- by 6-inch stock and
is similar to the window frames in form. Side and head jambs are
of pine and the sill is of oak. The frame extends approximately 2-1/2
inches beyond the outside wall surface. Inside door frames resemble
conventional frames except in width. The casing is of the type used
on the window frames.
All window and door frames are screwed to the rough frames installed
in the panels.
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Insulated Splines. The insulated splines for wall and floor panels were
made of 1/2-inch insulating board glued up to the desired core thickness and 3/8-inch Douglas-fir plywood facings. Their resilience was
sufficient to compensate for expansion and contraction of the core.
They were ripped to a finished width of 2-1/8 inches.
Miscellaneous Parts. -- The 3- by 3-inch corner posts were made of
Douglas-fir and cleats were glued to one side to receive the routed
edges of the end wall panels. End-wall cornices were preassernbled
for site erection as a unit. Water table, facia boards, and quarter
round molding were made in random lengths and installed during
erection of the unit.
Erection of the Unit
The superstructure of the experimental sandwich panel unit was erected
on the prepared foundation in 2-1/2 days, June 3 to 5, 1947 (fig. 7).
Thereafter, roofing, heating plant, flooring, and other installations
were made.
Exterior Painting
All bare wood, including exterior frames, Douglas-fir plywood, paper
overlays, facia boards, and trim, was given a prime coat of aluminum
paint. The frames were painted before they were fitted into the wall
panels, and exterior surfaces of the wall panels were painted before
erection. The aluminum-covered panels were given a zinc oxide
prime coat on the exterior surface.
All exterior surfaces were given two coats of a standard finish paint
consisting of titanium, lead, and zinc.
Research and Testing Program
The design of the sandwich panels and the construction of the FPL
experimental unit involved a great deal of research and developmental
work. As a research unit, not only were the panels subjected to
initial stiffness tests, but also prototype panels were tested to failure to establish both strength and stiffness. In addition, frequent
measurements were made to determine the effect of changes in relative
Report No. 2165
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humidity as related to moisture content and temperature on the
straightness of the panels. This was possible because the individual
panels were not rigidly joined as in an actual house structure, but
were left unrestrained to permit deflection. In addition, acclerated
aging tests were conducted on the core material and on sandwich
panel samples, and the effect of moisture was studied. Later some
panels were removed from the experimental unit after 16 months
and after 8 years of service.
Structural Tests
Tables I and 2 give results of bending tests on prototype sandwich
panels and stiffness tests of the panels used in the experimental unit.
Figure 8 shows the location of wall and roof panels in the unit. Tests
were made by supporting the panel on rollers near the end and slowly
applying load at the quarter points (fig. 9). Test spans were 90
inches on the wall panels and 144 inches on the roof and floor panels.
The tabulated values show that the equivalent uniform load S at
failure of the prototype panels exceeded by many times the design
loads of 20 pounds per square foot for walls, 25 pounds per square
foot for the roof, and 40 pounds per square foot for the floor. All except the aluminum-faced panels used in the experimental unit had deflections at design load that were less than 1/270th of the span.
Aluminum-faced panels showed more deflection at design load because they were designed for a lighter load (15 pounds per square
foot). These tests clearly show the relatively high strength and
stiffness values afforded by sandwich panels. Figures 10 and 11
show wall panels after failure in the bending tests.
Impact bending tests were made on prototypes of a plywood-faced wall
panel, an aluminum-faced wall panel, a plywood-faced floor panel,
and an aluminum-faced roof panel. Panels were supported near the
ends. Impacts were from a 60-pound sandbag dropped on the center
of the panel from increasing heights until failure occurred. Heights
of drop at failure were 8 feet in the plywood-faced wall panel, 7
feet in the aluminum-faced wall panel, and exceeded 10 feet in the
floor panel and 4 feet in the roof panel. There was no damage from
the 3-foot drop on wall or roof panels, nor from the 6-foot drop on
the floor panel. These values had been suggested as performance requirements in this test.
Concentrated loads of 50 to 200 pounds on an area I inch in diameter
caused less deflection of the aluminum-faced panels than that under
design load in static bending. Permanent denting of the 1/50-inch
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facings occurred at loads ranging from 190 to 290 pounds. Tests made
with a falling 2-inch steel ball on specimens of similar panels caused
dents 0.01 to 0.03 inch deep from drops of 4 inches. Dents of equal
depth were more noticeable in smooth, bright sheets of metal than in
materials like fiberboard, with a dull finish or texture.
Edgewise compressive loads up to 500 pounds per lineal foot caused
negligible deformation and no damage to plywood- and aluminum-faced
panels 8 feet long. Three aluminum-faced panels failed by buckling
of a facing at loads of 2,300 to 3,100 pounds per lineal foot. A 1- by
8-foot panel faced with 1/4-inch plywood developed a load of 19,000
pounds at failure.
Three aluminum-faced panels were tested under an edgewise racking
load. There was no structural failure at twice the design load of 60
pounds per lineal foot of width. Ultimate strengths were from 250 to
640 pounds pe r lineal foot when the panels were fastened and restrained
in a manner like that to be expected in service.
Accelerated Durability Tests
Paper Honeycomb Cores. --A number of paper-honeycomb cores
treated with phenolic or polyester resins were tested before and after
6 cycles of accelerated aging consisting of the following operations
in each cycle: immersion in water at 120° F. for 1 hour; spraying
with wet steam at 200 0 F. for 3 hours; storage at 10° F. for 20 hours;
heating in dry air at 210° F. for 3 hours; spraying with wet steam at
200 0 F. for 3 hours; and heating in dry air at 210° F. for 18 hours.
Some 72 core constructions were tested, mostly designs intended for
extreme exposure. A few representative results are summarized
in table 3. Strength was found to be reduced about 20 percent, stiff.
ness about the same amount, and shock resistance generally very
little. Because of the limited number of tests, the results show
some inconsistency and are considered to be conclusive only in a general way.
Sandwich Panels. --Sandwich panel specimens 3 by 36 inches in size,
with paper-honeycomb cores 1 inch thick and a variety of wood fac
ings similar to those in the FPL experimental unit, were subjected.
to the same accelerated aging cycles. Bending tests were made before and after aging to determine if the strength properties were
changed. The reduction of shear stress developed in the cores of the
aged specimens was about 20 to 30 percent. The reduction of stiffness was about 20 percent. No visual defects or warpin g were observed
Report No. 2165
-15-
in the aged specimens. Similar tests of sandwich panels 2 inches
thick and consisting of expanded paper honeycomb cores and aluminum faces showed a 28 percent average reduction in bending strength
after accelerated aging. In similar tests, sandwich specimens made
with facings of porcelainized steel bonded to 1/8-inch hardboard
lost about three-quarters of their bending strength. In another similar series, compression and shear strength of the expanded paper
honeycomb cores were but little reduced by aging.
Small specimens of a commercially manufactured 2-inch sandwich
with resin-treated paper-honeycomb core and aluminum faces were
tested in tension perpendicular to the faces after a variety of aging
exposures. The exposures and the results of the tests are summarized
in table 4. The tests showed appreciable softening of the adhesive
bonding the core to the facings when exposed to a temperature of 180°
F„ The adhesive bond also was seriously affected when soaked in water
for 48 hours. Exposure to high humidity or to cyclic conditions had
less severe effects.
Moisture and TL ,per
Effects
Moisture and temperature are important agents affecting the structural properties of sandwiches made of wood or wood-base materials.
They may have an immediate effect on the facings or the core, and
they are major factors in producing aging effects on facings, core,
or adhesive bond.
Facings of wood or wood-base material are hygroscopic; that is,
they take on or give off water vapor until they are in equilibrium with
the surrounding atmosphere. With an increase of moisture, the
dimensions are increased, while structural properties are generally
reduced. This can and often does happen to a building. Since the
properties of sandwich are largely controlled by the facings, these
effects are important. Table 5 gives the structural properties of a
number of common facing materials, both wet and dry.
Table 2 shows the moisture effects, both on dimension (columns 7 and
8) and on strength and stiffness (columns 12, 14, and 16) of a number
of facing materials. Plywood expands by 0.1 to 0.2 percent of its
original length and loses about 18 percent of its strength and stiffness when soaked. Shock resistance is little affected. Hardboards
and insulating boards have more expansion than plywood. The reductions of strength and stiffness follow the same order.
Report No. 2165
-16-
Moisture also affects the strength of the paper core. Honeycomb cores
A and B in table 1 were tested for compressive and shear strength
when dry and when wet. The wet values were about 30 percent in
compression and about 45 percent in shear, compared to the dry values
given in table 1.
Temperature effects on strength are generally not important in sandwiches for building construction. Most wood materials have 0.33 to
0.5 percent decrease or increase of strength for each 1° F. increase
or decrease of temperature. Adhesives that become plastic at high
temperatures should be used with care where there is a possibility of
high temperatures in service. On the other hand, thermosetting adhesives that have not been fully cured may become hardened and
strengthened by exposure to high temperature; this was shown in tests
of sandwich specimens with phenol-resin-treated paper honeycomb
cores bonded to aluminum facings.
The effect of severe temperature differences was shown by previous
laboratory tests on six sandwich panels, 20 by 72 inches in area and
3 inches thick. The core was paper honeycomb, and the facings were
various combinations of Douglas-fir veneers and plywood, mostly
with paper overlay and one with aluminum paint on the warm side.
The panels were built into a wall between two rooms, one at 70° F.
and the other a refrigerated room at -20° F. Bowing due to temperature occurred immediately; it was toward the warm side and was observed to range from practically nothing up to 0.06 inch in the, various
panels. With continuing exposure, the bow was reduced because of
expansion in the facings on the cold side due to absorption of moisture.
Tests of smaller panels placed near the floor in the same wall showed
about 5 percent of moisture in the facing on the warm side, 4 percent
in the core, 5 percent in the facing on the cold side, and an additional
5 percent as frost crystals on the inner surface of the cold facing.
Bow of the panels was not measured. The low moisture content in the
cold facings was due in part to incomplete air circulation in the cold
room and in part to the resistance of paper overlays and finishes to
the transmission of water vapor through the facings on the warm side
of some of the panels.
Effect of Temperature and Moisture Changes on Bowing
Sandwich panels have large surface areas that may change appreciably
in dimension with variations of temperature or moisture content.
When used in exterior walls of buildings, the two facings are generally
Report No. 2165
-17-
exposed to different conditions and thus assume different dimensions;
the resultant imbalance causes bowing or cupping. Defects in materials or manufacture can cause warping or twisting. Tests have
shown that the change in dimension of a sandwich panel with equal
facings and exposed to the same condition on both sides is practically
the same as that of a free facing. Table 5 gives linear-expansion
values for a number of facings.
Heat Transfer
A variety of sandwich-panel joint types were tested at the Forest
Products Laboratory for heat conductivity from a temperature of 73°
F. in still air on the warm (indoor) side to -10° F. with moving
air on the cold (outdoor) side. The panels were 3 inches thick, with
XN-type paper-honeycomb cores and 1/4-inch plywood or 0.02-inch
aluminum facings. Under these conditions, the plywood-faced panel
had surface temperature on the warm side of 66.4° F. , and dropped
to 66.0° F. at a joint with a plywood-fiberboard spline. These surface temperatures would require a relative humidity of nearly 90
percent indoors to cause condensation of water vapor.
The aluminum-faced panel had surface temperature of 57.5° F. on the
warm side, 36.2° F. on the warm side of a joint with continuous metal
from outside to inside, and intermediate values from other joints designed so that the continuity of the metal was interrupted from cold
side to warm side. With a facing temperature of 57.5° F. , condensation would occur at an indoor relative humidity of 65 percent, and
with a temperature of 36.2° F. , at a relative humidity of 30 percent.
Condensation
If sandwich panels are used for exterior walls or roofs in cold climates,
temperatures of the indoor surfaces of the sandwich may drop low
enough to cause objectionable condensation of water vapor from the
interior air. The problem is most acute with sandwiches having metal
facings and heat-conductive cores, and at joints or around openings.
Report No. 2165
-18-
Studies of Performance of Experimental Unit
Strength Tests of Panels Removed After Service
Tables 1 and 2 also give test values on full-size sandwich panels removed from the FPL experimental unit after exposure in the walls of
a building heated during cold weather. The wood-faced panels showed
no reduction of bending strength or stiffness after 16 months of service. The aluminum-faced panels showed no loss of stiffness, and
although they lost about 30 percent of their bending strength after 8
years of service, the strength remaining was still about .7 times the
design strength. It was estimated that this loss of bending strength
showed that the adhesive bond of the core to the facings may have been
reduced to about 30 percent of its original value.
Bowing of Wall and Roof Panels
Observations of panels in the FPL experimental unit give an interesting picture of the tendency to bow when unrestrained under actual service conditions. Bow was caused by a difference in the expansion of
the outside and inside facings that resulted from temperature or moisture differences, or both. Observations over a period of about 9
years have been analyzed.
The bow was found to follow a cyclic pattern, panels generally showing
about the same values in the same seasons, year after year (fig. 12).
Monthly averages throughout the years of observation showed similar
cycles within each group of plywood- and veneer-faced panels, aluminum-faced panels or hardboard-faced panels. Plywood- and veneerfaced wall panels 3 inches thick reached a general maximum outward
bow of 0.25 inch and roof panels 4-1/2 inches thick about 0.3 inch in
late, winter, becoming nearly straight in summer. Aluminum-faced
wall panels 2 inches thick bowed inward about 0. 1 inch in winter and
were essentially straight in summer. Aluminum-faced roof panels
3 inches thick were straight in winter and bowed outward (upward)
about 0. 1 inch in summer. Hardboard-faced wall panels 3 inches
thick showed the most bow, nearly 0.5 inch in late winter. Wall panels
3 inches thick with facings of 1/8-inch hardboard bonded to a thin porcelainized steel sheet bowed a maximum of 0. 1 inch.
Winter bow in panels faced with wood materials was due to differences
in the moisture content of the outside and inside facings. The temperature was lower and the relative humidty thus higher on the outside, so
Report No. 2165
-19
that the outer facing reached a higher moisture content toward the end
of winter. Thermal contraction of the outer facing tended to reduce
the amount of bow, but was overshadowed by the expansion due to the
higher moisture content. Calculations by formula from the observed
amount of bow, with correction for temperature, indicated a maximum moisture content difference between outside and inside facings
of about 8 percent. Direct observations of moisture content were not
made, but moisture content values of about 18 percent in the outside
facings and 10 percent inthe inside facings seem reasonable.
Theoretical analysis shows that the bow is proportional to the square
of the length and inversely proportional to the thickness. Thus, if
the bow of an 8-foot plywood-faced panel in winter is 1/4 inch, that
of a 16-foot panel of the same thickness would be 1 inch. Longer
panels, applied with their length horizontal would bow still more. In
such long panels, however, the bow can be largely restrained without
excessive stress on the facings by means of suitable fastenings at
midlength to other structural elements.
When the outside temperature approached zero,sorne condensation appeared on screws holding the wood wall panels to the sole plate and partition cap. Enough moisture gathered to stain the plywood. During
these periods, condensation gathered generally on the inner facings of
the aluminum-covered wall panels, appearing in very small droplets.
From time to time these droplets would collect in one large drop and
run down the face of the panel.
Discussion of Structural Tests and Performance
From the standpoint of structural and performance requirements, at
least five criteria must be met in sandwich panels for house construction. These are strength, stiffness, resistance to surface indentation,
insulation, and durability in the sense of long-term service. These
five characteristics are given particular consideration here. Other
features are, of course, also important, such as acoustical properties,
surface appearance, maintenance, and resistance to decay and termites.
From the data presented in tables 1 and 2, it is obvious that it is possible to design lightweight sandwich panels with paper honeycomb cores
within practical thickness limits that generously exceed the usual criteria of strength and stiffness for roofs, walls, and floors. Resistance to surface indentation is readily obtained or controlled by the
characteristics and properties of the facings. Acceptable or adequate
Report No. 2165
-20-
insulation is provided in the types of cores employed for many regions
of the country. However, for cold climates tests have shown that paper
honeycomb sandwich panels with open cells may require greater thickness for insulation than for strength and stiffness in exterior walls
or roofs. Insulation can be of course further improved if desired by
filling or otherwise modifying the cell -openings.
Considerable data have been presented on the effect of temperature
and moisture content on the bowing of sandwich panels with different
facings when unrestrained, to permit free movement. The results reflect what would be expected with moisture changes in plywood and
wood-base materials, and temperature effects with metal facings. It
should be noted that such measurements of bow as were observed in
the experimental unit are, of course, related to pertinent conditions
at time of fabrication that are subject to control. It should be noted
also that the stresses induced by bowing of the panel in the degree observed are relatively small. The actual bowing of sandwich panels
when restrained as in a house or other structure would likewise be
relatively small. The intimate study of bowing has been mainly of
value in demonstrating this characteristic of sandwich panels when
subjected to unbalanced forces and of suggesting the need for consideration in design when confronted with extreme conditions. Bowing
of sandwich panels in prefabricated houses has not in any specific
instances been reported or observed, and unless specific information
develops to the contrary is not regarded as a serious factor adversely
affecting their use.
The principal purpose of the FPL experimental unit was to obtain information on long-range performance. Such information cannot
authoritatively be obtained by accelerated aging tests or in any other
way. It is intended that the experimental unit will be maintained indefinitely to obtain such experience, with occasional removal of a
panel or panels for test at intervals to obtain interim information. The
tests of panels with plywood and other wood-base facings that have been
removed have shown no reduction in bending strength or stiffness after
16 months of service. None of the other panels with wood facings has
since been removed. However, careful visual inspection of the unit
, has not shown any specific evidence of deterioration, such as glue
failure. The appearance is satisfactory and thepanels are believed
to be in satisfactory condition to withstand indefinite further exposure.
The aluminum-faced panels showed no loss of stiffness, and showed
a modest 30 percent loss in bending strength after 8 years of service.
However, the panels after this period still carried about seven times
the required load.
Report No. 2165
-21-
It may be noted that the floor panels incorporated hot water pipes for
radiant heating. In fabrication, the conducting pipes were laid on
the top of the core and were forced into it in a press when the facing
was applied and bonded. This was intended to afford a means of checking on the long-range performance of the panel, and whether the temperature conditions associated with low temperature radiant heating
had any effect on the properties of the honeycomb core and the integrity of the glue bonds to the facings. None of the floor panels has
so far been removed for test. It can be reported, however, that no
deterioration such as evidence of delamination of the facing has yet
been observed. A careful inspection has not indicated the presence
of any unbonded areas associated with separation of the facing. The
radiant panel heating system has been operating satisfactorily.
In recognition of the condensation problem sometimes encountered
in roof structures, three of the sandwich roof panels were constructed
with vents to afford air movement across the roof and through the
panel. Observation of the performance over the years indicated no
appreciable difference in performance of the vented and unvented
panels with respect to bowing or moisture accumulation under prevailing interior humidity conditions.
Effective bonding of the sandwich panels was obtained by process con.trol in all of the fabrication details. The good results that can be obtained in this way were demonstrated by the tests and the performance
record. However, it is recognized that it would be particularly helpful to have some simple and rapid device by means of which the integrity
of glue bonds could be evaluated nondestructively in sandwich and
glued laminated constructions. Some progress has been made in such
development, but as yet no fully satisfactory instrument is available
for this purpose. An efficient and simple nondestructive test for
evaluating the integrity of glue joints would be an invaluable aid in
promoting the potential of adhesive bonding. Such a device would be
useful, not only to evaluate new construction but also to prove the continuing quality of the bond throughout its service life.
Conclusions
The experimental and developmental work on sandwich panel construction, particularly with honeycomb cores, has furnished information
for the basic engineering design and fabrication techniques. The numerous tests have shown that sandwich panels of the nominal thicknesses
and constructions that can be satisfactorily used for housing construction
Report No. 2165
-22-
have much more than the minimum strength and stiffness necessary
to meet the general requirements usually applied to such construction.
Also it appears, particularly with paper honeycomb cores, that
reasonable consideration of insulation requirements for dwelling construction results in a sandwich panel that more than meets the structural requirements.
With the advent of synthetic resins, the tests have demonstrated the
techniques of adhesive bonding that will afford adequate strength and
insure freedom from moisture problems at the bond. Furthermore,
the incorporation of synthetic resins in the honeycomb material affords a degree of moisture resistance that insures adequate stability
and strength even when completely immersed.
The panels removed after limited service showed very little evidence
of any deterioration or reduction in strength and stiffness and observations of the demonstration unit give no indication of any limitation of
degree of serviceability that may be encountered.
The experiments with floor radiant heating, while not yet common in
commercial practice, have shown the feasibility of this type of heating with sandwich panel construction in that no deterioration of the
panel with respect to integrity of bonding or other adhesive characteristics with water at 140 0 F. have been observed.
While the sandwich panels in the experimental unit exhibited a degree
of bowing during extreme weather conditions because of the method
of installation, permitting free individual movement, this characteristic on the basis of present information is not thought to have any
serious implications in sandwich panel house design as the tendency
to bow would be largely restrained by the structural methods incorporated to provide integrity or continuity as a complete structure.
The strength and durability tests have shown that effective adhesive
bonding in sandwich panel construction can be achieved by careful
fabrication techniques and quality control. However, the development
of an efficient and simple nondestructive test for evaluating the integrity
of glue joints would be an invaluable asset in developing the full potential of adhesive bonding for structural sandwich construction.
The research already performed and the structural service record of
sandwich panels gives promise of their continuing and increasing importance in house construction.
Report No. 2165
-23-
Bibliography
(1)
American Society for Testing Materials
1955. Standard Method of Test for Thermal Conductivity of Materials by Means of the Guarded Hot Plate. ASTM Designation. C177-45.
(2)
1955. Standard Methods of Fire Tests of Building Construction
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(3)
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Tentative Method of Test for Thermal Conductance and
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Bruce, H. D. , and Miniutti, V. P.
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Fahey, D. J. , Dunlap, M. E. , and Seidl, R. J.
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Korsbeid, John F.
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Kuenzi, E. W.
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1957. Mechanical Properties of Glass-Fabric Honeycomb Cores.
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Report No. 2165
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(10)
Libove, Charles and Batdorf, S. B.
1948. A General Small-Deflection Theory for Flat Sandwich
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Report No. 2165
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Report No. 2165
-27-
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Table 3.--Effect of a,ing on pa)or-honeycombl sandwich coreb
Treating resin
Type
:
: Ratio of property after aging to
property before aging2
:Amount : Compression parallel to flutes
:
Static : Impact : Modulus of
strength : strength : elasticity
:Percent: Percent : Percent : Percent
Water-soluble phenolic
Do.,.
:
48100***046 *40 **00.:
:
Alcohol-soluble phenolic :
:
D OO*00.040000000.•0.000:
Polyester
:
D000000.106.41000.0.410006:
20 :
79
:
125
:
107
:
.
20 :
81
:
80
:
86
80
:
..
loo
70
:
no
loo
:
.
:
20 :
64
:
86
:
223
:
100
:
7o
..
70
35
35
35
60
•
!Weight of paper was 90 pounds per 3,000 square feet.
2
—Accelerated
aging consisted of 6 cycles of the following: immersion in water at 120° F. for 1 hour; spraying with wet steam at
200 0 F. for 3 hours; storage at 10° F. for 20 hours; heating in
dry air at 210° F. for 3 hours; spraying with wet steam at 200°
F. for 3 hours; and heating in dry air at 210° F. for 18 hours.
Report No. 2165
Table .--L2=1"21T1111;2211.12110110 tests on pye_elmons of sandwich wall ..)anelog
Exposure conditions
Average of four panels
Exposure
: Time or
Tensile :
Failure in:
strength:
: Glue
Paper
cycles
before
testing
010644... 0.7014900.
.. ..ter am
►
warn .....
.
1. Conditioned at 80° F..65 percent R.H. Tested dry
48 hr. in water at 80° F. Tested wet
P.s.i.
---
44
3. 1 hour at 180° F. Tested at 180° F. 4. Continuous exposure to 97 percent R.H.:
►
Percent : Percent
42
58
75
MIM
28
82
94
6
70
85
78
88
69
88
69
54
58
39
60
46
31
46
42
61
40
54
1 cycle
2 cycles:
3 cycles:
4 cycles:
6 cycles:
91
74
71
95
80
49
51
59
63
31
34
41
37
69
66
1 cycle
2 cycles:
3 cycles:
cycles:
5 cycles:
6 cycles:
49
50
66
38
41
32
79
91
77
96
86
94
21
5 cycles:
at 40° F. and repeat 10 cycles:
15 cycles:
20 cycles:
92
82
82
83
42
33
57
30
58
67
88
63
80
83
50
35
59
52
28
53
at 80° F.
1 cycle -- 2 weeks at 80° F. and 97
percent R.H. 2 weeks at 80° F. and 30
percent RA/. and repeat
1 week
2 weeks
4 weeks
8 weeks
12 weeks
16 weeks
►
Y
2
6. cycle -- 1
3 hr. in wet
at 10° F., 3
wet steam at
at 212° F.
hour in water at 122° F.
steath at 200, F., 20 hr...
hr. at 212° F., 3 hr. in t
200° F. 18 hr. in dry air:
7. 1 cycle, 24 hours at 158° F., 24 hours: 8. 1 cycle, 2 days in water, 12 days at
80° F., 30 percent R.H. and repeat
1 cycle :
2 cycles:
3 cycles:
4 cycles:
6 cycles:
:
9
23
4
14
6
43
70
8
65
4i-
48
72
►
-Each value is the average from 10 specimens for each panel subjected to exposures 1,
2, or 3, and , from 5 specimens for each panel tested at the end of each period after,
being subjected to. exposures 4, 5, 6, 7, or 8.
2The wall panels consisted of 0. 020 inch aluminum faces bonded to 2-inch–thick
honeycomb core of resin-treat ed paper.
Report No. 2 165
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Figure 10. - -Final failure in static bending test of paper
honeycomb wall panel. Failure occurred at a load of
250 pounds per square foot, and consisted of a tension
failure at the bottom plywood facing and a compression failure at the top facing. The failure occurred
about 10 inches from the center of the span.
ZM 78996 F