Paper-honeycomb cores for structural sandwich panels


Material Information

Paper-honeycomb cores for structural sandwich panels
Physical Description:
21, 8 p. : ill. ; 26 cm.
Seidl, R. J
Forest Products Laboratory (U.S.)
University of Wisconsin
United States Dept. of Agriculture, Forest Service, Forest Products Laboratory
Place of Publication:
Madison, Wis
Publication Date:


Subjects / Keywords:
Core materials   ( lcsh )
Honeycomb structures   ( lcsh )
Sandwich construction   ( lcsh )
bibliography   ( marcgt )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


Includes bibliographical references (p. 21).
General Note:
Caption title.
General Note:
"March 1952"--Cover.
General Note:
"In cooperation with the University of Wisconsin"--Cover.
Statement of Responsibility:
by Robert J. Seidl.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029392981
oclc - 757390798
System ID:

Full Text


March 1952

5-T 5. C jiRil.

No. IP1918

Madison 5, Wisconsin
In Cooperation with the University of Wisconsin

Digitized by the Internet Archive
in 2013

a fin ~-

. --t -


i ondit of :-a r-Wi: 2 *o"' or: wri. thin an r. faci .' t: r

to. ( on V (ha- a-; l .
'an T, y 1 V t

' -.. papeyr s;unar'i-s br-iof]I, sonre oJ' .d
cr'vn .,r t, o n n t.. .. ..
corru:ate r, Dapel of lo7., K ,'1*ir. contn't.

Ilk.1 CO. 1 4- n' i..
expoc0%oi, ano a. "onr.iid ra'P ," ...1....... ai 'n o,.' <';
structubra"l desi g-nofans. O[)?r~S ":'.'- i~ri
indicated that< a Lnjitaco sa,... ..... ich a
hour, construction in bc-,_in str'-nrt ai

"oper~~~~~ 'rannjti ore ha' : uasn

de'- ind '. th t 1 -c o w, t ina.c. nt,
trfr... vcd b-, filling- cells wit nateia

: research at the( r o. Forest

-' V f" '- I r

n:a at e.n oat:a to ai in te
dwe li:;"o unit:;, streni~t:t'' tests ha^^r
*nal i)s c on a t <..,o conventionl
ad~ ~~4 ru3' 1tnot ct:" o:s '--it
''1,' e, d.>" 2c-S t- 0 nal

"novIlation vY"u-'- can be furth:r
7uch as Ioan, or leese- inulion.

nowu.. of ano. ls ue to -di ffe.rtial -oasture an rnal .ont ion:
found c to '-o of v aLTa4... "el the s'.; ordir a.s 4 .at ) ., st -
facirn pa -el ma>h of n'- orod ;'lu Le to w o _, 'i r ', s tanc o

S'ented at a mIThtinpl of thei r]1avstics dor',t- i of t Tech a Association
at.. an,,. .... Fl C-. T c 1 7 )- __ at j
o)f te P'ulp and i:r I industry, Gracus;- Y.., Tov. 9, lhl.
-Acknowlr r't nt is mad of inf'orma-tion iused i tis rpr :r as et l .-, on th 'ork
of various inv sti Ctors at th' 1" .'st 10r(u'tK7)oratory and on .,ork don-
'" te 'o oratory in cooo ration -with l> Housin ` o one 'i'nance, /",ncv.
--..dKiKnd at adison Wis., in ooo-pration with the kniv Uity of
i 0sconsin.

* ~ >1

ho..s comb panels is appreciably higher than nearly hollow panels with like
fcings. Filling of' cells with foamed resin further increases fire
r Sistance. 7 sults of accelerateL agin. t(-sts are favorable. Durability
and bowin unde r out oor conditions are undr study in a sad..ich-p l test
unit, which was built in l,9?7 and is still in very good condition.


The principle of the sandwich panel was undou tedly nut to effective use for
many ear. befre it was def'ined b, engineers and recognized as a separate
ty)e construction dominated by certain mathematical principles. General
recognition occurred during the accelerated search for high-strength, licht-
wei!ht materials for aircraft in orld 'ar II. Simply stated, a sandwich
panel consists of an appreciably thick core of a low-density material bonded
on each side to a thin sheet of strong, stiff material. A great variety of
facing materials, such as wood, hardboard, asbestos board, aluminum, and
plastic lmeinates, have been be:,ied to rmany lightweight core materials
such as alsa.. ood, rubber, plastic foa.s, nd formed sheets of cloth,
metal, or naper.

The new and Irs-enious materials, constructions, r.d ideas for use multiplied
rapidly. Stringent demands for strength and special characteristics were set
by des n(ners to meet needs for wartime aircraft. As the development progressed,
a distinction between highly specialized aircraft panels and other general
building' panels became evident. Obviously, a radical change in the outlook
regarding materials, properties, and economics was indicated, although the
*principle remained unchanged. Demands for strength appeared less exacting
and requre.,ients for durability and thermal insulation were perhaps more
critical for building panels than for aircraft panels. Problems of supply
and cost were paramount for building panels. Cores made of sht materials,
such a: cloth, nonwoven fabric, or paner formed into cellular configurations
of onr ind or another became more important. hen careful consideration
was given (o economics, availability, and other properties, it became in-
creasingly clear that paper would play a very Lmnortant part in the large-
scal o developmnt of honeycomb. It ma- indeed be only a slight oversta.ement
to say, that paper is almost the only material apparent at present for the
large-scale level n nt of hi-strngth honcomb anels.

In l,717, followin- several years of research with lightweight, paper-core
constructions and sandwich panels at tVe Forest Products Laboratory, the
status of the work was reviewed from the standpoint of panel applications in
building. any of the most promising ideas were gathered together and
crys tallized in Lrin form of sandwich-panel test building unit for outdoor
exposure (11)


Undcrlined numbers in parentheses refer to literature cited at end of report.

Since the 1r ction of tc. t t u 5it, c Uonini i.... f t' '"it.
Ti na'. IXs 1 'v i
rc ,.v': from n U.'ll V:" O" :)"rc -.a:r' IL int:. r" t<: V I: on]"

c < n ,.2 . .; '" lL 'h'c t 7 '*. V ...r A. "

....... *' o n A'co : a 1 ;onv... n :'."
V : ,L .% ] O ;: ] : : f ] ,' : .... 4 t +p.. .

< o2r. IO.'- O' ". 2,
U ] IL ... .... .. . 2; ,[i P ; ' 9" ; 2 .'-,0 .Co- T" I '"I" .. ..-':
!' ' S !.;: CO r,' t:1 ": ~ q C ,..,,. .:" !' "S. .. C;"21o 1">,

in:'' O t 2" .. C2 0 :.i' . '\r "O : + }=b," "1[' P < r: '2.'1 ;

" .... :.. 20%CollO" b

* :'* a-i ; n : 2
P.i . .. _

. ; ." -V: .... . .

4 ~, .~ V ,,., -~ 2.

'I I.'. ~'
2. .*. '-~ 1

'. -'Vi
,*-1 .~' VT.. -~ . .. V* -. V
V ~ -4
V -V

< 1:1 LJ2L .

u' "or :. ir ;''or
fi .be ;.oa ,., J.a.a., -.
doors. :**r .- 'rc
h'ct,-sfioaa'I", sta'nct


17 '.~

1+o. 1 nuo vir"' rl ]:-,

.... 3 <. W

2. ~


honey[Ct rf':" o:: .... '} -all of Ifr.t '" l' . :' '... ... -. ) .':' cr '" fo '"- .:.- : ,. '* "< *_ .p o
i 0 .- -" j .., 1 4. :- 'm .- 1,.

~... ...... > f'.

a ;
X wt,} .b s ', / ^ U .. . .. : . -.> l.\J. ,oitlj .. ... I ,

.. .i:. 'r. v 'U '.' to -' .:c *'. '' ,. 1 7" ." -''r" '" "j .

> IOd d," 'T'fifur a'.:nd (2' :'..... "',..-" o a o n c'an t ,
tc -2 't p 1r< ir p d < ...I ... na, "4'. '2 t. 'c'
stri-s of adhe'L iv. 3ucc* 22.> .ve rt:,'i of" ac'1 K :9 N'' '7"' -la..vd :c ti.,;t
Ct-ntfr ... .."f a..2. on,:' 1< "c' are -- ,:'o c t ; ;ti 1 : r",1.-oi .1 ":,' ''L" tL ;' L'

K 'V
21 2.~
I '

a( 7 inra V ]-,, l,. 7" i;t. ,
( ** *?" *"

.79, -,'-+. V' ' V.'

1-"' -,

RI )t. K. 1'19

r r L C' I

I':P t '''; ( i ... ..

cor' f.-




To produce the figure-8 core, a ribbon of paper is looped and bonded to form
circular cells resemblin a fiPure 8 in cross section, as shown in figure 2.
For the prodJuction of either the figure-6 type or the expanded type, special
machines are required.

To produce the corrugatedu type of honeycomb, the cell or flute is made by
hot forming, paper between flute' rolls on equipment of the type used in making
corrugated container board. The corrugate_ sheet, with or without uncorrugated
int.rleavin, sheet-, can be as-embled in many ways. The three types discussed
ciefl_ in ts report are hown in fim'ures , and .

rach t.ype of core has advanta-es and d isadvantae2 oeculiar to its construc-
tion. Tie differences lie in ouipmt needed for roduction, limitations in
reins and a hesivrts, relative difficulty of accurate cutting, strength, thermal
insulating properties, case of hipm nt, an' otl-er factors. As far as the base
material and resin treatment are concern such -roblers as fiber, resin, w;t
st'rnth an, durability are substantially corsion to all types. Itwas there-
fore intendeI that the results of this work should aT 1 to an,, honeycor struc-
ture made from paper treated with phenolic_ resin, although. most of the data
were obtained from sandwich panels having corrugated paper cores,

This report suminmarizes some of the research that has been conducted at the
Forest Products Laboratory on the structural type of corrugated-naner-honeycomb
cores for use in building panels, as distinguished from use in aircraft.
Consideration is given to the resin treatment of the base material and to
the fabric-cion of the core _.and sandwrich pa nels in the interest of low-cost
building material. Strength prooerties, bowing, thermal prooerties, dura-
bility, and fire resistance of the sandwich panels are discussed. Figure 6
shows a structural sandwich wall pn7el.

Base Pacer

For most ewr rimental work on sandwich panels at the Laboratory, an unsized,
neutra-l Praft p .pr was selected, because it was strong, potentially available
in large quan't 'es, as hd f.voratle indications of permanence. The paper
contained no sizinr in order to permit better resin penetration and to fa-.-or

A 50-pound (weight per 3j000 square feet) paper was usually employed for cores
havin' calls of' A-flute si-,e. Tests were mad to explore the possibility of
reducing the weio i of paper withoutt un-dlue sacrifice of strength in the sand-
wich pan l. The us.e of iightor-weight papers results not only in an economy
of base material but also an re, auction in the amount of saturating resin
required to produce a given voluIe of core. Results showed that sandwich
panrels wit surprisingly high strength proprties could be made with paper
weighing onl, 3) pounds per 3,00n s uarc ft'et, which yielded core of a
dornsity of -about 2.cI pounds per cubLic foot. V rmal insulation data obtained
oi these core constructions showed that, e'or a oprticular core construction,
an im1rovem -nt in thermal v-lues could be achieved by reducing the weight of
the. papr. Th' lower limit of p-'crn wei ht was not determined, as it was felt

RItN. No. T1918

that puroer of much lower w t t-.n 3 potnd re r 3,000 souvre wo'Oi .
difficult to handle, in th corru-tin, ir '. fabric-ti proc

':o information is availal 1 on the mirrLml str. ngth r equremt of
sheet for sati3factor 'or sructr. r, fatiton dtw n or'n,
oron rties of >1nqcr and th, pro?'e ths o'' coruis i: ,ocur' a''
should bo a to )vV r-o( thi, defici:n t str ngth cK r indi-, t2
effe ctvire ss o the ias1i tr at..l, r.r om.. on ot ..,' t '-: h.
Dnone l:; \n. C eane a da"I,1***'t" e n in Lc e' --"c e 'vS
SOP~: S aNc'. !'C p Ic' r1" t l l a Ct^ to a 'a, ri"LO' r ."a', T ....; L-, 2" o' t q r~ 't "-l :'.-'.

Until the ro"uiro:,'nt o ".n. "or '; ir. cor r'n b dfind. i a r ore
usable form, it will rena1n ifficult *'o t'r the ;4 p Co ar-

involvl,-Y short ti.nno oth "os wc n.norot hI na,
,Zql~l :0 .I C'Ci K 41] fr'' null:_ m% > i c 't O"Il .. " '= :0 ""'- tI 4'

... zle or seiich ni.c-.t oul n ma b. cc.t i .. f ....
structural ui.e. RHc>i pap may ..o 'rv .. ... -' a-e .". for cc-r-
tain ane ls. Few data ar;- v '.... 1 orn td'' rr" ., c f" e h7-c" .l oil
sciichc.'icl c pul. Th, nrnnc0" ',: .r :- not, how.r0v r, "t, crit cal,
since he core mit ri' "I th:-. ..n. pL -ne i-. p:rote-ct d"ro. ] tra'- olt
r"'.s, which contrib'2ute- to th.K lfs of sthingth of ',,-'r urpon ain.

hesin-treated Paper

One of the first problems co-non to any panoer core is that of resin treat ment
of the paper. Although in some cases paper without resin might be accYptable,
it is assumed that for general usage only resin-treated pancrs can be con-
sidered. Since panels are likely to be subjected to dame or wet conditions,
the presence of re. stin i'h r aTer is necessary to yield a r- odut that is
permanently strong a.n stifff Iin the wet c'. i tion. vwn s mall amounts of cer-
tain resins can be eff cctJve in this res pect. In terms of t.nsile strength,
it is not difficult to produce a np:-pr t... rct:in or 7 rnt o it
~~~ ~~ o ':) e a n ov~cr 71 ,: .n of-: ikts
streak -th when dry after water so-kinv th-at would c a -u untreated paper to
lose almost all of its str, ,.-th.

Th work reported relates to the use of reenol-formaldehyd'' sinss o, sv^ral
t~q:c ", f t-- s are wafte.r-s'olu1b) an '
tys... ~. : o' these arc watr-solublc and a] cohol-soluble ty',s an:d, although
both are suitable, t1" have inherent dif"erenes tht o be considered
.,-'n 'J are used with cellulose,. -With solution; of water-soluble resins,
the swelling eff --t of the w-ater on the -fiber permit: the "'a, to penetrate
the fiber itself and through a "bulkin effect" (10 to oaitain the fiber
in a swollen state. This also c au.. ses a reduction T he equ ilibrium moisture
cont ent of the sheet with water vapor. Then alcohol-solu'b]. r-:in is appli d
in the a-s:ence of a swellin solvent, the eff ect appears to chi'-y onI of
coa tii- the fibers. The equilibrium -:,isture content of such paper, when
based on the oven-dry weig-'t of fiber alone, is nearly as hirh as that of
untreated paper (3). Both may have high levels of strength in the wet
condit ion.

RP- r't. "o. R1918

_ c-

Ad-itional evidence that the water-soluble resin penetrates the fiber better
than alcohol-solule resin was indicated by dccay tests made on paper treated
with otlh typs of resin (. The results showed that a paper treated with
115 ercont of wat,; r-soluble resin has considerable resistance to decay funmgi,
as miasa3rod br strength, while the paper containing an equal amount of alcohol-
soluble resin lost most of its strength under similar exposure conditions.
"ie the r war r-soluble phenolic r .sins are not especially good as fungicides,
they can red.p>ci t'.: :-Iuilibri:u moisture content of the pnapor and thus dis-
oi .e th growth. 0" fu-n7. Inherent brittlen ss m'1y b a disadvantage of
p r i' )regnat d with wae r-olul re sin.

A resin tr aten t of 1o percn1t of water-soluble resin (based on weights of
resin and "fiber; was found to be adequate "or providing paper of -ood strength
wewet, decay resistance, an alin c aateristics during: corrugation
an, subseq.e.t fabr. cation. k:O'in content in e::ce. of about 1S percent does
not seen lo o ireuce a gain in strn gth commenurate with the increased
,-uantitv of rcsin reqi. c .re. The tensile strength of a kraft p, r- treated
w(ith 1 percent of water-soluble. phenolic resin mi, be greater when wet
aft.r Droloned soaking in water than the strength of the untreated paper
when dry.

Although more than 1]- percent of resin ma, be required for certain uses, it
is promising that even a lower resin content will be acceptable for most
applications. In one scri es of tests, raer -with as little as percent of
water-soluble resin was satisfactory in strength but showed less resistance
to dcay tV n paper with l. percent of resin, In this case, the
acdditionp of 2 rfent of pentachlorohenol in the prapr overcame the
c ficiecy in d::ca res stane. Th e irtahloronhenol was dissolved in a
smrll ai]nunt of ethyl alcohol and added to .h.e resin. T. concentration of
the solution was further r-educed by' adding. a mixture of 0 percent each of
water ard alcohol. Uci:T- this mixture, a krrft paper was treated with 5
percent of a w-ter-solubhle oh.nolic resin and 2 ie-rcent of fungicide. The
pentachlorophcnol did not seem to affect the cure of the resin as indicated
-by the 'oo' strenth values of th treat pan par arnd honeycomb structures
wh rn wet. V lgible losses in strength resultecd from rap r im.regnated with
rp perc-nt of rosin plus 2 percent of entochloronhinol after 2 months' e:cro-
ure to two -es of woodI- strr c yi7 ungi. n o cata are available to indi-
cat.e te inimua am mount of entachlorophenol rec7uired for good decay resist-
ance, -ut it s likely to be l!:ss 2ta 2 percent,

Since resin is he costly- itL ,in te honeycomb structures, a series of tests
was made to apre ... c. "L'e 't of cores contr0a:i 44- 10, and I' percent
of wat r-soluble phenolic r :in a. 1r nrcut of alcoho'-soluble resin.
'hse cores of t t fr ) w a sity of a'out 2. p ounds per
c foot, wr oond o .o.. facin s, an the anel-s ,tested for shear
Cil _c foot,) heT o n'ei d 1o .. 1, ' L
and comopressive strength in >,oth the dr ;nd wet condition. ,,_.:n tested dry,
panels with oercent resin content had shear stress of n2 ounds per
square- inch a.nd a compressive strength of 35 pounds per square inch. In-
cr sesin saturating resin content of th core resulted in an increase in
bolh strength pro crti s of the p.-n-,ls. Increasing the water-soluble resin
co 'nt of the :ore from 5 to 1 percent increased the shear strength of the
2anuls about L) porc nt in the dry condition and about 300 percent in the wet

P Kt. 1o. RlOl8

- 6-

condition. The effect of res in content on ,o restive strength h was no"
great as on shear strong th. The se t o the alTohol-soluble resin a. a
satura.nt resulted in slightly hiher shear stress developed in benain< and
impressive str ength "or I-:y panels 'ut 1o>wer str-.'-t for par s I ts ted wet,

T'-,' r--crnatin7 -paoper with resin on a s a ,'. i:."re(mating machine is li'el to
1-e somewhat costly, a5 it involves a ;ona.',, operation, Beca':s tie rce;in
content of the p,,- r in the honeoeI in r it I' .a.-t
ably 5si2ple to apply resin during t, !roce: o paper auf'. r s
means of reducing cos s iC : tonn an L' ire can jusfif, such a roe .
It is possiJle to a, d resin dAurin, ' a r'nt' by'

1. Apin.'ng it at the 'a ,r-mach.ino s' 1r ... siilar ovie.

2. >lcndin of r and rei in wa's ',',, in >. } bat:r 'a v
ine: thiis resin in the sheet durin oaper nahin,.

3. Continuous ad-ition 4of liquid resi a .... na ... d...

On the: ex'ri-mental paper K:,,ine a.. or.;s i' Kr ... a''otory,.. s
with a wide ran e of resin content a.: real , b s. of tha/' sis"-
,r..-ss e.q iaoent. T jese par s were co-r a e in most r c t to
match sheets treated on a sI arate r aion ma'hi wh the sa' e
resin. F'o-n numerous excerLa-.ntal trials, it app, i-ar th't h size-r ss
treatment was one lofical step toward t' realization of a lower-cost core
mat ,--ria1.

In recent years, suitable resins were dcvclopeod for addition to both the
beater and the paper-machine headhox. The retention of early: beater-added
phenolic resins was very poor, operatin diffiC.lti s were numerous, and
large chemical addition was required to properly re ciroitat the resin.
%.cral resins exist toda, that are suitable for this t'o of action, and
ex,:'r '-ntal paper-machine runs '-ave bee mad..' with coming results. In
on, series, the addition of 12 or 20 prc-nt of this resin to the pulp slurry
tended to froe the stock on he machine wire and no oporat-
ik,, difficulties. This is in sharp contrast to te-n "slowing" of stocks by
resin addition in earlier y-ears. The paper aft, r cure of the resin was con-
siderably less brittle than sheets treated with w'ator-soluile phenolic rosin
on a resin- impregnating machine or size press. An indication of the tough-
ness of the sheet was given by sharply creasing the sh.d by hand and then
obtaining a tensile strength value acr,-o t i fd a"a. The ape -
tained a surprisingly hi-h perc.ntat-e of its oriapinal str n tn. ric roscopic
,aminn ion ofi' the papers containing "-t- .-add re in, using an n Intro'a
mcrosco quipd with fluorescent liti-, s7w ola fa nt hae o0
resin, indicotinS very mll particle size' d >.xcell'nt disp rsion.

Core Fabr' cation

Paper for the corrugated type of hon'cUo sed in thi tudy was corru.ted
on a 5O-inch machine at the Forest rodu .cts Laborat ory. 'he A-size fluted

Rept, No. R1915

rolls common to the box industry were usec. The procedure and weight of
pae(r were varied as needed, depending on the nature of th- core desired.
Simplest from the standpoint of adaptability to existing equipment is the
PLT typ of construction (fig. 3). This consists of alternate corrugated
and flat sheets and is made by cutting the continuous web of single-faced
corrugated paper and bondinF these sheets into blocks of considerable thick-
ness, almost exactly as is now done in making blocks for insulation or
cusV ionin, in packacing. The only difference, in fact, is in the use of
treated instead of untreated paper. In order to make the P TI. type of core
with i flutes and a density of 2.5 to 3 pounds per cubic foot, it was neces-
sary to u: a paper of about 30-pound basis weight (per 3,000 square feet).
This core had certain attractive features, such as excellent strength, a
slipht compressibility in one direction, which aids in fitting pieces of
core into a oanel of predetermined width, anj ease of manufacture.

The : tpq'e of core (fig. I) was used for most of the data given in this
report. To fabricate this type, corrugated sheets were assembled with the
princi al flute directions of adjacent sheets at right angles and bonded at
the crests to form a block of core material. '0o uncorrugated facing sheet
was used for this construction. The base paper, a O50-pound (per 3,000 square
feet) kraft paper imprernated with anproximi7tely 15 percent of water-soluble
phenolic resin, yielded cores with a density of about 2.5 pounds per cubic
foot. To assure a good bond between adjacent cross-bonded sheets of corrugated
paper in the core, a phenolic adhesive was used in an amount equal to about
10 percent of the wei-ht of the core.

The core was cut and assembled in the sandwich panel with one-half of the flutes
perpendicular to the facings. The flutes in the other half of the corrut-.ted
sheets were parallel to the facinrs "nd contributed very little to the strength
of the core, but served rather as soacers for the flutes that carry most of the
load. It wIs deronstrcted that the basis weight of the spacer sheets could be
reduced from 50 pounds to 30 pounds per 3,000 square feet with very little
effect on strength properties of the panel. A further economy in cost could
also be realized by reducing the r sin content of this lighter paper.

The production of large volumes of this type of core would require certain
changes in existing commercial ccreugating equipment. Pasting of a flat
sheet to the corrugated web is omitted, and more care is needed in handling
the web without stretching the corrugations. The crossing of the corrugated
sheets would require special but relatively simple equipment.

Either the P'IL or the XN type of core can be assembled in the panel so that all
the flutes are parallel to the facings of the panel instead of perpendicular.
This results in a great irnrovement in the thermal insulation but a loss in
strength. The principal disadvantage of this flatwise construction is that
the integrity of the panel itself depends on each glue line between the sheets
as well as on the glue line between core and facings.

The P! construction illustrated in figure 5 is similar to the P'IL type except
that the flabt sheet is omitted. I,'- I''T type is probably the most difficult of
the various corrugated types to fabri-cate, since it involves placing all
corrugated sheets parallel, the crests of each sheet being in direct contact

RCpt. "10. R1918

with the crests of adjacent sheets, ako exprie.tal aon. c .re,
seT. :-.ts of ordinary soda+ st.raws .rtc. at the four crners of Ic. Iet
were used to achieve good crest-t)-crest alincent. tN e of core as
certain ideal feature for s tudies of' materials and Cesi' fa j rs 2. A co -
siderable amount of s wtuy wa,:s basa on tis type of core, particularly as
related to ,-!cialized aircraft its (6, ).

Most of the early work in making ccrPs involved i use cf costly "a a:iv .'
to bond paper sheets to eaci other. In or,.er to tter act nr, a; erist.r
rrmachine and to provide e -reater ecoco. a study of' tle effect c" ht quality
of the bond bet.'weer. shee t of papcr on the properties of the sa.d ih was a :dc.
In the assembly of ordina: corruig'ated board, a nonwater-resisjt+'t adhsi- is
usually e+p loyed. It was hyyothesized t at the bond between individual s h.ets
within the o ."'crtb c "re "' not critiLcal, th ends of ac lutre
bonded to both fac'ins will, a hih--aualI*, vater-resistint adhesive. eost
of the adhesive and the equirent necesary to use suc an adesiv ar' un co-
nomical features in the process, unless tKe igh-quaility bn, ir required.

A series of tests v-as made to comnpar aeanels having cores in 'hic ind'ivi ual
sheets vere bonded w- I (a) phenolic resin, (b) ura resin, (c) sodiu
silicate, and (d) no adhesive. Vlt'r. '4 th e as:erblr of 'orr'rated shets in
a panel without the ise of a sheet-to-sheet adesi"e iS .ipra..a, i ..-
rea.-soned that the resultant .ro:,erties v:ould establishl the l':'er li:it o1
stry -th, I.-' nature of the crest ad 'esive appeared to Ihave only a sl- .ht
effect on the shear strength of panrels testd i n either th dry or vw et -
tionr if the web of the corr'. -.-:ttion ras parai]l t tthe span. ane l ':th l '
silicate adhesive had slicltly h.i her strenativs when r and sihtly lCwv'er
stretL-ths when wet than tbose rith tle liienolic-resin. adl .e.v.. ese tes
e.:rcnstrated that it '."-; desirable to ,ave the web of' the csorrr:tion anaraliel
instead of perpendicular to the sp:an to otlain maximum trcnrtl, re, -ads
of adhesive used.

Sufficient commercial trials -v been rmade to demonstrate the feasibility of
r', i-.mir:gnated papers onr corruratin anachine, ad s. pa ave ben
corrugated at ordinary commercial s' -.. The ordinary size of flues, used in
the box industry produces satisfactor core,, but, for economy, it is desirable
to use a much liter i .per +.than tle usual cmorruatinr me ium for boxe .
.:.other -.' possibly better approacY ,,'old be to use larger flutes and heavier
.aiers, resulting in a faster bAuili-up of core of a :ivn density. : '
......-tir machines have larl-er flute sies,; anrd, any new o acine :esiner
expre.ssl for t+his purpose undoubtedly should ue flute,

ranel ianufac ture

A" r, c yom. cor e is a "t s, factc v
An' 1w'. armb core ie sat isfact ry only in e littio t th facing. it supNorts
and, conversely, the uit ability of) iy fa cin: may 'epIend on Ihe cre. r r-
haps one of the best ic .--ter.m a, vantags of t'he sanwic pane i ti.e l11:eat
latitude it Irovides in choice of facJ ..d the oppo rtunity to use thin

Rept. No. RlQla-

sheet materials because of the nearly continuous support by the core. Th,
stiffness, stability, and, to a large extent, the strength of the sandwich
are detemined by the characteristics of the facings. T':re are a wide
variety of she,_t materials suitable for facings or skins on honeycomb cores.
3one of the different types that have been used include pl-:o.)od or single
veneers overlaid with a resin-treate paper; hardboards; asbestos board;
metals, such as aluminum, enameled steel, stainless steel, or magnesium :.-'t;
wallboards; fiber-reinforced plastics or laminates, ana veneer bonded to metal.

PYlood is a versatile facin- material, and the performance of panels with
such facing's can now be well predicted. It has good. dimensional stability
. -rac ad strength proetis r.,. can be dependably bonded to core
.th rv. n oiv 7n exose to out,'oor conditions, some Dl0woods,
such a- aougas-fir, ma check ons.ieral,, and grain raising becomes
apo)'atre. These defects may be eliminated to a large extent 1by applying a
r sn-tret d ,e paer-overlay sheet to the out r face of ,he panel, resulting
in .,smoot, surface and uniform base for painting.

Since the facing is securely bonded to the core in the sand-wich pamel, a two-
ply veneer facing ; with or without !-n overlay, cn be used as in r. ir- flush
doors. Although each facing is unblmnced, the nanel itself is in balance.
5-ndwich panels with dissimil-r facin,,s may also be suitable for certain uses
if the proper unbalance is selected. These might be considered in cases where
the exposure conditions on each side are not in balance. Untalanced panels,
however, should be used only -ith considerable caution. Panels with facings
of hardboard or veneer with paper overlays are promising panels from the stand-
point of beingg lightweight and eoC:O" .cal. Excess dimensional movement may
limit use of this type in some cases.

Cutting, of Core

In assembling the core for fabrication of the sandwich panels, the cutting of
strips of core material to accurate thickness is essential to avoid difficulty
in -.aking or using the oanIs. Core material may Le reduced to the desired
thickness by, sawing on a circular saw, band saw, vibrating knife, or, in some
type s, a guillotin; cutter. It may also be made to the desired thickness
to avoid cutting. 4n accurate cut can usually be made witi a circular saw,
but this requires a rather thin section nci therefore more cuts. 'With the
proper blade, speed, andc technique, a band saw will reduce the core blocks to
proper si e w'ithiin +.01^-inch tolerance, whlc probably sufficient for
panis of 2 ince-s or more in thickness. A band saw with five teeth per inch,
or rating at a blad speed of' 31-," f'et pe.r minute, was found to be suitable
for most of the t st panels. A vibration -knife ty ne of cutter may be suit-
ale for cuttin- honeycomb core, because *.t elimin-tes the saw-cut losses.
For comrterci'al production, It may be desir-.l, to cut the core initially to
" broad ,,er tolerance, and control the thickness accurately with a sander or
other mr'chinc. It m' y b sufficient to simply roughen or slit the edges of
the core, so that, whun pr assure is "-onlicd to the ponel in the press, the
effective thickness will bo uniform.

Fep"t. No. RlP l-


LatUsfac4 '; f 'r':nc o'' th' :. to a 7-a + ;1 : t. rr-

t i.d pOSji e,, I r. fa't,, t jh 1 1.... + i of h -irl ' a7.i 4r.
r cet a.:a'rS

A s', ws rca na' of te tu-*raii "+

ri.' a n a T ..; DI.oRu i to l' '' o.. . l
' r'' ^* ^ t ^^ -^ ^ 1^, ^^)^. ^ -p 'a a t
. .. ..... +r f_:, . ..U -

.f a r~um].<,' c''" -1c;il *xt~'rior a I' .
*; r+r O~i' **. +; 'T*". .t,"( .. ; i. + .' .' p c

.. ....... '+ '- ( IO. .
.7 r+ or .=

... +,, ,. Oc +. ++%' ] qi+++.` ++o oF:0

a-' An ac'T ; -'" ? -
(z At+ aI':aI . -_., a... a -(1 n

...n ++_ . r '
(c) An alk .. -cal o'i root-to n;an r urv- t.r ,i ,r pr, *na-r ."-

(dc) An al'- *,'"l catalyzed, i -_-t .er .au' r stt f rh n l-p Ln ad i ;,.

h-+ bona ni of" +iost cf' the e>:p crue al >-nd-'7 'K pan l's-_ .... . ...... ..- ';
equipment. F 1 oray a v he short t.r'e- r. c cle , ... a ..;.i...
time favor d t. he ci -c(+t." .d,> hiJ.h-te- nr'i : .ture-s:'!ttin .+, r/ .u--rci.. 7. Ua .-
sire. Ad he iv-e w'as ap,.+ ed :o iot;'I +h t ';<; er+ and fa . ; inro. w l ^i +t i rit.ic'r roll er
or an crdJ nary pahi t 'o]. ,-- '.....r ... r.. .. t 2-', p r' 'a t of
surface, o>. -UL)f t'o t cofa +.nda or..o-holf-.. to t-'' t'>.; s. t .a] a.;o
demonstratId tli't i,. inay not': bo no..... to apl ad,+ v II t.b. ;oru. in.
ordi.r to r Ior uci a ,s i af'o" .. v. o. a f aiFG +-., +oe + s+t-nd
aft<:r :r :'ad of ?d.o'.cio' to r.r i-nt tr h s a'ora''icn o f ta'to; Go'a rt.. {''..r.
c*".~:. aorn Ua r'". then as.embl.d and pla d in a hot ar ss at +mr'
of 2?Y F. ( asle
Pressures r+",i-r f"OT'a 1. to :0 noun.aS per sqre: ncwh .: 'r. us1'd da-,;d i.
on de sit a ..d ev, of core construction" '::;ct. ,a q ur;, to 'll f" ir
] a ... in e . t ' a a aa i a V V .- .... 'a >r to y or''

net hod.

nr c -. al v o**-ir',d
r ijd are usually 1oIr ta n tatr
control on or'+ 'r 9nywoow or pla tce -.
F;+/I: l ]OWP S"~ "'" '2 mN< a, :; "" 1 3 i .I.;1],. a~'
UOcalu vOlI" -oSlen a ris in-oe pe'iiro sn ar V +
,ture Ls basically not coplicated.a
4-u~ t:. aalynot u1latci

i" n''B 't'4' l;e +press''-
.n h... raa of )o( rressure
,;a..-. oresure requlr-
C col e 'Ont In-
alo tae. I etaiI
'K u'n I s, 'ut ti.Lr .Iaiufac-

Fr-ssing of sandwich panels havir, diss. "'i -r fa, .7 in hot :r sses has 'cn
difficult becaus- of *junqual dimensionarl noven'e.t of' rh faciings due to moli-
turw or ther'mal csar" ,. In uch cases col,-r:'. ira a 27...aU-.. This
of o'e "'e itIs o o'r.- cv.' 1n with w re.-"ct to 'Kh ,ive li iatioas,
assembly t ime, wat< r introced I with a-dh'es l'i^ ij.J; under p:e'ssur- a ;ad,
in so00e caseu, duability f the ,ond. 0cdinr .... cld-sttinp a dhesCv :I,

Pept. Ko. hlia -'-

however, are undoubtedly adequate for a host of prospective sandwiclh-
panel applications.

Onie of the most persistent difficulties in thfe use of sandwich panels is
in tle problems c-used by the necessity for edp es, inserts, and connectors
for panels. In some cases, the problem involves tying together tlhin fac-
ing materials without severe stress concentrations and, in otl er cases,
suc as furniture, the problems is caused by t"show-t-rough11 of core or
in. erts through decorative facings. These proble!-ms, probably as much as
any other factor, -haxe restricted the development and use of honeycomb
core on a larger scale an- should be studied from ' Tie differential dimensional movement between core and insert materials
should be at a minimum, including the rate as well as the degree of move-
Ment. Adicsive.s ad moisture intro uced by adhesives would be a factor in
this stady. Examples of materials to consider for edges or inserts would
be end-grain wooc, plyVwood on ede part honeycomb and part ,-ood, metal,
ense o.ney comb, and mastics or fillers, Anoter approach wouldd be to
sIu'- engineering design factors for getting panels into the ultimate
product wdthout tie use of molded-in inserts; this has certain ideal
features from t'e standpoint of sa:id- ich-panel manufacture, and it
SsiTlifies pres sin.

Properties of Sand-ich Panels

otr- .-nth

strength data were obtained on both large and small sandvdwich panels compris-
ing paper-honeycomb core -d'th facings of veneer, plywood, hardboard, asbestos
board, aluminum, or otier materials. Th-se tests included static bending,
impact bending, and column tests. Tie most common test conducted was the
static bending test, which consisted of supporting thie panels at the ends
and applying an increasing load at tw-o quarter-span points. The amount of
deflection was recorded at various loads to the design load or until failure
oecurre(d. Tlis test not only produced useful information on stresses
'devl, op in the facirigs and on stiffness of the assembly but gave infor-
L4tion i n(icaiC, v e rs-_t of the cores ....
maio indicate of td shear str gth of the cores and the quality of the
bond betv'eqn core and facing s.

In the impact bonding test, a 10-inc -dLam ter sand 'ac, eighing 60 pounds,
'is idroppI'. d on te ce-nter of a large s'i.-ichi p-an 1, Ibe-1inning at a height
of 1 foot 'inc increasing by increments of 1 foot to a hiJgt of 10 feet or
until failure occurred. Pan-is were supported-. at ech .. nd, and both
inst Lnit n... *i nd rp rmannt d(fi,.ctions -..'.r muasurd. T}e, vi.rtical load
Itest- vIs m'ido to dI:t.rmin_ thi ability of 'i pncl to meet c-rtain column
Structure r quirementst. The short+cninr' of thc p-an(l in thi vertical
dnire-ction arid its literal dcfle'ctions wiVre mcasurtd.

Strength tests conducted on larg--size sand,'ic.h wll panels, 3 inches
t hik Uind S-vini ply-ood faclin's, indicated higher sihear strengths developed
in -< dinJ and gr.atlr reistance to vertical loads than found in conventional

r pt. ho. R.1 91il


houe o 'r. -tion. h1ys, rane-ls pn1 ',+ -;s ia" t'

fo r ho u e ra.a .

" :A .d t th- : tr.:.gth f "a4',:4':' .' ';>a4
*t ..'t .. ..'0 "

cf th: }- cli.- -tL:,.; ;: ia

sta:id fL.' f; ea"'in" r; :.,0 o ;;''.
itIL 0.1

lhe f 01'. oy, f : rimul a- r 1 7" -or nr-0.
struction:; *1.u'cir d to va i'r.U:3 ioad',. :
wt l a ,,oi. +oat r^ ",j- r ..i,;'<' at ji.!i~vL's .... f

mitt-h:# loads 'iiid it.:~ :trend

pt' 1:~

h;a;; I' ,'' c,1'air-rd 1 :> nake it r'.o
**1; c: ^ .*;' =. 'I] ih.,Vm to *^* At .*:'

a .. : o'-'i-Tr 'r .. i,

-i1, : t" c t -.::.


~ 'j~,4

4a 4

"h r ' wfI r i -u""rm + w i '"WA 2" 10;" ; '"

P i !& [' r *'0 '20 ,0 4 ),'" .,; ... ... 22 ]c' 2 + 4' '3 ._,; "V D'' k'O f"' ; : '+ '2{; '

4 2 . . .

whr c = fa i-,' In 'clrf,;1 o
c- u
a nt ..... r. -
1, 2

M "" r~~. El'.-


F 'Z'!.
4' 4 4

4"l, 4- *\ i~

= *, (I "I

-O 0-4 4'' .. '


whero 7 = cor(e -,s'r stn'-.

The stif'ness of sandwich construction having facings of the same mat-rial and
equal in thLckrneoss is Lyven by

E.- ,(h<-c")

n = -- -(3)
12 ^

w'-re st ffneses

= modulus of elasticity of the facings

S= 0.99 for wood or plywood facin-s

= 0.9 for isotropic facing's

Thr maximum deflection of a sandwich construction having facings of the
same mat rial and equal in thi kness under a uniT orml anplied loa and
simply supported at :,..e ends is given by

r 2 -i
F 2,aq
2 C3
3 `, 3 + T 6 T
a >*<' ______ ___j \
3-'l f 59) -r

where A\ = maxim. deflection

P = total loa on the ,and7,. ich

C = shear modulus of the core material
a span

Note. An approximation of the deflection mal, be obtained by neolectiiv the
last term in the brackets in formula i. This term will be less than 10 percent
for most constructions on long spans but nay become appreciable for short scans.

Strength data ,or sadi .. h -. of. vario o..s thicknesses and comprising
different facinm;s are -1ven in table 1. Most of the inform-Ition on the 3-inch-
thi, e ae s was obtained y xriml nation, h "-"lues for t' 1- and 2-
inch thicknsss i:cr< cal clatedJ roat hearicall, with the abore formulas. Values
in the ta-1-e sow t't reduc i the t>iLckss of the pwnel from 3 inches to
1 inch would d cr.. its sdfne I to L. ms -nd decreasc the h--uems
lod, tt i >t wil aunrot 3 to times, -o et a soan-,f election ratio of
270 or ore Undo 11 a un iorm load of 20 on ) per square foot, a structural
sndwcrh walL pal o-n a n of 6 inche:s ,,oud 'ave to be more than 2 r., s
t.i if is .a : ..r. of ./ ,I-inch. 0o<1I a-fr ply,,ood. Certain properties
of the heyc 'b ore ra 3e var id considrably with only a mild deviation in
stiffnees oIf ;'eult at sanul 'c pane 1.

H t. ~'~. ~i I


i:. '1 ~
2 "f. ~
44 ('4-4 .jfl :.''
14. r, r r.
~'i' :1.,,

4- 4' 4

'(.7. '.1
I" ~r 1' ~,'

rn (', ; -* .;.* 1 L i r'" ; '' '". ~i."l
'itr- i d 'ar -. I4 ':lm ".4. -~

4 1i~!% '" .. -io, P
O ,l '1 1:. 3 inbcr't'.:, '

OP'n t ". : tL ..

I.,}7 o 4

.. .'C . m r:i r- 4 2 %

-, '" r" f 1 '
.... T '.c *- u.,_T t--d cf ossi'r 1.: ij .


A" .' f t : fT 'i

1. .* :
ur :': i


.4 4

"*, ., '1 > >*
4, ^r i r-

r .:, ; "o

-4. ,,. 4 4
4 4
''.1., '.
'1 4-

'I' .4 4 ,, 4' 4'
'1 '.

1. 4~2'i'

Fov'r'4 '.-i 'ol 1. i n .'-43'i '. 4xo' ii ^ '1 hou ':1
4- -l *. .o "-. iorpJ !~*:f b' 4' 7 4'(.4ll o 1' 4: n
,-on: l id T'it" io^7 ;...'. l;=, *, :<-)C:" r ...... ir,, ''.ou:-inp; *:h~c

2u n 1',' -' ,:" ~ ~c .' r1-- i :i- ~ -. ; 'u .1

th.. OU. 1c:' .'"Lc- rr0' c-: c.h'- :.: -^ OV- .'

i 4o .. .; j.'r "7. 0 0 . .* .or,... ..

.i .ti-" hc, }S ,inOr'.O 21 ; p .' r i .... I, ....^ ..i n ..+
w o d nr .. : 7'. s .riii'. 4 ; ... "n "' '" . ..... "

c" .1 n '..... :.+ i "r 'S t:..' to btri "o .tu ; r. '' t -r

4:, .:. .:

" -. T1 '

1_ .:

i -, L 7 iS lj ^ n,

'o .... }] o. :-".

4 1 .: 44 .

F-, I 7 +



AiA''Z. -
14 -


'4..' , '1
4 El

I:. coh,,rt"Lell sonrstr"on tior it ": r c
t ,: : i .. ... .. ..

"'.ccl'l i'i ^ of a ^o l V'Lr'" u:" ,v?',?'

I In nun 1 1
L"r'n,'r c c '4 t'r } ti'I : r

' L ti1 o lu "-i ,j!j j P ', v,'i >- j'
1 '. .. 17 ".. . ,' '- ;i r V

not b(. re sor tec' to *:..n i.*: ti 'r, r^.;,! .
14 ) t1' '4(~ *-, ',4<'C } ,IO

foil, '-0 :tlooK t-th; ^*[^rr'if '* 1.' vopr;:' to
*r'i'r,2 tc w^ i. C:, or'ulCt,; :1 or' rrci0:' ):':"' to

4--.'. r. c^ \'.r : 4 i. of r'o- n '
.4 : :o"\. o .* '1.. O ;Cl

t 4' .. ." ' I !

,'-7 r 4--:. -,, ;P q f ... '1t ..

4.. ,2,S O( ,-4it' 1.4-4" ;"

in 0
t. lr .. r) 4L r
.. .4 .... '14. O t t 4. ."I_ "
.,: -]IZ ... I i' : 0 V IL '1, ,.:iLl' i~ '.) 18 .

'it1 'r d l 4 ro.,
''. 1 ',. '', -. '7 1 4 ^,,


4 ~

Dimon: ionatl ta lity

consisted of thrcu-ply, 1/4-inch Doudlas-fir plywood with and without paper
overlay; two-ply, 1/5i-inch plywood with paper overlay; and overlaid, 1/8-inch
veneer. One of the plyw.ood-faced prirn-ls had two coats of aluminum paint on
the warm side only.

All panels deflected slig-htly tc .a,.J the warm side immediately after being in-
stalledc, a: the facing, on the cold side underwent thermal contraction. As the
test progressed, the deflection becan-e less prominent, because the moisture
content of the cold side increased. The overlaid, three- and two-ply construc-
tions after 9 weeks' exposure becanve straight. At the end of the exposure, the
overlaid veneer panel showed the greatest deflection, although the overlaid,
three-ply panel showed the greatest initial change. After the panels were re-
m .ved from the ;il1 and exposed to 70 F, on both sides for 3 hours, the panels
reversed themselves, bowing toward the cold wall. This was probably caused by
the plywood on the cold side undergoing thr;rrial expansion and possibly, in part
by the absorption of moisture from the melting of the frost crystals that had
formed inside the panels during the exposure. Although these test conditions
are not exactly typical of ordinary service conditions, the amount of deflec-
tion observed in these panels would not be considered objectionable. Accurate
data on the dimensional change of sheet facing materials due to moisture or
thermal changes is very irrportant to a study of panel bowing.

It is possible to calculate mathematically the bowing of a sanddwich construc-
tion if the percent expansion of each facing is known. The maximum deflection
due to bowing caused by the expansion of one facing resulting from tempera-
ture or moisture differential is given amr'itely by

w-I,-re k1 = percent expansion of one facing as c3-.arcd to the opposite facing

a = length of panel

h = total s'.'t"ich thickness

Using this fermtv]a, the approximate bowing deflection was calculated for
hardboard- and p y.r ood-faced sandi ci. panels of varlou: -,hkne:oes. This
information is given in table 1. T-,u bowin- deflection due to moisture or
temperature d difference is greatly 0 ndont on thi:kne:' of the panel. As
a, Cxa:ple, a 9(-inch-long, pl- ,-faced psnel 3 inches thick bows about
one-third as much as a similar panel 1 inch thick.

Tkerwil Con''uctivity

The bacic values determined in esta- li:hiw' the thermal conductivity of a
-at -ria] or combination of materials used in a structure can be defined as

n t. ~o. .

k therm:al con>uctivity: T'"e ti e rat, o e fl'w t.fro rfi
ai hoo,-cnous m. *ria or on o"f unior structurr a.d o
stats conditions, xpr ssod in "i trmal unjts nor hour
par s quare foot per ;r ch of thion:ns r er dffence in
.- rature betwee'a f-ion fc a C ial.

U ovoe-al coefficient lf ,,at transmiss:icrn, air to air:
The tine rate of h'at floi ,xressd in 'tj t" al i t
vr hour per square foot prir dcre if.e'rer.ce in t1mprature.

Th.e t r'- allies to the c','ination of ,aterials useI in a
construction: for exa" 0.,, both fac '.. and Core of a sa.'....
and include standard aluesI for f and f for stil air on
t:e warn side and air moving at 1 miles per hour on the 1cold

usual method of comparin- insulatinr values for wall construction is by
comparison. of eat transmission coefficients or U values. The U vaue for a
construction is founJ from the relation

nT -- J

where the value of 1 are for the i ve ral -on the thicknes-s of t'h s: con: ti.tunts.

To cor.r ::lai'. char"actri +." o7' var; .....c c,'t"c1 t'-
were nade on 1-inch-,-1 ck spcimns u+s-ra a -ua c,. ht'l., ar re
data ar .-T...i... i. tai e 2. Th" co .c-''t val:e o' 4 he core .s
affected by :c:l -, CYQnc-y, c.. te t, ons......on, '..i tye
material. ruas in elation wa.- sV-rnlfica 9Th af'' te b the o
construction; values for va:nied fron a-, lo: as 0.* +o as hi ,- a .. h
variation.: -n co2e str" '4'r, A 1: value o"; aud *;'.; .......
units r r hour '.)rr s:uarr foot a '' :nch o7 -'- r.,u, n:r -. -tno
with cross-corruated (X7) or the ',ral-l-corr..' (?") strtur h
flutes per- en, :uliar to facins, T nso laJn '.t structures 1n the test
so that flutes were parallel instead of Ic-nd>cu ar u o th nte value was reduced to ).30 to 0'.3 rut thee strcturcs !:,a v, ot> r disadvan-
ta{es, as mentriorned e -rlir in this rno:'t. For any narticuar cer cor-struc-
tion, the densi t- of t' e structure aiff-cts I is r.,. I'on I ". xue. As an e,-
amp le, a Icore h(vring a densi r ,' f ou, t r5< pOurd ",r ,ubi foot had a k
value of -'out w. ', whl:e a siilaly a..m. 1ed ore, wt a density of .
pounds per cubic foot, ave a k value o. 0 about C.47.

An actual TT value of ..1L British thermal units per square foot per hour per
F. was obtained on a 3-inc-thick:, lar-' panel a:i: the cross-
corrugated ( ) tyre of core and li --in ', three-ly rou'las'-i plywood
fac ns. TIis rane1 was ex'YM in Ia wall betIee. two rooms, 0m controlled
at 72?' F. and 14 nT recent rel.ative huadit and the other at -2 F. 7 Ts
0.1 O value -o".'rres reasonably W: ll with a ,,. a]'d 11 vale of '.I! for
the sa:e p, anel. If t( t'ici-n.s e his: 'ne' was reduce M to 1 ., the U
value could be e _,Aected to incr- ase to a-out "..' 2.

An improvement in the insulation value of the sandwich construction can be
realized by fillin' the honcycom'r core with insulation or a foamned-in-place
resin. A reduction in the h value of a corrugated core from O.46 to O.4O
British thermal units per hour per square foot per inch of thickness per F.
was obtained when a phenolic resin was foamed into the core. A slightly
lower value was obtained through the use of fill insulation. Foaming of
resins into honeycomb appears very promising as a means of improving thermal
insulation and fire resistance.

Fire Resistance

Sandwich panels were tested for fire resistance by two methods d-sirned
for housing materials: (1) Exposing one face of the panel to a standard
flame that appro:.:m-ites conditions of fire in a house in which furnishings
are being burned; and (2) introducing flame through a hole in the facing, as
might occur at openings for electrical conduit or other house equipment.

The first method gives fire-resistance values for sandwich construction that
are comparable with those accepted for other types of house construction.
The critical factor in this test is the ability of the facings and the bond
between the facin-s and core to resist the higih temperature without developing
construction failures. Obviously when the facings or bond have failed, the
construction is gravely weakened, since the facings are the principal load-
carrying elements of the sandwich. At the Laboratory, this test was con-
ducted in a gas-fired furnace accordinS to the exposure conditions specified
in American Society for Testin. 1Katerials (ASTIi) Specification No. E-119-47.
r:!," fire resistance of the wood-faced sandwich panels was appreciably higUer
than hollow panels faced with the same of plywood. When aluminum-
faced panels were exposed to the gas, a rather rapid buckling of the facing
and failure of the bond usually occurred. In most cases, the cores were badly
charred after the test '-ut retained their ori-inal form to a considerable

The fire resistance of the sandwich panel can be increased considerably by
incorporating in the core foamed resin or an intur-.i: scent coatin- material,
such as certain types of sodium silicates. It is conceivable that a material
could be developed for deposition in the core that would serve the triple
function of bonding the paper sheets, providing foam for fire resistance,
and also improving th riiEl insulation.

By the second method, the likelihood of flame s -preil in the core was investi-
gated. This was done by cutting a small hole in one facing, holding the panel
vertically, and applying a gas flame to a small area for 4 minutes. In panels
having flutes perpendicular to the facings, such as in the expanded figure-8
and the corrugated PN and P'L tv?-s of core, only slight flame spread
occurred. Burning was restricted to the honeycomb material in contact with
the flame. '4'Jen the flame was removed, flamin[ stc.',ed immediately. Some
gIoew persisted '.or an additional 1 to 2 minutes. In the case of the cross-
corrug ated hi type of core in which one-half of the flutes are parallel to
the length f ti' panel, the spread of flane occurred in the vertical direction
due to t'ie open channels. This could no doubt be readily improved by placing a


ent. C, o. T Il

barrier sheet at the top of the nrn'.-l or at intervals in the panel h1g uht, or
perhaps by ly turning the length of the core ulochs at 9)' to the vertical

The resin-treeted core in Itself is not fire-re distant, but its use between
sandwich facings does not see to be hazar os.. it is possible to addi fire-
resistant chemicals to the paper or to din or pray the core as- ,,t
such c.'- calls; ut this is believed to he iunecessary when wood-based fac:.e :
are employed in the nanel construction. Such treatments could e'-fe tctd-, if
necessary, but might create gluinr .. moisture-absorption problems, and their
effects on long,-time aging characteristics of paper are not well understood.

?,.-r...ich-panel Exterior Test Unit

in 1947, an experimental sandwich unit about 12 by 40 feet in size was built
on the Lab'oratory grot.,,;1 for long-time exposure tests. incorporated in this
unit are various types of sandwich .*, floor, partLtion, and roof panels.
All have paper-honeycomb cores and are faced with veneer, plwood, overlaid
plywood, -.rI:oard, asestos board, or aluminum for comparative purposes.
The unit on the inside is equiipped with heatinp coils an'd is controlled dur-
ing the winter at a temperature of 72 F. and a relative humidity of 0 p-
cent. The outside of the unit is exposed to the variable outdoor temperatures
of the ad.son, His., area.

Before the wood-faced and aluminum-faced nanels were "-:,tailed in the test
unit, they were tested to Jetermine their deflection and sran-deflection
ratios at design lead. The -inch-tick, woo(-faced wall panels met
requirement th; t the spn-deflection rati: be not les: than 272 under a design
load of -3 pounds ner square foot. Floor panels of 6-inch thickness wLth fac-
ings of 3/8-inch, five-ply Douglas-fir plywood had a s'-an-dcflection ratio of
about 5") 1,,, :r a load o" 40 pounds er sqor aro foot (1-.).

After l- .,..nthst expov' ., four wall panel: were r vcd from the e:,crLmental
unit for test. panels had of i/1s-inch Doud-las-fir yoo, and
two had facings of 1/-inch Doiuglas-fir ener overlaid naper. crcrs were
of the cross-corrugated type. On r, --)val, the panels -howed no visLible
signs of deterioration in either the core or the facing s. Th'-, Stiffnes of
the exposed panels was equal to or slightly after than their stiffness prior
to installation. This increase in may possibly be due to tIe
further cure of the resin upon a'-in-. axim ,m loads nobaincd on these Pan els
varied from 13 to 20 times the desi, n lo' of 20 pounds per square foot.

To obtain additional information on th e -f'rc+C of continuous weath1 ring on
a'.earance and strength, sandwich panels having 1-linch-thick, paner-hone,, comb core
and various wood facinrs with 'vi without paper cverla', were subjected to the
following conditions in the Lal-oratory:

1. Immersed in water at 122 F. for 1 hor.
2. Sp--c -d with wet steam at 1 L' to 20 ". for 3 hours.
3. Stored at 10c F. for 20 hours.
4. Heated in dry air at 2]?1"' 1'. for 3 hovrs.

Rept. 'To. R19]]8

-3 -

5. Sprayed with wet steam at 19h* to 200 F. for 3 hours.
6. Heated in dry air at 212 F. for 18 hours.

This sequence of exposures was continued through six cycles, after which
appearance of the specimens was noted, and bending tests were made to determine
any change in strength properties. Results of these tests were compared with
those of tests made on control specimens not subjected to the aging tests. The
reduction in shear stress developed in the cores of the aged specimens was
about 20 to 30 percent as compared with that of the control specimens. Reduc-
tion in stiffness was about 20 percent as obtained from a comparison of load-
deflection ratios. r.? sandwich specimens were exceptionally straight, and
no visual defects were apparent in the core. Although accelerated aging tests
are never completely satisfactory, the performance of such specimenrs plus the
observations made on the actual exposure unit indicate that good performance
could be expected.

One of the reasons for building the test unit was to obtain measurements of
the actual bowing under outdoor conditions. Some data were obtained during
the first year of exposure of these panels. Shortly after the unit was con-
structed in June 19147, deflection data showed a tendency of the panels with
wood facins to bow slip'htly inward. In "ovember when the heating system
was turned on, the wood panels reversed their movement and bowed outward.
The outward bowin- was due to the lower outside temperature, which caused an
increase in moisture content of the outer facing, and the shrinkage of the
inner facings due to the heat. The bowin. increased progressively in the
wood-faced panels as the average outdoor temperature decreased and continued
until late in Narch when the outdoor temperature began to rise again. The
ri-r:irinum bowing recorded in olywood-faced panels was about one-fourth inch.
Aluminum-faced panels, not being affected by moisture, bowed toward the in-
side as the temperature of the outside dropped. On a hot day with a hi-h
surface temperature on the outside, an outward bow could be noted. Results
of the first year indicated that bowing of sandwich panels with facings of
three-ply plywood is consistent with that of panels with three-ply stressed
facings. Whether the inner surface was untreated, had aluminum paint, or had
an overlay appeared to be unimportant in panels with three-ply facings.

As the development of the sorn--ich structural panel grows, the need for more
data on the behavior of such panels under ordinary exposure conditions will
increase. It is hoped that this test unit will be helpful in resolving some
of the problems regarding large-scale use of srrdxuich panels. Although there
are many other uses for sandwich panels, the evaluation of these panels under
housing conditions provides information thliat applies to most other products
in which use of sandwich panels might be considered.

No attempt is made in this report to detail actual or proposed uses for sand-
wich panels, but these include partitions, doors, spandrell panels, and other
constructions in houses, trailers, shelter buildings, warehouses, and farm
buildings, lightweight shipping containers, and furniture. Because of the
inherent structural strength of these panels, the greatest total benefit can
probably be realizeC by usin{ them to carry the principal loads in a construc-
tion, not just to provide coverage. The g general trend toward the use of sheet
materials, both on the inner and outer surfaces of buildings, also points to
long-term importance of sandwich panels as building materials.

Rept. 'o. R1918


Literature t ed

.11 '.!"AL P1- .TC Bakelite Review, July.


1- +. "SYCO:." ., OF AGE. odern Plastics Vol. 2, "-. II

K., 3EIDL, R. J., A'::) FYY, D. J.
Al Forest products Lab. et. Irn 10 pp., illus.
, AID~l. Forest Iroducts LaL, Wrt. I7+_., 1 pl~~

91. H C.. TY.' CO GIVES 'T TC(HT-VTnE`7 RATIO. Vener rs
Plywood Vol. h, No. P.

(5) L,71; J. D.
19U6. PRO

N 7W

(7) 1

. odern Plastics Vol. 23, 'lo. 9.

T T 7, H" !' Yg
' n I^ A ''1 .. ..Tr f t ,''TM ".. .. P +' +T
. ..... ,. s' < .... ;,*), OF : '[, LC I .: q + T :_ O n Y :
t 8 A I 7.TITC7I;C" O 'U I IF UTE: HCY .7
T... A.. O rP. I E l. Natl. Advisory
for A,:ronautics. Tech. -ote ''o. I52Q.

!L" T ..T'T T L C 7 -
l.. 9+ ....... C" T} TF -j Tj -M
192:0. EF1TC? 0:'' CLLL IAFE Op, C 7T's ...., TP"TI F
> T,-. S UCT T T ... Cli',al. Advisory Coirnittee Por
A e onauticI. Toch. sotc 'lo. 223.

(S) K IDL, R.


'T ... T I, T !r +,, .... - r "*n T- (I
l1Y, 7 -I: T 'T-YCC: u 7J T F C*JC J 1 ,P T)L T

o. R1796, 15 p., illus.
I. I U -,I A
STiE 7" 1 A'TjD TTSISTA ICE TO'- 1)1CAY. Forest Products l,ab. Rent.
';o. 1-179.", 16 pn. illus.

FA'::Y D. J, -- 'l S, A. *.
C IE"TATION, ES':C TYPE AKM AO?:. Natl. Advisory Committee
for Aeronautics, Tech. 'te No. 296o.

(10) T. 0',!TO : UAROLD
1I.'9. "- :: ITT 7I"'fl '' A S :A :'+l' TEXT:, '7
A- T'I CC. 0. Tappi Vol. 32, No. T-.
AN T'; I'R 0-"-: 0!+. Tappi Vol. 32, ,Jo. 5.

(II) U. F 1
i9UK .

;T I?( ''T- LAPORATC '.:.Y
I 7I IT ) R DE3 A. t D'T FABRICPTON i,: u F IXP, 'AL
O'n.:'CC_-CCL ..DIC. H(` FP. A .,D3. Housing H -omo Finance
Agency Tech. aper No. 7.

Rept. No. :sr9Il

Conrmitte e

( 1 ) A U T : . *
U ',6 EXP A "'::

(3) BAIRj, P.
1914i .

,T IO N O F "'-'E/ C S T:' "


Table 1.--Stiffness and strength of structural sandwich panels on a 96-inch

: 'anel : Center : Span- :1 maximum
: thick-:deflection2 :deflection uniformn
* ne : : ratio : load



:L. er

- Approximate
: b ic,,ir. 3
* deflectior-

: In.

1/ -inh KD ou-las-fir : 3

: 1

T-o !/-inch Douls- 3
fir vcnc,--rs with : 2
paper oY.-;rila, : 1

One 1,/-inch Dour2as-
fir veneer with
par er ov, rliy on
each side
l1!;-inch to':'T" er, d

1/3-inch tcmired
hrdbo o.rd

: 0.1q
* 1 /
23 ',jn

I 207
2. 1)0

!L. I0

: 202
: .h0>8
2.b 00


: 21
: 212
: ho

Y. i"'

* 0~3?

: 251
: 175


: 2h2

t 193
: 3';




: 269
: 176
: 82

1. 50

* .. a a *0****** a *

All cores of tle cross-

'or rug'atd XI tyne were made from

treated w i'h 1i pe rcnat of re in.
cub< c foot.

The core dcensity' was

50-pound paper
2.5 pounds per

DFlTctio..n und] r a un form loaK, of 20 pounds n-oMr square foot on a span of

4 i 'idn< deflection compute-; from he d fferontial e:capnsion of the two
fac s obtained exposure of on facin to a rclativc hunidit- of 97
0nt, ;. ot, to 'v hc :- 'ity of 30 percent for 30 da-,s.

Facin ,s

0, . .. .. .,


Table 2.-----f '-- -t of co _- 'U-- -- -
aL --- ---I I 5

Cq '<: ?]i1 'v
>2 112t'" 't 20 : 2 A>'<


-- -. 8 -. -

0L ; J Y, S ; i^.i .

. .. . . . .

. ... .'l .. .. .

... J o.. ..

S S .

7 ~

*~ 5

t. .2. 191

9 [ '

k 4.



Figure l.--Expanded type of core consisting of sheets of paper inter-
spaced with parallel strips of adhesive and expanded to form hexa-
gonal cell sections.
ZM J'7220 F


-AL AV -A* -0

A,2K f a,.,,i..P:il N..,J-1,
~D: ~: _i,I

-A 81
Il_ _


L 1

A ~~

Figure 2.--Looped or figure-6 type of core consisting of sheets of
paper looped and bonded to form circular cells.
Z M 97221 F

., 1 *
4' 4

>< . q

,.; ,, ;. ,; .,

" ' "

..' ,

Figure .-- ed y,- of core
corrugated sheets ofI pa.-r asem
separated b. a B ,, 1. treai-.

(d as .It consists of
ie to each other and


Figure 4.--Corrugated type of core designated as XN. It consists of
corrugated sheets assembled with principal flute directions of adja-
cent sheets at right angles.
Z M S7222 F

1. *h I :, 1 *
I b b

,' I & i., ib i j *
,,,u' ^


V ".

ZM 8887 F

Figure 5.--Corrugated type of core deE_, -.' a : It consists of
corrugated sheets assembled parall.- to a ti. bonded at
the crests.



:iI !

S .......

i" /-

Figure 6.--Structural sandwich wall panel tested
under vertical load.

Z M 79770 F


3 1 262 08866 6044I II I I III I
3 1262 08866 6044