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Issued In Cooperation with the
AcIRCIRaT VIESIGN CRITRIA
Under ~the: Sup~erwhionI of the
VNITED SSTATES IEPARhENT OF AGRICUTURE
FORESTI.POD UCTS LA BORATORY
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RESULTT OF SCf USTS ON
RESULTS OF SOME TESTS ON LOW-DENSITY MATERIALS -
E. C. 0. ERICKSON, Engineer
This report presents the results of exploratory tests made at room
temperature to determine certain of the mechanical properties of a number of
low-density materials examined for possible use as cores for sandwich con-
structions. In general, the results are to be regarded only as indicative
since the number of tests is small.
The study included such tests as edgewise and flatwise compression and
tension, flatwise static bending, and shear modulus tests, to provide the
strength data currently recognized as the most significant in qualifying ma-
terials for core usage.
In referring to the application of loads in test, the following defi-
nit ion~s apply:
IFlati&ise: Load applied in a direction normal to the plane of the
panel as manu-factured.
Edgewise: Load applied to the edge of the original material, that is,
in a direction parallel to the length or width of the panel.
Test Mt erial s
Test material consisted of two materials received from The Ministry
of Air Production, British Air Commission, "0nazote"l and "Calcium Alginate,"
and two domestic materials, "Hycar or Ebonite FEX50036,11 received from The
Sponge Rubber Products Company of Derby, Connecticut, and cellular or fibrous
cellulose acetate, an experimental material made by the DuPont Company. It
is understood that this identical material is no longer, available and that an
improved cellular cellulose acetate has since been developed by DuPont.
The 0nazote was in two nominal thicknesses, 1/2 and 3/4 inch. The
calcium alginate varied in thickness from 0.4 to 0.8 of an inch. The 18- by
"'This mimeograph is one of a series of progress reports prepared by the
Forest Products Laboratory to further the Nation's war effort. Results
here reported are preliminary and may be revised as additional data become
Mimeos, Ra4 150
12-inch Hycar panel was 0.8 inch thick. The cellulose acetate sample was
taken from a sandwich panel received from the Consolidated Water Power and
Paper Company about October 1, 1943. It was composed of fine fibers oriented
in one principal direction, parallel to the 3/4-inch thickness of the test
Onazote and Hycar are black materials of quite similar appearance.
The surfaces of both, as formed during manufacture, are fairly smooth. The
material adjacent to these formed surfaces appears to be much denser than the
intermediate structure; otherwise both materials appear to be homogeneous.
Test Specimens and Test Methods
All specimens were tested under ordinary room conditions and were pre-
sumably in equilibrium with such conditions. Nio investigation was made of
the effect of moisture on the strength properties or of the behavior of the
materials with respect to absorption of water from humid atmospheres or when
soaked. No determinations were made of moisture content. Specific gravity
as given includes whatever moisture was present.
Test specimens of the Onazote and Hycar materials had two original
surfaces and four cut edges. The original or formed surface of these mate-
rials was not removed prior to test, except as noted in the table of results.
Equal numbers of edgewise compression specimens were taken from each
of the two principal directions of the samples, lengthwise and crosswise
(00 and 900).
The majority of edgewise compression tests were conducted on 1-inch
by t (material thickness) by 4-inch specimens as laterally supported columns.
A 2-inch gage length Marten's mirror compressometer was used to supply the
deformation data. The cellulose acetate specimens were 1 by 3/4 by 3 inches
long. Deformation data for these acetate specimens was obtained by averag-
ing the readings of two dial gages recording the movement between the fixed
and movable heads of the testing machine. A few tests were conducted on
1/2- by 1/2- by 2-inch specimens of nominal 1/2-inch Onazote, using a 1-inch
gage length extensometer to obtain deformation data.
Flatwise compression tests were conducted on specimens 1 inch or
1-3/4 inches square and with a height equal to the thickness of the material.
The elastic properties were obtained from deformation data taken between the
fixed and movable heads of the testing machine.
Lengthwise tensile specimens of the 3/4-inch Onazote were 1 by (t) by
16 inches long, shaped to have a 2-1/2 inch long central section 1/2 inch
wide. The taper followed a 30-inch radius on each edge. These specimens
were tested in self-aligning Templin grips. Deformation data was obtained
by means of a Tripolitis extensometer equipped with a Last Word (1/10,000)
-iAmeo. No. 1509 -2-
Flatwise tension tests were made on dumbbell-shaped specimens in the
standard tensile grips used for molded plastic specimens. These specimens
were prepared by gluing wood strips to each face of a 1-inch strip of the
test material. Test specimens having a 1- by 1-inch net section were then
cut to shape on a band saw. These specimens were tested for maximum load
Flatwise static bending specimens 1/2 to 1-1/2 inches wide were tested
as a simple beam with center loading, over spans equal to 16 times the thick-
ness of the material. Center deflections were obtained by means of a
(1/1, 000) dial gage.
Modulus of rigidity data were obtained by means of the standard Forest
Products Laboratory plate shear test, developed for measuring the shearing
module in wood (.rL Mimeo. No. 1301); and by torsion-pendulum tests of long
thin strips having a width of at least three times the thickness.
Average property values resulting from a specified number of tests,
together with the range of values of the four materials, are presented in
All of the materials are believed to be essentially isotropic in the
flatwise plane, because of the good agreement between the 00 and 900 data
for edgewise compression. The edgewise compression properties reported in-
clude the results of tests at both 00 and 900.
Modulus of elasticity for flatwise compression are affected by the
inclusion of end conditions in the deformations recorded between heads.
Typical edgewise and flatwise compressive stress-strain curves of the
four materials are presented in figures 1 and 2, respectively. A typical
stress-strain curve for 3/4-inch 0nazote in lengthwise tension is shown in
figure 3o, These curves are based on actual load-deformation data for indi-
vidual test specimens whose properties agreed fairly well with those of the
average for the group. The end points indicated for each curve represent the
ultimate stresses at failure, unless the numerical ultimate is given, in which
case, the end points indicate the extent of the deformation data only.
It may be noted that the properties of 0nazote In the 1/2,-inch thick-
ness (specific gravity 0.172) were greater than the corresponding properties
of the 3/4-inch thickness (specific gravity 0.135). However, on a specific
basis, the 3/4-irnch material is superior to the 1/2-inch material in flatwise
Mimeo, No, 1509
bending but inferior In edgewise compression. This behavior reflects the
presence of denser material adjacent to the surfaces. Further evidence of
this was found in testing an edgewise compression specimen taken from the
formed edge of the 3/4-inch material. In comparison with similar rectanglar
specimens having two formed surfaces and four cut edges, this 1- by 3/4- by
@-inch specimen with three formed surfaces and three cut edges differed as
follows: 9 percent increase in specific gravity; 90 percent increase in spe-
cific modulus, but no change in specific ultimate.
Platwise tensile specimens tested normal to the formd surfaces of the
3/4-inch material failed in a fairly smooth plane parallel to and very close
to the original surface of the material. Localization of the failure plane
in the denser material near the formed surface would appear to indicate that
a plane of weakness exists within the finer structure of the dense surface
Hycar or Ebonite EX5036
The characteristics of Hycar with respect to densification at the
formed surfaces and types of failure are similar to those of Onazote.
Calcium alginate machines readily by itself, but is difficult to ct
when in combination with harder materials, because it hat a tendency to spIl
and fracture along the edge being worked.
Cellular or Fibrous Cellulose Acetate
The cellulose acetate is readily compressible perpendicular to the
principal fiber direction, but has considerable strength in the fiber direc-
tion. The reported crushing strength, flatwise, represents the breakdow
strength or first crushing failure. The average ultimate or maimum compres-
sive strength flatwise (parallel to fibers)'was 290 pounds per square inch,
with values ranging from 240 to 320 pounds per square inch.
It may be pointed out that the minimum flatwise crushing values of
all the reported materials represent the limiting cold press pressures that
may be employed in bonding face materials as in sandwich construction.
Mimeo. No. 1509
Material : 1/2 Inch Onatote : 5/4 inch Onazote : Hycar or Ebonite (Ex50036): Calcium alginate :Cellular cellulose acetate
,------------------------------ ---------- - - - - - - - - - - - -- - - - - - - -- - - - - - -- - - - - - - -
Specific gravity : 0.172 (0.163 to 0.196) : 0.135 (0.127 to 0.145) : 0.094 (0.092 to 0.099) 0.11 (0.097 to 0.112) : 0.086 (C.064 to 0.0e
------------------------------ ---------- - - - - -;- - - - - - - -- - - - - - - -- - - - - - -- - - - - - - -
:Humber : :Number:- :Number.: :Number:' :Number:
of :Average: Range : of :Average: Range : of :Average: Range : of :Average; Range ; of :Average: Range
: tests: tests: : tests: : tests: :eats :
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --: -- -- -- -- -- -------- :------- - - - ---------------------- ;----- - - - - -
Table 1.--Mchanical orooerties of low-density materials at room temperature
: 3, 400
: 3,100-360 :
8 2' q : :
160 : 145-165
40 : 35-Z1
: 0 : 70-8 :
465 : 460-470
: 260 : 225-270
99 : 97-104
9,400 : 9,300-9,500
170 : 160-160
100 : 90-120
2 : 4,70s : 4,600-4,900
3 : 4,OC : 3, bOO-4, 01
: 74-91 :
245 : 230-290
85 : 70-110
1 0 170-230
300 : o,400-9,400
240 : 200-260
16b : 140-6b :
30 : 260-370
190 : 170-220
BS : 30-150
Modulus of elasticity
Stress at proportional limit
Stress at 0,07 inch per
Modulus of elasticity
Strees at proportional limit
Static bending (flatwise)
Modulue of elasticity
Modulue of rupture
Stress at proportional limit
Modulue of elasticity
Stress at proportional limit
Modulue of rigidity2(a)
1Load applied parallel to the
4 : 44-46
19 : 16-22
length or widt.n ?T we panels.
?(a) Modulus associated with shear distortion in the original surfaces and p
(b) Modulue associated with shear diatortion in the original surfaces an.d :
specimen with wider faces parallel to original surface. In tte core
(c) Modulus associated with shear distortion in planes owcren,!dicular to *1
wider face perpendicular to original surface.
(d) Same es(c) but with about 1/32 inch of each akin Itmovr.1
hLoad applied cerpendicular to the fiber direction.
!Load applied parallel to the fiber direction.
kLorfd applied parf11el Lt) fiber direction (fibers perpendl c.i*r oofn).
parallel tnerete Plate shear tests (mimeograph 1301) on full L;icrress of material
a rllel tliereto determined from rate of vibration of Loreion pendulum cois ,ating or
.1<, spec:ns were *:-em central portion of the tnicaness all., densified "ss.-n" removed.
. :ice Determined fr.m rate of viorilsio: of torSion pendulum caslatng of secn site
Z M 53662 F
5T/?AIN (IN'CHE,5 PER INCH)
N 54757 Y Figure l.--Compression (edgewise)stes tr ncuv.
5TR'f55 (POtY/VD5 PfI SQO/ARE INVCH-)
[l -L 1T
Figure 2.--Compression (flatwise) stress-strain curves.
M 54?56 F
5TRA/N INCHES5 PEA' INCH)
Figure 3.--Tension (lengthwise) stress-strain curve.
M 54758 F
DIVERSITY OF FLORIDA
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