MECHANICAL PINDPEPTIIES
PLASTIC LAMINATES
At'it
No. 1820-1)
January 1958
\>13/R:4)
A 28 MB
PR
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This Report is One of a Series
Issued in Cooperation with the
ANC-17 PANEL ON MASTICS FOR AIRCRAFT
of the Departments of the
AIR FORCE, NAVY, AND COMMERCE
Jl
FOREST PRODUCTS LABORATORY
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
MADISON 5 . WISCONSIN
In Cooperation with the University of Wisconsin
Supplement to
MECHANICAL PROPERTIES OF PLASTIC LAMINATES1
By
GORDON H. STEVENS, Engineer
Forest Products Laboratory, ? Forest Service
U. S. Department of Agriculture
Summary
This report presents the results of a series of tension, compression, and bending
tests of a parallel-laminated silicone laminate reinforced with 181 glass fabric.
The data supplement those of Forest Products Laboratory Reports 1820, 1820-A,
1820-B, and 1820-C of the same title, in which results of tests of polyester,
phenolic, and epoxy laminates were given.
Tension tests were made after normal conditioning; compression and flexural
tests were made after both normal and wet conditioning. From the results of the
tension tests, the panel shear strength was calculated. The average strength
properties are presented both in tables and as curves.
In general, the mechanical properties of the silicone laminate at room temperature were lower in value than those of comparable polyester, phenolic, and epoxy
laminates. In tension at 0 degrees, however, the mechanical properties of the
silicone laminate were of approximately the same values as those of polyester
and phenolic laminates.
1 .
–This
progress report is one of a series (ANC-17, Item 55-6) prepared and
distributed by the U. S. Forest Products Laboratory under U. S. Navy,
Bureau of Aeronautics No. NAer 01683 and U. S. Air Force Contract No.
DO 33(616)-56-9. Results here reported are preliminary and may be revised
as additional data become available.
?Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
Report No. 1820-D -1-
Agriculture-Madison
Introduction
This is one of a series of reports on basic mechanical properties of various
types of laminates. This report presents values of some of the basic mechanical properties of a silicone-resin laminate reinforced with glass fabric. The
particular laminate reported here is a parallel laminate of 181 heat-cleaned
glass fabric that was impregnated with EC 2106 resin. DC 2106 is a silicone
resin of the type required by Military Specification MIL-R-25506 (USAF) and is
commercially available. The glass fabric is generally preimpregnated by commercial treaters. The data from this research may be considered as typical of
laminates fabricated by a technique similar to the one reported here.
This investigation was conducted at the U. S..Forest Products Laboratory,
Madison, Wis., during 1956 and 1957, in cooperation with the ANC-17 Panel on
Plastics for Aircraft.
Description of Material
For this investigation, one panel
pared. The panel was made with a
glass fabric. The preimpregnated
merit Company and was reported to
resin in this fabric was DC 2106,
Corning Company.
1/8 inch thick and 36 inches square was predry lay-up of preimpregnated 181 heat-cleaned
glass fabric was furnished by the U. S. Polycontain 39 percent of resin by weight. The
a silicone resin manufactured by the Dow
The preimpregnated fabric was cut to size, and 12 plies were laid in parallel
between cellophane-covered aluminum cauls. The assembly was then placed in a
press at 347° F. and cured at a pressure of 100 pounds per square inch for 30
minutes. The panel was cooled under pressure, removed from the press, and
stored at a temperature of 73° F. and a relative humidity at 50 percent for 7
months before it was postcured.
Before it was postcured, the panel was cut into smaller sections, as shown in
figure 63. These sections were then placed in a small oven. The temperature
was raised to 195° F. and kept there for 16 hours. This step was taken to minimize any moisture that might have collected in the laminate during storage.
The sections were then postcured for 1 hour at each of the following temperatures: 250°, 300°, 350°, and 420° F. The temperature was then raised to 480° F.
and maintained there for 112 hours.
After the postcuring was completed, a brown discoloration was observed in the
central portion of each section of the laminate. The resin manufacturer was
consulted but did not regard this discoloration as an indication of damage to
the laminate.
Report No. 1820-D
-2-
General information on the panel:
Before postcure
Average thickness (In.) Specific gravity Estimated resin content (percent by weight) Barcol hardness 0.119
1.81
33.7
32.6
After postcure
Barcol hardness 59.0
The estimated resin content, based on the weight of the laminate and the
average weight of 181 glass fabric, was determined by the formula:
B.C. .
W L
F
x 100
WW
L
when R.C. is the estimated resin content, WL is the weight of the laminate, and
Wt, is the weight of the glass fabric. The glass fabric was estimated to weigh
8.90 ounces per square yard.
Testing
After the panel had been postcured, specimens for the various tests were cut
from the panel in the manner shown in figure 63.
All specimens were conditioned at 73° F. and 50 percent relative humidity for at
least 2 weeks. Normally conditioned specimens were then tested. Wet tests were
made after 30 days' immersion in distilled water at 73° F.
Tension tests were made only after normal conditioning, according to procedures
outlined in Report No. 1820. 2 Compression and bending tests were made after both
normal and wet conditioning, according to the procedures outlined in Report No.
1820-B.11
Werren, Fred. Mechanical Properties of Plastic Laminates. Forest Products
Laboratory Report No. 1820, February 1951.
Werren, Fred. Supplement to Mechanical Properties of Plastic Laminates.
Forest Products Laboratory Report No. 1820-B, September 1955.
Report No. 1820-D
-3-
Presentation of Data
The results of the tension, compression, and bending tests are shown in tables
23, 24, and 25. These are average values. Ratios of wet strength to dry
strength are shown in tables 24 and 25. The calculated value of panel shear is
included in table 26. Values of 0.2 percent offset yield stress in compression
and bending were identical to the maximum stress in compression and to the modulus of rupture, respectively, hence are not given in this report.
Average stress-strain curves in tension and compression and average load-deflection curves in bending are shown for this laminate in figure 64. The relationship between tangent modulus and stress in compression, based on the average
curve in figure 64, is shown in figure 65.
Discussion of Results
The results of this investigation may be considered as typical of silicone laminates made by similar techniques. The previous reports of this series present
similar data for some polyester, phenolic, and epoxy laminates. Comparison of
the laminates with each other, however, should be limited to similarly reinforced laminates tested at normal conditions. Properties of dry laminates may
be compared with reasonable assurance that differences are primarily due to the
different types of resin used. Comparison of properties of wet laminates, however, is subject to several limitations. Properties of wet laminates are affected not only by differences in resin but by differences in fabrication, conditioning, and fabric finish.
In the basic study, –3 properties of the wet laminates were determined by testing
specimens that had been conditioned for at least 2 months at 100° F. and 100
percent relative humidity. This is probably a more severe conditioning than the
30-day immersion in water at 73° F. used in the present study.
In general, the mechanical properties of the normally conditioned silicone laminate were found to be lower than those of the polyester, phenolic, and epoxy
laminates previously reported. This was particularly true for compressive and
flexural strength.
Tension
All tension tests were made after the specimens had been conditioned in an atmosphere of 73° F. and 50 percent relative humidity. The specimens were tested at
angles of 0 degrees, 45 degrees, and 90 degrees to the warp of the reinforcement.
Six specimens were tested at each angle of loading.
Report No. 1820-D
-4-
As in previous studies of laminates similarly reinforced, the average stressstrain curves show two moduli of elasticity with corresponding proportional
limit stresses. The double moduli and proportional limit values were not found
at the 45-degree angle.
The ultimate value of tensile strength parallel to the warp direction was substantially higher than that required by Specification MIL-R-25506 (USAF). At
45 degrees and 90 degrees to the warp direction, the respective tensile strength
values were about 25 and 82 percent of those at 0 degrees. These values were
proportionately lower, based on the strength at 0 degrees, than those generally
found for polyester, phenolic, and epoxy laminates. When a 181 parallel laminate is made with with most resins, the tensile strength at 90 degrees is usually
about 90 percent or more of the strength at 0 degrees; at 45 degrees the strength
is 50 percent or more of the strength at 0 degrees. The low tensile strength of
the silicone laminate at 45 degrees indicates a weakness in shear strength, as
will be noted later. Severe twisting and delamination were observed in the 45degree specimens as failure occurred.
The moduli of elasticity of the silicone laminate at 0 and 90 degrees were quite
similar to those of the polyester and phenolic laminates but significantly lower
than those of the epoxy laminates. At 45 degrees, however, both the modulus of
elasticity and stress at proportional limit were substantially lower for the
silicone laminate than for the laminates made with the other types of resins.
In general, the tensile strength of the silicone laminate at 0 degrees and 90
degrees was slightly lower than those of polyester and phenolic laminates and
considerably lower than the strength of epoxy laminates. At 45 degrees, however,
the silicone laminate had a tensile strength of about 50 percent or less of that
expected for laminates made with other resins. These results show that the angle
of loading has a greater effect on the strength properties of a silicone laminate
than it has with the polyester, phenolic, or epoxy laminates.
Compression
Specimens tested in compression were loaded at 0 degrees and 90 degrees to the
warp direction, and were tested after both normal and wet conditioning. Six
specimens were tested at each angle of loading for both normal and wet conditions.
All the specimens were carefully weighed before and after they were conditioned.
The average weight increase during conditioning was 0.53 percent at 0 degrees
and 0.58 percent at 90 degrees.
During tests of the wet specimens, erratic strain readings resulted from two
specimens --one for each angle of loading. The maximum stress values appeared
to be satisfactory and were used in determining average values, but the strain
readings were omitted.
Report No. 1820-D
-5-
One wet specimen at 0-degree loading had a modulus of elasticity about one-half
that of the other specimens and a maximum stress about 80 percent of the average.
Another specimen of this group also had a maximum stress of about 80 percent of
the average. The stress-strain data, failures, and test procedures were carefully reviewed, and it appeared that the results were due to the properties of
the specimen rather than to substandard testing procedures. Hence, the test
results were included in the averages and are reflected in the values of standard deviation for the group.
The average compressive strength parallel to the warp direction, after both
standard and wet conditioning, exceeded the requirements of Specification MIL-R25506 (USAF).
In general, the dry compressive strength was about two-thirds or less of the
compressive strength of the polyester, phenolic, and epoxy laminates. The values
of modulus of elasticity were also lower than those of the other types of laminates. In general form the stress-strain curves in compression were similar to
those of previously tested laminates.
At 90 degrees to the fabric warp, the compressive properties varied between 82
and 86 percent of the respective properties at 0 degrees. The ratio of compressive properties at 90 degrees to those of 0-degree loading has been observed to
be quite variable in previous tests and the ratios of the silicone laminate
were about on the order that might therefore be anticipated.
Ratios of wet to dry strength are given in table 2 1t. It will be noted that the
maximum reduction due to wet conditioning was 12 percent.
Bending
Bending specimens were tested after both normal and wet conditioning; tests were
made with the warp direction of the laminate at 0 and 90 degrees to the span.
Six specimens were tested at each angle of loading and for each exposure condition. All the specimens to be wet conditioned were carefully weighed before and
after conditioning. The average weight increase was 0.50 percent at 0 degrees
and 0.5I percent at 90 degrees.
The modulus of rupture parallel to the warp direction was about 1 percent less
than that required by Specification MIL-R-25506 (USAF) at standard conditions,
but exceeded specification requirements after wet conditioning.
In bending, the average strength properties of the specimens tested at 90 degrees
varied from about 64 to 88 percent of the comparable properties of specimens
tested with the warp direction at 0 degrees to the span.
Ratios of wet to dry strength properties are shown in table 25, and the maximum
reduction due to wet conditioning was 11 percent.
Report No. 1820-D
-6-
In general, the dry bending strength of the silicone laminates was two-thirds or
less of the bending strength of polyester, phenolic, and epoxy laminates.
The moduli of elasticity in bending obtained for specimens tested with the warp
at 0 and 90 degrees to the span were similar to those of the polyester and phenolic laminates but significantly lower than those of the epoxy laminates.
Shear
No panel shear tests were made in this study, but the shear strength along the
orthotropic axes was calculated, using equation (3) developed in a previous report. 2 When the tensile values for 0-, 45-, and 90-degree loadings are inserted
in equation (3), the calculated shear stress for the silicone laminate is 5,150
pounds per square inch. This value is only about one-half or one-third of those
reported previously for polyester, phenolic, and epoxy laminates reinforced with
181 glass fabric.
Comparative values of shear strength for four types of laminates, as calculated
by use of equation (3), are presented in table 26. The low shear strength of
the silicone laminate indicates that the directional strength properties will
vary more than for polyester, phenolic, or epoxy laminates.
Conclusions
The following conclusions are based on room-temperature tests of a silicone laminate reinforced with 181 glass fabric:
(1) The mechanical properties of the silicone laminate were generally
lower than those of comparable polyester, phenolic, and epoxy laminates. This
was especially true with respect to the epoxy laminates.
(2) Wet conditioning resulted in approximately a 5 to 10 percent reduction in compressive and bending strength; modulus of elasticity was affected
only slightly.
(3) The theoretical panel shear strength of the silicone laminate along
the orthotropic axes was only one-half or less that of polyester, phenolic, and
epoxy laminates reinforced with 181 glass fabric.
(4) Directional properties of the silicone laminate vary more than those
of polyester, phenolic, and epoxy laminates because of the low shear strength of
the silicone laminate along the orthotropic axes.
(5) Average stress-strain and load-deflection curves of the silicone
laminate had the same general form as those of polyester, phenolic, and epoxy
laminates.
2:Youngs, Robert L. Supplement to Mechanical Properties of Plastic Laminates.
Forest Products Laboratory Report No. 1820-C, November 1956.
Report No. 1820-D
-7-
Table 23.--Tensile properties of a parallel laminate made of 181 heat-cleaned
glass fabric and DC 2106 resinAngle of :
loading : Modulus of elasticity : Proportional limit stress : Maximum
: stress
Secondary
Initial : Secondary : Initial : 1,000 :
p.s.i. :
1,000
p.s.i.
:
:
: Av.
,2
.
„,---
2,720 :
2,540
: 11,610
55 :
45
: Av.
S
:
2,330 :
99 :
2,010
Degrees :
0
90
45
: Av.
S
246
1,080
170
:
P.s.i.
P.s.i.
33,080
:
1,797
961
:
740
:
:
25,240
2,891
: 34,220
: 1,192
•
1,370
120
: 10,210
:
306
Within plot standard deviation based on range of plot.
Report No. 1820-B
: 41,980
: 10,990
1,255
:
-Each value is based on six specimens
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Table 26.--Theoretical shear strengthl values for
dry laminates reinforced with 181
glass fabric
Resin
Silicone (EC 2106) Shear strength values
•
P.s.i.
•
5,150
Polyester Phenolic 11,820
•
10,810
Epoxy (Epon 828-D)
12,420
Epoxy (Epon 1001)
17,560
1Shear stresses along the orthotropic axes calculated from tension data with equation (3),
Forest Products Laboratory Report No. 1820-C.
Report No. 1820-D