CULTURE ROOM
EFFECT OF TENSILE AND COMPRESSIVE
PPROADING ON TENSILE, COMPRESSIVE,
AND SITAR PROPERTIES Of A
GIASS4APRIC4ASE EPDXY LAMINATE
May 1959
No. 1870
This Report Is One of a Series
SEP 22 1959
Issued in Cooperation with the
ANC-23 PANEL ON COMPOSITE CONSTR ION
FOR FLIGHT VEHICLES
of the Departments of the
All? FORCE, NAVY, AND COMMERCE
FOREST • ODUCTS LABORATORY
MADISON 5, WISCONSIN
bi
UNITED STATES DEPARTMENT Of AGRICULTURE
FOREST SERVICE
ciopermtion wh
withthe Univerutty of
Wtoconain
EFFECT OF TENSILE AND COMPRESSIVE PRELOADING ON THE
TENSILE, COMPRESSIVE, AND SHEAR PROPERTIES OF A
1
GLASS-FABRIC-BASE EPDXY LAMINATE —
By
KENNETH E. KIMBALL, Engineer
2
Forest Products Laboratory, —
Forest Service
U. S. Department of Agriculture
Abstract
Tests were made to determine the effects of tensile and compressive preloading on the tensile, compressive, and shear properties of a typical epoxy laminate. It had previously been shown that tensile preloading of polyester laminates may result in marked changes in the tensile stress-strain relationships.
Effects of preloading on modulus of elasticity and strength of epoxy laminates
must also be known for design of such material for structural applications.
The data in this report show that tensile preloading will affect certain tensile
and compressive strength properties. Of particular interest is the fact that
the dual characteristics of the usual tensile stress-strain curve•for laminates
are essentially erased after the application of a single preload to a fairly high
stress. For design, the modulus of elasticity in tension can be closely approximated by the secant modulus from the usual stress-strain curve. Tensile preloads generally have no appreciable effect on tensile strength but do result in
lower compressive strength under some conditions-.
Compressive preloading generally has little or no effect on the compressive
or tensile strength properties.
! This progress report is one of a series (ANC-17, Item 57-4) prepared and
distributed by the Forest Products Laboratory under U. S. Air Force Contract No. DO 33(616)58-1, and U. S. Navy, Bureau of Aeronautics Order
No. 01835. Results here reported are preliminary and may be revised as
additional data become available.
?Main
tained at Madison, Wis. , in cooperation with the University of Wisconsin.
—Maintained
Rept. No. 1870
-1-
Crazing of the resin caused by relatively high levels of tensile preload may
result in more moisture sorption, with accompanying reduction of strength
properties, than occurs with laminates that have not been previously stressed.
However, resin crazing due to stresses parallel to the warp has but little effect on the dry shear strength of the laminate.
Introduction
Knowledge of the effect of preloading on the mechanical properties of a material is important in its proper structural application. Information on the
effect of preloading on glass-fabric-base plastic laminates is therefore required for better structural utilization of these materials.
,
Tests on the effect of preloading have previously been reported3— 4, 5— on the
tensile, compressive, and flexural properties of polyester laminates. Data
were needed on the effects of preloading epoxy laminates. This study, therefore, was designed to obtain information on the effects of various degrees of
preloading on the mechanical properties of a typical epoxy laminate.
The work outlined in this report was undertaken at the request of and in cooperation with the ANC-17 Panel on Plastics for Flight Vehicles. All laminating of materials and testing was done at the Forest Products Laboratory;
work was started in 1957.
Description of Material
Preparation of Panels
Three wet-layup epoxy laminates containing 13 plies of 181-Volan A glass
fabric were made for this study. Each was parallel laminated in panels about
1/8 inch thick by 36 inches square. The resin, Epon 828, contained 14 percent by weight of Curing Agent CL. In its preparation, the resin and curing
3
—Werren, Fred. Effect of Prestressing in Tension or Compression on the
Mechanical Properties of Two Glass-Fabric-Base Plastic Laminates.
Forest Products Laboratory Report No. 1811. September 1950.
4
—Werren, Fred. Supplement to above. Forest Products Laboratory Report
No. 1811-A. June 1951.
5Youngs, Robert. Effects of Tensile Preloading and Water Immersion on
Flexural Properties of a Polyester Laminate. Forest Products Laboratory Report No. 1856. June 1956.
-2Rept. No. 1870
agent were heated in separate containers to 150° F. in a water bath at 160° F.
The curing agent was then mixed with the resin in a mechanical mixer. Meanwhile, the glass fabric was being heated for 30 minutes in an oven at 200° F.
When resin and fabric were ready, the stack of hot fabric was removed from
the oven and placed in the center of a large sheet of 600 PC cellophane. The
hot resin mixture was poured on top of the stack of hot glass fabric and distributed evenly over the entire area. The resin was left to diffuse throughout
the fabric for 4 or 5 minutes, and the layup was then covered with a sheet of
600 PC cellophane. The edges were rolled and sealed on all four sides, forming a bag. This bag, containing the impregnated fabric, was turned over and
voids and excess resin were worked out with a hardboard squeegee. The assembly was again turned over so that the original top surface was up, and the
operation of removing voids and excess resin was repeated. Working out of
voids and excess resin required about 10 to 12 minutes. The laminate was
then cured in the hot press. The initial resin application would give a content of
about 60 percent, but considerable resin was removed during the above process.
Aluminum cauls 1/16 inch thick were placed on each face of the laminate. The
assembly was placed in the hot press with pieces of 0.027-inch chipboard, one
above and one below, serving as cushions between the cauls and platens of the
press. Contact pressure was maintained for 7 minutes, followed by 25 pounds
per square inch for 53 minutes with the temperature of the platens at 215° to
217° F. At the end of the curing period, the hot laminate was removed from
the press and cooled to room temperature. The laminates were not postcured.
The general information in table 1 is based on the fabrication data and observations on the cured panels.
Cutting of Test Specimens
The respective panel-cutting diagrams are shown in figures 1, 2, and 3, with
the arrow representing the warp direction of each laminate. Panels 650 and
651 were used for the Part 1 portion of the study, while panel 649 was used
in Part 2. Depending on the test requirements, the specimens for Part 1
were cut parallel to or at,45° to the warp direction. Part 2 required specimens to be cut parallel, at 45°, and at 90° to the warp.
The specimens were cut by a 1/8-inch emery wheel which rotated at 1,770
revolutions per minute in the arbor of a variable-speed table saw. The tensile specimens were finished to the desired shape and curvature by use of an
emery wheel mounted on a shaper head. Ends of compression specimens
were ground flat, square, and parallel to each other with a surface grinder.
Rept. No. 1870
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Specimens for Part 1, cut from the various areas of the panels shown in
figures 1 and 2, were randomly distributed into several groups before testing.
All specimens tested to failure in tension or compression were cut and tested
according to the requirements described in Methods 1011 and 1021.1, respectively, of Federal Specification L-P-406b, "Plastic, Organic: General Specifications, Test Methods, " dated September 27, 1951.
Key to Specimen Identification
The classification and test of each specimen, except the controls, was indicated by the numbers and letters assigned to it. As an example, a Part 1
specimen with the code designation 650-TC-3-B-4, classified it as follows:
650 - Panel number
TC - Preload in tension, final load to failure in compression
3 - Dry (normally conditioned) specimen preloaded to a designated
level 10 times, exposed at 100° F. and 100 percent relative
humidity for 90 days, then tested to failure
B - Specimen was cut at an angle of 45° to the warp
4 - Specimen number of a group
Part 2 classification of letters and numbers on the specimens was as follows:
649 - Panel number
N - control Specimens (no preload)
12, 24, or 36 - Preload level of 12,000, 24,000, or 36,000 pounds
per square inch
0, 45, or 90 - Direction of applied load in relation to the warp
direction
Final number - Specimen number in series
An example of the Part 2 numbering system is as follows: The designation
649-12-45-1 shows that this specimen (1) was cut from panel 649, (2) had been
subjected to a tensile preload of 12,000 pounds per square inch, (3) was cut at
a 45° angle to the warp direction, and (4) was the No. 1 specimen in the series.
Testing
All specimens were conditioned for at least 2 weeks in a normal atmosphere,
maintained at 73° F. and 50 percent relative humidity, before preloading or
Rept. No. 1870
-4-
testing in the dry condition. Specimens to be tested wet were further conditioned by exposing them for 90 days in an atmosphere maintained at 100° F.
and near 100 percent relative humidity.
A single preload involved loading a specimen to a designated stress level and
then unloading to the initial load; 10 preloads involved 10 cycles of this type.
Preloading was done only on normally conditioned (dry) specimens. Furthermore, all tests were made on dry specimens except where it is specifically
stated in the report that the specimens were wet.
Test Design - Part 1
Four series of tests were conducted to evaluate the effects of preloading either
in tension or compression on the mechanical properties of the laminate.
Control tests in tension and compression, both after normal and wet conditioning, were made on specimens from the respective panels. The extent of
these tests depended on the tests to which specimens cut from the given panel
were to be subjected.
The various series were identified by the type of preload applied and the type
of final load as follows:
Tension-Tension. --This series of tests required a tension preload and a final
tension test. The series was further divided into three different groups.
(1) Twelve standard specimens cut parallel (0° ) to the warp direction were each
preloaded to a different specified stress level, then loaded to failure. Six
other standard specimens cut at a 45° angle to the warp were preloaded to
different stress levels, then loaded to failure.
(2) Six standard specimens cut parallel to the warp and 6 cut at an angle of 450
to the warp were each preloaded to a different specified stress level 10 times
before being loaded to failure.
(3) Six standard specimens cut parallel to the warp and 6 cut at an angle of
45° were each preloaded to a specified stress level 10 times. The specimens
were then wet conditioned before loading to failure.
Tension-Compression. --This series of tests required a tension preload and
a final test in compression. The series was further divided into the following three groups:
(1) Strips 1/2 inch wide by 9 inches long were subjected to specified tension
preloads then compression specimens, 1/2 inch by 3-1/16 inches, were cut
Rept. No. 1870
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from the middle of the strips. After further normal conditioning, the specimens were loaded to failure in compression. This group of tests consisted of
12 parallel-to-warp specimens and six 45°-to-warp specimens.
(2) Six strips 1/2 inch wide by 13 inches long of both 0° and 45° -to-warp ma-
terial were preloaded to different specified stress levels. The preloading consisted of loading to the specified level 10 times. After preloading, the strip
was cut so that 2 compression specimens, 1/2 inch by 3-1/16 inches, were
obtained from near the middle section. One of these compression specimens
from each strip was then again conditioned in a normal atmosphere and tested
to failure in compression. The other compression specimens were used in
the group 3 test.
(3) This test used the 6 compression specimens of each of the 0° and 45° strips
that were cut from the preloaded strips of group 2 (above). These specimens
were wet conditioned in an atmosphere of 100° F. and 100 percent relative
humidity for 90 days before testing to failure in compression.
Compression-Compress i on. --These tests required a compression preload
and a compression load to failure. The series was divided into three different types of tests similar to the Tension-Tension series.
(1) Six specimens, 1/2 inch by 3-1/16 inches, from each group of 0° and 45°
specimens were each preloaded in compression to specified stress levels and
tested to failure in compression.
(2) Six specimens from each group of 0° and 45° specimens were each preloaded in compression to specified stress levels 10 times before being loaded
to failure.
(3) Six specimens from each group of 0° and 45° specimens were each preloaded in compression to specified stress levels 10 times then wet conditioned
for 90 days in an atmosphere of 100° F. and 100 percent relative humidity before testing to failure in compression.
Compression-Tension. --This series of tests required a compression preload
and a tension load to failure. The series was also divided into the same three
types of tests as the other series were.
Sets of 6 strips, 2 inches wide and 9-3/8 inches long, were cut for each
group of 0° and 45° specimens. Each strip in the set was preloaded to a difa tension blank 3/4 inch wide
ferent specified stress level. After preloading,
specimen
and 9-3/8 inches long was cut from the strip. The standard tension
(1)
was prepared from this blank and tested after further normal conditioning.
Rept. No. 1870
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(2) Sets of 6 strips, 2 inches wide by 9-3/8 inches long, were cut for each
group of 0° and 45° specimens. Each strip in the set was preloaded to a
specified stress level 10 times. After the preloading cycle was completed 2
standard tension specimens were cut from each strip. One of these specimens
was tested to failure in tension after further normal conditioning, while the
other tension specimen was used in the group 3 tests.
(3) This group of six test specimens from both the 0° and 45° groups had been
preloaded in compression with group 2 above. These standard tension specimens were tested to failure after conditioning for 90 days in an atmosphere
at 100° F. and 100 percent relative humidity.
Test Design - Part 2
Tension stresses applied parallel or perpendicular to the warp direction of a
laminate may result in crazing of the resin that could possibly reduce the
shear strength of the laminate. It has been shown that the shear strength of
an orthotropic laminate along the 0° and 90° axes
can be calculated from
6
the tensile strength of the laminate at 0', 45°, and 90° to the warp direction. —
This part of the work was designed to determine the effect of tensile stresses
parallel to the warp direction on the shear strength of the laminate.
Panel No. 649 was cut into four pieces (fig. 3), each 9 by 36 inches in size,
the 36-inch dimension being parallel to the warp direction of the laminate.
One of the pieces (B) was used as the control section. Five standard tension
specimens were cut for each of the three directions (0°, 45°, and 90°) to the
warp. After normal conditioning, the specimens were tested to failure in
tension.
The other three pieces were subjected to tension preloads parallel to the warp
direction of the laminate before the various test specimens were cut. Section
A was preloaded to 12,000 pounds per square inch, Section C to 24,000 pounds
per square inch, and Section D to 36,000 pounds per square inch. After preloading, the standard tension specimens were cut in the same manner as those
described for the control section. After normal conditioning, the specimens
were tested to failure in tension.
6
—Freas, Alan D. , and Werren, Fred. Directional Properties of Glass-FabricBase Plastic Laminate Panels of Sizes That Do Not Buckle. Forest Products
Laboratory Report No. 1803-B. November 1955.
Rept. No. 1870
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Test Methods - Part 1
All of the tensile loads were applied with a universal testing machine. Each
specimen was gripped at the ends in self-alining Templin grips and loaded at
approximately 0.02 inch per minute during the first preload. The unloading
and successive cycles of preload made at a head speed of about 0.08 inch per
minute. The final tension-to-failure load was applied at a head speed of 0.02
inch per minute.
The compression specimens and the compression-tension specimens were also
tested or preloaded in a universal testing machine. The standard compression
specimens were tested using a supporting jig of the type described in Federal
Specification L-P-406b. Specimens that were first preloaded in compression
and then tested in tension were preloaded in a special jig that supported the
2- by 9-inch blanks, so that they could be compressed to a specified stress
level without buckling. The speed of loading during the first preload and during the compression load to failure tests was 0.05 inch per minute. After the
first loading, the unloading and successive cycles of preload were made at a
head speed of about 0.2 inch per minute.
Test Method - Part 2
The 9- by 36-inch pieces that were subjected to tension preload levels of
12,000, 24,000, or 36,000 pounds per square inch were mounted in special jigs
and loaded in a universal testing machine at a head speed of 0.02 inch per
minute. The ends of the pieces were attached to heavy steel plates with an
adhesive (Bloomingdale FM-47 Liquid-film) and mounted in the testing machine as shown in figure 4. The fixtures were designed so that reasonably
uniform tensile stress would be applied to the area from which specimens
would be cut.
After preloading, the separate standard tension specimens were cut from the
zones as described above.
Analysis of Results
General
The Marten's mirrors used in this study were calibrated with a Zeiss comparator before and after the series of test runs were made. The calibration
curves showed that the tensile and compressive strains indicated by the Marten's mirrors were 5 percent larger than the true strains. Modulus of elasticity data reported in the respective tables have been corrected to account
Rept. No. 1870
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for this difference. The curves, however, were plotted using the data as
they were collected, with no correction factor applied, since they are to be
used merely to show trends.
Low proportional limit values in tension have been indicated for most types
of glass-fabric-base plastic laminates. The initial straight-line portion of
the tensile stress-strain curve may be in effect to about 15 or 20 percent of
the maximum stress for a typical polyester laminate when tested parallel to
the warp, at which point the curve assumes another straight-line portion of
a different slope...1, 1 This dual characteristic was observed in the tension
stress-strain curves of the epoxy laminate with the initial straight-line portion of the curve ending at about 25 percent of the maximum stress value.
When these two straight-line portions appear in a stress-strain curve, initial
and secondary values of modulus of elasticity and proportional limit are obtained.
The exact reasons for the dual characteristics obtained during the first loading in tension are not completely understood. There is considerable evidence,
however, that the break in the stress-strain curve at the lower proportional
limit is associated with crazing and failure of the resin in tension, and hence
the stress at which the transition takes place is determined in part by the
properties of the resin. However, this is but one of the many factors influencing the properties of laminates with which this report is not concerned.
Where a test specimen has been preloaded and subsequently tested to failure,
the correlation of the properties obtained during preloading and final loading
are not subject to specimen or material variables. Thus, it is possible to
state with considerable certainty what effect preloading has on the strength
properties. Unfortunately, it was not possible to eliminate all of the material
variables in some parts of the work. For example, some specimens were
preloaded dry, then wet conditioned and tested to failure. For such specimens,
data from the test to failure must be compared with control data from the same
panel, which, of course, introduces material variations, even though a
special effort is made to match the control and preload specimens.
Control Tests
The results of the control tests in tension and compression are shown in
tables 2 and 3. Some load-deformation curves of typical control specimens
are shown in figures 5, 6, 7, and 8 for comparative purposes.
The wet tensile control values for ultimate strength and modulus of elasticity
parallel to the warp were less than the requirements of Military Specification
MIL-R-9300 for an epoxy-resin laminate reinforced with 181 glass fabric.
This deficiency may be due to the severity of the wet-conditioning treatment
Rept. No. 1870
-9--
used in this study. The specification wet-conditioning treatment is for 30
days' immersion in water at room temperature, while the wet conditioning
in this study was for a period of 90 days at a temperature of 100° F. and a
relative humidity of near 100 percent.
Dry tensile control specimens tested parallel to the warp had an initial proportional limit value at about 25 percent of the ultimate strength. At that point,
the slope of the stress-strain curve changed sharply so that the secondary
modulus of elasticity was about 75 percent of that of the initial E. The wetconditioned tensile control specimens reacted in a similar manner, with the
initial proportional limit at about 30 percent of the ultimate strength and the
secondary E being about 80 percent of the initial E.
The wet compressive strength values were all slightly below those given in
the specification for the parallel-to-warp tests. Again, this difference was
probably due to the severity of the conditioning treatment used in this study.
The specifications do not list any requirements for compressive properties
at a 45° angle to the warp. The values of modulus of elasticity of the 45°
control specimens were erratic because the wet-conditioned control specimens had a higher modulus of elasticity than the normal-conditioned specimens. A close examination of the test procedures and data did not reveal any
tangible reason for the unexpected reversal of these control values. The proportional limit and ultimate strength values on the other hand are in the anticipated direction, with the wet values being lower than the corresponding dry
values.
Effect of Tensile Preloading on
Tensile Properties
The effects of tensile preloading followed by tensile loading to failure are
shown in tables 4 and 5 and figures 5 and 6. Table 4 and figure 5 show the
results of tests parallel to the warp, while table 5 and figure 6 show the effects at 45° to the warp.
Loading parallel to warp. --Although the dual characteristics of this material
were not completely erased by preloading, they were markedly changed. When
a dry specimen was preloaded to a level between the initial and secondary proportional limit, the initial E of the second run was lower than the initial E of
the preload or first run. The secondary E's of both runs were about the same.
If, however, the preload level was above the value of the secondary proportional limit, the secondary E' of the second run was higher than the secondary
E of the preload run but the initial E was again lowered by preloading.
Rept. No. 1870
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When the specimen was preloaded to some level less than the secondary proportional limit, the initial proportional limit of the next run was found to be
about the level of the preload. At preload levels approaching or above the
secondary proportional limit, the initial proportional limit of the next run was
slightly less than the secondary proportional limit of the preload run but the
slope of the stress-strain curve was nearly a straight line to the preload level.
For practical purposes, the slope could probably be considered as a single
straight line to the secondary proportional limit when preloads to or above the
secondary proportional limit have previously been applied.
The ultimate strength values of the preloaded specimens and the controls were
about the same.
An examination of the tensile stress-strain curves for the 16 specimens that
were preloaded and tested in the dry condition indicates that the modulus of
elasticity at any stress can be closely approximated by using the secant modulus. On a typical tensile stress-strain curve, the secant modulus at any stress
will be nearly the same as the secant modulus at the same stress after 1 to 10
preloads. Data from the available curves show that the secant modulus based
on the usual stress-strain curve will be slightly conservative. That is, after
loading to a selected stress, the secant modulus will generally be between 3
to 6 percent higher than was estimated from the usual load-deformation curve
obtained at first loading.
Preloads that apply stresses approaching the secondary proportional limit do
not completely erase the dual characteristics up to the preload level. Nevertheless, the slopes of the initial and secondary portions of the stress-strain
curve are much more nearly alike than those of the first loading. It appears
that the proportional limit as well as the modulus of elasticity for preload
stress levels up to the secondary proportional limit can be approximated by
using the secant modulus. The estimated proportional limit will be the preload level.
Specimens that had been preloaded 10 times in tension, wet-conditioned, and
tested to failure still showed dual characteristics during the final test run, but
these were not nearly as pronounced as in the wet control tests. The initial
modulus of elasticity of the wet run was only slightly greater than the secondary E of the wet controls. The initial proportional limit values were about
twice as great as the initial values of the wet controls. The wet ultimate
strength values for the preloaded specimens and the wet controls were about
the same.
Loading at 45° to warp.--Data in table 5 and figure 6 for the dry specimens
loaded to failure in tension at an angle of 45° to the warp indicate that, in
general, the modulus of elasticity and ultimate strength values were not
Rept. No. 1870
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affected by preloading. The proportional limit values were increased considerably by preloading.
The series of tests in which specimens were preloaded to a predetermined
stress level, then wet-conditioned and tested to failure, showed a slight loss
in ultimate strength when compared with the wet control values. The modulus
of elasticity values were slightly higher than those of the controls, while the
proportional limit values remained about the same.
Effect of Compression Preloading on
Compressive Properties
The effects of compression preloading and compression loading to failure of
the glass laminate are shown in tables 6 and 7 and figures 7 and 8. Figure 7
shows the effect of preloading and loading to failure parallel to the warp,
while figure 8 shows the effect at 45° to the warp.
Loading parallel to warp. --From the values in the table and the typical loaddeformation curves of dry specimens, there was no appreciable change in the
modulus of elasticity or proportional limit values due to preloading, even at
levels as high as 94 percent of the ultimate strength. The ultimate strength
values, when compared with the control value, were, in general, not affected
by prestressing. There is no apparent explanation from the test data for the
low ultimate strength value found for specimen 650-CC-1-A-1.
Compressive preloads, followed by wet conditioning and a compressive loadto-failure test, did not result in any appreciable changes in the modulus of
elasticity, proportional limit, or ultimate strength values when compared to
the wet control values.
Loading at 45° to warp. --The values in table 7 and figure 8 for the dry specimens show that the modulus of elasticity and the proportional limit values for
the load-to-failure run are about the same as were found for the preload run.
Preloading evidently did not affect these values. It is significant to note that
the values for the modulus of elasticity in the preload and load-to-failure runs
were generally higher than the control values. Perhaps the average modulus
of elasticity of dry control specimens was low, but the actual reason for this
difference could not be determined. The ultimate strength values, however,
compared favorably with the values found for the control specimens, with
higher values being noticed in only some specimens.
Specimens that were preloaded, wet-conditioned, then tested to failure had
lower modulus of elasticity and proportional limit values than those of the wet
controls. The ultimate strength values, however, are slightly higher than the
corresponding wet controls.
Rept. No. 1870
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Effect of Tension Preloading on
Compressive Properties
Tension preloading effects on the compressive properties of the laminate are
shown in table 8 and 9 and figures 9A and 9C. Table 8 and figure 9A show the
results when both loads are applied parallel to the warp and table 9 and figure
9C show the effect when the loads were applied at an angle of 45° to the warp.
It is reasonable to expect that tensile preloads causing crazing of the resin
might result in lower compressive strength properties than would be obtained
from unstressed material. This expectation was shown to be partially true.
The effect was quite pronounced on the wet compressive strength of the material tested parallel to the warp. Effects of wet conditioning would be expected to be more damaging if the resin of the laminate is crazed before wet
exposure.
Loading parallel to warp. --There was a general reduction of about 5 percent
in the compression modulus of elasticity after a tension preload and a dry
compression test to failure. The proportional limit values, however, were
generally slightly higher than the value of the controls. Ultimate strength
values fluctuated between a low of 92 percent and a high of 102 percent when
compared with the compression control value. The number of preloads (1 or
10) in tension did not appreciably alter the compressive properties. The
general trend, however, as can be seen in figure 9A, is that as the tension
preload level is increased the compression ultimate strength tends to decrease
slightly.
Preloading specimens 10 times in tension, then wet conditioning and testing
in compression, showed a marked loss in ultimate strength when compared
to the wet control value. These values ranged between 74 and 91 percent of
the wet control (table 8 and figure 9A). The modulus of elasticity and proportional limit values were also lowered appreciably.
Loading at 45° to warp. --Table 9 and figure 9C show the results of tension
preloads and compression loads to failure on specimens when the loads are
applied at an angle of 45° to the warp. The dry compression test data, after
preloading in tension, shows a marked increase in modulus of elasticity, one
being as much as 30 percent over the control values. The proportional limit
values do not show any definite trend, some being exceedingly high, others
low, when compared to the control. These dry control values, as mentioned
earlier, appeared to be low and may not have been well matched with the preload specimens. The ultimate strength values ranged up to about 28percent
higher than the control values, with a possibility that they might have been
even larger if the compression jig had not contacted the machine head. No
reason can be suggested for this apparent increase in compressive strength
properties.
Rept. No. 1870
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The wet tests of the material after preloading in tension, then testing in compression after wet conditioning, showed large losses in modulus of elasticity
values on the basis of the wet control values. The average modulus of elasticity of the wet control specimens, however, appeared to be high. The proportional limit values were much higher than those of the controls. The ultimate strength values were only slightly lower than the wet control values.
Effect of Compressive Preloading
on Tensile Properties
Compressive preloading effects on the tensile properties are shown in tables
10 and 11 and figures 9B and 9D. Table 10 and figure 9B give the results
when preloading and loading to failure parallel to the warp; while table 11 and
figure 9D give the 45° -to-warp values.
Loading parallel to warp. --Preloading in compression, either 1 or 10 times,
before loading to failure in tension did not show any evidence of impairing the
dry tensile properties of this laminate. The dual characteristics were still
evident, and the tensile strength properties were about the same as the control values. The wet strength properties in tension, after preloading 10 times
in compression, then wet conditioning, showed the initial modulus of elasticity
to be about the same as, and the secondary modulus of elasticity about 5 to 10
percent higher than, the controls. The initial proportional limit values were
about the same as the controls, while the secondary values were slightly lower.
There was no appreciable change in the ultimate strength when compared to
the wet controls.
Loading at 45° to warp. --Preloading in compression, followed by a dry tension test to failure, had practically no effect on the modulus of elasticity after
1 preload but appeared to result in a slight reduction after 10 preloads when
compared with the control data. Proportional limit values varied considerably but were generally about the same as the control values, while ultimate
strength values after preload appeared to be slightly higher than the control
values. The wet strength properties in tension, after preloading in compression and wet conditioning, showed a reduction of about 15 to 25 percent in the
modulus of elasticity when compared to the wet controls. The proportional
limit values were about the same as, and the ultimate strength values slightly
less than, the control values.
Effect of Preloads on Moisture Sorption
The effects of tension and compression preloading on moisture sorption could
be determined from only a few compression specimens that had been preloaded
Rept. No. 1870
-14-
uniformly throughout their length prior to wet conditioning. These preloaded
specimens were compared with the wet-compression controls. The moisture
sorption percentages as related to different tension and compression preload
levels are shown in table 12.
From the data it is apparent that the parallel-to-warp tensile preloads caused
about 40 percent more moisture pickup during wet conditioning than occurred
in the controls. The increase in weight for specimens preloaded in tension at
45° to the warp was about 20 percent more than that of the controls. Compression preloading caused a smaller gain in moisture sorption than tensile
preloading. This is as expected, because crazing of resin during tensile preloading would provide additional paths for moisture sorption. Though crazing
is not expected from compressive preloading, some damage might occur that
would affect moisture sorption. These data are limited and insufficient to
establish any precise relationships, but they are presented to show the trends
that occurred.
Effect of Tension Preloading on
Shear Strength
The calculated shear strength value of the epoxy laminate was practically unchanged when the panel was preloaded in tension parallel to the warp to stresses
as high as 40 percent of the tensile strength (table 13). However, when preloaded to 64 percent of the tensile strength parallel to the warp, the calculated
shear strength was 6 percent less than the calculated strength for the panel
that was not preloaded.
With the preloads applied parallel to the warp and then tested to failure at the
different directions to the warp, the tensile strength of the material tested at
90° to the warp was decreased the greatest percentage when compared to the
control group. It is not known whether this is a characteristic and significant
factor in the behavior of reinforced plastics or whether it is due to experimental
variables.
Observed Set After Preloading
The curves on figure 10 show the effect of one preload on the observed set of
a 181-Volan A epoxy laminate. The observed set is considered as the deformation retained after a specified preload has been applied and released to the point
of just holding the specimen in the test position (usually 25 pounds). The observed set would no doubt have been less than shown if all of the applied load
had been removed or if a longer period could have been allowed for recovery.
As can be seen in figure 10 and figures 5 through 8, the observed set after one
Rept. No. 1870
-15-
preload varied with direction and type of loading as well as the level of preloading. Further preloads tend to add only slight additional set, such set
being in the order of 0.00005 to 0.00010 inch per inch.
In tension the maximum ratio of observed set to elongation at failure was 5
percent on the parallel-to-warp tests. The 45° to warp tests showed a larger
set; however, set cannot be expressed as a ratio, since the total strain at
failure was generally well beyond the range that could be read with the Marten's mirror gages.
The maximum set after the first preload in compression, in relation to the
total compression strain, was only 3 percent on the parallel-to-warp specimens; on the 45° to warp specimens it was almost 10 percent for the specimen that was preloaded to 80 percent of its ultimate strength.
Conclusions
Application of tensile or compressive preloads to a reinforced plastic laminate may result in strength properties that are substantially different than
those that are obtained during the first loading of the material. If such preloading results in increased strength values, the effects of preloading are conservative and usually of little concern to the designer. If, however, preloading results in lower strength or stiffness, design must be based on the lower
values.
The analysis of results of various combinations of tensile and compressive
preloads and tests to failure of the 181-Volan A epoxy laminates revealed
a number of detailed observations that are discussed in this report. The following are general conclusions of primary interest.
Tension Preload - Tension to Failure
When loaded parallel to the warp, modulus of elasticity and proportional limit
values are changed markedly by preloading. These changes depend upon the
level of preload. In general, however, preloading to a stress between the
initial and secondary proportional limits indicated by the tensile stress-strain
curve results in a new stress-strain relationship having (1) a single modulus
to the level of preload and (2) a proportional limit at about the level of preload.
For general use, the modulus of elasticity for such preloaded material can be
closely and conservatively approximated by using the secant modulus of the
usual stress-strain curve to the desired stress level. The tensile strength of
both dry and wet specimens is not appreciably changed by preloading.
Rept. No. 1870
-16-
At 45° loading, all strength properties of dry specimens are about the same
or increased slightly due to preloading. The same is true for wet specimens,
except that the tensile strength decreased slightly.
Compression Preload - Compression
to Failure
Compressive preloading to levels as high as 94 percent of the compressive
strength has no appreciable effect on any of the dry-strength properties at 0°
and 45° loading or on the wet-strength properties when tested parallel to the
warp direction. At 45° loading in the wet condition, there appears to be a
significant decrease in modulus of elasticity and proportional limit with preloading when compared with the controls. This is not a positive conclusion
because the control data seemed to be erratic.
Tension Preload - Compression to Failure
When loaded parallel to the warp, compressive strength properties generally
decrease after tensile preloads have been applied. This is particularly true
after wet conditioning, when wet strengths of preloaded specimens are only
74 to 91 percent of the control strength. Crazing of the resin due to tensile
preloading can reasonably be expected to cause reductions in some compressive strength properties. This is especially so after wet conditioning because
the crazed resin permits moisture to enter the material more readily than is
possible in noncrazed material.
At 45° loading, it was observed that modulus of elasticity and compressive
strength of dry specimens increase with preloading while similar properties
of wet specimens decrease with preloading. This inconsistency of data does
not permit any firm conclusions; at least part of the inconsistency is believed
due to erratic control data.
Compression Preload - Tension to Failure
The dry or wet tensile properties of material loaded parallel to the warp are
not appreciably reduced by preloading in compression. Similarly, dry tensile properties at 45° loading are practically unchanged by compression preloads. Tension tests at 45° loading after wet conditioning, however, indicate
approximately a 20 percent decrease in modulus of elasticity and about a 5 to
10 percent decrease in strength due to compression preloads.
Rept. No. 1870
-17-
Tensile Preload Effect on Shear Strength
Shear otrength along the orthotropic axes is not appreciably affected when
tensile preloads up to 40 percent of the tensile strength are applied parallel
to the warp. There is some evidence that higher tensile preloads cause a
slight reduction in shear strength.
Observed Set after Preloading
Application of a single preload to a stress level in the order of 30 percent or
more of the ultimate stress results in an observed set. The set is more pronounced in tension than in compression; also it is substantially more pronounced at 45° loading than at 0° loading. The magnitude of the set parallel
to the warp is small, being in the order of 0.0005 inch per inch or less after
preloading to 50 percent of the ultimate strength. Additional set resulting
from more than one preload is much less than the set induced by the first preload.
Preloads and Moisture Sorption
Preloads resulting in crazing of the resin will permit more moisture sorption
than will occur in nonstressed laminates. Limited data of this report indicate
that the additional moisture has a deleterious effect on some strength properties.
Rept. No. 1870
-18-
1. -42
Table l.--General information on three parallel laminates made with 181Volan A glass fabric and Epon 828 resin with Curing Agent CL
Panel No. :Number of:
Thickness
:Specific: Resin : Barcol
1
plies :
:gravity :content–:hardness
:of fabric:Minimum:Maximum:Average:
:
:
649
650
:
:
:
:
..
651
: In.
•
: -In.:
In. :
—
:Percent :
13
: 0.113 : 0.128 : 0.122 :
.
.
: .120 : .132 : .128.
1.81
:
36.0
:
69
13
:
1.80
:
36.6
:
68
13
.121 :
.135 :
.130 :
1.84
:
33.8
•
:
70
•
1
–Based on the weight of the panel and the predetermined weight of the
fabric.
Rept. No. 1870
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trN
aloNo-1-c-n
ooNoo\mcr,
tr\
•
•
•
•
•
•
•
•
••
••
•• • • • • • • •• • •
oNcnain\om
0\0\OCAHM
-1- N\oNOH
000Ncro
•
00.HNOw1
HHPHH
•
nnnnnn
•
N 0000N
tre\re\K-No-NrrN
H
H Hi
H
OHNONNO
rrsreNtrNNN■prN
HHHHHH
•
•
•
•
•
•
4.
8
o'
o
4.
01
•
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0
t4
'
4-i
0
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0)
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P
A
a)
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r'0h)
co
0
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hi) co
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O
PI
4,
111
.
O
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rf,P3
E4
1H LC\ 01 10 tr‘
C
10 o•
a
Rept. No. 1870
00ra0
OM
cfN 61:1
H
0 I>
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a,
0.
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C.)
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4
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•r
Table 8.--Effects of preloading in tension on compressive properties of a 181-Volan A epoxy laminate. Preloads and
tests to failure were applied parallel to the warp.
.•
.•
Specimen
No.
.•
.•
•
•.
•
•
:
.
: Maximum
: applied
: stress
: applied :Modulus of:Proportional-: Ultimate
:elasticity:limit stress : stress :
: stress
.
.
:to control:
.•
.•
: strength :
.
:
:
.•
.•
(1)
Final compression run
:Thickness: Width : Preload :First tension: Ratio of :
: maximum :
run
: level :
.
(2)
:
(3)
:
(4)
In.
: In.
:
.
Lbs. :
(5)
(6)
(7)
:
(8)
:
: Percent : 2,222
: 22,1, :
P.a.i.
(9)
: Ratio of : Ratio of
ultimate :E of final
: stress to: run to E
ultimate :of control
: stress of:
: controls
:
(1o)
:
:
:
: Percent :
(11)
P.s.i.
: P.e.i.
26,610
• 53,040 •
100.0
22,820
• 39,440 •
100.0
24,450
24,280
30,230
51,740
18,740
100.6 :
24,48c
30,48o
: 53,350 :
; 54,250 :
: 52,100 :
: 52,550 :
: 53,550 :
: 51,000 1
: 52,250 :
: 50,68o :
: 52,550 :
1 50,700
: 50,000 :
94,3 :
.94
25,680
30,300
53,33o
32,96o
36,580
: 53,450 :
: 52,280 :
: 50,390 :
: 52,75o :
: 53,800 :
100.8 :
.96
.92
: 48,980 :
92.3 :
.90
: 29,96o :
76.0 :
.98
88.5 :
1.02
.
TENSION CONTROLS (DRY)
: 0.127 : 0.251 •
Panel 650
Average
•
• 100.0
58,810
COMPRESSION CONTROLS (DRY)
.131
Panel 650
Average
•
.504 •
3,440 •
COMPRESSION CONTROLS (WET)
Panel 650
Average
:
•
.133 : .502
3,210 •
PRELOADED ONCE
650-TC-1-A-1 :
2 :
3:
4:
6:
7:
8:
12 :
11 :
9:
10 :
.131 : .501 : 1,210 :
.131 : .505: 1,420 :
.132 : .5o6 : 1,640 :
.131 : .504 : 1,830 :
.131 : .504 : 2,23o :
.131 : .504 : 2,43o :
.131 : .504 : 2,640 :
.130 : .502 : 2,830 :
.130 : .5o5 : 3,000 :
.131 : .50 : 3,040 :
.130 : .505: 3,195 :
18,460
21,550
24,600
27,65o
33,800
36,900
40,000
45,350
45,700
46,150
48,65o
31•4 :
3,170 :
3,380 :
36.6 :
3,220 1
41.8 :
3,280 :
47.o :
3,250 :
57.5 :
3,260 :
62.7 :
3,140 :
68.o :
73.7 :
3, 240 :
77 . 7 :3 , 560 :
3,250 :
78.5 :
3,220 :
82.7 :
:
:
:
:
:
:
:
:
22,680
55,520
30,710
28,940
102.3 :
98.2 :
99.1 :
101.0 :
96.2 :
98.5 :
95.6 :
98.7 ;
0.92
.98
.94
.95
.94
.95
.91
.94
.98
PRELOADED 10 TIMES
650 -Tc - 2 -A - 1 :
3
2 :
:
4:
5:
6:
.132 : .498 : 1,215 :
.132 : .499 : 1,620 :
.132 : .501 : 2,040 :
.133 : .503 : 2,47o :
.132 : .501 : 2,84o :
.131 : .503 : 3,240 :
18,460
:
31.4 :
30,800
36,900
43,000
49,200
:
:
:
:
52.4 :
24,600
:
41.8 :
62.7 :
73.1 :
83.7 :
3,290 :
3,150 :
3,100 :
5,170 :
3,090 :
3,100 :
38,020
98.6 :
95.o :
99.5 :
101,4 :
.90
.92
.90
PRELOADED 10 TIMES AND WET CONDITIONED
650-TC-3-A-1 :
2 :
6
3:
4:
5:
:
Rept. No. 1870
.132 : .498 : 1,215 :
.132 : .499 : 1,620 :
.132 : .501: 2,040 :
.133 : .503: 2,470 :
.132 : .501 : 2,84 q :
.131 : .50 : 3,240 :
18,460
:
30,800
36,900
45,000
:
:
:
24,600
:
49,200
:
31.4 :
41.8 :
52.4 :
62.7 :
73.1 :
83.7 :
3,140 :
2,950
3,060
3,000
3,280
:
:
:
:
2,620 1
18,150
: 29,180 :
: 35,290 :
91.3 :
74.o :
89.5 :
: 34,24o :
86.8 :
21,090
: 36,000 :
18,010
: 34,900 :
18,040
18,010
14,950
.92
.95
.93
.82
Table 9.--Effects of • reloading in tension on compressive properties of a 181-Volan A epoxy laminate Preloads
,
were applied at 5° to the warp.
'
Specimen
No.
(1)
•
•
:Thickness: Width : Preload :First tension: Ratio of :
Final compression run
: Ratio of : Ratio of
: level :
run
: maximum :
: ultimate :E of final
.
:
: applied :Modulus of:Proportional-: Ultimate : stress to: run to E
.
: Maximum
: stress to:elasticity:limit stress : stress : ultimate :of control
; control :
. : applied
.
.
: stress of:
: stress
•
: strength :
.•
.•
: controls :
:
(2)
:
(3)
:
(4)
:
In.
: In.
:
Lbs. :
.
.
.
.
:
(5)
P.s.i.
:
(6)
:
(7)
: Percent : 1,000
: P.s.i.
(8)
:
P.s.i.
.
:
(9)
:
(1o)
:
(11)
P.s.i. : Percent :
TENSION CONTROLS (DRY)
Panel 650
Average
: 0.128 : 0.251 •
•
23,520
• 100.0
1
COMPRESSION commas (DRY)
Panel 650
Average
.131 : .501 •
Panel 650
Average
.132 : .500 •
650-Tc-1-B-1
4:
2:
5:
3:
6:
130 494 :
480
.131 : .505 :
660
.131 : .501 :
740
.131 : .500 :
82o
.131 : .506 :
915
.132 : .505 : 1,040
• 1,650
7,310
• 23,470 • 100.0
•
5,150
• 19,720 • 100.0
•.
:
9,460
11,340
5,380
5,340
6,420
5,260
: 27,5901
T : 117.6
. 26,4601 : 112.7
: 29,990 : 127.8
: 26,6844- : 113.7
: 27,16c4 ; 115.7
: 23,690- : 100.9
:
:
1.07
.99
1.22
1.17
1.10
.93
5,800
4,760
5,240
4,700
6,010
3,140
: 24,0201 : 102.3
: 23,820- : 101.5
: 24,150, : 102.9
: 24,700= : 105.2
: 28,050 : 119.5
: 23,5101 : 100.2
:
:
:
:
:
:
1.3o
1.19
1.19
1.09
1.18
1.05
: 18,880 :
5.7
97.9
: 19,30o :
: 20,730 : 105.1
: 19,180 . :
97.3
96.4
: 19,020 :
91.6
: 18,070 :
:
:
:
:
:
:
.81
.75
.83
.72
.73
.62
•
COMPRESSION CONTROLS (WET)
• 1,860
PRELOADED ONCE
:
:
:
:
:
:
7,500
10,000
11,250
12,500
13,750
15,620
:
:
:
:
:
:
31.9
42.5
47.8
53.1
58.5
66.4
:
:
:
:
:
:
1,770
1,630
2,020
1,930
1,820
1,540
:
:
:
:
PRELOADED 10 TIMES
650-Tc-2-s-1
4
2
5
3
6
:
:
:
:
:
:
460 :
.125 : .491 :
640 :
.127 : .5o4 :
710 :
.126 : .502 :
.128 : .505 :
810 :
875 :
.126 : .506 :
.129 : .501 : 1,010 :
7,500
10,00o
11,250
12,500
13,750
15,620
:
:
:
:
:
:
31.9
42.5
47.8
53.1
58.5
66.4
:
:
:
:
:
:
2,140
1,960
1,960
1,800
1,940
1,740
:
:
:
:
PRELOADED 10 TIMES AND WET CONDITIONED
650-TC-3-B-1 :
4:
2:
5:
3:
6:
460
.125 : .491 :
.127 : .5o4 :
64o
.126 : .502 :
710
.128 : .505 :
810
.126 : .506 :
875
.129 : .501 : 1,010
:
:
:
:
:
:
7,500
10,000
11,250
12,50o
13,750
15,620
:
:
:
:
:
:
31.9
42.5
47.8
53.1
58.5
66.4
:
:
:
:
:
:
1,50o
1,390
1,540
1,34o
1,350
1,160
:
:
:
:
1
6,420
6,220
7,040
6,900
6,940
6,180
-Machine head contacted jig -- recorded values were taken just prior to contact and assumed to be the ultimate stress.
Rept. No. 1870
II
1]
.-.118
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H
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Table 12.--Effect of tension and compression preloads on sorption of
moisture
45° to warp
Parallel to warp
Specimen : Preload : Increase :: Specimen : Preload : Increase
1 :in weight
1 :in weight ::
level–
:
No.
level–
:
No.
: Percent : Percent
: Percent
------- : Percent ::
..
:
::Compression :
:
Compression :
0.61
0
:
:
0.52 :: eontrca pg :
0
controls? :
TENSION PRELOAD'
TC-3-A-1
2
3
4
5
6
:
:
:
:
31
42
52
63
73
84
:
.69 ::TC-3-B-1
:4
2
.7741 ::
.
.82 ::
:
5
..
.67 ::
3
6
.77 ::
•.
:
:
:
:
:
32
42
48
53
58
66
:
:
:
:
:
•82
.7o
.71
.66
.7o
.75
D–
COMPRESSION PRELOAD
Cc-3-A-1
2
3
4
6
5
:
:
6o
66
72
78
83
87
:
.66 ::cc-3-B-6
1
.64 ::
5
.53 ••
2
.6o ::
4
.54 ::
.64 ::
3
:
:
46
52
59
67
71
78
:
:
:
:
.6o
.62
.68
.61
.68
.61
1
–Percent of preload applied stress to ultimate strength of control.
-Average of 5 specimens.
wry stress preloaded in tension, cut into specimens, and wet conditioned.
–Dry specimens preloaded in compression and wet conditioned.
Rept. No.
1870
Table 13.--Calculated shear strength values of a 181-Volan A
epoxy laminate after preloading to various stress
levels in tension parallel to the warp direction
Preload stress : Tensile strength—1 at
angle to warp of -:
P .s .i.
0°
:
45°
: Calculated shear
strength2
—
: 90°
P.s.i. : P.s.i. : P.s.i. :
(F 0-90 )
P.s.i.
None
: 59,010 : 26,590 : 55,430
13,670
12,000
: 57,920 : 26,940 : 52,470 :
13,900
24,000
: 58,65o : 27,220 : 51,150 :
14,o6o
36,000
: 56,580 : 24,970 : 50,920 :
12,840
1
—Values are the average of 5 tension test specimens in each of
the directions to warp listed.
2
—Shear strength, F 0_ 90 , calculated from the equation
1
F 0-90
2
1
= 4
F45 2 F 0 2
1
—
—
F907 + F0F90
where F 0 , 'F45 , and F90 are the tensile strength values at
0°, 45°, and 90° to the warp direction respectively.
Rept. No. 1870
Figure 4. --Part 2 test piece mounted in machine prior to tension preloading.
Z M 113 294
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