'ACTORS ARMING THE STRENGTH
Of PARREG
EFFECT OF ACCELERATED WEATHERING ON CERTAIN
STRENGTH PROPERTIES OF PAPREG
January 1945
This Report is One of a Series
Issued In Cooperation with the
ARMY-NAVY-CIVIL COMMITIZE
on
AIRCRAFT DESIGN CRITERIA
Under the Supervision of the
AERONAUTICAL BOARD
No. 1521-A
UNITED STATES WARTMENT OF AGRICULTURE
ILO REST SERVICE
12REST 6 RODUCTS LABORATORY
Madison, Wisconsin
In Cooperation with the University of Wisconsin
FACTORS AFFECTING THE STRENGTH OF PAPREG 1 '
3
Effect of Acceleratedd-Weathering-3 on Certain Strength Properties of Papreg
By
H. R. MEYER, Engineer
and
E. C, O. ERICKSON, Engineer
• n•••••••••n•
This report presents the results of strength tests to determine the
influence of accelerated weathering on the tensile, compressive, flexure,
bearing, and hardness properties of papreg. It covers one of a series of
investigations to determine the effect of aircraft service conditions on
certain strength properties of papreg.
Since exposure to natural weathering would require considerable
time, an accelerated weathering test -4 was employed, designed to produce in
a short period of time effects comparable to those resulting from sun and
moisture.
In this series of tests, the papreg was exposed continuously for
240 hours by repetition of the following 24-hour schedule: (1) 2 hours in
a fog chamber, (2) 2 hours irradiation, (3) '2 hours in a fog chamber, and
(4) 18 hours irradiation.
•
In general, the strength properties of papreg were improved
slightly by this accelerated weathering exposure.
1
-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 available.
gA laminated paper plastic made by the Forest Products Laboratory
(Improved Standard - June 1943).
3
-This report is the second of a series of reports presenting the effect of
aircraft service conditions on certain strength properties of papreg.
-Federal Specification for Plastics, Organic: General Specifications
(Methods of Tests) L-P-406, December 9, 1942, paragraph B-14.
Mimeo. No. 1521-A
-1-
Test Material
All test material for this investigation, identified as Improved
Standard -- June 1943, was made by the Forest Products Laboratory from a
commercial Mitscherlich sulfite pulp. The preparation of the papreg relative to base paper, impregnation, assembly, and molding was identical to
that described in Forest Products Laboratory Mimeo. N. 1521, The specific gravity of the papreg, based on weight and volume at 75° 7. and 50
percent relative humidity, was 1.40.
Preparation of Test Panels
Ten parallel-laminated and six cross-laminated panels, (1/8-inch
nominal thickness And trimmed to approximately 11 inches square) were
selected from material groups I and II, respectively, 'described in Forest
Products Laboratory Mimeo. No. 1521. Five of the parallel-laminated
panels (numbered 78 to 82, inclusive) and three of the cross-laminated
and B, as
panels (numbered 58, 59, and 60) were cut into test panels, shown in figure 1. Test panels 0 and D (fig. 1) were cut from only two of
the parallel-laminated panels (Nos. 78 and 79).
Two test strips, 9 inches long by 3/4 inch wide, (not shown in
fig. 1) were cut from each of five group I panels (numbered 62 to 66, inclusive) and from each of three group II panels (numbered 46, 47, and 48)-.
One of each pair of these strips was cut lengthwise and the other crosswise to the fiber (machine) direction of the paper when parallel laminated, but parallel and perpendicular, respectively, to one edge of the
panel when cross laminated.
Holes were drilled in the test panels (A, B, C, and D) in positions
' such that they would be centered in the width and two and one-half times
their own diameter from the end of bearing specimens to be cut from these
panels after exposure (fig. 1). The holes in panels A from parallellaminated material were 1/4 inch in diameter; f those in the other panels
were 1/8 inch in diameter. All holes were fitted, prior to exposure, with
snug fitting brass bolts provided with brass nuts and washers so that
service conditions might better be simulated. In addition, all edges of
the panels and strips were sealed with two coats of aluminum paint to retard movement of moisture through these edges.
Factors Affecting the Strength of Papreg. Some Strength Properties at
Elevated and Subnormal Temperatures.
f Federal Specification L-P-406 requires determination of bearing value at
a deformation equal to 4 percent of the hole diameter. When the bearing specimens from panels with 1/4-inch holes were tested following exposure, it was found that failure occurred before this deformation was
reached. Consequently, holes drilled in the remaining panels were 1/8
inch in diameter.
Mimeo. No, 1521-A
-2-
The papreg, both before and after it . was cut into panels A, B, C,
D, and test strips, was stored at ordinary room temperature and humidities,
and was not otherwise conditioned prior to the accelerated weathering exposure. Panels A, B, C, and D and the 3/4- by 9-inch strips were weighed
and measured prior to and after exposure to the weathering test.
Exposure Apparatus
The irradiation and fog apparatus shown in figures 2 and 3, respectively, was designed essentially as outlined in Federal Specification
L-P-406. The turntable consisted of a piece of 16-gage galvanized sheet
metal, 20 inches in diameter, mounted on a 3/4-inch round hardwood shaft
in a wood frame, and driven at 35 revolutions per minute by means of a
small electric motor and a speed reducer. Two sheet metal ribs 3/16 of an
inch high were soldered to the disc in two concentric circles of 3-3/4- and
6-inch radii to support the test material. Several small machine screws
attached to the disc, prevented lateral movement of the test material
during rotation. A commercial sun lamp, equipped with a 15-inch diameter
reflector and an S-1 bulb supplied the irradiation.
The fog chamber consisted of a large topper boiler with a tightfitting wood cover in which was an inspection window, a baffle, and a wood
rack with glass rod supports. Fog was created by an aspirator operated by
air from the compressed air system of the Laboratory and drawing distilled
water from a supply in the bottom of the boiler.
The irradiation apparatus and fog chamber were placed in an open
space in a large room having natural air circulation. The temperature in
this room varied from 74° to 83° T. (average of 78° P.) during the test
period.
Exposure Procedure
In conformance to Federal Specification L-.P-406, the panels were
subjected to ten 24-hour cycles of irradiation and wetting. Each cycle
consisted of 2 hours in the fog chamber, 2 hours irradiation, 2 hours in
the fog chamber, and 18 hours irradiation.
Preparation of Test Specimens
At the end of the exposure period, each panel (A, B, 0, and D) was
cut into specimens, as shown in figure 1, to provide compression, bending,
and bearing specimens. Compression specimens (No. 3, fig. 1) for the determination of ultimate stress were 1 inch wide by 1/2 inch long (1/r
ratio = 13.9). Those for the determination of the modulus of elasticity,
proportional limit, and yield strength (No. 2, fig. 1) were 1 inch wide by
Mimeo. No. 1521-A
_3_
4 inches long. The edge distance (circumference of the hole to the near
end of the specimen) of bearing specimens (No. 4, fig. 1) was 2.5 times
the hole diameter.
Each of the 3/4- by 9-inch strips was machined to the proper contour of a tension test specimen.
All specimens were machined to size with high-speed steel cutting
tools in such a manner as to be virtually free from tool marks or any evidence of overheating. No further finishing was done after machining. The
type and dimensions of specimens conformed to those specified in Federal
Specification L-P-406.
Conditioning of Specimens
All specimens were conditioned at 75° F. and 50 percent relative
humidity for at least 48 hours, prior to testing,
After the specimens were conditioned, they were placed in an airtight container and removed, a few at a time, for testing. Except for t'he
few minutes required for weighing and measuring, the specimens were not
otherwise exposed to test-room atmosphere until placed in testing machines.
Testing Procedure
The specimens were tested at room temperatures and humidities
(78° J.- 30 F. and 46 ± 10 percent relative humidity) according to the procedures outlined in Federal Specification L-P-406. Detailed descriptions
of each type of test are presented in the appendix.
Test Results
Maximum, minimum, and average property values from the indicated
number of tests of each strength property investigated are presented in
table 1. 2 Presented also, for comparison, are corresponding strength
properties of the same material subjected only to normal conditioning at
75° F. and 50 percent relative humidity (hereinafter referred to as controls). The tensile and static bending values of the control specimens
7
-As stated previously, the specimens from the cross-laminated papreg were
oriented in both the principal directions. Results from these tests and
those reported in Forest Products Laboratory Mimeo. No. 1319, revised,
showed no significant difference between these two orientations. Consequently, lengthwise and crosswise results are combined in table 1.
Mimeo. No. 1521-A
-4-
8
were taken from table 1 of Forest Products Laboratory Mimeo. No. 1521.
The compression and bearing values were obtained from Forest Products Laboratory Mimeo. Nos. 1521-P and 1319, revised, respectively. Strength
properties of control specimens are based on dimensions at time of test.
Those of weathered specimens are based on width measurements made at time
of test and on original thickness as measured prior to exposure.
In general, the various properties were higher after exposure to
the 240 hours of accelerated weathering than in comparable material not so
exposed. Table 1 consistently indicates higher strength values for weathered than for control specimens. No change in ultimate bearing strength
is indicated in two instances, and a definite decrease in proportional
limit in compression is shown in one instance. All other ratios of weathered to control are above unity. In tension, for instance, the indicated
range of improvement, based on average values, is as follows: 4 to 13
percent in ultimate strength, 4 to 7 percent in modulus of elasticity, and
4 to 6 percent in yield stress at 0,7 percent strain. In compression the
improvement is: 5 to 10 percent in ultimate strength; and 2 to 3 percent
in modulus of elasticity. In flexure the improvement is 3 to 19 percent
" in modulus of rupture. No change in the average hardness number due to
accelerated weathering, was found.
Typical tension stress-strain curves are shown in figure 4. Deformation was observed immediately before fracture but not in the range
indicated by the broken lines. Figure 5 shows typical compression stressstrain curves. The end points are arbitrary and do not indicate the ultimate stresses. Typical load-deflection curves for flatwise static bending
tests are presented in figure 6. All curves are based on actual loaddeformation data for individual test specimens chosen because their properties agree closely with the averages of those for the group.
The physical change resulting from exposure to accelerated weathering was principally one of color. The color changed from a light yellow
brown to a mottled dark-reddish brown. The surface condition was good.
There appeared to be no checking, warping, blooming, crazing, or apparent
changes in the bonding resin and fibrous structure. There were slight
changes, however, amounting to a decrease of 2 percent in weight and between 1 and 2 percent in thickness.
The surface temperature of the material during rotation under the
sun lamp was 1840 F., while the temperature of the air 1/4 inch above the
material was 140° F. Temperatures were measured by means of a thermometer.
The surface temperature during rotation was obtained by placing the thermometer . on the papreg with the bulb in contact with the exposed surface.
Readings were taken without halting the rotation of the papreg. To obtain
the temperature of the air during rotation, the thermometer was held stationary 1/4 inch above the papreg.
13-Factors Affecting the Strength of Papreg. Some Strength Properties at
Elevated and Subnormal Temperatures.
Mimeo. No. 1521-A
-5-
Appendix
Tension, compression, bending, and bearing tests were conducted as
hereafter described.
Tension
Tension specimens were tested in self-aligning Templin grips in a
motor-driven, 10,000-pound capacity, universal testing machine. Theilaachine was operated at a ho load speed of 0.042 inch per minute to approximately 75 percent of ultimate load, during which time load-elongation data
were obtained, and then increased to 0.157 inch per minute and maintained
at this rate until failure. A 2,-inch gage length separable nonaveraging
type extensometer, equipped with a spiral staff type 0.0001-inch dial, was
used to measure elongation.
Compression
Compression tests to determine ultimate stress were made on specimens 1 inch wide by 1/2 inch long. To overcome the difficulty in accurately placing a single specimen of such small dimensions in position to
assure axial loading, two specimens placed 1 inch apart and parallel to
each other were centered by means of a jig under a spherical head in a
10,000-pound capacity universal testing machine. The machine was operated
at a no load speed of 0.023 inch per minute.
Specimens 1 inch wide by 4 inches long were tested in compression
as a laterally supported column by means of the pack apparatus shown in
figure 7, The assembly was centered under a spherical loading head on the
table of a 10,000-pound capacity hydraulic testing machine. The machine
speed was constant at 0,012 inch per minute. Deformations, to approximately 0.02-inch strain, were measured at equal increments of load with a
2-inch gage length Martens mirrors compressometer attached to the machined edges of the specimen.
Bending
Specimens 1 inch wide by 4-1/2 inches long were tested flatwise in
a bending jig adjusted to a 2-1/2-inch span (span-specimen thickness ratio
of 20:1). The load was applied at the center of the span by means of a
loading block attached to the cross head of a 30,000-pound universal testing machine equipped with . a hydraulic capsule and a load indicator. The
machine was operated at a no load speed of 0.049 inch per minute until approximately 50 percent of the maximum load was reached and then increased
to 0.146 inch per minute, Deflections at the center of the span were
measured with a 0.001-inch dial gage.
Mimeo. No. 1521-A
-6-
Bearing
Bearing tests were made on specimens 15/16 inch wide and 4-3/4
inches long with a bearing hole 0.25 or 0.125 inch in diameter centered
in the width dimension at a distance of three times the hole diameter from
the end of the specimen.
Specimens were tested in a bearing jig similar to that described in
Federal Specification L-P-406 and shown in figure 8. The jig was suspended from a fixed cross arm of the testing machine. The load was applied
by means of a Templin grip attached to the movable head of a 1,000-pound
capacity universal testing machine. The machine was operated at a no load
speed of 0.016 inch per minute.
Deformation of hole diameter was measured by two spiral staff type
0.0001-inch dial gages attached to the jig. The plungers of the gages
were actuated by the movement of a collar in which were set knife edges
bearing against opposite edges of the specimen in a line tangent to the
circumference of the hole nearest the end of the specimen. Deformations,
at equal increments of load, were observed to failure.
Mimeo. No. 1521-4
-7-
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long tested as a laterally supported column.
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Figure 6.--Typical flatwise flexure load-deflection curves for papreg
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inch wide by 1/8 inch thick by 4-1/2 inches long.
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Figure 7.--Specimen ready for test in the apparatus used for the
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S, specimen; Y, yokes.
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Figure 8.-j9lPparatus assembly for bearing test of papreg (tensile
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edge; E, specimen; F, 1/8 inch diameter steel pin.
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