TRENGTII OF tAIPRE G
SOME STRENGTH PROPERTIES AT ELEVATE D
AND SUDNORMAL TEMPERATURE S
formation 'eviewed and Reaffirmed
LOAN COPY
Please return to :
Wood Engineering Researc h
Forest Products Laborator y
Madison, Wisconsin 53705
2
FACTORS AFFECTING THE STRENGTH OF PAPREG11 -
Some Strength Properties at Elevated an d
Subnormal Temperatures3
By
:6
H. R . MEYER, Enginee r
and
E . C . O . ERICKSON, Engineer
a
Forest Products Laboratory, Forest Servic e
U . S . Department of Agricultur e
This report presents the results of tension, compression, bending, and impac t
strength tests on papreg conducted at temperatures of -69°, 75°, 158°, an d
200° F .
The study was undertaken to provide data on the effect of extreme temperature s
applied for a relatively short period of time (24 hours or less) on the strengt h
properties of papreg at the range of temperatures that might be encountered b y
aircraft under service conditions . Although 158° F . is the upper limit recommended by government specification, tests were also conducted at 200° F . a t
the request of the Army Air Forces .
Temperature strength relations were determined for both parallel-laminated an d
cross-laminated papreg, 1/8 and 1/2 inch in thickness . These strength relation s
were obtained in the two principal directions, lengthwise and crosswise, tha t
is, parallel and perpendicular to the machine direction of the paper when par allel laminated, and parallel and perpendicular to the edges of the panel whe n
cross laminated .
1
This is one of a series of progress reports prepared by the Forest Product s
Laboratory relating to the use of wood in aircraft issued in cooperatio n
with the Army-Navy-Civil Committee on Aircraft Design Criteria . Origina l
report issued Jan . 1945 .
laminated paper plastic made by the Forest Products Laboratory (Improve d
Standard -- June 1943) .
2This report is the first of a series of reports presenting the effect of Air craft Service Conditions on certain strength properties of papreg .
4
-Maintained at Madison, Wis ., in cooperation with the University of Wisconsin .
Rept . No . 1521
-1-
It was found that, in general, the strength properties of papreg tend to in crease at subnormal temperatures and to decrease at elevated temperatures .
Test Materia l
The paper used was made on the Laboratory paper machine from a commercia l
unbleached black spruce, Mitscherlich type, sulfite pulp . This paper wa s
impregnated with a Bakelite phenolic type, thermosetting resin identified b y
number as BV 16526 . The resin content was 36 .3 percent, based on the weigh t
of the treated paper, and the volatile content 4 .5 percent .
The impregnated paper was molded to form parallel-laminated and cross-laminate d
flat panels, approximately 12 inches square, and 1/16, 1/8, and 1/2 inch i n
thickness . In making the parallel-laminated panels, the machine direction
(fiber grain) of each lamination was placed parallel to that of adjacent lami nations . The cross-laminated material was assembled so that the machin e
direction (fiber grain) of each lamination lay at right angles to that of th e
adjacent laminations . Each of the 1/16 and 1/8 inch thick panels was made fro m
approximately 35 and 70 sheets of treated paper, respectively, and pressed fo r
12 minutes at 250 pounds per square inch . Each of the 1/2 inch thick panels wa s
made from approximately 280 sheets of treated paper and pressed for 25 minute s
at 250 pounds per square inch . The temperature of the hot press platens wa s
325° F . The panels were removed from the press immediately after pressing an d
allowed to cool in air at room temperature . The specific gravity of the papreg , ,
based on equilibrium weight and volume at 75° F . and 50 percent relative
humidity, was 1 .40 . The material is identified as " Improved Standard -- Jun e
1943 . a
A number of panels were manufactured for service condition tests and store d
under normal room temperatures and humidities . These panels were divided int o
six groups as follows :
Group
Group
Group
Group
Group
Group
I - Parallel laminated - 1/8 inch thic k
II - Cross laminated - 1/8 inch thic k
III - Parallel laminated - 1/2 inch thic k
IV - Cross laminated - 1/2 inch thic k
V - Parallel laminated - 1/16 inch thic k
VI - Cross laminated - 1/16 inch thic k
Preparation of Test Specimen s
Panels were selected from the first four groups for the experimental wor k
. covered in this study . Twenty-two panels were taken from group I, 13 from
group II, 4 from group III, and 2 from group IV .
Test specimens, exclusive of tensile and bending specimens tested at 200 0 F . ,
were cut from the selected panels as indicated in the cutting diagrams an d
selection tables of figures 1 and 2 . Specimens not shown in the selectio n
table (fig . 1) were used in tests of other service conditions .
Rept . No . 1521
-2 -
The selection and matching of tensile and bending specimens for tests at 200° F .
are not shown, since the request for tests at 200° F . was received after the
other temperature tests had been completed . Specimens for these tests wer e
selected as follows : Five tensile specimens were taken in the lengthwise an d
crosswise directions from two uncut panels of group I, and five from one pane l
of group II ; one lengthwise and one crosswise static bending specimen wa s
taken from the unused central portion of each of five group I panels allocate d
to another series5 of strength tests, and five cross-laminated static bendin g
specimens were obtained from three group II panels of the same series .
The type and dimensions of specimens conformed to Federal Specification fo r
Plastics, Organic ; General Specifications (Methods of Tests) L-P-406, Decembe r
9, 1942 .
Conditioning of Specimen s
The specimens for test at -69° F . were exposed to that temperature for a t
least 4 hours prior to test .
Specimens tested at normal temperature (75° F .) were conditioned at 75° F . an d
50 percent relative humidity for approximately 2-1/2 weeks prior to test .
All specimens tested at 158° F . were conditioned prior to test for 24 hours i n
a large chamber automatically maintained at 158° F . and 20 percent relativ e
humidity .
Those specimens tested at 200° F . were conditioned prior to test for 24 hour s
in an electric oven thermostatically maintained at 200° F . and essentiall y
zero humidity .
Description of Test s
Tension, compression, static bending, and impact (Izod) tests were conducte d
according to the procedures described in Federal Specification L-P-406 .
Tests at Subzero Temperature s
The subzero tests were conducted in a room maintained at -69° + 3° F . at th e
Army Air Forces Materiel Command, Wright Field, Dayton, Ohio, by the personne l
of the Materials Laboratory and on specimens supplied by the Forest Product s
Laboratory from the same material as was used for tests at other temperatures .
`"Factors Affecting the Strength of Papreg . The Effect of Accelerated Weather ing on Certain Strength Properties of Papreg . Forest Products Laborator y
Rept . No . 1521-A .
Rept . No . 1521
-3-
These tests were conducted as follows : Standard (dumbbell type) tension speci mens, were tested in self-aligning Templin grips in a 20,000-pound capacit y
universal testing machine, using the 5,000-pound load range . The machine wa s
hand-operated to obtain an average rate of grip movement of approximately 0 .01 '
inch per minute . Deformation data were obtained by means of a 2-inch gag e
length knife edge averaging type extensometer, equipped with a spiral staf f
type 0 .0001-inch dial gage .
Compression tests were conducted in the previously described testing machin e
on specimens placed lengthwise in a special compression jig described an d
pictured in the Army Air Forces Technical Report No . 4648 . Deformation wa s
measured with a 0 .0001- or 0 .001-inch dial gage mounted on the base of th e
jig and actuated by the travel of the vertical plunger of the jig . The averag e
rate of head travel of the plunger was approximately 0 .01 inch per minute .
Flexure tests were made on a bending jig described and pictured in Report No .
4648, adjusted to a 3-inch span . The load was applied at the center of th e
span by dead weights . Deflections at the center of the span was measured wit h
a 0 .001-inch dial gage .
Izod impact specimens were tested in a pendulum type impact testing machin e
using a'50-inch-pound capacity hammer for specimens notched flatwise and a
25-inch-pound capacity hammer for specimens notched edgewise .
Tests at Normal and Elevated Temperature s
Normal- and elevated-temperature tests were conducted at the Forest Product s
Laboratory .
The procedures of tests conducted at normal room temperatures and humiditie s
(76° F . and 23 percent relative humidity) were as follows :
Tension specimens, identical to those described in the subzero test, were test ed in self-aligning Templin grips in a motor driven, 10,000-pound capacit y
universal testing machine . The rate of cross arm motion under no load wa s
0 .042 inch per minute, while load-elongation data were taken to approximatel y
75 percent of ultimate load ; it was then increased to 0 .157 inch per minute and
maintained until failure . A 2-inch gage length separable nonaveraging typ e
extensometer equipped with a spiral staff type 0 .0001-inch dial gage was use d
to measure elongation .
Compression tests were conducted on specimens placed lengthwise between on e
fixed and one spherically seated plate centered in a 120,000-pound capacit y
hydraulic testing machine, using the 6,000-pound load range . The rate'of hea d
travel under load was 0 .012 inch per minute . _Deformations were measured wit h .
a 1-inch gage length Martens Mirrors compressometer attached to the machine d
faces of the specimen .
Flexure specimens weretested flatwise over a 2-1/2-inch span . Center loading
was applied by means of a universal testing machine, equipped with a hydrauli c
capsule and load indicator . A rate of cross-head motion (under no load )
Rept . No . 1521
-4-
of 0 .0+9 inch per minute was maintained to approximately 50 percent ;'of the
maximum load and then increased to a rate of 0 . 1)+ 6 inch per minute and main - ,
tained until failure . Deflections,at the center of the span were measure d
,with a'0 .001-inch dial gage .
Izod impact specimens were tested in a 16-foot-pound capacity pendulum-typ e
' impact testing machine using the 8-foot-pound hammer capacity on specimen s
notched flatwise and the 4-foot-pound hammer capacity on those notched edge wise .
- The testing procedures, .of tests conducted at 158° + 5° F . and 200° + 5° F .
conformed to those made at normal room temperature, except as noted in th e
following descriptions . The high temperature tests were conducted in a ply wood box (12 by 24 inches) housing the specimen and necessary apparatus an d
equipped with the following : a door with a double'glazed window (to permi t
deformation observations), a 60-watt light bulb for illumination, a thermome ter, two heating coils, and a thermostat . The box-with a tensile specime n
in test position is shown in figure 3 .
.For the tension tests, the box was placed on the table of a-100,000-poun d
capacity, hydraulic testing machine in a manner which permitted normal axia l
alignment of the Templin grips through small openings in the top and bottom of
the box .
Compression tests were conducted on specimens placed lengthwise in a compressio n
jig similar to that used in the subzero tests . The jig was placed in the ply wood box on the table of the hydraulic testing machine in such a manner as t o
permit normal axial alignment of the plunger of the jig through the small open ing in the top of the box, with the spherical seat attached to the center o f
the movable head of the testing machine .
.r
In conducting static, bending tests, the bending jig was likewise placed i n
the plywood box centered on the table of the hydraulic testing machine . Load
was applied through the medium of a loading block attached to the movable hea d
of the testing machine by means of a .pipe sleeve and bolt extending through the
small opening in the top of the box .
Izod impact specimens were tested in the Laboratory-at ordinary roomtempera tures and as soon as possible after removal from the conditioning mediums .
In the tension, compression, and static bending tests at 158° F . and 200° F . ,
several minutes were usually required to attach and adjust the deformatio n
indicators and place the specimen in the testing machine . In order to com pensate for any change in specimen temperature during these operations, . tes t
loads (except for a small initial load) were not applied until several minute s
after the temperature within the box had reached the proper level .
Rept . No . 1521
-5 -
Results of Test s
Maximum, minimum, and average values for a specified number of tension, com pression, static bending, and impact strength tests at four temperatures, ar e
given in table 1 . All values reported are based on dimensions taken a t
normal temperature .
The results indicate that certain properties of papreg tend to increase a t
subnormal temperatures and to decrease at high temperatures . Impact strengths ,
however, did not follow this general behavior, but indicated instead a sligh t
decrease in strength, with respect to strength at normal temperature, at both
temperature extremes .
The data obtained from these tests serve to confirm the results of simila r
tests of papreg made from Mitscherlich base-papers impregnated with Bakelite 6
resin 16303 and with Resinpx 465, conducted by the Monsanto Chemical Company .Temperature-strength relationships of parallel-laminated (lengthwise) an d
cross-laminated papreg are shown graphically in figure 4 . It may be noted that
the relationship for some properties is almost linear between -69° and 158° F .
and that the trend lines for the elastic modulus and yield strength in tension
and compression are almost parallel . The data show the greatest departur e
from a uniform trend at the two extremes of the temperature range explore d
(-69° and 200° F .) .
Variations in strength properties at the extreme temperatures with respect t o
corresponding properties at normal or room temperature, expressed as a percent age, is of interest . Such a comparison, giving the range of differences amon g
the direction of load-to-grain relationships for the majority of the strengt h
properties, is presented in table 2 . This comparison shows that the greates t
increase in strength properties occurred in the compressive yield strength a t
0 .2 percent strain offset, and the greatest decrease occurred in ultimate ten sion .
Typical tensile and compressive stress-strain curves at each of the severa l
temperatures are shown in figures 5 and 6 . The end points of the compressiv e
curves terminate at approximately 80 percent of the ultimate stresses . Typica l
load-deflection curves for flatwise static bending tests up to maximum load a t
fracture are shown in figure 7 . All curves are based on actual load-deformatio n
data for individual specimens, having properties in close agreement with th e
average of the group . It may be noted that the general shape of the curves i s
similar for all temperatures . The fact that the length of the initial straigh t
line or elastic portion, however, becomes less as the temperature increases ,
indicates that the material has probably become more plastic .
6
Forest Products Laboratory Rept . No . 1321, and " Special Report on the Influenc e
of Temperature on the Physical Properties of High Strength Paper Laminates . "
By D . Telfair and R . H . Haslanger, Research Department, Plastics Division ,
Monsanto Chemical Company, dated April 9, 194 3 .
Rept . No . 1521
-6-
Conclusion s
The results of these tests indicate that the tensile, edgewise compression , ' and
static bending properties of papreg are adversely affected at elevated tempera tures but'tend to improve at subnormal temperatures .
7
i
Rept . No . 1521
-7-
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1
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1
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showing tensile specimen in test position .
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