n
h
r
FACTORS AF'F TING Till
OF RAIR.EO
EFFECT OF I E'PEATf CYCLES OF FREEflM AN
THAWING ON CERTAIN STEt 'M h1 .PROPER .
OF PAPREG
INFORMATION REVIEWE D
AND REAFFIRMED.
1962
No. 1521-C
T
COPY
Please return to- ;
Wood Engineering Researc h
Forest Products Laborator y
Madison, Wisconsin
5370 5
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FOREST PRODUCTS LABORATOR Y
UNITED STATES DEPARTMENT OF AGRICULTUR E
FOREST
MADISON 5 , WISCONSIN
In Cooperation with the
5ErVi i r
_SW
FACTORS AFFECTING THE STRENGTH OF PAPREG-'
2
Effect of Repeated Cycles of Freezing and Thawing- on Certai n
Strength Properties of Papreg
By
H. R . MEYER, Enginee r
and
E . C . O . ERICKSON, Enginee r
Forest Products Laboratory, 4 Forest Servic e
U . S . Department of Agricultur e
Summar y
This report presents the results of approximately 800 strength tests t o
determine the effect of repeated cycles of freezing and thawing on th e
tensile, compressive, bending, and bearing properties of papreg .
The tests included cyclic changes in temperature of approximately 110 °
F . on relatively dry, normal, and wet papreg . The number and type of
cycles varied from a few cycles consisting of a short freezing period an d
a long thawing period to a large number of cycles of short, equal period s
of freezing and thawing .
-This is one of a series of progress reports prepared by the Fores t
Products Laboratory relating to the use of wood in aircraft issued i n
cooperation with the Army-Navy-Civil Committee on Aircraft Desig n
Criteria . Original report published January 1945 .
?A laminated paper plastic made by Forest Products Laboratory (Improve d
Standard -- June 1943) .
-This report is the fourth of a series presenting the effect of aircraf t
service conditions on certain strength properties of papreg .
-Maintained at Madison, Wis . , in cooperation with the University o f
Wisconsin .
Rept . No . 1521-C
-1 -
Agriculture--Madison
The results of the tests indicated that the freezing and thawing action ha d
a negligible effect upon the strength properties of papreg covered in thi s
investigation .
Introductio n
5
Previous investigations- designed to simulate certain aircraft service conditions have indicated the level of certain strength properties of papre g
over a wide range of temperature and humidity . Moisture effects an d
temperature effects at a specific level of moisture content were each deter mined by separate studies involving only the single variable, moisture o r
temperature .
This investigation was designed to simulate, in part, the combined effec t
of moisture and temperature as produced by rapid and repeated exposure s
of aircraft materials to conditions that varied from hot, humid atmosphere s
at ground level to frigid, dry air at high altitudes .
Since all such possible conditions could not readily be experimentally produced, the effects of the following two freezing and thawing conditions o n
papreg were investigated : (1) cyclic changes from normal to subzer o
temperatures, on relatively dry papreg ; and (2) similar cyclic changes i n
temperature on relatively wet papreg, to determine whether the repeate d
expansion and contraction associated with the alternate freezing and thaw ing of the moisture particles in the wet papreg would decrease certai n
strength properties beyond that attributable to moisture alone .
Test Material
The test material, which is identified as Improved Standard -- June 1943 ,
was manufactured by the Forest Products Laboratory from a commercia l
-Forest Products Laboratory Rept . No . 1521 . Factors Affecting th e
Strength of Papreg . Some Strength Properties at Elevated and Sub normal Temperatures .
Forest Products Laboratory Rept . No . 1521-A . Factors Affecting th e
Strength of Papreg . Effect of Accelerated Weathering on Certai n
Strength Properties of Papreg .
Forest Products Laboratory Rept . No . 1521-B . Factors Affecting th e
Strength of Papreg . Effect of Moisture on Certain Strength Propertie s
of Papreg .
Rept . No . 1521-C
-2-
Mitscherlich type black spruce sulfite pulp . The preparation of the papre g
was identical to that described in Forest Products Laboratory Rept . No .
1521 .6 The test material consisted of both parallel- and cross-laminate d
flat panels, approximately 12 inches square and 1/8 inch in thickness .
Selection and Preparation of Test Panel s
As in previous related investigations, the papreg panels were selecte d
from groups I and II (parallel- and cross-laminated material, respectively )
which are described in the aforementioned Rept . No . 1521 . Twelve panel s
were taken from group I and six from group II .
These panels were divided into three lots numbered 1, 2, and 3 . Each lo t
consisted of four panels from group I and two panels from group II .
The panels were trimmed to provide clean-cut edges, which were the n
sealed with two coats of aluminum paint to retard the movement of moisture through these edges . In addition, all panels were weighed and measured before and after conditioning and at the conclusion of the last freezing
cycle . Subsequent weight and dimensional changes are based on data take n
before conditioning .
Conditioning of Panel s
The various lots were conditioned as follows :
Lot 1 -- 80° F. and 30 percent relative humidity .
Lot 2 -- 75° F. and 50 percent relative humidity .
Lot 3 -- 80° F. and 97 percent relative humidity .
One-half of the panels from each lot were conditioned for approximatel y
100 days and the remainder for approximately 130 days . The 30-day interval permitted a preliminary evaluation of test data to determine th e
course of further testing .
6
-Factors Affecting the Strength of Papreg . Some Strength Properties a t
Elevated and Subnormal Temperatures .
Rept . No . 1521-C
-3 -
All conditioning was done in large rooms in which temperature and humidit y
were automatically controlled .
Freezing-thawing Cycle s
The freezing-thawing cycle consisted of 1 hour of freezing followed by 1
hour of thawing . The freezing was done by placing the conditioned panel s
in air in a mechanically refrigerated chamber in which the temperatur e
varied between -20° F . and -30° F . At the end of the freezing period, th e
panels were thawed in air by returning them immediately to their respective conditioning chambers .
These procedures were repeated 36 times on those panels conditioned fo r
approximately 100 days and 95 times on those panels conditioned for approximately 130 days .
Preparation of Specimen s
At the conclusion of the 36 and 95 cycles of freezing-thawing, all panel s
were cut into specimens as shown in figure 1 . Six lengthwise and fou r
crosswise specimens of parallel-laminated material and five specimens ,
lengthwise and crosswise of the cross-laminated material were provide d
for each of the following tests : tension, compression (edgewise), stati c
bending, and bearing .
The type, dimensions, and cutting of the specimens conformed to Federa l
Specification for Plastics, Organic ; General Specifications (Methods o f
Tests) L-P-406, December 9, 1942, and were identical to those describe d
in Forest Products Laboratory Rept . No . 1521-B . i
Conditioning of Specimen s
The preparation of test specimens was performed in a relatively dry atmosphere . In order to insure a uniform and specific moisture content ,
?Factors Affecting the Strength of Papreg . Effect of Moisture on Certai n
Strength Properties of Papreg .
Rept . No . 1521-C
-4-
each specimen, following preparation, was reconditioned for at least 2 4
hours prior to test in the previous conditioning atmosphere of the respective panel from which it was taken .
Except for the few minutes required for the weighing and measuring operations, the specimens were not exposed to test room atmospheres unti l
placed in a testing machine .
Testing Procedure s
All tests were conducted at normal test room temperatures and humiditie s
according to procedures specified in Federal Specification L-P-406 . Tes t
details were identical with those described in the appendix of Forest Products Laboratory Rept . No . 1521-A .
a
Test Result s
The results of tests to determine the effect of repeated cycles of freezing thawing on various strength properties of papreg are presented graphicall y
in figures 2 and 3 by means of comparison with identical papreg similarl y
conditioned but not subjected to freezing and thawing action . The solid line curves9 in figures 2 and 3 show the relationship between moisture an d
several strength properties of papreg, while the plotted points represen t
the average results of 4 to 6 tests at specific moisture levels after 36 an d
95 cycles, respectively, of freezing and thawing action . Hence, the difference between the points and the curves (beyond the variability of the papreg )
reflects the effect of repeated cycles of freezing-thawing .
The plotted points do not depart greatly from the curves at specific moisture levels . In fact, the magnitude of the departures is largely within th e
limits of variability indicated in Forest Products Laboratory Rept . No .
1319 .l
An exception to this behavior occurred in the modulus of elasticit y
8
Factors Affecting the Strength of Papreg . Effect of Accelerated Weather ing on Certain Strength Properties of Papreg .
9
-Reproduced from Forest Products Laboratory Rept . No . 1521-B .
10
-Strength and Related Properties of Forest Products Laboratory Laminate d
Paper Plastics (Papreg) at Normal Temperatures .
Rept . No. 1521-C
-5-
in tension for both parallel- and cross-laminated materials after 95 repetitions of freezing-thawing . The departures in these instances amounte d
to a reduction of 23 and 19 percent, which is beyond the limits of variability . In general, however, the effect of repeated cycles of freezing thawing is not significant .
An inspection of the plotted data reveals that the increase in the numbe r
of repeated cycles (open versus solid symbols) did not produce a trend i n
the values of any specific strength property. That is, increasing th e
number of repeated cycles from 36 to 95 did not bring about a significan t
change in strength properties .
Figure 4 shows the change in the interior temperature of the papreg durin g
a freezing-thawing cycle . Observations were obtained by means of a
thermocouple inserted in the edge of a panel at midthickness to a depth of
about 2 inches . It may be noted that the length of time required to lowe r
the temperature of the papreg to 32° F . was relatively short -- only abou t
7 percent of the total time in the cold chamber . At the beginning of th e
thawing period, a similar length of time was required to raise the temperature of the papreg to 32° F . From these facts, it is evident that the tim e
element of the cycle was of sufficient length to freeze and thaw the papreg ,
Freezing and thawing produced no visible change in the surface conditio n
of the papreg . Likewise, the type of failures appeared no different tha n
normal .
Conclusion s
The study indicated that the several strength properties of papreg covere d
in this investigation are not appreciably affected by alternate freezing an d
thawing . These properties included tension, compression, bending, an d
bearing .
Rept . No . 1521-C
-6-
APPENDIX
Exploratory freezing-thawing tests were conducted on a group of specimens immersed in water at room temperature for periods of 2, 4, an d
8 days, respectively, for which the objective, the test material, preparation, conditioning, and testing were identical to those described in th e
appendix of Forest Products Laboratory Rept . No . 1521-B . ?
The specimens selected constituted the majority of those referred to i n
the report as "used in tests of other service conditions . "
Freezing-thawing Cycle s
The freezing-thawing cycle consisted of 6 hours of freezing followed by 1 8
hours of thawing . The freezing was done in a mechanically refrigerate d
chamber in which the temperature varied between -25° F . and -30° F .
The thawing was done in a room maintained at 40° F . and 95 percent relative humidity .
All specimens, with the exception of half of those conditioned by immersio n
in water for 8 days, were subjected to six consecutive cycles of freezing thawing immediately following their respective conditioning period . Th e
remaining specimens were subjected to 12 cycles .
Testin g
Tension, compression, and static bending tests were conducted at normal
room temperatures and humidities at the conclusion of the designate d
number of freezing-thawing cycles according to the procedures outlined i n
Federal Specification L-P-406 .
Test Result s
The results of these exploratory tests showed no significant changes in th e
stress at ultimate, proportional limit, modulus of elasticity, and modulu s
of rupture for the tensile, compressive, and static bending propertie s
when compared with like papreg not subjected to freezing and thawin g
action . In addition, it was also found that increasing the number of cycle s
from 6 to 12 did not change the strength properties .
Rept . No . 1521-C
-7 -
.3-12
MACH/NE D/RECT/ON Of LAM/NATIONS
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LEGEND : T .TENSION SPECIMENS -BY9-/NCHES (BEFORE SHAP/NG)
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B :-BEAR/NG SPEC/MENS -BY 4 -INCHES .
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FROM THE EDGE OF THE HOLE TO THE EN D
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PANEL /Y05.28,29,33,34,40,4 /
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CUTTING DIAGRAM FOR CR055 L A MINATED (GROUP II) PANEL 5
Figure '1 .--Cutting diagrams for specimens taken from 1/8-inc h
papreg panels after exposure to repeated cycles .of freezing
and thawing .
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A-36 CYCLES OF FREEZ/NG-THAWING AFTER /00 DAYS COND/T/ON/NG AT 50°F
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0-36 CYCLES Of FREEZING-THAW/NG AFTER /00 DAYS COND/T/ONING AT DOE -F-9797PERCENT
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CONDIT/ON/NG MED/A
EACH PO/NT REPRESENTS THE AVERAGE 06' 6 TESTS
Figure 2 .--Comparison between the freezing and thawing-strengt h
relations and the moisture-strength relationship of parallel laminated papreg (Improved Standard - June 1943) teste d
lengthwise .
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.
REPRESENTS
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.
h
.--Comparison
Figure
between
the
freezing
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thawing-strengt
3
relations
and
moisture-strength
relationship
of
cross
.
laminated
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1943)
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TIME (M/NUTE5,)
FREEZ/NG /NTERVAL---+-THAW/NG /NTERVAL
Figure 4 .--Temperature change at center of a 1/8-inch papre g
panel, conditioned at 80° F . and 30 percent relative humidity ,
during a freezing and thawing cycle of 60 minutes at -30° F .
followed by 60 minutes at 82° F .
Z M 57933 F
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