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EFFECT CF ,ELLIVATIEV'TESPIEIPATIVIQIES
ON'il-li STIAENGTI1S CF SMALL SPECIMENS
Of SANDWICH CONSTRUCTION Of THE
AIRCRAFT TYPE
(GIASS7C10-111 TACINGS, IBAILSA CORE -- TESTED IMMEDIATELY
AIFTER THE TEST TEMPERATURE WAS REACHED)
•
April 1952
INFORMATION REVIEWED
AND REAFFIRMED
1958
This Report is One of a -Series -IssUed In Cooperation with the
AIR FORCE-NAVY-CIVIL SUDCOMMITTEE
on
-AIRCRAFT DESIGN CRITERIA
Under the Supervision of, the
- AIRCRAFT COMMITTEE
of the
MUNITIONS 130ARD
No. 1804-A
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
EFFECT OF ELEVATED TEMPERATURES ON THE STRENGTHS OF SMALL
SPECIMENS OF SANDWICH CONSTRUCTION OF THE AIRCRAFT TYPE"(Glass-cloth Facings, Balsa Core - Tested Immediately After the
Test Temperature Was Reached)
By
EDWARD W. KUENZI, Engineer
Forest Products Laboratory,? Forest Service
U. S. Department of Agriculture
Introduction
The purpose of this study was to determine the behavior of a sandwich construction with facings of glass-fabric-base', phenolic-resin laminate bonded to a
core of end-grain balsa wood, at temperatures from 75° to 600° F. (23.9° to
315.6° C.). The experimental work paralleled the work reported in Forest
Products Laboratory Report No. 18042 on other sandwich constructions of the
aircraft type. Details as to testing apparatus and procedures were identical
with those described in that report.
Test Material
The sandwich construction was manufactured according to Chance-Vought Aircraft
Process Specification CVA 270. The facings were of five plies of glass cloth
112-114 hand-impregnated with approximately 50 percent, by wet weight
(approximately 30 percent of dry resin by weight) of resin 9. The core was
1/2-inch end-grain balsa wood with a density of 5 to 8 pounds per cubic foot.
It was sized with a spray coat of thinned resin 9 and immediately dusted with
2This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U. S. Navy, Bureau of Aeronattics Order No.
NAer 01237 and 01202 and U. S. Air Force No. USAF-(33-038) 51-4062E.
Results here reported are preliminary and may be revised as additional data
become available.
Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
Atbemzi , E. W. Effect of Elevated Temperatures on the Strengths of Small Speci-
menu of Sandwich Construction of the Aircraft Type. Forest Products
Laboratory Report No. 1804, 8 pp., illus., 1949.
Report No. 1804-A Agriculture-Madison
powder (the solid component of adhesive 34). After an open-assembly period
of approximately 2 hours, the facings and core were assembled on an aluminum
and
mold and cured for 1 hour under fluid pressure in 4 bag at about 260°
a pressure of 75 pounds per square inch,
r.
Testing Apparatus and Procedures
The testing apparatus, procedures, and temperatures and the specimen sizes
were identical with those described in Forest Products Laboratory Report
No. 1804.3 The tests were begun as soon as the desired temperature was
reached in the test oven. This took from 10 to 20 minutes, depending on how
much apparatus adjustment was necessary. Tests were made in compression on
short and long specimens. Tests were also conducted in shear and in flatwise
tension.
Results of Tests
The properties of the sandwich construction, as determined by the tests at
various temperatures, are presented in table 1 and in graphical form in
figures 1 to 5.
Colimn-compression specimens failed first by bending and finally by compression of the facings. The nominal facing stresses at failure (fig. 1)
decreased from 17,000 pounds per square inch at 75° F. to 12,000 pounds per
square inch at 450° F. and to zero at 600° F.
Short specimens in compression (edgewise compression) failed by crushing of
the facings at Stresses of 24,700 pounds per square inch at 75° F. This
stress was reduced to 16,000 pounds per square inch at 350° F,, to approximately 3,000 pounds per square inch at 450° F., and to zero at 600° F.
(fig. 2). It is not known, at present, why the strength of the short
compression specimens at 450° F. is less than that of the long column specimens.
The effective modulus of rigidity, as determined by the frame shear test,
decreased almost linearly from 21,200 pounds per square inch at 75° F. to
approximately 4,000 pounds per square inch at 450° F. (fig. 3). At 600° F.,
the strength was so low that no readings could be taken to determine modulus
of rigidity values,
The shear strength dropped from 330 pounds per square inch at 75° F. almost
linearly to approximately 15 pounds per square inch at 600° F. (fig. 4).
Some of the failures occurred in the balsa cores and some in the facing-tocore bonds
Tensile strength decreased almost linearly from approximately 780 pounds
per square inch at 75° F. to 70 pounds per square inch at 600° F. Failures
Occurred in the bonds. Several attempts were made to bond the loading'
blocks to the facings, but failure occurred in these bonds at temperatures
of 250° and 350° F.
Report No. 1804-A
-2-
The balsa cores of all specimens charred throughout to a light brown at
F. and charred black and brittle at 600° F.
450°
Comparison with Other Sandwich Constructions
Although comparisons with previously tested sandwich constrUctions l are
difficult to make, a few general comments may be worth considering.
At 450° F., the column strength of the sandwich construction having glasscloth facings on a balsa core was approximately three times as great as
the strength of comparable constructions with glass-cloth facings on glasscloth honeycomb cores. The column strength values at 75° F. were approximately equal.
The edgewise-compressive strength of the sandwich construction having
glass-cloth facings on a balsa core was somewhat higher than that of some
of the other constructions with glass-cloth facings at moderately elevated temperatures, but was lower at 450° and 600° F. It is not known
whether this is due to. the types of resin used in the facings or for bonding the facings to the core.
The shear properties of this sandwich construction are comparable to the
properties determined for a construction having aluminum facings on a
balsa core.
The tensile strength of the sandwich construction having glass-cloth facings on a balsa core was approximately two-thirds of the strength of aluminum facings bonded to balsa at 75° F. At elevated temperatures of 350°
to 600° F., however, it was greater than the tensile strength of aluminum
bonded to balsa.
Report No. 1804-A
-3-
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Table 3.--Results of tension tests at elevated telperatUres of a
sandwich construction with facings of five-ply glass
cloth 112-11 bonded to a core of 1/2-inch end-grain
balsa with the solid colvonent: stf adhesive. T#
Specimen :
No.
Tension tests
Core streSs at failure at -
:
.
75° It' : 160° F. : 250° F. : 350° F. : 450° Y. : 60o . F.
P.6 i • Pos.i.
:
P.s.i.
: P s i.
1
•:
1
• 1
-685 : -470
1
: -515
• 2
: -330
2
;
X625: 1522
: 242o
: .2365
3
:
-812 : -842
2
2
: -37o
4
:
5
: -Boo : '550
6
-705 : -595
: 1705
1
1
-1,020 1 1548
1
' 1
:
-435
2
2
: -44o : -58o
' 2
: P.s.i.
;
; P.8.1.
-
: 218o
: 2 so
2
: -325
• 1
: -105
— go - 2
2
: -500 : 1 65
:
' 2
: -485
: -415
2
• -52o
-g340
85
1
15
:
: - 35
:
• 1
1
: -315
: - 42
•
775 •
Av....•..:
588
:
469
:
367
-Bond failure between facings and core.
-Bond failure between facings and "Ceding plate.
- Core
failure.
Report No.
1804-.A
:-
299
:
69