SHEAR-FATIGUE PROPERTIES OF
VARIOUS SANDWICH CONSTRUCTION
July 1952
Information Reviewed and Reaffirmed
No. 1837
INFOrr rI N PFTEVVE5
LiaiRMED
AI
1965
1958
LOAN COPY
Please return to:
Wood Engineering Research
Forest Products Laboratory
Madison, Wisconsin 53705
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UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN
FOREST SERVICE
In Cooperation with the University of Wisconsin
1
WEAR-FATIGUE PROPERTIES OF VARIOUS SANDWICH CORSTRUCTIOND -
By
FRED WE • • Di
Engineer
2
Forest Products Laboratory, - Forest Service
U. S. Department of Agriculture
IND WI MD NI
Summary
Shear-fatigue curves for eight different sandwich core materials are presented
in this report. These curves are based on Forest Products Laboratory tests of
sandwich constructions in which failure was not influenced by weakness in the
core-to-facing bond. They are therefore considered to be indicative of the
shear-fatigue properties that might be expected from these core materials when
adequate bonding methods are employed.
Introduction
If plates of sandwich construction are designed so that their facings are
elastically stable under the intended loads, the most critical stress to which
the core is subjected is shear. The consideration of the effect of repeated
shear stresses on the core material and on the bonds between the core and
facings is therefore important.
The purpose of this report is to summarize the results of the shear-fatigue
studies of various sandwich constructions made at the Forest Products Laboratory in cooperation with the ANC-23 Panel on Sandwich Construction for Aircraft. Sandwich constructions made of nine different types of cores, with
aluminum or glass-fabric-laminate facings, were tested. The core materials
were: Paper honeycomb, end-grain balsa, glass-fabric honeycomb, cellular hard
rubber, cellular cellulose acetate, perforated aluminum honeycomb, foamed-inplace alkyd isocyanate, expanded aluminum honeycomb, and a waffle-type material
of resin-impregnated glass-fiber nat. Separate reports have been issued on
each sandwich construction tested.2
1
_This
report is one of a series prepared and distributed by the Forest Products
Laboratory under U. S. Navy, Bureau of Aeronautics Order No. NAer 01319 and
U. S, Air Force No. AF-18(600)-70. 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.
.See list of detailed reports on individual sandwich constructions at end of
this report.
Report No. 1837
Agriculture-Madison
Fabrication of Sandwich Constructions
All of the sandwich constructions tested. were assembled by procedures that, at
the time of fabrication, were considered to be representative of procedures
used in industry to produce similar sandwich constructions. Most of the test
panels were assembled at the Forest Products Laboratory, but two of the sandwich constructions were purchased elsewhere.
Recent developments and improvements in techniques may provide better bonds in
thoso sandwich constructions that failed largely in the core-to-facing bond.
Testing;
All fatigue specimens were tested in a direct-stress fatigue machine. The
ratio of minimum to maximum stress (range ratio) was 0.10, and loads were
applied at the rate of 900 cycles per minute. Static shear tests of control
specimens were made in a testing machine at a head speed of 0.01 inch per
minute. The details of a typical frame shear specimen, as used in these tests,
are shown in figure 1.
Further details of testprocedures are given in Forest Products Laboratory
Report No. 1559.-A:4
Presentation and Discussion of Data
A brief summation of data on the various sandwich constructions tested is
given in table 1. Information is included on the density of the core material,
the shear strength as determined from the control tests, the type of fatigue
failure, and a reference to the report wherein details regarding tests of each
construction are given. If the failure was primarily or entirely a core
failure, the fatigue results are representative of the fatigue characteristics
of the core material. If failure was due to an inadequate bond between the
core and facings, it is possible that the shear strength and the fatigue
characteristics may be changed considerably by a different method of bonding.
It may be noted from table 1 that there is sometimes considerable variation
between the static shear strengths for a particular material. This may be due
to differences in the core material or in the core-to-facing bond. The
average shear strength given is the average of all control specimens tested
for the particular typ e of sandwich and core direction.
Most of the sandwich constructions failed completely or primarily in the core
(table 1), and the bond between core and facings was thus considered to be
satisfactory. In some constructions, however, the specimens failed largely or
completely in the bond. This shows that some difficulty might be expected in
producing a satisfactory bond for these constructions by the techniques
Report No. 1837
-2-
recommended at the time of these studies. Methods may now be available that
permit fabrication of such sandwich constructions so that the properties of
the core material may be developed.
Typical S-N (stress versus number of cycles to failure) curves for sandwich
constructions made from the various core materials are shown in figures 2 and
3. These curves are based on specimens that failed completely or predominately
in the core material and thus are considered representative of the shearfatigue properties of the core material. Curves for specimens that failed predominately in the core-to-facing bond are not shown in this report, since they
reflect the effect of an inadequate bond between core and facings and are thus
not indicative of the properties of the core material. The average fatigue
strength of a core material, in pounds per square inch, can be obtained from
figures 2 and 3 by multiplying the percent of control strength by the average
strengths Given in table 1.
No fatigue curves are shown for the perforated aluminum honeycomb in the LT
plane, the waffle-typo core material, and the expanded aluminum honeycomb in
the LT plan.
List of Detailed Deports on Shear-fatigue Tests of Sandwich Constructions
The following Forest Products Laboratory publications discuss the shear-fatigue
properties of different sandwich constructions, under the general title,
"Fatigue of Sandwich Constructions for Aircraft":
(1) 1559-A, "Aluminum Face and Paper Honeycomb Core Sandwich Material," 1947.
(2) 1559-B, "Aluminum Face and End-grain Balsa Core Sandwich Material," 1948,
(3) 1559-C, "Aluminum or Fiberglas-laminate Face and Fiberglas Honeycomb Core
Sandwich Material," 1948.
(4) 1559-D, "Fiberglas-laminate Face and End-grain Balsa Core Sandwich
Eaterial," 1948.
(5) 1559-E, "Aluminum or Fiberglas-laminate Face and Cellular-hard-rubber Core
Sandwich Material," 1948,
(6) 1559-F, "Cellular Cellulose Acetate Core Material with Aluminum or
Fiberglas-laminate Facings," 1948.
(7) 1559-G, "Fiberglas-laminate Facing and Paper Honeycomb Core Sandwich
Material," 1949.
(8) 1559-h, "Aluminum Facing and Aluminum Honeycomb Core Sandwich Phterials"
1949.
Report No. 1337
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