Supplement No. 2 to
STRESSES INDUCED IN A SANDWICH
PANEL 13Y LOAD APPLIED
AT AN INSERT
No. 1845-13
INFORMATION REVIEWED
AND REAFFIRMED
0962
February 1956
LOAN COPY
Please return to:
Wood Engineering Research
Forest Products Laboratory
Madison, Wisconsi n 53705
S
This Report is One of a Series
Issued in Cooperation with the
ANC-23 PANEL ON SANDWICII CONSTRUCTION
of the Departments of the
AIR wpm NAVY, AND COMMERCE
111111111miiii
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FOREST PRODUCTS LABORATORY
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
MADISON 5, WISCONSIN
I n Cooperation with the University of Wisconsin
SUPPLEMENT NO. 2 TO
STRESSES INDUCED IN A SANDWICH PANEL BY LOAD
APPLIED AT AN INSERT—1
By
W. G. YOUNGQUIST, Engineer
and
EDWARD W. KUENZI, Engineer
Forest Products Laboratory,? Forest Service
U. S. Department of Agriculture
Summary
Results of previous research have indicated that theoretical formulas for
determining the stresses produced in a sandwich panel loaded on a central
insert and normal to the panel faces are satisfactory for use in the design of sandwich panels with isotropic or orthotropic cores. The results
of the present study indicate that the formulas are also satisfactory for
use in the design of sandwich structures in which the load is applied to
the insert at angles other than 90 degrees, with respect to the faces, if
the formulas are modified to utilize the normal component of the inclined
load.
-This progress report is one of a series (ANC-23, Item A-6) prepared and
distributed by the U. S. Forest Products Laboratory under U. S. Navy
Bureau of Aeronautics Order No. NAer 01628 and U. S. Air Force No. AF18 (600)-102 and DO (33-616) 53-20. Results here reported are preliminary and may be revised as additional data become available.
2
—Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
Report No. 1845-B
Agriculture-Madison
Introduction
Forest Products Laboratory Report 1845 deals with the general problems
of the design of load-carrying fittings in sandwich panels with isotropic,
solid-type cores. Supplement A to the above report extends the study to
panels with orthotropic cores of aluminum honeycomb. In the experimental
work involved in both previous studies, the central test loads were
applied normal to the facings of the panels. In practice, loads are
frequently applied to fittings at angles other than 90° to the sandwich
facings. The present study attempts to check experimentally the validity
of the previously developed formulas for use in the design of fittings
carrying loads at angles to the sandwich facings.
Experimental Procedure
One of the previously fabricated, aluminum honeycomb-core test panels was
used for these tests. Shear strains in - 6
. he panel core were determined
with the central load on the insert applied normal to the facings and at
angles of 30 and 45 degrees to the facings. The values obtained were
checked against the previously developed formulas. Facing strains were
also determined under similar loading conditions.
Materials Tested
Panel A-1 (Forest Products Laboratory Report 1845-A) was used for these
tests. The panel was 20 inches in diameter and had a solid aluminum insert 2 inches in diameter. The faces were of clad aluminum, 0.032 inch
thick, and the core consisted of aluminum honeycomb of 0.003-inch-thick
foil and 1/4-inch cell size. The total panel thickness was 0.321 inch.
A complete description of the test panel is contained in Report 1845-A.
Method of Test
The method used for testing the panel under an inclined load is illustrated in figure 1. The same heavy base with the 18-inch-diameter hole
used in the previous tests was used to support the panel. Two supporting
frames for the base were used, 1 supporting the panel at 30 degrees with
the horizontal and the other at 45 degrees with the horizontal. A
heavy plywood supporting ring was attached to the top of the base to
prevent lateral movement of the panel under load. A tensile load was
applied to the panel insert through a flexible cable attached to the
lower head of the testing machine on one end, and to a 3/8-inch-diameter
Report No. 1845-B
-2-
steel ball on the other end. The steel ball transmitted the load to a
shoulder at the center of depth of the panel. This method of loading
was used in an attempt to avoid undesirable bending stresses in the
panel.
The shear strains in the panel core were determined in the manner
described in Report 1845-A. After the first set of measurements had
been obtained, the test was repeated with the panel in the same plane
but turned at angles of 90, 180, and 270 degrees from the original
position in order to determine any possible effect of gage location on
the magnitude of the indicated shear strains. Essentially straightline relationships between load and differential movement between faces
were obtained in all cases.
Facing strains were determined on both faces of the panel by means of
SR-4 gages of 1/4-inch gage length. The gages were glued to the panel
at distances of 1-1/4, 2-1/4, 3, and 4 inches from the center of the
insert, in both the radial and tangential core directions.
Formulas Used
The formula used for the determination of shear stresses in the core
under normal load and the formula for determining the facing stress at
the edge of the central insert of a normally loaded sandwich panel are
Presented in Report 1845. In the present study, the normal component
of the inclined load was used in these formulas.
Presentation of Data
The measurements of shear stresses in the panel at the arbitrarily
selected total load of 140 pounds are tabulated in tables 1, 2, and 3.
The values in columns 1 to 5, inclusive, and the test data for the horizontal position were obtained from the previous series of tests, and are
included here for the sake of completeness. Column 8 shows the vertical
load applied to the panel, and column 9 shows the normal component
acting on the panel. In each case, the normal component was used in
computing the shear stress values shown in column 12 and the K values
shown in column 14. The shear stress values shown in column 11 were
obtained from the indicated movements in column 10 by the method outlined in the appendix of Report 1845-A.
The data tabulated in tables 1, 2, and 3 are shown graphically in
figure 2. Table 4 presents the observed direct radial facing strains at
various points on the panel. The computed facing strains at the rim of
the panel insert and the observed direct radial facing strains at
various points on the panel are presented graphically in figure 3.
Report No. 1845-B
-3-
Results of Tests
The data presented in figure 2 show good correlation between the computed and observed values for the parameter K for all 3 loading conditions. In general, the values obtained in the LT direction are somewhat higher than those in the LE direction. Almost identical values
were obtained when the tests were repeated after rotating the panel at
angles of 90, 180, and 270 degrees in the same plane. It thus appears
that, under the test conditions used, the shear stress in the core is
not affected by the angular location of the core with respect to the
applied central load.
The data presented in figure 3 show that the observed strains in the
top and bottom facings are proportional to the magnitude of the normal
component acting on the panel. If the load on the insert is applied at
any angle other than 90 degrees between the load and facings, direct
tensile and compressive stresses are introduced in the facings by the
load component parallel to the faces. Under the loading conditions
used in this test, these direct stresses were evidently of small
magnitude.
Of principal interest to the designer is the stress at the edge of the
insert. The formula for the determination of the facing stresses at
the edge of the insert is presented in Report 1845. Under certain
loading conditions, the direct stresses in the facings, rather than
stresses due to a bending of the panel, may govern the design. Formulas for the design of fittings under such conditions are presented in
Part II of Bulletin ANC-23.2
Conclusions
Inclined-load tests on the central insert of a single sandwich panel
with aluminum facings and an aluminum honeycomb core indicate that the
design formulas for normal loading previously presented in Report 1845
for the determination of core shear stresses are also valid for the
design of panels carrying inclined loads, if the formulas are modified
to utilize the normal component of the inclined load. The data also
show that the strains in the top and bottom facings of the panel are
proportional to the magnitude of the normal component of the load on
the central insert.
2Sandwich Construction for Aircraft. Bulletin ANC-23, May 1951;
Munitions Board, Aircraft Committee.
Report No. 1845-B -4-
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Figure 1. --Circular sandwich panel under test, with load applied
at the insert at an angle of 45 degrees to the faces. Load is
being applied to the central insert by means of a 3/8-inch
diameter steel ball attached to the pulling cable.
Z M 107 142