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Tests of 10 Inch 24S-T Aluminum Alloy Shear Panels with 1.5 Inch Holes - II - Panels Having Holes with Notched Edges

SUMMARY

In a previous investigation of shear panels of
24S-T aluminum alloy, it had been found that yielding of
the material almost eliminates the stress concentration
around small holes before failure takes place and that
reinforcing rings consequently effect no significant
improvement in the static strength of the panel. The
present tests established the fact that the stress con-
centration around such holes is increased very materially
by the presence of a notch on certain parts of the circum-
ference of the hole and that reinforcing rings effec-
tively reduce this additional stress concentration.

INTRODUCTION

A considerable amount of theoretical work has been
done on the analysis of the stresses around holes with
or without reinforcement of the edges. These theoretical
analyses often indicate high concentrations of stresses.
Tests of square shear panels of 24S—T aluminum alloy with

l%—inch holes showed, however, that yielding of the

material almost eliminates the stress concentrations in
this particular material before failure occurs, and rein-
forcements consequently have a negligible influence on
the ultimate strength (reference 1).

The present paper describes tests made of specimens
similar to those of reference 1 in order to discover
whether a notch at the circumference of the hole
decreases the strength of the panel and, if so, whether
reinforcements are effective in overcoming the delete—
rious effect of the notch, Such a notch may be considered
2 --- - . .NACA RE No. L4D01

as a laboratory'facshnile of a.bad_crack or of accidental
damage that might exist in an airplane structure.

SYMBOLS

D diameter of hole, inches

ultimate load acting on shear frame, kips

a length of side of shear panel, inches
t thickness of sheet, inches
0 average diagonal-tension stress in net section
at failure, ksi
T aveiage shear stress in net section at failure,
s

TESTS AND TEST RESULTS

Material.— The material used was 24S-T aluminum
alloy nominaIly 0.051 inch thick. It was cut from the
sheet from which the 0.051-inch-thick specimens for the
investigation of reference 1 had been cut; the test
results may therefore be compared directly with those of
reference 1. The stress-strain curve of the material
may be found in reference 1.

Test specimens.- The tesL specimens were sheets
12 inches square with a l%-inch hole in the center. The

edges of the holes were plain or reinforced. oThe rein-
forcement was provided either by forming a 45° flange
on the edge of the sheet or by riveting rings to the
sheet as shown in figure 1.

On one control specimen, the edge of the sheet at
the hole was left smooth (specimen 1, table 1). The
ultimate stress obtained from this control specimen was
averaged with that from a similar specimen of reference 1
(specimen 2, table 1). On all other specimens except
specimen 12, the edge of the hole was notched with a
triangular Jewelerls file. On specimen 12, scratches
NAGA RB No. L4D01 5

were madenacross the,sdge of the.hole with.a double-cut
file'having 36 teeth per inch. For the main series of
tests, the notches were located at the two ends of the-
diameter coinciding with the compression diagonal of the
test Jig. For an auxiliary series of tests, the location
of the notches was varied. The depth of the notch was
measured with an optical micrometer and the shape of the '
notch was determined from a photomicrograph. The shape
and dimensions of the notch are shown in figure 2.

Test rocedure.- The specimens were bolted into a-
shear Jig of the picture-frame type (fig. 1) and were
loaded in a hydraulic testing machine at a rate of
4 kips per minute until failure occurred. As far as

could be observed, failure always took place immediately
after the sheet began to tear.

Evaluation of test data.- The ultimate shear stress
on the net section Sf’a specimen was calculated by the
formula