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National Advisory Committee for Aeronautics, Technical Notes - Effect of Rivet and Spot Weld Spacing on the Strength of Axially Loaded Sheet Stringer Panels of 24ST Aluminum Alloy

Eighteen 24S—T aluminum—alloy sheet—stringer panels
were tested in end compression under carefully controlled
edge conditions. The stringers were fastened to the sheet
by brazier-head rivets spaced 0.5 inch to 6 inshes between
centers for nine of the panels, by spot welds spaced 0.5
inch to 4 inches between centers for six of the panels,
and by round—head rivets spaced 0.5 inch to 2 inches be-
tween centers for the other three panels.

In the tests of the panels with stringers fastened
to the sheet by brazier—head rivets and by spot welds,
measurements were made of the stringer strains and of the
buckling deflections of the sheet. In the tests of the
three panels with round—head rivets only the buckling
loads and ultimate loads were measured.

.The buckling load and the deflection of the sheet
between rivets«and spot welds were compared with Rowlandrs
theory. The buckling load of the sheet between stringers
and the deflection of the sheet between stringers were
compared with_Timoshenko‘s theory. Most of the observed
buckling loads and deflections were in agreement with
these theories and indicated that the two types of buck—
ling were substantially independent of each other for the
specimens tested.

Four of the panels with brazier—head rivets and
three of the panels with spot welds failed by separation
of rivets or spot Welds at stringer stresses of 24.2 to
39. 5 kips per square inch. The other panels failed by
stringer instability at a stringer stress between 37. O
and 42 .0 kips per square inch.

The observed effective widths of the sheet between
stringers were from 8 percent lower to 20 percent higher
than those calculated from Marguerre's approximate formula
up to an edge strain at which buckling occurred between
rivets or spot welds. The sheet load remained approxi-
mately constant after buckling of the sheet between rivets
or spot welds.

A nomogram was devised for calculating the load for
failure by stringer instability of panels of the type
tested as a function of rivet or spot—weld spacing,
stringer spacing. reinforcement ratio. and critical
stringer stress.