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National Advisory Committee for Aeronautics, Technical Notes - Comparison of Static Strengths of Machine Countersunk Riveted Joints in 24S-T, X75S-T, and Alclad 75S-T Sheet

Flush—riveted joints are often prepared by machining
a conical recess in the sheet orlflete into which the
countersunk head of the rivet is fitted. When this machined
recess goes completely or nearly through the thickness of
the sheet, there is a knifelike cutting edge on the sheet
bearing against the shank of the rivet which tends to
reduce the effective shear strength of the rivet. This
cutting action seems to become more pronounced when using
the "harder" sheet alloys and when using the "softer“ rivets.
In order to obtain some definite information on this subject,

static tests were made as outlined herein on machine~
countersunk riveted joints in 245—T, X758—T, and Alclad
7SS—T sheet using A17S—T and 24s—s rivets. In order to
intensify the cutting action, all specimens were made with
the depth of countersink just equal to the thickness of
the sheet.

It has been noted that the results of the tests in
reference 1 show similar weakening of the riveted joints
when the depth of the countersink is Just ehnal to the
thickness of the sheet and that the shear strengths in
reference 1 check those reported here in the few cases
that can be directly compared. —4r

The object of this investigation was to compare the
static strengths of machine—countersunk riveted Joints in
24$~T, X758—T, and Alclad 758—T sheet using A17S—T and
245—T rivets in order to evaluate the differences in cutting
action of the sharp edge of the sheet when the machine—
countersunk hole was the same depth as the thickness of the
sheet.

The type of specimen used in this investigation is
illustrated in figure 1. It will be noted that two sizes
of rivets were used, 1/8 and 3116 inch, and that the
corresponding thicknesses of sheet were nominally 0,040'
and 0.064 inch. Special care was taken in the shop to
ensure a feather edge at the bottom of all machine~
countersunk holes used in this investigation.