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National Advisory Committee for Aeronautics, Technical Notes - The Fatigue Characteristics of Bolted Lap Joints of 24S-T Alclad Sheet Materials

Data have been'reported_on the fatigue properties mL
steel plates Joined by steel rivets and on the properties
of sheets of light*metal alloys joined by alumi_num~allov
rivets. (See, for example, references 1 and 2.) However,
little information is available concerning the fatigue
properties of sheets of light~metel alloys joined by hard
steel bolts} This report presents’the results of an investi-
gation on the behevibr, under repeated loading, of bolted
lap joints of ass—r Alcled sheet materials.

The'main objectives of the investigation have been to
find out to what extent the fatigue strength of bolted lap
Joints is effected by such factors as: bolt fit, number and
arrangement of bolts, and sheet gage (for a given bolt
diameter). "Fatigue strength" here means the load sustained
to complete fracture at some lifetime under tensionwtension
(at B = m: ni mum/maximum lo_a_d = +~O 25) Or under tension~
compression- (st R = O 50).

The important factor of bolt fit has been examined bv
testing otherwise similar specimens with bolt holes of
different clearances, Nominal clearances of —0. 001, +0.002,
+0.010, and +0.0?5 inch have been used. It_has been a major
objective to learn whether extremely close bolt fits are
needed to obtain maximum life of bolt Joints under repeated
stresses.

Several types of bolt patterns have been tested: single—
bolt specimens, specimens with two bolts in line with the
load, specimens with three bolts in line with the load,
specimens with a single.row of three bolts, specimens with
two rows_of three bolts each, and specimens with three rows
of three bolts each. In general, all bolts, in anv multi—
bolt Joint , had the same fit. Nearly all test pieces were
Joined with S/Q—incb—diameter steel bolts of aircraft
quality (ANS). The unthreeded length of the bolt: was such
that the bearing was on the smooth shank of the bolt.