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Figure 4. MIL-STD-1312-16 Pre-load Test Schematic
Figure 5. Several Pre-load Specimens after Testing
A series of pre-load test was performed for each fastener
diameter. The voltage setting of the EMR on the collar
side controls the amount of the swaging force. A range
of acceptable EMR swaging voltages was found for each
fastener diameter. Table 1 below lists the fastener
manufacturer’s minimum pre-load tension, minimum
ultimate load, and the airframe manufacture’s load for
tension fatigue test.
lockbolt
diameter
Minimum
pre-load
Min. tensile
ultimate
Tension
fatigue load
in N (lbf.) N (lbf.) N (lbf.)
1/4 6670
(1500)
13300
(3000)
5560
(1250)
5/16 11100
(2500)
22200
(5000)
8450
(1900)
3/8 15600
(3500)
31100
(7000)
13300
(3000)
7/16 22200
(5000)
44500
(10000)
17800
(4000)
Table 1. Lockbolt Tension Targets and Loads
Figures 6 through 9 show the relationship between
swaging voltage and pre-load. Taking Figure 7, for
example, the minimum pre-load tension is 11,100 N
(2,500 lbf.) and the minimum ultimate load is 22,200 N
(5000 lbf.) Collar side voltages from 235 to 275 volts
yield acceptable pre-loads for these 5/16 inch lockbolts.
In production, acceptable pre-load for the minimum and
maximum stack thickness for a given grip narrows the
actual range of voltages used to approximately ±10 volts.
EMR charging voltage is controlled within ±1.5 volts. This
testing also identified a need to change the philosophy
for establishing the swage gauge design.
1/4 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
150 160 170 180 190 200 210 220
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Huck
HG113
fail
Huck
HG113
pass
Figure 6. ¼ Collar Pre-load Tests
5/16 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
200 210 220 230 240 250 260 270 280
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Huck
HG113
fail
Huck
HG113
pass
Figure 7. 5/16 Collar Pre-load Tests
3/8 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
250 260 270 280 290 300 310
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Figure 8. 3/8 Collar Pre-load Tests
7/16 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
280 290 300 310 320 330 340
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Figure 9. 7/16 Collar Pre-load Tests
TENSION-TENSION COLLAR FATIGUE TESTS
Tension-tension fatigue of lockbolt collars was performed
to MIL-STD-1312 test 11. Cyclic tension is put on the
lockbolt head and the swaged collar to the loads listed in
the far right column of Table 1. Twelve collars were
EMR swaged, three for each diameter 1/4”, 5/16”, 3/8”
and 7/16”. All tests were stopped after 130,000+ cycles
without failure. Collars were then tested to ultimate load.
All tests exceeded the minimum required ultimate load of
the collar. Tension fatigue life is a function of good
fastener design. These tests again verified that the EMR
collar swaging process provided adequate performance
compared to existing processes.
HIGH-LOAD LAP SHEAR FATIGUE TESTS
These tests compared the lap shear fatigue life of 1/4-14
lockbolts with EMR installed collars to the existing
hydraulic squeeze (Drivmatic) process. The test
coupons conformed to the airframe manufacturer’s
drawings. In addition, Electroimpact tested additional
coupons with two bolt holes as shown in Figure 10. See
Figure 11 for a photograph of a specimen in the fatigue
test fixture. In practice, the jaws (plates) clamping the
coupon were shimmed to center and parallel within 0.12
mm (0.005 inch.)
These coupons were subjected to a net alternating
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