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stress of 100, 150, or 210 MPa (14.5, 21.7, or 30.5 ksi)
with a mean stress of zero. (Fully reversing tension and
compression loads). Over 30 fatigue tests were
performed by Electroimpact. 16 tests with specimens
provided by the airframe manufacturer. 20 preliminary
tests were also carried by Electroimpact with 7075-T6
aluminum coupon material.
Figure 10 High Load Lap Shear Fatigue Test Coupon
Figure 11. Fatigue Test Fixture – Loaded High Lap Shear Coupon
A comparison of the fatigue results for hydraulically
pulled and swaged lockbolts versus pneumatically
driven, EMR swaged lockbolts is given in Figure 12. For
example, at a stress level of ± 150 MPa (21.7 ksi), both
the hydraulically swaged and the EMR swaged
specimens cracked around 100,000 cycles. Similar
results can be seen at the higher and lower stress levels.
Control specimens and further tests conducted by
Airbus UK Limited at Filton verified the integrity of joints
assembled by this process.
STATIC LAP SHEAR TESTS
These tests were carried out to verify the static shear
strength of lockbolts with electromagnetic set collars.
The specimens were two rectangular aluminum plates
with a two lockbolts through the lap joint to the airframe
manufacturer’s drawings. The plates are pulled, causing
shear stresses in the lockbolts. Laboratory control
coupons were provided to the airframe manufacturer. In
addition, two coupons with 7075-T6 material were tested.
Both coupons failed by shearing the (1/4 inch nominal)
titanium alloy lockbolts at 50.7 kN (11,000 lb.) and 50.5
kN (11,350 lb.) respectively. The EMR swaging process
provided good static lap shear strength.
PRESSURE FUEL RETENTION TESTS
A pressure chamber test was used to verify the fuel
retention capability of pneumatically driven/EMR swaged
lockbolts. This test was per MIL-STD-1312-19. Airbus
UK Limited provided the test plate as shown in Figure 13.
Electroimpact applied sealant to the countersink,
pneumatically installed the lockbolts, and EMR swaged
the collars. Huck International performed the test at their
Irvine, California test facility. Pressure in the test
chamber was cycled to 50 psi for 1000 cycles. After the
successful test, Figure 14 illustrates the top of plate with
bolt heads under die penetrant developer.
METALLURGICAL EXAMINATION
Electroimpact provided (14) specimens to the airframe
manufacturer’s Technical Centre for metallurgical
examination. Lockbolt sizes were 1/4-4, 1/4-15 and
5/16-7. Typical microsections are shown in Figures 15
and 16 in the following illustrations. Lockbolts were
pneumatically/percussively driven and EMR swaged.
Specimens were cut from the plates, potted in polymer,
faced, polished, and photographed under a microscope.
Microsections verify the integrity of EMR swaged collars.
CONCLUSIONS
Pneumatically driving and EMR swaging lockbolt collars
met or exceeded the current Drivmatic (hydraulic)
installation in terms of pre-load, lap shear fatigue life,
and static shear strength. These and the other airframe
manufacturer’s tests provided confidence to invest in
equipment for the electromagnetic swaging of lockbolts.
Pre-load tests also identified a need to change the
swage gauge design philosophy when using this type of
process. A wide range of acceptable voltage settings can
be used for EMR swaging. In addition, these tests give
design engineers confidence that the allowable loads,
strength, and fatigue life of these installed fasteners
exceed the lockbolt manufacturer’s minimum values.
Electromagnetic riveters (EMRs) have now installed
millions of lockbolts in production on large commercial
aircraft. They have been used on lockbolts from 1/4 to
7/16 inch diameter.
ACKNOWLEDGMENTS
The authors acknowledge Airbus UK Limited, Filton
Design Centre for allowing the publishing of this test
information The authors also wish to acknowledge the
assistance of Huck International by providing fasteners
and test assistance.
REFERENCES
1. “Low Voltage Electromagnetic Lockbolt Installation”,
John Hartmann, et.al., SAE Aerofast 1992 paper
922406
CONTACT
For more information, contact Sam Smith at
Electroimpact, Inc, 4606 107th St. SW, Mukilteo, WA,
98275, telephone 425-348-8090, fax 425-348-0716. Email
sams@electroimpact.com
DEFINITIONS, ABBREVIATIONS
CNC: computer numerically controlled, any machine or
process controlled by a computer, where the computer
controls the machine position and processes.
Collar: a cylinder (typically of aluminum alloy) that is cold
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