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Assembly Cells in Broughton, UK. Testing compared the
percussion insert/EMR swaging of lockbolts with existing
hydraulic installation methods. Tests included pre-load
tension tests, ultimate tension load tests, tension fatigue
tests, high-load lap shear fatigue tests, static lap shear
tests, a pressure leak test, and metallurgical
examination.
Fastener configurations tested covered diameters from
1/4, 5/16, 3/8, and 7/16 of an inch. Joint materials
conformed to ABM3-1031 (7150-T651 plate), stump-type
lockbolts to ABS0550VHK (Huck LGPS4SCV), and
collars to ASNA2025 (Huck 3SLC-C). Some pull-type
lockbolts to ABS0548VHK (Huck LGPL4SCV) were also
tested as noted.
INTRODUCTION
Airbus UK Limited manufactures wings for the Airbus
family of large commercial aircraft at their Broughton
facility in the U.K. It is desirable and practical to have
automated CNC machines install the lockbolts between
stiffeners and panels for such aircraft. Please see
Figure 1. When a new process was proposed for
installing lockbolts at the facility, testing was needed to
assure that the new process would provide a joint
performance that meets or exceeds existing processes.
ı Prior (baseline) processes: hydraulically pulled
(manual pin-tail) or pushed (Drivmatic) lockbolts
with hydraulically swaged collars.
ı New (now current) process: pneumatically
percussion driven lockbolts with EMR swaged
collars.
Low-voltage EMRs have been developed to install
lockbolts by Electroimpact since 1991. (Please see
reference 1.) For production aircraft, test requirements
were defined by the Materials and Process Group of
Airbus UK Limited, Filton. As part of the risk share
agreement, Electroimpact performed testing of the
percussion insert/EMR swage of lockbolts. Material
specimens, fasteners, and collars were provided by the
airframe manufacturer. Joint materials conformed to
ABM3-1031 (7150-T651 plate), stump-type lockbolts to
ABS0550VHK (Huck LGPS4SCV), and collars to
ASNA2025 (Huck 3SLC-C). Some pull-type lockbolts to
ABS0548VHK (Huck LGPL4SCV) were also tested as
noted. This test work verified the integrity of the
electromagnetic installation process, helped set voltages
of the EMRs for collar swaging, and extended the
process for lockbolt diameters up to 7/16 inch.
Figure 1. CNC Machine Fastening of Stiffeners to Panels
Unless noted, lockbolts for these tests were installed with
a pneumatic percussion tool. Some pull-type lockbolts
were installed with a hydraulic installation tool. EMR
(electromagnetic riveter) swaging of the lockbolt collars
was done on a test bench as similar to Figure 2. Fatigue
testing of specimens was done on the author’s MTS
fatigue tester, see Figure 3. Fatigue coupons were also
assembled for the airframe manufacturer’s Technical
Centre as laboratory controls.
Figure 2. Test Bench for EMR Collar Swaging
Testing was carried out to verify pre-load, ultimate tensile
strength, tension-tension fatigue, high-load lap shear
fatigue, static lap shear, fuel retention and metalluragical
properties.
Figure 3. Fatigue Testing Machine.
TESTS PERFORMED ON LOCKBOLTS
A series of tests were used to verify the quality of the
pneumatically driven, EMR swaged lockbolts. Although
all tests are needed to verify different characteristics of
the installed lockbolt/collar combination, special
emphasis was placed on the pre-load and fatigue
specimens.
PRE-LOAD TESTS
When a collar is swaged on the serrated end of a
lockbolt, the entire shank of the lockbolt is put in tension.
The amount of tension is known as “pre-load” and is a
critical parameter for ensuring the integrity and the
fatigue life of a bolted joint. Tests were performed for
determining the optimum voltage setting range for 1/4,
5/16, 3/8 and 7/16 inch nominal diameter lockbolt collars.
Tests were per MIL-STD-1312-16 as shown in Figure 4
below.
Inserting a lockbolt and swaging a collar between two
thimbles with clearance fit holes as shown in Figure 4
makes the pre-load test specimens. A thin friction
paddle is sandwiched between the two thimbles. There
is hole through the center of both thimbles and the
paddle. The ends of the thimbles are installed in a
tensile test machine and axially loaded at a known rate,
such as 6700 N/min (1500 pound/min). The paddle
moves when the axial force matches the pre-load
tension. Further, the axial load is increased until the
collar fails in tension. The aluminum of the collar is
sheared in the lockbolt grooves as shown in Figure 5.
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