SAMPE Journal - September/October 2020 - 20

FEATURE / OPTIMUM NUTPL ATE PERFORMANCE
Figure 4. Aircraft Plasma Cleaner with 1" wide hexagonal showerhead plasma source.

of this device is presented in Figure 4. The handle
contains integrated timer and status lights to assist
the user by presenting all system information necessary to operate the tool. A built-in timer function
allows the user to initiate a 5 second blinking light.
The width of the hexagonal showerhead pattern is
1". Standoff pins around the plasma head maintain
a 2 mm distance when the APC is pressed against the
material being activated. The plasma was operated
at 165 W of RF power using 30 LPM of helium and
0.58 LPM of oxygen. To prepare the material surface,
plasma is generated inside the source and the reactive gasses flow out onto the substrate below. The
showerhead end of the device was placed directly
onto the substrate and centered around the nutplate
installation hole. The plasma was kept over each installation site for a period of 5 seconds.
Hand Abrasion
A subset of the nutplates and structure coupons
were prepared using manual abrasion. Abrasion of
the substrates was achieved using Scotch-Brite™
7447 pads. Prior to abrasion, the solvent wiping
step was performed, and the surface was abraded
by hand until a matte finish was visible. Following
abrasion, another solvent wipe step was performed
and repeated until no residue was observed on the
wipe. It was then given a final dry wipe.
Bond Procedure
All samples were bonded within 20 minutes of the
surface activation processes. A set of at least five
bonded nutplates was produced for each test. Adhesive was applied to the nutplates in a single bead
around the elastic fixture of the nutplate. The fixture was then pulled through the installation hole
in the structure coupon until the nutplate sat firmly
against the surface with a visible bead of adhesive
around the edges. Coupons were then allowed to
cure for a minimum of 72 hours before testing.
Water Contact Angle
Water contact angle (WCA) of the test coupons
was measured using a Surface Analyst™ (model
SA3001) manufactured by BTG Labs. The WCA of

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the nutplates before and after surface treatment
was obtained using a Krüss DSA15B EasyDrop digital goniometer with DSA3 software package.
Push-Off Testing
Push-off testing followed the requirements of
NASM 25027 (reference para. 4.5.3.5)13. Mechanical
push testing was performed on an Instron (model 3369). A custom push off tool sized for the nut
element of the nutplate was attached to a 50 kN
load cell. It was lowered into the nutplate until it
engaged the nut. Load was applied at a rate of 0.05
inches per minute until failure occurred.
Torque Out Testing
Torque out testing followed the requirements of
NASM 25027 (reference para. 4.5.3.4)13. The substrates were placed into a test fixture which consisted of a channel to hold the test coupons and
fixturing for mounting a 0-100 ft·lb torque wrench
with memory needle made by CDI (model 1003LDFNSS). The torque wrench was attached to an extractor tool which allowed it to engage with the
nut element to perform the toque-out test. Prior
to testing, the torque wrench memory needle was
set to zero, and then torque was applied to the nutplate until the joint failed.
Failure Mode Analysis
Following mechanical testing, the failure mode
of the disbanded assembly was analyzed. Images
were taken of the bond region for each substrate
and nutplate. Four different failure modes were
identified: interfacial/adhesive failure at the substrate, interfacial/adhesive failure at the nutplate,
cohesive failure within the adhesive and mechanical failure of the nutplate. Images of each substrate
were processed through an internally developed
computer algorithm, which overlaid the various
failure modes as different colored pixels on the
original image. These results were visually checked
against the substrate and nutplate to confirm accuracy. Finally, the number of pixels of a given color
was compiled and divided by the total number of
pixels to yield the percentage of each failure mode.

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SAMPE Journal - September/October 2020

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