SAMPE Journal - September/October 2020 - 24

FEATURE / OPTIMUM NUTPL ATE PERFORMANCE
different epoxy adhesives were used for the torqueout testing. One set of samples was bonded using
Click Bond CB301 adhesive while the other was
bonded using Epibond 1544-1. For each nutplate
test coupon, substrate and nutplate were either
both abraded or both cleaned with plasma. Both
adhesives exhibit similar results with the sanded
sample having a maximum torque of 100±18 in·lbs for the CB301 adhesive and 90±18 in·lbs for the
Epibond 1544-1. Plasma activation results in a significant improvement in performance. The average
torsional load at failure was 159±27 in·lbs when using CB301 and 162±12 in·lbs using Epibond 1544-1.
As a result, plasma processing proved to be a viable
way to maximize the bond performance of these
nutplates.
Following torque-out testing, failure modes for
each nutplate were determined. Figure 9 shows two
examples of the bond failure interface observed
with Epibond 1544-1. It is apparent that the lower
bond strength for the abraded samples reported
in Table 3 can be attributed to weak interfacial interactions on the nutplate surface. These bonded
nutplates exhibited complete interfacial adhesion
failure at the nutplate. Examining both the nutplate and aluminum coupon together shows no
adhesive remaining on the nutplate surface and
the aluminum coupon shows adhesive still present
over the entire bond area. The image on the right
in Figure 9 shows a dramatic difference in the failure mode exhibited after using the plasma device.
Significant amounts of adhesive material are observed on both bond surfaces. This is indicative of
cohesive failure and means that in these regions,
the interfacial strength of the nutplate surface is no
longer the weakest point in the bondline. The interfacial strength generated by the plasma surface
preparation is high enough to force failure into the
adhesive itself.
The failure modes observed in Figure 9 are consistent with those that were obtained from push-off
testing. Torque-out testing has shown that sanded
nutplate surfaces often exhibit interfacial failures
at the nutplate indicating that the bond strength
can be increased by improving the surface preparation process for the nutplate. Bonding the plasma cleaned nutplates with either of the epoxy adhesives yields high quality bonded nutplates. The
mechanical strength exceeds that achievable with
abrasion as shown in Figure 6 and Table 3. Also
beneficial, is the ability of plasma activation to shift
bond failure away from the interface.
The push-off and torque-out results are in
good agreement with the trends observed during
previous testing when plasma was used to clean
nutplates prior to bonding on BMI and epoxy
composites6,9,10. Plasma cleaned nutplates bonded

SAMPE JOURNAL

to BMI composite produced bonds with 3.6x the
push-out load and 1.9x the torque-out load as a
nutplate bonded immediately after removal from
its individual packaging. The push-off and torqueout testing showed that plasma cleaning produces
a bond strength which far exceeds industry standard minimum requirements13. This shows that the
nutplate surface preparation is robust and can be
applied to nutplates bonded to structural elements
made from a wide variety of materials.
CONCLUSIONS
In this study, two novel plasma devices were used
to prepare the surfaces of stainless steel nutplates
and aluminum structural coupons prior to bonding. Using these devices, namely the Nutplate
Plasma Cleaner (NPC) and Aircraft Plasma Cleaner
(APC) from Surfx Technologies, eliminates operator
variability from the surface preparation process.
The entire process, including plasma activation of
the aluminum substrate, plasma activation of the
nutplate, application of the adhesive and nutplate
installation, can be completed in less than 30 seconds.
This work has shown that atmospheric pressure plasma treatment is a desirable method for
replacing solvent wiping and hand abrasion when
preparing fasteners and the corresponding substrates for bonding. Using the NPC removes surface contaminants in a matter of seconds with no
other cleaning steps required. The fastener is rendered active for bonding and converted to a high
surface energy, hydrophilic state. Plasma significantly reduces interfacial failure modes and leads
to a dramatic improvement in bond quality. This
resulted in nutplates which exhibited cohesive failure within the adhesive or mechanical failure of
the fastener after push-off and torque-out testing.
This means the bond created by the plasma can be
stronger than the mechanical limits of the fastener!
In addition to improving warfighter readiness,
plasma surface activation offers numerous manufacturing advantages including increased bond reliability, reduced rework, lower materials costs, and
better labor utilization.
ACKNOWLEDGMENTS
This effort is the continuation of multi-year programs dedicated to understanding the bonding
mechanism and the bond properties of nutplates.
This article includes references to previous work
which was funded through an SBIR with the Air
Force Research Laboratory and a program with
Navy ManTech.

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SAMPE Journal - September/October 2020

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