JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020 - 43

The Bondline Control Technology

neous as described before. The subsequent
application of the repair patch by adhesive
bonding can be carried out with a high
degree of process reliability and is certain to
not result in the formation of non-detectable
weak bonds.
With this method at hand, boltless bonded
repair of FRPs can be performed with high
confidence and can possibly be certified for
safety-critical cases.
Fig. 7: Application of the BCT on a stepped CFRP surface in a repair scenario

In brief
New methods are developed at DLR
that enable proof of adhesive bond
quality and bonding of hard-wet interfaces with the same process reliability as in wet-wet interface bonding.
Applications are the reliable surface
pre-treatment of FRPs for structural
bonding, proof of bond quality in repair and production, process control
specimens for parts manufacturing
and mechanical test coupons for the
assessment of adhesive bond.

Applications in FRP repair
In repair and retrofit scenarios, the bondability of an FRP structure is not entirely
predictable. This is due to the fact that the
material has already been subject to the
in-service environment. The contact with
adhesion-inhibiting media, the manual surface preparation and pre-treatment as well as
the execution of the bonding process under
non-optimum conditions can result in local
and global bonding defects that cannot be
detected via conventional NDT.
By applying the BCT proof test to the entire
joining surface prior to the application of the
repair patch, the bondability of the structure
is verified. This is illustrated by Figure 7, in
which the test is performed on a stepped
repair surface. The mesh used for this application is applied in a large piece that can later
on be peeled in strips of a defined width. The
properties of the mesh result in relatively low
peeling forces that prevent any damage to
fragile structures, without compromising the
detection sensitivity.
If the test result is positive, the joining surface
is covered with a coating of fresh adhesive
that has a proven bond strength with the
structure. At the same time, the surface of
the coating is pre-treated, clean and homoge-

Summary and outlook
The various applications of the BCT have
positive effects over the entire lifecycle of an
aircraft.
In the development phase, the novel test coupon is an inexpensive and effective means of
studying new material combinations and
manufacturing processes for bonding. With
the reliability and robustness of the BCT surface pre-treatment, boltless bonded assemblies can be realised that enable advanced
structural designs. These bonded structures
require less material and fewer assembly
steps than bolted structures, thus saving energy and resources. The better aerodynamic
performance and lower structural weight
have a positive effect on fuel consumption
and emissions during operation. And the extension of advanced repair processes ensures
maximum service life with constant efficiency. After decommissioning, the absence of
metallic fasteners in bonded structures helps
with dismantling and supports separation by
material type.
All in all, it can be said that the improvements
achieved by the BCT contribute to more
affordable and sustainable mobility while at
the same time improving industrial competitiveness.

N°135 July - August 2020

More information:
www.dlr.de

/ jec composites magazine

Feature Aeronautics

pre-treat ment methods such as grinding or
blasting, or technologically sophisticated
methods like laser ablation or plasma treatment. None of these surface pre-treatment
methods achieves the formation of mechanical interlocking that is strong enough to be
considered a secondary load path.
The optimum properties of the surface
created by the BCT make it compatible with
epoxy resins curing at ambient, moderate
and high temperatures and pressures alike.
In fact, no incompatible combination of
epoxies has been identified so far.
Due to the simplicity and robustness of the
BCT surface pre-treatment, technological
and economic advantages are achieved. The
method is particularly suitable for applications with high demands on the quality and
safety of the produced joints, but also for

those manufacturing processes that require
time-consuming pre-treatment processes
due to current technological limitations.

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JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020