JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020 - 40

FEATURE
bonding

The Bondline Control Technology

Dipl.-Ing. Lennert Heilmann,
Research Associate
German Aerospace Center (DLR)

oining fibre-reinforced plastics (FRPs)
via adhesive bonding enables optimum
material utilisation and optimised designs
in terms of manufacturing, aerodynamics and
weight. In aviation, however, the use of this
joining technology is restricted since adhesion defects cannot be reliably detected with
non-destructive testing methods (NDT). In
the case of critical joints, fail-safe design is
implemented in the form of additional bolts
or structural redundancy, resulting in reduced
efficiency in the manufacture and operation
of the aircraft. To solve this issue, DLR invented the BCT, a novel method for bondline testing and surface pre-treatment (international
patents pending). The proof test is capable
of detecting any defects present in a bonded
interface with a very high sensitivity. And the
surface treatment produces a joining surface
that is guaranteed to be bondable and that is
highly robust against the formation of critical

The Bondline Control Technology (BCT) comprises several
innovative methods that substantiate proper bond quality
in adhesive joints. It unlocks the use of adhesive bonding in
safety-critical applications and offers a variety of benefits for
lab testing, production and bonded repair of FRPs.

bonding defects. The proof test and surface
pre-treatment can be performed in combination or separately to meet the requirements of
various application scenarios.
Novel proof test
The quality and properties of a joint between
an adhesive and a solid adherent are assessed
by means of the novel proof test. The mechanical test is performed by bonding a mesh
fabric onto the adherent and by successively
peeling it out of the cured adhesive, see Figure
1. The peeling of the fabric generates a high
mechanical stress that reveals any bondline
defect present, regardless of its cause. Defects
are recognised by an invalid fracture pattern
or as a deviation from the nominal peeling resistance. In case of a proper bond, the peeling
resistance reaches a reference value and a thin
layer of tested adhesive remains on the adherent. Due to the flexibility and drapeability of
the mesh fabric, the test can be performed on
rigid materials with a flat or curved surface
and even on complex-shaped surfaces such as

Fig. 1: Crosssectional view of the
BCT peel test

jec composites magazine / N°135 July - August 2020

Fig. 2: Test results of defective bonds: BCT vs. DCB

stepped repair geometries.
In the framework of the FACTOR project
supported by the German Federal Ministry
of Economics and Technology, several test
programmes were conducted to assess the
sensitivity of the BCT with respect to the
detection of different types of bonding
defects. Figure 2 illustrates the results of a
comparative analysis between the BCT and
the double cantilever beam test (DCB or "GIC
test"), which is deemed the most sensitive
state-of-the-art test for interfacial defects.
The tests were performed on CFRP panels
with different degrees of pre-bond contamination of the joining surface (S1 to S4). The
contaminant was a silicone-based release
agent that was applied homogeneously via
the dip-coating method at Fraunhofer IFAM.
The results showed that for each degree of
contamination, the amount of adhesion
failure detected is comparable between the
two methods. Similarly positive results were
obtained for other types of bondline defects
such as cohesion defects of the adhesive and
surface-near defects in the substrate.
The sensitivity of the BCT peel test was
found to be so high that it detects defects in

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

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