JEC COMPOSITES MAGAZINE - Issue #135 - July/August 2020 - 42

FEATURE bonding
Applications in material
and process qualification
A high-performance test coupon was developed based on the novel proof test. This
Bondline Control Specimen (BCS) is intended for lab testing of adhesive bonds and
has many technical and economic advantages
over state-of-the-art test methods such as the
DCB test or the floating roller peel test (EN
2243-2). The specimens are produced in the
style of the floating roller peel test, see Figure
6. However, a mesh fabric is bonded onto
the rigid substrate instead of sheet metal.
The specimens are peeled manually or with a
testing device according to EN 2243-2.
The BCS can be manufactured with any rigid
substrate material such as FRP or metal, using film adhesives, paste adhesives and liquid
adhesives. In comparison to state-of-the-art
methods, it is possible to test curved, stepped
and scarfed surfaces of unidirectional and
quasi-isotropic laminates. This is especially
relevant for the development of pre-treatment and bonding procedures for the repair
of FRPs.
The new test coupon requires only one substrate panel, is easily and rapidly manufactured and therefore grants significant savings
in the specimen preparation. Manual testing

of the specimens gives a fast indication of
the test result and instrumented testing in
accordance with EN 2243-2 is inexpensive
and simple compared to testing of DCB
specimens.
A statistical analysis of 18 test series comprising a total of 94 specimens showed that the
peeling resistance has an exceptionally low
scatter. In several test programmes, it was
found that due to the high consistency and
reproducibility of the test results, even minute influences on the bonding process can be
analysed extremely well. On top of that, the
interpretation of the fracture pattern is easy
and straightforward. While DCB specimens
often yield mixed-mode failure including
substrate damage due to bending stresses,
BCS result in pure cohesion failure in case of
proper bond quality, see Figure 6.
However, one of the most important advantages of the new coupon is the significantly
reduced time required for media saturation.
The bondline of a BCS is not shielded by a
second adherent and is thus exposed to the
environment. While metal peel specimens
require 6 weeks and DCB specimens usually
require around 30 weeks of time for full
saturation with moisture, BCS are ready for
testing after just 2 to 4 weeks of conditioning
in a constant climate chamber. This advan-

Fig. 5: Illustration
of a co-bonded joint
formed via the BCT

Fig. 6: Bondline Control Specimen in the peeled and non-peeled state

jec composites magazine / N°135 July - August 2020

tage can significantly improve development
times for new manufacturing processes and
material combinations.
Applications in parts manufacturing
Besides material and process qualification,
the BCS is applicable as a means of process
control for quality assessment in adhesive
bonding processes on a production scale.
By implementing representative panels of
peel specimens into the manufacturing flow,
the production of bonded assemblies can
be checked against the occurrence of systematic bonding errors. In order to achieve
representativeness between the BCS and the
part, the specimens are produced from the
same material batches, with the same surface
pre-treatment and under the same curing
conditions as the actual part.
In order to achieve an even greater representativeness, the peel test can be performed
directly on the structural part. In this in-situ
process control, pieces of mesh fabric are
bonded onto the part surface adjacent to
the structural joint. The test bond and the
structural bond are produced in parallel
under identical conditions to achieve high
representativeness. The quality check is performed by peeling the mesh fabric manually
or with the aid of a mobile testing device. A
very thin layer of tested adhesive is left on the
structural part as evidence of proper bond
quality.
The surface pre-treatment via BCT can be
implemented into the production of bonded
assemblies in order to achieve an unmatched
degree of quality assurance. It makes the
bonding process very robust and can exclude
the occurrence of non-detectable weak
bonds. Fabric specifications optimised for
this application scenario are laminated onto
joining areas of the part as an outer layer.
Before the adhesive bonding operation, the
fabric is peeled off to generate the optimised
joining surface. With conventional surface
pre-treatment by peel ply, strong and durable adhesive bonds fulfilling aerospace
requirements are usually only achieved when
high-temperature, autoclave-curing adhesive
systems are applied. Epoxy adhesives curing
at ambient or moderate temperatures and
in cost-saving out-of-autoclave curing processes usually require other time-consuming

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

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