SAMPE Journal - January/February 2017 - 22

Feature Article
T. Löbel, D. Holzhüter, C. Hühne
German Aerospace Center (DLR), Braunschweig, Germany

Disbond-Stopping Concepts for
Bonded Composite Joints
Abstract

Bonded composite joints have many advantages in comparison to conventional fasteners. However, adhesive bonding of
primary aircraft structures is still a certification issue. One promising way to achieve airworthiness for bonded composite joints
in accordance with the authority requirements is the establishment of disbond-constraining design features. Consequently, two
novel design features are developed. On the one hand, the hybrid bondline is introduced as a combination of adhesive bonding and
local thermoplastic welding. On the other hand, an array of small diameter pin elements is used as out of plane reinforcements.
Enhanced manufacturing concepts are developed for both features. Their mechanical performance is evaluated by means of static
and fatigue tests. Both principles are proven successful since crack growth is fully stopped for both technologies in fatigue crack
lap shear (CLS) tests under relevant loading conditions. Thus, it can be concluded that these technologies are a promising step
towards certification of bonded composite joints.

Introduction

Today's Usage and Limitations of
Bonded Composite Joints
The implementation of more
composite
load-bearing
parts
demands efficient solutions in
terms of joining technology. Bolting
and adhesive bonding are the two
major joining techniques available
for thermoset composites which are
the majority of composites used for
aeronautical applications. From a
mechanical perspective, adhesive
bonding is the favorable joining
technique for several reasons.
Adhesive bonds lead to weight
reduction, offer a more uniform
load distribution, are capable
of joining thin-walled parts and
minimize
material
weakening.
Fasteners have a noticeable impact
on the part design and could even
be a key dimensioning factor. Thus,
fasteners hamper the full lightweight
potential of composites1. Therefore,
the development of adhesive bonds
being capable for certification is of
high interest.
For civil aircraft, bonding of
composites
is
well-established
for various secondary joints. For
instance, for the latest aircraft of the
Airbus family (Airbus A350XWB),
a large share of stiffeners are
(composite to composite) bonded
joints leading to an overall bondline
length of about 5 km per aircraft2.
22

However, due to certification
requirements the implementation of
bonded joints in aircraft composite
structures is still limited to secondary
joints or combined with so-called
"chicken rivets" if used for primary
joints. Those additional fasteners
have to be capable of carrying limit
load in case of a global failure of the
bondline2. Design benefits that come
along with adhesive bonding do
not come into effect, since fastening
elements must be taken into account
for part design. Thus, up to now
adhesive bonding has not been used
to its full extent. The main reasons for
this limitation are briefly discussed
below.
Reasons for Certification Limitations
The
manufacturing
process
of structural bonded joints is
sensitive to many factors, e.g.
surface treatment, adhesive curing
conditions, entrapped humidity,
and many more. These factors may
affect the long-term durability of the
joint3. Judging their impact on the
joint's performance is complex and
still subject of current research and
scientific discussions.
In addition to manufacturing
uncertainties, aging and fatigue
life of bonded composite joints
are still challenging to predict
and also influenced by many
factors (e.g. load level, strain rate

and environmental conditions)4-6.
Thorough investigations of the
interaction between those factors
and their impact on the joint's longterm durability are hampered by
the necessity of cost-consuming
and time-consuming experimental
fatigue studies. Furthermore, inservice damage (e.g. impact events)
could hardly be avoided and may
also lead to a noticeable decrease of
the joint's strength4,7. Eventually, all
these factors have led to a significant
scatter in the performance of bonded
composite joints in the past with
some working well and some failing
after short time in service [8]. These
experiences have caused distrust
towards adhesive bonding as a
joining technology.
Certification Requirements of Bonded
Composite Joints
Based on experience in the past
and the uncertainties mentioned
above, the certification authorities,
namely the Federal Aviation
Administration (FAA, USA) and
the European Aviation Safety
Agency (EASA, Europe), specify
two major prerequisites that have
to be met to achieve certification of
bonded composite joints for primary
structures3,9. The manufacturing
process must be specified, controlled
and monitored and has to be carried
out in a pre-defined manufacturing

SAMPE Journal, Volume 53, No. 1, January/February 2017

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