JEC COMPOSITES MAGAZINE - Issue #112 - April/May 2017 - 41

AERONAUTICS
self healing

Skin-stringer component
with self-healing functionalities
X. tSilimigkra1, a. kotrotSoS1 ,
S. tSantzaliS1, g. SotiriadiS1, v. koStopouloS1, S. Florez2, a.W. boSman3
1
Department of Mechanical
Engineering & Aeronautics,
University of Patras
2
Industry and Transport Unit,
Tecnalia
3
SupraPolix BV

Skin-stiffened composites have been extensively used in aerospace to produce
lightweight structures. Stringer debonding is one of the most critical types of
damage in this type of structure. Implementing a self-healing functionality
within the bond line would benefit the service life of the component by repairing it when damage occurs.

Extrinsic and intrinsic sampling
The encapsulation strategy was implemented using non-toxic poly(methyl
methacrylate) (PMMA) capsules combined with a scandium triflate catalyst.

Experiment settings
The ultrasonic C-scan technique was
applied at the bond line of the stiffened panel to secure its initial quality.
Due to the nature of the experiment
(compression) and the geometry of the
component, the way of transferring the
compressive loads to the component
was crucial. If anvils alone were to be

Fig. 1: Skin-stringer a) schematic, b) composite component

used, local crushing of the contact area
of the component to the anvils would
have resulted in early failure, and not
in the desired way. A technique called
potting was utilized for this purpose.
It consists in a liquid resin casting
procedure that finally delivers a solid
resin block that surrounds the ends of
the component. It provides a robust
support, while transferring compressive loads through the shear forces that
develop around the component in the
resin block. The potting edges were
then machined to provide perfectly
parallel surfaces for the subsequent
compressive test.
Damage was induced with an air gun
capable of firing a spherical (1/2''
diameter, 8.3 grams) steel ball. For
the present experiment, an impact
energy of 13.5 J was selected in order
to achieve substantial damage (visible
impact damage, VID) to the skin and
to produce bond line damage (delamination). A single shot was delivered

Fig. 2: Skin-stiffened composite integrated with a supramolecular prepreg

No112 April - May 2017

/ jec composites magazine

Feature Aeronautics

everal conceptual approaches
have been developed for the
integration of self-healing functionalities in composite materials.
The capsule approach is an extrinsic
method while supramolecular and
thermoplastic materials are an intrinsic technological approach. Both
intrinsic and extrinsic solutions were
investigated, first for their ability to
be incorporated within the adhesive
at the bond line of the demonstrator
(skin-stringer) (Figure 1 a, b) and
secondly for comparative reasons
regarding the knock-down effect and
their healing efficiency.

The capsules were homogeneously
dispersed in the adhesive at 25% wt.
content. Using a syringe, the adhesive
was placed at the bond line and cured
at 60°C for 1 hour. The healing cycle
was set at 120°C for 12 hours.
For the reversible polymers strategy,
a supramolecular material in prepreg
form was incorporated on the top layer
of the skin to provide the self-healing
functionality (Figure 2). In this case,
the healing cycle was achieved through
the pressure of two claps at the bond
line and heating at 100°C for 15 minutes.

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #112 - April/May 2017