fast and robust joining process for aerospace components by local heating of paste adhesives

308 Pages·2014·6.54 MB·English

ETH Library
Fast and robust joining process for
aerospace components by local
heating of paste adhesives
Doctoral Thesis
Author(s):
Sánchez Cebrián, Alberto
Publication date:
2014
Permanent link:
https://doi.org/10.3929/ethz-a-010164453
Rights / license:
In Copyright - Non-Commercial Use Permitted
This page was generated automatically upon download from the ETH Zurich Research Collection.
For more information, please consult the Terms of use.
DISS.-No. ETH 21842
FAST AND ROBUST
JOINING PROCESS
FOR AEROSPACE COMPONENTS
BY LOCAL HEATING
OF PASTE ADHESIVES

ALBERTO SÁNCHEZ CEBRIÁN
2014

DISS.-No. ETH 21842

FAST AND ROBUST JOINING PROCESS
FOR AEROSPACE COMPONENTS BY
LOCAL HEATING OF PASTE ADHESIVES

A thesis submitted to attain the degree of

DOCTOR OF SCIENCES OF ETH ZÜRICH

(Dr. sc. ETH Zürich)

presented by

ALBERTO SÁNCHEZ CEBRIÁN

Dipl. Industrial Engineer
Universitat Politècnica de Catalunya (2009)

born 20.07.1985

citizen of Spain

accepted on the recommendation of

Prof. Dr. Paolo Ermanni, examiner
Dr. Markus Zogg, co-examiner
Prof. Dr. Heinz Voggenreiter, co-examiner

2014

Allá donde estés, esto va por ti.

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Summary (English)
Since the 70s, industry is doing research in new adhesive sys-
tems for joining composites for structural applications. Adhe-
sive bonding permits having lighter structures and thus more
efficient solutions than today’s aerospace state of the art, rivet-
ing. Additionally, the use of adhesives does not require drill-
ing the components, which is especially beneficial for joining
composite materials.
Typically, the curing of paste adhesives is carried out in forced
convection ovens, applying isothermal heating cycles recom-
mended by the supplier. The variation of the curing tempera-
ture during the process is today not considered due to its high
energy consumption. To avoid this limitation, the use of in-
duction heating to heat locally the bondline is considered in
this doctoral thesis. This technique allows heating the assem-
bly faster and thus saving energy resources and time. In this
context, the acceleration of the curing process of a paste adhe-
sive by increasing the heating temperature during the process
is investigated.
However, the increment of the curing temperature leads to
void formation and thus to the thermal degradation of the
paste adhesive. This phenomenon is caused by the evapora-
tion of moisture, the degradation of chemical components and
the expansion of air trapped during the mixing process. The
increment of void content affects the mechanical performance
of the bonded system. To deal with this limitation, a novel
modeling of the void formation during the curing process con-
sidering the degree of cure is developed in this doctoral thesis.
The fundamental idea of this approach is that the evaporation
of the paste adhesive that leads to void formation decreases
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with curing progression. Therefore, a novel thermal degrada-
tion model for the evaporation of paste adhesives is defined as
a function of temperature, time and the degree of cure. Once
the mass evaporation is modeled, the relation between evapo-
rated mass and void formation is experimentally established,
completing the void prediction model for paste adhesives.
This model is then used to find the optimal strategy to acceler-
ate the curing process of paste adhesives without increasing
the void formation. This approach is applied to two different
paste adhesive systems in order to ensure the repeatability of
the research findings. The results show a reduction of more
than 85 % of the processing time compared to the reference
curing cycle without affecting the mechanical performance of
the joint. Finally, the methodology investigated is applied to a
real aircraft component, defining the guidelines to ensure the
robustness of the process under real component geometries.

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Zussammenfassung (Deutsch)
Seit den siebziger Jahren forscht die Industrie an neuen
Klebstoffsystemen zur Verbindung von Faserverbunden für
Strukturanwendungen. Kleben erlaubt leichtere Strukturen
und damit effizientere Lösungen als der heutige Standard in
der Luft- und Raumfahrt, das Nieten. Zudem erfordert die
Verwendung von Klebstoff kein Bohren der Komponenten,
was besonders vorteilhaft für das Fügen von
Faserverbundwerkstoffen ist. Typischerweise wird das
Aushärten von Pastenklebstoffen in Öfen mit erzwungener
Konvektion durchgeführt, wobei vom Hersteller empfohlene
Heizzyklen zur Anwendung kommen. Die Änderung der
Aushärtetemperatur während des Prozesses wird heutzutage
wegen ihres hohen Energieverbrauchs nicht berücksichtigt.
Um diese Beschränkung zu überwinden, wird in dieser
Doktorarbeit die Anwendung induktiven Heizens betrachtet,
um die Klebenaht lokal zu erwärmen. Diese Technik erlaubt
schnelleres Aufheizen und damit Ersparnisse an Ressourcen
und Zeit. In diesem Kontext wird die Beschleunigung des
Aushärteprozesses eines Pastenklebstoffs durch Erhöhung der
Heiztemperaturen während des Prozesses untersucht.
Die Steigerung der Aushärtetemperatur führt jedoch zu
Porenbildung und damit zur thermischen Degradation des
Klebers. Dieses Phänomen wird durch die Verdunstung von
Feuchtigkeit, die Degradation chemischer Komponenten und
die Ausdehnung während des Mischvorgangs
eingeschlossener Luft verursacht. Die erhöhte
Porenkonzentration beeinträchtigt die mechanischen
Eigenschaften der Klebeverbindung. Um mit dieser
Beschränkung umzugehen, wird in dieser Dissertation eine
neuartige Modellierung der Porenbildung während des
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