| Περίληψη: | Adhesive bonding is a desirable joining technique as compared to
conventional mechanical fastening, especially for aircraft components made
from composite or polymeric material, as it presents some significant
advantages including uniform distribution of the load, attractive strength to
weight ratio and design flexibility. However, the application of this
technology is limited to the assembly of less critical aircraft structures due to
limited strength of bonded joints, the requirement to redesign the joints so
that they are subjected to shear, difficulties in inspecting bondline quality,
sensitivity of bondline integrity to environmental attack as well as the
condition of the adherend surface prior to bonding which may result in the
formation of weak bonds at the interface. Particularly for the case of
aeronautical applications, the parameters which may act as contaminants,
thus leading to weak bonds, have not yet been fully identified and their
effects on the integrity of the bondline have not been quantified.
The present work aims to contribute on identifying these
contaminating factors and quantify the effect of weak bonds on the mode I
fracture toughness of adhesively bonded composite joints. The investigation
focuses on contaminating factors and resulting defects which are not
detectable by means of conventional non destructive testing techniques, such
as ultrasonic inspection or radiography. To accomplish this, an extensive
experimental investigation has been conducted. The contaminating factors
have been classified with regard to their origin, i.e. whether they are related to
the manufacturing process of the parts to be joined or to the in-service
conditions of the joined structure. Five different parameters have been
considered, namely pre-bond moisture, release agent, thermal degradation, a
hydraulic fluid and curing temperature. The above factors were found to
generally degrade the fracture toughness of the bonded joints; that was also
confirmed by the statistical ANOVA analysis that followed. Where feasible,
identification and quantification of the defects by means of advanced/hybrid
NDT techniques was carried out and a correlation between the results
obtained from these techniques and the mechanical testing results for every
investigated contamination scenario was made. To support and understand
the experimental results, an effort was made to relate the obtained results to
the underlying physics of the contamination process. Finally, a representative
aircraft service-related scenario concerning the synergistic influence of the quality-reduction factors studied herein and an environment of a specific
relative humidity and temperature, which is likely to happen during aircraft
service, was considered. The specific environmental conditions examined
herein were found to enhance the fracture toughness rather than degrading it.
The results of this study will contribute to i) the identification of a
number of parameters, related either to the manufacturing process or to the
in-service conditions of the aircraft structure expected to cause contamination
ii) the quantification of the degradation of mode I fracture toughness of the
composite bonded joints, iii) evaluation of the suitability of advanced/hybrid
NDT techniques for the detection of weak bonds caused by the factors
considered herein and assist on their further development, iv) understanding
of the underlying mechanisms resulting to the observed degradation of the
mechanical behavior, v) motivating the development of numerical models
capable of predicting the mechanical behavior of bonded joints under realistic
aircraft-related conditions, and finally vi) emphasizing the need for the
establishment of standards targeted at realistic aeronautics conditions under
which an adhesively bonded joint is loaded.
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