Experimental investigation of the effect of defects, non-detectable by means of conventional non destructive techniques, on the mode I fracture toughness of adhesively bonded composite joints

• Κύριος συγγραφέας: Μαρκάτος, Διονύσιος
• Άλλοι συγγραφείς: Παντελάκης, Σπυρίδων
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2016
• Θέματα: Adhesively bonded joints; Kissing bonds; Συνδέσεις με κόλλα; Μη ανιχνεύσιμες ατέλειες; 629.134 6

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.