| Περίληψη: | The use of adhesive bonding in aircraft structures is continuously increasing due to the numerous advantages it provides over conventional joining techniques. However, the use of adhesive bonding technology is limited to joining and patch-repairing of secondary structures that are not load-critical, as the existing airworthiness certification requirements allow the use of adhesive bonding in combination with mechanical fasteners for primary structures.
According to these requirements, for any bonded joint, the failure of which would result in catastrophic loss of the airplane, the limit load capacity must be substantiated by one of the following methods:
1. the maximum disbonds of each bonded joint consistent with the capability to withstand the loads must be determined by analysis tests, or both. Disbonds of each bonded joint greater than this must be prevented by design features or
2. proof testing must be conducted on each production article that will apply the critical limit design load to each critical bonded joint or
3. repeatable and reliable nondestructive inspection techniques must be established that ensure the strength of each joint.
The 2nd and 3rd means of compliance were the subject researched in the present PhD dissertation, which was part of the ComBoNDT European research project (Quality assurance concepts for adhesive bonding of aircraft composite structures by advanced NDT).
The main objective of the present thesis was two-folded: (a) to evaluate experimentally the detailed effects of contamination scenarios related to the production and repair processes as well as the effects of hygrothermal ageing on the strength of composite bonded joints and repairs, and (b) to investigate the potential of the novel centrifuge testing method to characterize strength of bonded joints and to capture the effects of contamination.
To meet this objective, the subsequent specific objectives have been defined:
• Experimental characterization of the fracture toughness (Mode-I and Mode-II) of pre-bond contaminated composite bonded joints.
• Experimental characterization of the combined effect of pre-bond contamination and hygrothermal ageing on the Mode-II fracture toughness of composite bonded joints.
• Characterization of the effect of pre-bond contamination on the lap-shear strength of scarf composite bonded joints.
• Characterization of the effect of hygrothermal ageing on the bulk mechanical properties of the adhesive and the CFRP adherends.
• Centrifuge testing of reference and pre-bond contaminated composite-to-metal bonded joints.
• Application of a thorough methodology for the interpretation of the experimental results and the evaluation of the effects of pre-bond contamination and hygrothermal ageing on the strength of composite bonded joints by exploiting data from NDT and E-NDT (Extended-NDT) methods.
• Upscale of the effects of pre-bond contamination to bonded CFRP parts by numerical simulation.
• Development of a numerical model based on explicit FE simulation to simulate the centrifuge testing process and to apply the model to assess the role of loading, material and geometric parameters on the predicted adhesion strength.
As conditions on the aircraft production line and in the maintenance/overhaul shed are different, defects are categorized as either production-related or repair-related: Moisture (production), Fingerprint (production & repair), Release agent (production), release agent + Fingerprint (production), Faulty curing (repair), Thermal degradation (repair), De-icing fluid (repair), De-icing fluid + Fingerprint (repair). For each scenario, three different levels of contamination were pre-set and applied, namely low, medium and high level. These contaminants cause weak bonds which are not detectable by the conventional NDT methods.
The experiments were conducted following the requirements of international standard procedures in order to obtain reliable and consistent results; except from the centrifuge testing which is yet to be standardized. The experimental results were evaluated by implementing a methodology comprising the pre-bond contamination procedure, the statistical analysis methods and the data from the conventional and extended NDT (namely X-ray photoelectron spectroscopy, Optically Stimulated Electron Emission, Laser Induced Breakdown Spectroscopy, Computed Tomography and Ultrasonic Inspection).
For most of the investigated disturbances, a significant reduction of the mode-I and mode-II fracture toughness of the CFRP bonded joints was found. The reduction of the bond performance was greater under mode-II loading than under mode-I loading. The above results were also endorsed by the ANOVA statistical analysis and the failure surface characterization. The NDT and ENDT experimental results, added to the fracture toughness results by confirming the presence of the contamination on the CFRP adherend surface and on the bondline.
After-bond hygrothermal ageing under the prescribed conditions had no significant influence on the moisture absorption and load caring capacity of the joint as well as of the bulk adhesive. However, it influenced the fracture toughness of the CFRP adherends and the bondline integrity as it led to a drastic reduction of the mode-II fracture toughness of the CFRP sample and of the joint, respectively. These findings lead to the conclusion that the combined effect of the pre-bond contamination and after-bond exposure to hygrothermal environment is more detrimental than each contamination separately and can affect mainly either the CFRP adherend or/and the interface between the adhesive and the adherend.
Furthermore, the results of the tensile mechanical testing revealed that the thermal impact enhances the joint performance, whilst the additional presence of deicing fluid lead to a reduction of the lap-shear strength.
Additionally, the adhesion strength values measured by the centrifuge tests compare very well with both the fracture toughness tests and the adhesion strength values measured by the established and standardized butt joint test. This indicates that the centrifuge testing technology shows a great potential to be established as a test method for the characterization of pristine and defected bonded joints as it is a fast testing process which generates repeatable tests capable of describing the strength of the joints.
Numerical simulation of a composite panel stiffened with two T-stringers under compression was performed using the LS-DYNA FE platform in order to upscale the investigation of the effect of the contamination on realistic structural parts. The debonding growth was simulated using the CZM method and the parameters (GIC and GIIC values) used in the model of the panel were those of the contaminated bonded coupons measured experimentally. The comparison concerning the maximum load for the contamination scenarios showed that all contamination scenarios had a negative influence on the load bearing capacity of the stiffened panel.
Finally, the present thesis focused also on the development of a FE model in order to simulate the behavior of composite-to-metal adhesive joints loaded under centrifugal force. The debonding growth was simulated using the CZM method. A parametric/convergence study was performed to assess the effect of the explicit analysis time and select the optimum analysis time. Overall, the model presented adequate accuracy and agreement with the experimental results, simulating effectively the centrifuge experiment and contributing to its optimization by conducting parametric analyses different adhesive area and thickness values. The numerical results showed that there is a small effect of bondline area, while there is a significant effect of adhesive thickness. Specifically, increase of the adhesive thickness lead to a considerable decrease of adhesion strength.
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