| Περίληψη: | In the present thesis a thorough investigation regarding the mechanical response of several types of composite coupons, composite or hybrid fastened joints and a hybrid large-scale panel with several fasteners, is conducted under dynamic (impact) and quasi-static loading conditions with the aid of efficient experimental and numerical methodologies developed for these purposes.
With respect to the experimental activities, the quasi-static and dynamic (medium strain rate regime) tests are performed using a conventional servo-hydraulic frame and a drop tower machine in conjunction with special devices and jigs that are designed and manufactured in the frame of the current work. Special efforts are made on the development of a novel tensile testing apparatus which has the ability to transform the compression loading of a drop weight system into tension loading on the specimen. Due to the complexity of the impact phenomena that are related to the tensile apparatus function, the sizing of its members, the identification of the optimal positions of the force and displacement sensors, as well as the investigation of the critical dynamic testing conditions which should be satisfied, are all performed with the support of a three-dimensional finite element model of the apparatus. The experiments, which are conducted in a range from quasi-static to 4.6 m/s impact velocity, show small to medium loading rate sensitivity concerning the ultimate load values for the laminate and joint level configurations, while in some fastened joint cases, pronounced differences in the failure evolution and modes among the various loading rates are observed.
Within the finite element framework, efficient numerical modelling methodologies are developed for simulating the dynamic response of the laminated specimens and the composite/hybrid fastened configurations. These numerical methodologies, the development of which is the main scope of this thesis, can be classified into two main categories, depending on their applicability on the various structural levels that are investigated in the current work. The first modelling methodology is based on the stacked shell approach for the representation of the laminate part of the simulated coupons, while the metallic components (e.g. fasteners) are modelled using solid elements. Although this approach is very time efficient, it can predict the laminate and the fastened coupons impact response in a very accurate and detailed manner; in particular, the developed stacked shell models are validated against the relative experimental data, which are extracted using the unique devices mentioned in the above paragraph, showing very good correlation in terms of stress – strain and load – displacement curves, as well as on the prediction of the failure initiation, the damage progression and the final catastrophic failure modes. In addition to this modelling technique, a second finite element modelling methodology, which is referred as ‘macro-modelling approach’ and is more efficiently applied in large- and full-scale structural components, is developed in the frame of this thesis. In the latter approach, the fastened areas are mainly represented by three components, i.e. the two plates (laminate or metallic) meshed with one layer (each plate) of shell elements and the fastener which is modelled with linear beam elements. The macro-modelling approach is evaluated against a three-dimensional numerical solution and analytical spring-based macro-models using a benchmark single-lap single-bolt composite joint configuration. Afterwards, this approach is adopted for the modelling of a hybrid composite-metallic panel (representative part of a rear fuselage section) with eighty fasteners in a multi-row and multi-column arrangement, in order to predict its behaviour under quasi-static and transient compressive loads; the model is validated against respective experimental and reference numerical results, in terms of load – displacement curves and buckling mode shape/location prediction.
|
|---|
