A Vibration-Based Method for assessing the integrity of welded structures is presented. The proposed technique relies on fitting theoretical predictions of a lumped mass-based model of the welding with experimental measurements obtained from modal analysis. Experiments are performed in various test platens that are soldered at one or two locations and exhibit distinct welding qualities, achieved by using different control parameters in the welding process. The inertia added during the soldering execution is modelled by a lumped mass that connects to the structural element via linear springs. The associated rigidities play the role of fitting parameters and are adjusted to reproduce the (three) measured natural frequencies by means of optimisation algorithms. This method is successful in characterising the platens tested and provides an accurate and consistent (with the expected welding quality) quantification of their effective mechanical properties at the bond. We note that this is beyond the qualitative evaluation achieved with other extended Non-Destructive Testing techniques. Such characterisation is possible even without particular knowledge of modal shapes, whose experimental determination is difficult in any real structure, although different resonant frequencies must be considered to achieve reliable estimates. Based on the present results, we suggest that this technique can be used for assessing the overall integrity of welded structures in a quantitative and a reliable manner.
In this work, the aeroelastic stability of an aerial refueling system is investigated. The system is formed by a classical hose and drogue, and the novelty of our work is the inclusion of a grid fin configuration to improve its stability. The unsteady aerodynamic forces on the grid fins are determined using the concept of a unit grid fin (UGF). For each UGF, the unsteady aerodynamic forces are computed using the Doublet-Lattice Method, and the forces on the complete grid fins are calculated using interfering factors obtained from wind tunnel measurements for the steady case. The static equilibrium position of the system influences the linearized perturbed unsteady motion of the ensemble. This effect, together with the phase lag angle introduced to account for the unsteady aerodynamic forces in the hose, makes the flutter computation of the complete system a non-typical one. The results show that, by adding the grid fins, the stability of the refueling system is improved, delaying or annulling flutter occurrence.
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