Abstract. The present paper explores the capabilities of a tensegrity-inspired tower with regard to frequency tuning by shape morphing. To change the configuration of the proposed structure, shape-memory-alloy actuators are used. This actuation principle also takes advantage of the variation of the elastic modulus of shape-memory alloys associated with the martensitic transformation. The temperature modulation of the shape-memory-alloy wires is successfully achieved by Joule heating, through a proportional-integral-derivative controller, to change between a low-temperature shape and a high-temperature shape. The implementation of a short-time-Fourier-transform control algorithm allows for the correct identification of the dominant input frequency, associated with the dynamic excitation. This information is used to automatically change the configuration of the structure in order to shift its natural frequency away from that of the dynamic excitation. With this frequency tuning, one obtains a reduction of the accelerations throughout the structure up to about 80%. The good performance of the proposed control approach gives promising indications regarding the use of tensegrity systems, in combination with shape-memory alloys, for shapemorphing applications, and, in particular, for self-tuning structures.
Increased demands on the capacity of the railway network gave rise to new issues related to the dynamic response of railway tracks subjected to moving vehicles. Thus, it becomes important to evaluate the applicability of traditionally used simplified models which have a closed form solution. Regarding simplified models, transversal vibrations of a beam on a visco-elastic foundation subjected to a moving load are considered. Governing equations are obtained by Hamilton's principle. Shear distortion, rotary inertia and effect of axial force are accounted for. The load is introduced as a time varying force moving at a constant velocity. Transversal vibrations induced by the load are solved by the normal-mode analysis. Reflected waves at the extremities of the full beam are avoided by introduction of semi-infinite elements. Firstly, the critical velocity obtained from this model is compared with results of an undamped Euler-Bernoulli formulation with zero axial force. Secondly, a finite element model in ABAQUS is examined. The new contribution lies in the introduction of semi-infinite elements and in the first step to a systematic comparison, which have not been published so far.
This paper addresses the Caughey Absorbing Layer Method (CALM) performance in the one-dimensional problem and its implementation in commercial software, with possibility of direct extension to twodimensions. The adequacy and numerical efficiency is evaluated using three different error measures and five different variations of the damping profile. Other parameters that are subjected to evaluation are the length of the absorbing layer in relation to the wavelength to absorb, the value of the loss factor at the end of the absorbing layer, and the ratio of the load to layer frequency. The problem is firstly analysed theoretically, resulting in estimates for the wave reflection due to transition and truncation of the model. In order to confirm that no spurious waves will be present in the finite element solution, the numerical implementation is validated by comparison with the analytical solution. The analysis of the error measures on the numerical results obtained for various combinations of the model's parameters lead to the conclusion that CALM is effective at mitigating waves reflected from the boundaries. The optimum loss factor as a function of the ratio of the length of the absorbing layer to the wavelength to absorb is determined through parametric analyses. Although the optimal damping is frequency dependent, it was shown that the CALM's effectiveness can be extended to a wider range of frequencies by increasing the smoothness of the damping profile.
This paper deals with a multi-objective optimization of geometrical and mechanical properties of a high-speed railway track when subjected to a moving load representative of a train axle. The model of the track employs several simplifications; it was implemented in two dimensions on the explicit dynamics module LS-DYNA for the finite element commercial software ANSYS. The design space is formed by the ballast height and by parameters representing the dynamic properties of the rail-pads. The objective function covers minimization of the maximum ascending and descending displacement and velocity in the main structural elements, namely the rail, the sleepers and the ballast. A genetic algorithm implementation is used to optimize these functions, which proved to be effective in finding quasi-optimal solutions with a low search effort. The tools presented can give insight into how the behaviour of the railway track is influenced by various design parameters.
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