By means of PZT pickups, we experimentally measured the natural frequencies of an aluminum thin-walled open profile under centered compression, and investigated the effects of end warping constraints on free vibration and buckling loads. The specimen was mounted on a MTS testing machine, controlling the axial displacement imposed to a beam end by the hydraulic jack. The free vibration frequencies were detected for different values of the compressive force. The experimental results are compared to those provided by an in-house numerical code, which investigates the elastic stability of thin-walled beams in a dynamic setting. Indeed, the code is able to follow the paths of natural frequencies versus the applied load, accounting for the effect of cross-sectional warping on both natural frequencies and buckling loads. The experimental results show that piezoelectric pickups, like the disk adopted, can be efficiently used for the experimental modal analysis of engineering structures.

Compressed thin-walled beams with open section are prone to torsional, or flexural-torsional, buckling. Here we present the results of several studies where, by piezoelectric pickups and a universal testing machine, we experimentally detected the natural frequencies and buckling loads of centrally compressed aluminum thin-walled beams with open cruciform section, exhibiting remarkable warping stiffness. We detected the free vibration frequencies for different values of the compressive force and for both free and (at least partially) restrained warping of the end sections. We compared the behavior of integer elements to that of analogous beams, where we introduced a localized damage, i. e., a sharp variation of a cross-section. For the integer elements, we compared the experimental results with those provided by an in-house numerical code, which investigates the elastic stability of possible non-trivial paths of thin-walled beams in a dynamic setting. The results show that, on the one hand, piezoelectric pickups can be efficiently used to extract modal parameters of structural elements; on the other hand, the numerical code proves to be robust and accurate in the determination of the buckling loads of the integer elements, in all the analyzed configurations.

Piezoelectric disk buzzers are commonly used on stringed musical instruments to acquire the sound in the form of a voltage signal. Aim of the present investigation is to assess the possibility of using these transducers for experimental modal analysis. Piezoelectric disks were therefore used in the laboratory to extract the natural vibration frequencies and mode shapes of an aluminum cantilever beam and of a steel arch. The results are compared with theoretical predictions and with other experimental values obtained using a laser displacement transducer and accelerometers. Due to their high accuracy, small dimensions, low weight, easy usage, and low cost, piezoelectric disks seem to be an attractive tool for experimental modal analysis of engineering structures.

During the last few years, some investigators reported interesting results regarding neutron emissions from ultra-sonic cavitation in liquids and solids. In the present paper, the described experiments were conducted in order to evaluate neutron emissions from liquids subjected to hydrodynamic cavitation by an hydraulic circuit prototype. In particular, different aqueous iron salt solutions were tested in order to correlate neutron emissions and evolution of chemical element concentrations after different operating hours. The experiments were conducted using an hydraulic circuit fine-tuned by the authors. The pilot plant, which also includes the power supply and the electronic control of the recirculation pump, was realized entirely by plastic material (with the exception of the centrifugal pump and the hydraulic cavitator). The pump will be equipped with a system of six stages and with a maximum flow rate of 6 m3/h. The maximum working pressure is equal to 10 bar. The evidence obtained during the tests returned an appreciable neutron emission, about 30 % greater than the background level. A significant decrement in Fe concentration was detected at the end of the test, whereas a considerable amount of aluminum—previously absent—was found on the internal walls of the pipe.

We investigate the effects of steady aerodynamic loads on stability and natural frequencies of long-span suspension bridges through a simplified analytical model. The single (central) span suspension bridge model is considered, and the linearized integro-differential equations describing the flexural-torsional deformations of the bridge deck-girder are adopted as starting point. Thus, taking into account the second-order effects induced by a constant transverse wind in the bridge equations of motion, we derive a generalized eigenvalue problem in which all configurations intermediate between those of pure lateral-torsional buckling, pure torsional divergence, and pure free vibrations can be investigated. We show that the natural frequencies of a suspended deck-girder depend upon the mean (quasi-static) wind loading. As a consequence, the input parameters to the aeroelastic stability analysis result affected by that dependence, suggesting the possibility of modifying the dynamic stability analysis in order to take into account the mentioned influence. Based on this fact, possible implications for the flutter analysis of long-span suspension bridges are discussed.

The influence of applied axial loads on the fundamental vibration frequency is strictly connected with the stability analysis of elastic slender beams. For this reason, the correct evaluation of the fundamental frequency is of primary importance in designing new structures and components, as well as in monitoring existing ones. At the same time, if an internal axial load arises in a slender element as the consequence of an imposed (static) axial end displacement, then a different dynamic structural response is encountered respect to the case in which a beam end is free to slide, during transverse vibration, and a (constant) axial load is applied externally. This difference is due to the change in the axial boundary condition. Moreover, the presence of an initial curvature of the beam axis may significantly affect the aforesaid response. The experimental study proposed in the present paper investigates the dependence of the fundamental frequency on the axial load in slender beams subjected to imposed axial end displacements. The considered specimens presented different geometrical imperfections (initial curvatures), and were tested in two different constraint conditions (hinged–hinged and hinged–clamped). In addition, the behaviors observed during the experiments were reproduced by numerical simulations offering a valid confirmation for test results and contributing to understand the evolution of the fundamental frequency in the analyzed slender elements subjected to imposed axial end displacements.

The dependence of the fundamental frequency on the axial load in slender beams subject to imposed axial end displacements was experimentally investigated. The considered specimens presented different geometrical imperfections (initial curvatures), and were tested in two different constraint conditions (hinged–hinged and hinged-clamped). The natural frequencies were extracted in both conditions of forced and free vibration, using an electromagnet and a laser displacement transducer. In addition, the responses observed during the experiments, for the hinged–hinged case, were reproduced by numerical simulations, obtaining a good agreement between numerical and experimental results.

In the present paper, the interplay between vibration and stability in axially loaded beams and trusses is analyzed through experimental and numerical studies. Firstly, an experimental research on the fundamental frequency evolution in slender beams subjected to imposed axial displacements is presented. Different geometrical imperfections were analyzed, leading to distinguish two different phases in the fundamental frequency vs. axial load curves. We therefore present a multibody dynamic simulation of these experimental tests, showing a good agreement between numerical and experimental results. Lastly, we present results from finite element modeling of elementary space trusses subjected to a dead load, in terms of natural frequencies vs. load curves. In particular, we investigated the effects of direction and magnitude of the applied force on the natural frequencies and vibration modes of tripod-shaped trusses, comparing different constraint conditions and several geometrical configurations. Results show interesting aspects of the dynamic behavior of axially loaded elements, such as softening vs. stiffening structural responses and particular relationships between natural frequencies and buckling loads