Purpose. The purpose is to study free vibrations of thin-walled bar with combined bisymmetrical section allowing for the effects of shear strains. Methods. We used the main hypothesis of V.Z. Vlasov concerning thin-walled bars. Solving motion equations analytically, we used the method of initial parameters and the known methods for differential equations solution. In the natural experiment we applied Autodesk Inventor system. In the course of the natural experiment on shaker, we used the method of smooth change in the frequency of sinusoidal vibrations. Results. We provide the analytical solutions of the shapes and the natural frequencies of thin-walled bar with combined bisymmetrical section allowing for shears caused by bends and constrained torsion. We compared the results of the calculations conducted in CAE system and the results of the full-scale experiment to determine the frequencies of vibrations of box beam. Conclusions. The results showed that the analytical calculations are in good compliance with the results of numerical and physical experiments and can be used for dynamic calculations of thin-walled structural elements, in particular, for the prevention of the destruction of structures in the event of resonance phenomena.

The paper presents the development of approaches to the crack detection in metal rod structures based on the analysis of the lowest eigen-frequency modes. Full-scale experiments and numerical calculations are carried out, and the obtained results are compared. A vibration analyzer is used for full-scale experiments, and numerical calculations are performed by using Autodesk Inventor. With regard to the internal friction, the antinodes of various vibration forms were identified using a specially developed program. The model includes sensors for the the field experiment as masses affecting the frequency-response characteristics. The dependences are obtained for eigen-frequencies in the presence of cracks and for the crack locations. The polynomial dependences of the crack location on the lowest eigen-frequency modes of the rod can be used to analyze the crack position of in cantilever beams.

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Vibrations of a thin-walled cantilever rod of a box-shaped profile were studied. The natural vibrations frequencies of a rod with a crack of different depth and located in different places of the rod were determined. The method of conducting an experiment using a vibration analyzer is given. A variant of determining the location of the crack depending on the change in the natural vibrations frequency is proposed. The resulting expression for estimating the location of the crack has an error of no more than 2.7 percent of the length of the rod, which makes it effective in practical application. The puchnost points of each oscillation form of the cantilever rod are determined, taking into account internal friction, which allowed us to specify the locations of sensors for maximum efficiency in evaluating the influence of crack parameters on natural frequencies. We found that when the crack is located in the places where the puchnosty for the corresponding vibration form causes noticeable energy dissipation due to friction in the crack.