Calculated and Experimental Study of Free Vibrations of a Cylindrical Shell

At designing of thin-walled structures for aerospace engineering, it is vital to identify their frequencies and modes of free vibrations based on the results of frequency tests and the finite-element analysis. The paper defines calculated frequencies and modes of free vibrations of a cylindrical shell based on the finite-element model. Experimental frequencies and modes of free vibrations are obtained by the impulse excitation method. The authors used the Modal Assurance Criterion to evaluate matching of the calculated and experimental eigenmodes. However, such evaluation is complicated due to the limited number of sensors (to measure the mode and the frequency of free vibrations) and the presence of initial imperfections not taken into account in the calculated shell model. Based on the study of the Modal Assurance Criterion sensitivity to the difference of the relevant pairs of calculated and experimental vibrations of nodes, the authors have shown the influence of the initial imperfections on the eigenmodes. Based on the results of this study, the authors have identified nodes and areas of the shell with the maximum difference of the compared modes. By adjustment of the calculated vibration mode with regard to the experimental one in the circumferential direction, the authors have improved the values of the Modal Assurance Criterion. The location of the imperfection area and evaluation of its influence are used to specify the eigenmodes of the calculated shell model and to increase the modeling accuracy of the structure’s dynamic characteristics.