Modal Identification of Ultralow-Frequency Flexible Structures Based on Digital Image Correlation Method

Traditional modal testing has difficult accurately identifying the ultralow-frequency modes of flexible structures. Ultralow-frequency excitation and vibration signal acquisition are two main obstacles. Aiming at ultralow-frequency modal identification of flexible structures, a modal testing method based on Digital Image Correlation method and Eigensystem Realization Algorithm is proposed. Considering impulse and shaker excitation are difficult to make generate ultralow-frequency vibration of structures, the initial displacement is applied to the structure for excitation. The ultralow-frequency accelerometer always has a large mass, which will change the dynamics performance of the flexible structure, so a structural vibration response was obtained through the Digital Image Correlation method. After collecting the free-decay vibration signal, the ultralow-frequency mode of the structure was identified by using the Eigensystem Realization Algorithm. Ground modal tests were conducted to verify the proposed method. Firstly, a solar wing structure was adopted, from which it was concluded that the signal acquisition using Digital Image Correlation method had high feasibility and accuracy. Secondly, an ultralow-frequency flexible cantilever beam structure which had the theoretical solution was employed to verify the proposed method and the theoretical fundamental frequency of the structure was 0.185 Hz. Results show that the Digital Image Correlation method can effectively measure the response signal of the ultralow-frequency flexible structure, and obtain the dynamics characteristics.

Modal testing and modelling the dynamic characteristics of a plate with bolted joints

Modelling the dynamic characteristics of the bolted joints in a complex assembled structure with a high accuracy is very challenging due to the assumptions and uncertainties in the input data of the FE model. In this paper, the identification of the dynamic characteristics of the bolted joints structure using the CBUSH element connector is proposed. Modal testing and normal modes analysis are conducted on a thin plate assembled structure with bolted joints. In the simulation work, the CBUSH element connector is employed and the stiffness coefficient for six degrees of freedom is computed based on four flexibility formulae. The predicted natural frequencies and their corresponding mode shapes are compared against the results of the experimental work. A good agreement of the FE model is achieved by using the coefficient of stiffness as represented in the Swift flexibility formula. The study justifies that the dynamic characteristics of the bolt joints could be accurately modelled by using the CBUSH element connector. The obtained findings provided an alternative approach to modelling the dynamic characteristics of a thin plate assembled structure with bolted joints.