| Thin cylindrical shell (TCS) has the advantage of light weight, superior carrying capacity and good applicability, which is widely used in practical engineering such as outer shells of submarines, torpedoes and missiles, as well as rotary drums in steam turbines, gas turbines and aircraft engines. TCS is often working in complex-field coupling environment, such as vibration, noise, high temperature and high-speed flow erosion, which may easily cause high level vibration and fatigue failure and lead to severe security accidents. Therefore, it is of great scientific significance to study the vibration characteristics of TCS.Modal parameters, such as natural frequencies, modal shapes and damping ratios are crucial for further studying the vibration characteristics of TCS. These modal parameters play a key role in theoretical modeling, simulating calculation, response predicting, anti-vibration optimization, vibration mechanism studying and structural failure identification. Even though abundant research efforts have been focused on vibration-related problems of TCS, such as natural characteristics, damping characteristics, dynamic response, stress and complex boundary conditions, most of studies are done under the ideal free boundary, simply supported boundary or some kind of constraint boundary condition of cylindrical shells, and the majority of them are only at simulation level.Despite some scholars have done experimental researches, these researches are mainly for investigating the vibration response of TCS and validating some theoretical methods instead of systematically studying the testing methods related to modal parameters. In practice, due to the different boundary conditions, vibration excitation modes and response measurement techniques, the resulting modal parameters obtained by experimental test will be heavily affected. Meanwhile, during the vibration test of TCS, some nonlinear problems such as jumping, nonlinear stiffness characteristics are also found under coupling boundary and elastic boundary conditions, which makes the traditional modal test methods unable to meet the requirements of experimental research. Therefore, it is necessary to explore a suitable modal test method of TCS, and exclude those techniques as well as the related equipments which cannot meet the demand of the test, especially design and assemble appropriate test systems, propose reasonable test procedures, and improve the test efficiency as much as possible.On the basis of a good understanding of the structure and vibration characteristics of TCS, an advanced measurement method of modal parameters is proposed in this paper. Compared with the traditional modal test method, this method has significantly improved the test accuracy and efficiency and enlarged its application field. The contents of this paper are as follows:(1) Because it is difficult for traditional excitation equipments to effectively excite the high-frequency modes of TCS, a high natural frequency test system of piezoelectric ceramic is designed in this paper with considering the advatage of high-frequency excitation of piezoelectric ceramic. Meanwile, high natural frequency test method of TCS under piezoelectric ceramic excitation is also proposed. By using this test method, the natural frequencies of TCS with constraint at one end are measured, and Up to 60 natural frequencies have been obtained within the range of 0-12kHz. Therefore, the actual test results have proved that this system can well satisfy the requirements of testing high natural frequencies (above 6kHz) of TCS.(2) Because traditional vibration test method is of low efficiency and accuracy in the process of obtaining modal shapes of TCS with dense frequency characteristics, a laser rotating scan system for testing modal shapes is designed in this paper and a fast modal shape test method of TCS is proposed based on the above rotating scan system. The test results indicate that the modal shapes obtained with laser rotating scan method have higher accuracy compared with traditional modal shape test method which is based on the measurement of frequency response function. The curves obtained with laser rotating scan method are much smoother, less time-consuming and of high space test resolution, which can also be applied to efficient vibration response test field of shell structures.(3) For the problems existing in testing the time-domain and frequency-domain damping ratios of the shell structures, three high-accuracy modal damping ratio test methods of TCS under base excitation are proposed in this paper, such as sliding-envelop method, frequency bandwidth method and nonlinear stiffness bandwidth method. The sliding-envelop method greatly improves the test accuracy of damping ratios based on time-domain attenuation signal; The frequency bandwidth method and the nonlinear stiffness bandwidth method are optmized from the traditional half-power bandwidth damping test method, which enables it to effctively obtain the damping of shell structures with dense frequency or nonlinear stiffness characteristics. Comparing with the damping results obtained by half power bandwidth method, these three damping test methods have good repeatabilities, and the frequency bandwidth method have better repeatability in obtaining low-order dampings of shell structures. Therefore, experimental test on modal damping of TCS should give preference to frequency bandwidth method. AdditIonally, for the test on modal damping of shell structures with nonlinear stiffness characteristics, nonlinear stiffness bandwidth method should enjoy top priority, especially in the situation of obvious nonlinear stiffness characteristics.(4) The proposed advanced modal parameter test method of TCS is applied to experimental studies on different coupling boundary condition and elastic boundary condition. Modal parameters of shell structures are obtained under the previous two complex boundary conditions, and further research work has been done to determine the change of boundary condition as well as its influence on the natural frequencies, modal shapes and damping parameters of shell structures.This research can contribute to establishing an accurate dynamic model of thin cylindrical shell component, providing technical support for improving the caculation accuracy of theoretical methods under complex boundary conditions, and offering important references on anti-vibration optimization of shell structures under constraint boundary condition in practical engineering. |
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