On Equivalence Of Stress And Fatigue Damage Of The Typical Missile Cabin Under Triaxial And Uniaxial Random Vibrations

Random vibration test is often used to assess the environmental adaptability and structural strength of the rocket and missile. The traditional vibration test usually use three orthogonal axes to approximate the multi axis vibration test. However, with the increasing complexity of products, the defects of the traditional single axis vibration are more and more obvious. It is widely proved that the multi axis vibration test can find the latent filure which is not found by the traditional vibration test, and simulate the actual vibration environment more realistically. But the standard of the triaxial vibration is not mature, it is urgent to establish a scientific and reasonable standard of triaxial vibration test. Studies have shown that if the traditional single axis vibration test spectrum is loaded to three axis shaking table for vibration test,may make the specimen over-testing,even will cause damage to mature product by single axis vibration standard design. The stress response and fatigue damage of the three axis vibration can be considered to be equivalent to the single shaft vibration, so that the two kinds of vibration have the same measure effect, it is necessary to cut the three axis vibration excitation spectrum.Aiming at the equivalence of the stress response and fatigue damage of triaxial vibration to that of uniaxial vibration, this paper cuts the load spectrum of the uniaxial vibration to provide a basis for establishing the triaxial vibration testing standards. Details are as follows:1. A study of the basic theories of structural dynamics is made, which mainly studied the inherent characteristics and relationship between the random excitation and response in the frequency domain. The structural finite element simulation technology is discussed, as well as the reliability of Ansys Workbench simulation results are demonstrated by structural modal simulation and random vibration simulation, which lays the foundation for the following analysis.2. Analysis of uniaxial and triaxial random vibration on the external beam by using Ansys Workbench, and the differences of the stress response under the two conditions are comparated. According to the relationship between input rms and output rms, the conclusion that linear system input and output satisfies linear relationship is obtained, which provide the basis for the cutting of the load spectrum. According to the relationship between the random vibration response and load in the frequency domain, this paper put forward a method for identify the random loads spectrum, and verify it's feasibility by using a simple model.3. The repairing and simplifying of the CAD/CAE model of complex cabin structure is studied, according to the actual connection of cabin structure, the contact type and the restricted locations of cabin structure are set up, then the finite element model of the cabin is established. The natural frequencies and vibration modes of the cabin structure are determined through the modal analysis. According to the acceleration response spectrum of the control point, basic acceleration load spectrum is obtained by using the theory of load identification. The differences of the cabin structure stress response under uniaxial and triaxial random vibration are comparated. According to the relationship between input and output, the cutting coefficient of triaxial vibration spectrum is established, and the triaxial vibration spectrum cutting plan based on the equivalent stress response is presented.4. A calculation method of random vibration fatigue is studied. The fatigue life of a cantilever beam is estimated through the finite element analysis software, and the reliability of the method is verified by comparing with the actual test results. Also the fatigue lifes of cabin structure under uniaxial and triaxial random vibration are calculated, and the relationship of the fatigue damages caused by triaxial random vibration and uniaxial random vibration is studied, and the triaxial vibration spectrum cutting plan based on the equivalent fatigue damage is established.