On The Equivalent Theories And Methods For Responses Of The Uniaxial And Triaxial Random Vibrations Of Missile Cabins

As a strategic strike force in modern war,missiles have attracted great attention since their birth,and gradually developed into the important weapons in modern war.At the same time,with the development of science and technology,the complexity of missile structures,and increasingly stringent use environment,the requirements for their quality and reliability are also becoming higher and higher.As a traditional engineering test to guarantee the reliability of weapon equipment,environmental vibration test is an indispensable part of missile development work.It has very important practical significance in many aspects,such as finding production defects,improving equipment reliability and reducing maintenance costs.According to a lot of engineering practice experience,random vibration test is one of the most effective environmental test methods to find potential manufacturing and process defects.In the early stage,due to the limited test conditions,the traditional random vibration environment test can only use the uniaxial sequential vibration test method,that is,the uniaxial vibration test is carried out in three orthogonal axes to approximate the real threedimensional vibration environment of the product.Obviously,the traditional uniaxial sequential vibration test cannot reproduce the multi-axis random vibration environment of the structure,resulting in some products that have passed the uniaxial sequential vibration test have premature failure under normal service conditions.With the progress of science and technology,the multi-axis vibration table technology has been developed rapidly,so the triaxial vibration test has gradually become the development trend of vibration environment test.But there is no authoritative standard of triaxial vibration test in the current domestic and foreign public environment vibration test standards and specification,if the triaxial vibration test is carried out directly using the uniaxial vibration test standard,the mature products may be over-tested in the triaxial vibration test,and even cause damage to the structure,thus affecting the normal service function of the equipment.Therefore,how to use the triaxial vibration table reasonably to carry out the triaxial vibration test so as to avoid "over-testing" and "under-testing" has become an urgent problem to be solved.Based on the existing standards of uniaxial vibration test,it is an idea to obtain the corresponding control spectrum or test time of triaxial vibration by equivalent clipping the control spectrum or test time of uniaxial random vibration test,aiming at the equal dynamic response of the structure under uniaxial random vibration and triaxial random vibration.The purpose of equivalent clipping is to ensure that the responses of the key components of the structure are consistent in uniaxial and triaxial vibration tests,so as to avoid over-testing of triaxial vibration and screen out potential defects concealed in uniaxial vibration.Clearly,choose the maximum stress of the key component as the equivalent index is one of the most direct and conservative schemes,and if the test requires screen out more potential defects as much as possible,the limit of the maximum stress of the structure under triaxial vibration should be appropriately relaxed within the strength safety range.But the maximum stress under triaxial vibration may have been close to yield limit of the material,fatigue failure is likely to occur in the process of triaxial vibration test.Therefore,it is also necessary to determine the equivalent index from the perspective of ensuring that the fatigue damage equivalence of the key components.In this dissertation,a frequency domain identification method of triaxial random vibration loading is proposed,which lays a foundation for the equivalent clipping of uniaxial and triaxial random vibrations.In an uniaxial vibration test standard for reference,the equivalent methods of uniaxial and triaxial random vibrations are studied.Based on the maximum stress equivalent and fatigue damage equivalent under uniaxial and triaxial random vibration,the control spectrum preserving clipping method and the double clipping method of control spectrum and test time are proposed respectively to provide guidance for the triaxial vibration test of missile cabin structures on the premise of avoiding over-testing.The main research results are as follows:The research contents of the dissertation mainly include three parts below:(1)A identification method of triaxial random vibration loading is proposed,and the simulation response control of triaxial random vibration is realized in frequency domain.The differences of responses of structure under uniaxial and triaxial random vibrations are quantitatively analyzed,which lays a foundation for the maximum stress and fatigue damage equivalence under uniaxial and triaxial random vibrations.Based on the theory of multiinput multi-output random vibration,the transfer function matrix of the structure is obtained by loading unit white noise in different axial directions to the structure.Then the triaxial random vibration loadings are obtained by using the relationship between random vibration excitation and response.Finally,the triaxial random vibration loads obtained by inverse calculation is loaded,and it is judged whether the three-axis random vibration response control is realized according to the control error.Taking the missile cabin structure as the research object,the finite element modeling is carried out and the correctness of the model is verified by triaxial loading test.At the same time,the frequency domain simulation response control of uniaxial and triaxial random vibrations of the cabin structure is realized by using the proposed load identification algorithm,which lays a foundation for the subsequent equivalent clipping works.(2)A shape preserving clipping method of control spectrum is proposed to achieve the maximum stress equivalent under uniaxial and triaxial random vibrations,which not only ensures the maximum stress equal under uniaxial and triaxial random vibrations,but also avoids over-testing caused by the triaxial vibration,which guarantee the normal service function of the structure.Firstly,the transfer relationship between excitation and von-Mises stress response at a point under the triaxial vibration is deduced,and then the ratio that the equivalent stress under triaxial vibration to the maximum equivalent stress at the equivalent point under uniaxial vibration is used as the clipping coefficient to clipping the root mean square value of the control spectrum.Secondly,in order to avoid over-testing or undertesting of key components,four different equivalent clipping criteria are proposed to ensure that the maximum von-Mises equivalent stresses of key components before and after clipping are the same.Then,considering that the equal root mean square values may correspond to the control spectrums of different shapes,and the obtained triaxial vibration control spectrum should meet the requirements of environmental test evaluation standards,a shape preserving clipping rule of the control spectrum is proposed,so that the shape,frequency bandwidth and slope of the triaxial vibration control spectrums are consistent with the uniaxial vibration control spectrums.It also provides a useful reference for clipping other types of control spectrums.Finally,an example of maximum stress equivalent clipping of missile cabin structure proves the correctness of the control spectrum shape preserving clipping method.(3)A double clipping method is proposed to achieve the fatigue damage equivalence under uniaxial and triaxial random vibrations by clipping both the control spectrum and the test time.It not only realizes the equal fatigue damage of the key components of the structure under uniaxial and triaxial random vibrations,but also avoids the over damage caused by the triaxial vibration,and improves the efficiency of the test.By defining the stress margin,the vibration intensity of the triaxial vibration test is quantitatively evaluated,and the control spectrum of the uniaxial vibration is properly clipped to avoid strength failure.Then,the control spectrum obtained is used to carry out triaxial vibration.By studying the calculation method of fatigue life of random vibration,the test time of uniaxial vibration is clipped to avoid over damage by aiming at achieving the fatigue damage equivalence at the key point under uniaxial and triaxial random vibrations.Considering that the selection of appropriate key points is the prerequisite to achieve reasonable equivalence,and the damage of a component in a complex structure will indirectly affect the stress distribution of other components.Therefore,taking the stress relationship between different components as the key element and comprehensively considering the importance of different components in the structural function,the determination criteria of three key points are put forward.Finally,the correctness of the double clipping method of control spectrum and test time is verified by the fatigue equivalent clipping examples of the notched specimen and missile cabin structures.