Dynamic Load Identification Under Structural Uncertainties Based On Ellipsoid Convex Set

In practical engineering problems,dynamic loads acting on structures are always the important basis for system fault diagnosis,structural design and optimization,structural strength analysis and fatigue life estimation.Therefore,the acquisition of dynamic loads is of great significance in many engineering problems.However,due to some technical reasons or considering the relationship between measurement cost,it is difficult for us to directly get the dynamic loads acting on the structures,such as the wind load on the tall tower,the road excitation on the moving vehicle and so on.Therefore,based on the established structural dynamic model,the identification of dynamic load through the response information of any measuring point has gradually become a way to indirectly obtain dynamic loads.Traditional dynamic load identification technology in only suitable for deterministic structures.However,in practical problems,there is a certain degree of uncertainty in engineering structure because of various factors.If the traditional deterministic inversion method is still adopted,the results often lead to large deviations and the inversion result of the load will lose reference value.At the same time,the dynamic load on the uncertain structure in not a specific value,but a series of load histories about various uncertain variables and time.Because of a certain degree of correlation between uncertain variables,we also consider the effect of different degree of correlation of load identification,which makes the uncertain structure inversion of dynamic load identification more complicated than the one on deterministic structure.In order to solve these problems described above,on the basis of the existing theories of dynamic load identification for deterministic structures,the thesis adopts the first order linear approximation expansion method,performance measure approach to research the methods of dynamic load identification under uncertain structures aiming at the correlation between uncertain variables.The work of this thesis is as follows:(1)While the uncertain structure is weak linear or small uncertainty,a dynamic load identification method based on first order Taylor expansion is investigated.In the time domain,the dynamic load is expressed as a function of time and uncertain parameters and the correlation approximation modeling method is applied to deal with the correlation between the various uncertain variables.Afterwards,on the basis of the convolution integral relation of deterministic structural dynamic response,a positive problem model for dynamic load identification of uncertain structures is established.Through the discretization of convolution integral,first order Taylor expansion is used to transform the problem of load identification for uncertain structure into two kinds of certain inverse problems,namely the identification of dynamic loads at the middle point of all uncertain parameters and the recognition of the gradient of the dynamic load with regard to each uncertain parameter.(2)Aiming at the situation of large uncertainty or high degree of nonlinearity.,a method based on performance measure approach is proposed.It is usually that the linear transformation of the n-dimension ellipsoid space composed of uncertain parameters is converted to the standard unit sphere space,and then structures the ultimate equation of dynamic load on the surface of a unit spherical by the Lagrange multiplier method.The derivation shows that the vector direction of the extreme point and the origin is collinear with the direction of the load gradient of the extreme value point.We take advantage of this condition to calculate the upper and lower bounds of the dynamic load efficiently and accurately.(3)On the basis of the uncertain structure load identification theory,the impact load test of the composite plate is carried out.Firstly,modal experiments are carried out to get modal parameters of composite plates,such as natural frequencies,mode shapes and modal damping,which are applied to establish and modify finite element models in simulation software.Secondly,the information of excitation that is exerted by a force hammer and acceleration response is collected by some sensors,then we can get the Green function of the load point to the response point from the modified finite element model which is applied to impulsive load.Finally,we will identify the impact load of composite plate,and evaluate it by real excitation.