Broadband Dynamic Computation And Experimental Connenction Modelling With Finite Element Method

For satisfying ever raising demands on the structural dynamic computationaccuracy and the upper analysis frequency of spacecraft structures，methods forextending Finite Element Analysis (FEA) to the broadband frequency range andimproving the accuracy of conection structure modeling through experiments areinvestigated in this dissertation.One major difficulty in carrying out broadband vibration analysis for spacecraftstructure with conventional FEA is caused by the sharp increase in the number ofDegrees of Freedoms (DOFs). For avoiding this sharp increase in the number of DOFswith rational accuracy, an rthorder free interface modal synthesis method is proposed atfirst. With up to rthorder Residual Attachments Modes (RAMs) retained in thecomputation, sufficient influences of higher-order truncated modes can be included inthe computation to ensure the accuracy. Then a set of higher order interfacecompatibility conditions is introduced into computation to eliminate the lineardependence of the retained RAMs for ensuring computation accuracy and to deriveformula of complete structure only in terms of generalized modal coordinates forensuring computation efficiency. To reach a balance between the computation accuracyand the efficiency, a self-adapt algorithm for determining the exact value of r is alsosuggested. Finally, for broadband frequency response analyses, a modified modalcut-off criterion is proposed to reduce the number of the eigenpairs retained in thecomputation and hence improve the computation efficiency.To overcome the relative insufficient sensor difficulty caused by experimentalmeasurement noise, a non-model method for identifying the dynamic properties of pointtype connection structure with partially measured Frequency Response Functions (FRFs)is proposed. Four basic formulas are derived first and a method for estimatingunmeasured FRFs is developed as the second step of the identification procedure. Thento suppress the influence of measurement noise for overcome the relative insufficientsonsor difficulty, the four basic formulas are integrated together to form a united formformula for utilizing all available data, i.e. all measured and estimated FRFs. With thisapproach, the relative insufficient sonser difficulty can be overcomed and thus the identification accuracy can be improved significantly compared to exisiting methods. Anumerical example and an experimental example are provided to describe theidentification procedure and to show the effectiveness of the proposed method.For identifying dynamic properties of line type connection and area typeconnection structure, which usually encounters absolute insufficient sensor difficulty, anon-model method is proposed. The strategy of the method is to estimate the motioninformation, i.e. acceleration, velocity and displacement, of unmeasured DOFs andjunction forces firstly, and then connection stiffness matrix, damp matrix and massmatrix are constructed with these estimated motion information and junction forces.With this strategy, line type connection structure can be modeled; however, modelingarea type connection still faces challenge due to the dimension of the connectionstructure is too large to directly construct stiffness matrix, damp matrix and mass matrix.Thus an equal modelling method is developed to model such kind of connection withVirtual Structure (VS) and Virtual Connection Element (VCE). Compared to variouscurrent modeling methods with predefined or presumed connection models, theidentification accuracy of the proposed method does not depends on the accuracy of thepredefined or presumed models, and large-scale optimization procedure can also beavoided. An experimental example is provided to describe the identification procedureand show the effectiveness of the proposed method.