Experimental Studies Of FRF Based Substructuring Technique

Today finite element method (FEM) is widely used in many engineering fields due to its convenience and high efficiency. But the error inevitably exists in modeling process when using the FEM and this error may cause the difference between the finite element model and the true structure. In order to obtain the exact dynamic behavior of the structure, physical test is often taken into account. However, for large scale and complex structures used in aerospace, aviation or ocean engineering etc, it is very difficult to carry out the full size ground experiment. A possible way to deal with this problem is to use the experimental substructure technique. That is, firstly divide the original structure into several parts based on the geometry or the different function of the structure, then perform test for the substructure with small scale, and finally use a certain synthesis method to assemble the information of all substructures to obtain the dynamic performance of the whole structure. In addition, the substructure technique is not only efficient but also fit the time/segment sectionalized manufacture process of modern industry. Therefore, the study on the substructure method is important both theoretically and practically.Substructure method can be classified into two kinds according to the dynamic model adopted: time-domain method and frequency-domain method. The first one uses modal model, namely the component mode synthesis, while the second one directly uses the frequency response function, namely the FRFs(frequency response function) based substructuring technique. This dissertation focuses on the latter. Actually, compared with the component mode synthesis, the FRF based substructuring technique has no limitation of number of main modes and does not need extra attachment modes. It only needs the integral FRFs of the degrees of freedom of interface. So the frequency-domain substructure method is more suitable for practical application.This dissertation presents theoretical and experimental studies on the FRF based substructuring technique, and discuss the existing problems and the solving method. The research was funded by the National Natural Science Foundation of China (Grant No.'s 10772112, 11072146), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070248032), the Key Scientific Project of Shanghai Municipal Education Commission (Grant No. 09ZZ17) and the Research Project of State Key Laboratory of Ocean Engineering of China (Grant No. GKZD010807). Some achievements obtained in both theory and applications are as follows:(1) The development and application of the experimental FRF based substructure techniques are comprehensively reviewed. The research extent and contents of this dissertation are put forward.(2) Numerical method and indirect measuring technique are studied for the pathological matrix and the non-integrity of interface degrees of freedom in the application of the FRF based substructuring technique. Using a spatial truss structure as object, the validity of theoretical methods are numerically verified through finite element simulation example. The effect of integrity of degree of freedom of interface on the results using the substructure method is discussed.(3) In order to gain the precise test data and the FRF of integral degree of freedom of interface, this dissertation deeply studies the experimental modal analysis techniques and the identification method of modal parameters, and apply them together with the indirect measuring technique to the study of the experimental FRF based substructuring technique.(4) This dissertation presents an experimental study of the FRF based substructuring technique for a spatial truss structure. Synthesis results indicate that dynamic characteristics of the structure may be effectively obtained using the substructure synthesis method, and the introduction of rotational degrees of freedom may improve the estimating quality.The Application of FRF based substructuring technique is a challenging research topic, where many aspects need further study and more efforts. At conclusion, a summary of work done in this dissertation is given out and some problems of interest are also brought forward for future research.