Dynamic Load Identification Based On The Structure Of The Modal Analysis Method

Structural dynamic load identification on the basis of modal analysis is an indirect method to measure the dynamic loads on structures, if only the structure's dynamic characteristic and its response are available. This technique is very effective, especially when it is being applied to measure the loads that can't be measured directly. This paper covers the theory and application of identifying dynamic load by direct-inverse method in frequency domain.This paper begins with the theory of modal analysis for multi-degree of freedom vibration system. The matrix of frequency response function for proportional damping system is given.To analyze the dynamic characteristic of stiffened plate/shell structures, a four-node quadrilateral plate/shell element is developed. The element is based on the Reissner-Mindlin plate bending theory and is eccentrically stiffened with beam element. To avoid shear locking when it is used to analyze thin plate/shell structures, the method of non-isoperimetric interpose is employed. In order to improve the computational accuracy of natural frequency of Mindlin plate/shell structures, a method is given when forming the mass matrix of Mindlin plate: taking the shear strain as the effective rotation into the kinetic energy of the system, making the mass matrix of Mindlin plate include the effect of shear deformation.There are two main methods to identify structural dynamic load, i.e. time domain method and frequency domain method. The basic theory of dynamic load identification in frequency domain is introduced in this paper.With the analysis of the characteristics of the matrix of frequency response function, the effective factors to the precision of load identification is analyzed theoretically, and relevant measures to improve the precision of load identification is also presented. In a computer simulation experiment, the effect of the number of modal order, the number of response test point, the test point position and whether the load position is known to the precision of load identification are discussed respectively.Finally a modal test and a response test under harmonic excitation of a simple support rectangle plate are conducted. The modal test results show that the computation program in this paper is valid. With the structural damp identified from the test results and the response of the structure under harmonic excitation, the load is identified and is compared with the actual load.