Random Decrement Technique for Modal Identification of Structures

Abstract

An algorithm is developed to obtain the free responses of a structure from its responses, due to some unknown or known input or inputs, using the decrement technique without changing time correlation between signals. The algorithm is tested using responses from a ' 'generalized pay load model and from the model. The resulting free responses are then used to identify the modal characteristics of the two systems. The method is limited to structures that are linear or have small nonlinearities. N general, the experimental identification of structural modes of vibration is carried out by measuring the input (or inputs) to the structure under test and the resulting responses due to this input. Some vibration testing techniques, in order to simplify the identification procedure, use the free responses of structures. In such cases, although the input excitation need not be measured, some initial excitation is applied to the structure, and free responses are measured immediately after the initial exciting force is removed. There are situations where controlled excitation or initial excitation cannot be used. For example, if the structure to be tested is in operation, applying any kind of external force may cause undesirable interruption. Another example is the case of in-flight response measurements, where a complete knowledge of the excitation is usually not available. In such cases, the use of the random decrement technique (a special averaging procedure that is used to determine the step and/or impulse response from the response) to obtain the free responses is a promising technique. The decrement signature technique1 has been used successfully for failure detection and damping measurement of structures in single-statio n, single-mode response cases. Application of the decrement signature technique to a multiple of signals changes the time correlation between the individual signals. If the resulting responses are to be used to identify several modes of a structure, the decrement signature technique must be modified to keep the time correlation between signals unchanged. In this paper, an algorithm is developed to obtain the free responses of a linear structure from its responses, due to some unknown or known input or inputs, using the decrement technique without changing time correlation between signals. The algorithm is tested by ap- plying it to responses obtained from two real structures. The first structure is a generalized payload model previously tested using sine sweep method and analyzed by NASA Structural Analysis (NASTRAN). The second structure is the V& -scale space shuttle model with modal parameters previously determined using sine sweep method and fast Fourier transform (FFT). Only responses from four stations on the solid rocket boosters (SRB's) were considered in the case of the space shuttle model. The filtered responses from these two structures were recorded and