AN IMPROVED TECHNIQUE FOR SNAPBACK EXCITATION USING MULTIPLE FORCE INPUTS

摘要

This paper discusses an improved technique for snapback excitation that uses multiple static force inputs to individually excite the various modes of a test structure. The forces are applied such that the deformed shape of the structure approximates some mode shape of interest. When the multiple forces are simultaneously released, the desired mode dominates the free response of the structure. A series of tests was conducted on an eight-inch diameter piping system to evaluate the improved technique. The required static deflections for the first three horizontal modes of the piping system were obtained from the results of a finite element model, and from the results of an experimental modal analysis using impact excitation. Simultaneous release of all input forces was achieved with the use of explosive bolt release devices. Modal amplitudes, natural frequencies, and modal damping values were derived from free-response displacement data by using a simplex algorithm to fit the data with a multiple degree-of-freedom, time-domain model. Using a single force, only the first horizontal mode of the piping system could be excited in a dominant manner. Using multiple forces, it was possible to individually excite the second and third horizontal modes of the piping system, such that they dominated the free response of the system.