Publications

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Some initial investigations have been published which simulate nonlinear response with almost traditional modal models: instead of connecting the modal mass to ground through the traditional spring and damper, a nonlinear Iwan element was added. This assumes that the mode shapes do not change with amplitude and there are no interactions between modal degrees of freedom. This work expands on these previous studies. An impact experiment is performed on a structure which exhibits typical structural dynamic nonlinear response, i.e. weak frequency dependence and strong damping dependence on the amplitude of vibration. Use of low level modal test results in combination with high level impacts are processed using various combinations of modal filtering, the Hilbert Transform and band-pass filtering to develop response data that are then fit with various nonlinear elements to create a nonlinear pseudo-modal model. Simulations of forced response are compared with high level experimental data for various nonlinear element assumptions.

This work presents a modal test on a cylindrical bolted structure that initially appeared to be a routine model calibration experiment. However, while reviewing the test data the structure appeared to have two pairs of ovaling modes with identical shapes. Assuming this to be the result of an uninstrumented component of the test article, extensive efforts were conducted to identify this feature. When all options were exhausted, the interaction between the structure and the air contained within was investigated. Contrary to the typical assumption that the fluid-structure interactions are negligible for such a thick walled cylinder, analysis showed that for this test article the acoustic modes of the internal air significantly impacted the structural response. In this case, the acoustic and the structural modes coincided in frequency, causing the first ovaling modes to split into two pairs at different frequencies. Experimental and analytical results are presented that describe this structural-acoustic mode coupling phenomenon.

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