Modal and frequency domain based techniques for Finite Element Model correlation

Abstract

It is common for Finite Element Model (FEM) validation activities to be carried out in the modal domain; using finite element modal analysis results and modal parameters extracted from test measured data. In order to obtain the required mode shapes and natural frequencies from the test measured Frequency Response Functions (FRFs), a modal parameter estimation method must be employed. An alternative approach involves correlation based in the frequency domain. In that case the FRFs would be considered directly from the test, without the need to apply advanced modal parameter extraction, and compared to FEM FRFs. In this work, sine sweep test-FEM correlations of two full spacecraft have been considered. A variety of the aforementioned modal parameter estimation methods have been applied and compared in order to establish which approach is most suitable to the considered applications. Findings highlight the issues associated with applying the simple 'peak picking' approach to modal correlation, and as such indicate that more sophisticated parameter estimation techniques often need to be employed in modal correlation. Frequency based methods therefore have potential inherent advantages as the issue of modal parameter extraction, and the associated introduction of further errors, is avoided. Frequency domain method also have several drawbacks, however, including the inflexibility of the FRF based methods to account for a deflected shape being well matched between test and FEM, but occurring at a slightly different frequency.