Using Mode Shape Expansion Techniques for Pretest Analysis and Correlation

Abstract

When comparing analytical data from a FE model with experimental data from a modal survey test, the number of degrees-of-freedom in the analytical model is usually by far greater than the number of measured degrees-of-freedom. To overcome this problem, a reduction of the FE model is often considered to be the most reliable way. However, in some cases it is inevitable to expand mode shapes, for example in the presence of rotational degees-of freedom in the dynamic model. A situation which is frequently encountered in aircraft simulation. The paper evaluates the benefits of several mode shape expansion techniques like the SEREP method, a purely geometric interpolation scheme and a generalized static expansion method. This is done not only in view of correlation purposes after the test, but also for pretest analysis. Some of the methods can be used to assess the quality of the accelerometer plan and, thus, to optimize the set of measurement locations. Using two examples, a simple beam model and an aircraft model, it is shown that the generalized static expansion technique provides a good trade-off between accuracy, simplicity and robustness with respect to noise in the measured data and errors in the FE model. It also provides an excellent measure for the quality of the pick-up plan