Maximizing the Quality of Shape Extractions from Base Shake Modal Tests
The promise of extracting fixed base modes from structures mounted on shake tables is enticing since doing so allows a testing organization to save a considerable amount of schedule and money by reducing two traditionally separate tests into one. Oftentim
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Maximizing the Quality of Shape Extractions from Base Shake Modal Tests Kevin L. Napolitano
Abstract The promise of extracting fixed base modes from structures mounted on shake tables is enticing since doing so allows a testing organization to save a considerable amount of schedule and money by reducing two traditionally separate tests into one. Oftentimes, however, the modal analysis results are not of high quality because the test planning and conduct of base shake environmental tests are not conducive to performing a high-quality modal survey. This paper will discuss test planning and test conduct methods that can be used to maximize chances for successfully extracting high-quality mode shapes from structures mounted on shake tables. These methods are the same for any modal survey test; sensors should be placed to adequately observe and differentiate modes of a structure, and the excitation must be long enough in duration to adequately define high-quality frequency response functions (FRFs). Finally, methods to separate closely spaced modes by using multiple references, either with multiple-degree-of-freedom shake tables or a single-axis base shake test supplemented with modal shakers, will be discussed. Keywords Modal testing · Vibrations · Base shake · Environmental testing · Fixed base · Constraint shapes
12.1 Introduction There has been considerable literature discussing how to extract fixed base modes from structures mounted on shake tables [1–9]. Recently, methods have been developed to extract fixed base modes from structures mounted on flexible shake tables. One method applies a constraint equation to measured mass-normalized mode shapes to generate fixed base modes [10]. A second method uses base accelerations as well as constraint shapes as references to calculate frequency response functions (FRFs) associated with a fixed base [11]. However, in many cases these correction techniques are not required. In these cases, poor-quality modal results are often due to a perceived belief by a program that a modal test will take too much time or effort to conduct, and that “good enough” results can be obtained by analyzing frequency domain results from the environmental test itself. The reality, however, is that high-quality modal test results can be obtained with little extra on-site effort if the modal test engineer can advocate for a few small (or in some cases, more significant) changes in the base shake test. The modal test setup effort can also be performed in parallel with the environmental test setup effort to help reduce schedule. The objective of this paper is to provide the modal test engineer an outline of potential changes that they can advocate for in order to obtain high-quality modal results. Potential pretest analysis and test conduct adjustments to environmental tests will be discussed that can help a program extract high-quality modal information efficiently.
12.2 Pretest Analysis The objective of aerospace-quality modal tests of structures mounted on shake tables is to extract fixed base m
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