Some Considerations of Boundary Conditions in Dynamic Testing
Often times, tests are performed on structures to attempt to simulate a built in condition at the attachment point on the structure – or at least some type of fixity is desired. The actual implementation of this type of connection is not always easy to achieve. The use of a “large attachment stiffness” is one approach and is a possibility when only the lower order, first few modes are of interest and the fixture/attachment stiffness is much higher than the lower order modes of interest. But when this is not possible, then the use of a “large mass attachment” is another approach but when the structure does not have a low center of gravity (such as wind turbine blade applications), the mass inertia can be significant to simulate a built in fixed condition.
Several conditions are studied for the simulation of a built in condition using a large mass approach for a simple beam type structure. Models are presented to show the need for careful evaluation of the test set up so that the proper boundary conditions are achieved; in addition, tests are performed to confirm the model results obtained. Models are extended to consider wind turbine blades to understand the importance of identifying the proper test set up to achieve a particular boundary condition of interest; several test results are also included.
KeywordsMode Shape Beam Element Beam Model Mass Inertia Test Article
Some of the work presented herein was partially funded by NSF Civil, Mechanical and Manufacturing Innovation (CMMI) Grant No. 0900534 entitled “Dynamic Stress–strain Prediction of Vibrating Structures in Operation”. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the particular funding agency. The authors are grateful for the support obtained.
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