Doomed from the start, understanding the importance of boundary conditions in modal testing

Abstract

Modal survey tests are commonly performed in the aerospace industry to validate the structural dynamics behavior of the finite element model used in the flight loads predictions for both hardware structural capability and expected crew loads. During the planning phase, great effort is spent on the selection of accelerometer locations to properly capture the modes and satisfy orthogonality conditions. The same due diligence is not always spent on the design of the test’s setup and boundary conditions. Logistical and programmatic concerns may influence the technical approach taken, which, unfortunately, may result in test modes that are strongly coupled with boundary condition fixturing. The purpose of the paper is to present case studies of modal survey tests that were performed where the boundary condition influenced the test results. Additionally, best practices will be presented.