Chapter 3 Mechanical Environment Test Specifications Derived from Equivalent Energy in Fixed Base Modes Troy J. Skousen and Randall L. Mayes Abstract The main point of mechanical environment testing is to prove that designs can withstand the loads imparted on them while being exposed to in-service conditions. This is dependent not only on the test article construction, but also the loads imparted through its boundary conditions. Current practices for developing environment test specification are typically based on inadequate information reduced to single input point control with large uncertainty as compared to the field environment. Yet the test specifications are considered conservative, with the assumption that most of the adjustment for uncertainty is conservatism. For base mounted components, a modal model is presented that can be used to generate specifications with much lower uncertainty and with guaranteed quantifiable conservatism. In this method, the modal energies in the fixed base modes of the article due to the in-service loads are determined. Using the fixed base modes of the test article as a basis, the test specification is derived by determining what fixture motion is required to emulate the inservice environment. The specification method accounts for frequency shifts between the in-service and test configurations. Variability in nominal test articles can be included in the derivation of the test specifications. Real hardware under in-service environment loads and in a ground test fixture and loading configuration are considered. Keywords Vibration test specifications · Component qualification · Modal energy · Fixed-base modes · Multi-degree of freedom 3.1 Motivation The qualification of designs to dynamic mechanical environment loads requires specifications for laboratory testing. Often the test specification is provided as a simple straight-line acceleration spectral density profile that is drawn to conservatively envelope the raw acceleration spectral density from a sensor intended to represent the input to a component. The acceleration measurements from the system environment are often made at different locations than the component connection points, therefore, the measurements are not a good representation of the base input to the component. In addition, the connection response has three rigid body translations and three rigid body rotations and sometimes elastic motion, some of which are completely ignored. Thus, the correct input to the test has a great deal of uncertainty. The specification is generally considered conservative, but it is rare that the conservatism is quantified. We recognize that it is unfeasible to measure every degree of freedom of response that is needed to generate a complete description of a specification for a certain system environment. In many systems, sensors cannot be mounted at desired locations due to physical space and data bandwidth constraints. The approach implemented in this paper uses research hardware known as the Modal Analysis Test Vehicle (MATV) developed at the Atomic Weapons Establishment (AWE) with the Removable Component (RC) hardware from the Boundary Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. T. J. Skousen ( ) Mechanical Environments Engineering, Sandia National Laboratories, Albuquerque, NM, USA e-mail: tjskous@sandia.gov R. L.Mayes Structural Dynamics, Sandia National Laboratories, Albuquerque, NM, USA © The Society for Experimental Mechanics, Inc. 2021 A. Linderholt et al. (eds.), Dynamic Substructures, Volume 4, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-47630-4_3 27
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