Chapter 8 A Study on the Dynamic Interaction of Shock Response Fixtures and Test Payload Jesus M. Reyes and Peter Avitabile Abstract Shaker system physical characteristics (e.g. shake tables, modal shakers, and shock fixtures) vary from one laboratory to another. Parameters such as size and payload capacity, make every system’s dynamic characteristics unique. The commonly used control accelerometer cannot adequately adjust for the actual dynamic interaction/dynamic coupling effects between the test article and excitation fixture. Therefore, the payload/test article deformation will vary from test fixture to test fixture, making every test different—even with the same Shock Response Spectrum (SRS). In order to properly address the dynamic coupling and dynamic interaction that results, and to subject the test article to a vibration level that provides the proper excitation, customization of the shock response input is necessary. This work focuses on the identification as to how the article under test is affected by the attachment to the excitation fixture. Models are presented to show how the test article response is distorted by the dynamic interaction with the test fixture. Keywords Shock response spectrum • Mechanical shock environment • Shock • Mechanical excitation • Qualification testing • Dynamic coupling • Vibration exciter 8.1 Introduction Spacecraft structures, electronic components, packaging designs, automotive assemblies and military weaponry are a few examples of systems which are subjected to, in early design stages, dynamic testing because they are created/manufactured to accomplish great-risk tasks in high-energy environments. For instance, impact shock, transportation shocks, and, particularly, pyroshocks from multi-stage separation vehicles, naval and ballistics shock due to the extreme amounts of energy that they release [1]. Shock response data is obtained from these service environments and used for vibration testing; the frequency response of these environments is characterized by the Shock Response Spectrum (SRS) and is used to calculate the maximum dynamic response of structures. However, in practice, because the acceleration profiles are acquired while the structure is in service, the acceleration data does not contain any information regarding the dynamic characteristics of the excitation fixture. Depending on the particular dynamic characteristics of the test fixture (slip plate/shock plate) and if there are resonances in or near the frequency range of interest, the response of the Device Under Test (DUT) may be seriously altered [2]. In the past several decades, efforts have been made to account for the dynamic coupling and dynamic interaction between the DUT and the test fixture. McConnell [3] stated that a general theory of vibration testing is necessary, because very different laboratory results are acquired when field data is used as the excitation input; collected field data is needed for design engineers but the dynamic interaction between the test article and shaker/test fixture must be properly addressed. Several years later, Varoto [4] defined which measured responses and modal parameters are required to be used by the design engineers to account for the dynamic interaction between the test article and shaker/fixture system such that the actual field vibration is properly represented in the test. Oftentimes, in vibration testing, the correct level of response at the DUT is not obtained by just using the measured acceleration profile recorded during the shock event. In addition, the SRS can be formed from many different signals and may not expose the test article to the proper in-situ environment. Also, the DUT dynamic characteristics can be altered due to any dynamic interaction between the DUT and the shaker/test fixture; the changes, if any, in the dynamic characteristics J.M. Reyes (*) • P. Avitabile Structural Dynamics and Acoustic Systems Laboratory, University of Massachusetts Lowell, One University Avenue, Lowell, MA 01854, USA e-mail: jesus_reyesblanco@student.uml.edu #The Society for Experimental Mechanics, Inc. 2016 A. Brandt, R. Singhal (eds.), Shock & Vibration, Aircraft/Aerospace, Energy Harvesting, Acoustics & Optics, Volume 9, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30087-0_8 77
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