Understanding the Effect of Preload on the Measurement of Forces Transmitted Across a Bolted Interface Charles Butner1 Research Assistant Douglas Adams1 Professor Jason Foley2 Research Engineer 1Purdue University, Center for Systems Integrity, 1500 Kepner Drive, Lafayette, IN 47905 2Air Force Research Laboratory Munitions Directorate, Fuzes Branch, Eglin Air Force Base ABSTRACT Often in the study of bodies that undergo shock loading, it is desirable to measure the response of such bodies with an instrumentation package. This instrumentation can be separated from the external housing by several preloaded interfaces. To better understand the effects of preload on the nonlinear dynamics introduced into the measurement, a simple preloaded interface has been fabricated that consists of an upper smaller mass body bolted at three locations to a lower, larger mass body. A finite element model of the fixture was used to study the modal characteristics of the individual components and the coupled bodies. A series of modal impact tests were used to analyze the effects of a varying preload between the bodies on the linear and nonlinear features observed in the dynamic response of the coupled bodies. High amplitude shock loading was also used to understand if the laboratory results would be indicative of a more realistic loading scenario. The results of these measurements lead to the determination that a simple two degree of freedom model could be used to explain some aspects of the system. INTRODUCTION AND BACKGROUND A common problem in vibrations testing is to identify and manage uncertainties associated with the mounting condition of the sensor. In a particular representative problem currently being studied, a tri-axial sensor mount is bolted to a larger body. The goal of the sensor is to measure the response of the body to which it is attached, but the bolted interface between two such bodies introduces both linear and nonlinear features in the sensor data, and leads to errors in the measured response of the body of interest. To study this particular interaction, a test fixture was designed and built to simulate the interface. The fixture design is illustrated in Fig. 1. Cross Section of Bolted Interface Fig. 1 Bolted Interface Test Fixture Used for Experiment T. Proulx (ed.), Modal Analysis Topics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series 6, 567 DOI 10.1007/978-1-4419-9299-4_48, © The Society for Experimental Mechanics, Inc. 2011
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