Effect of Fastener Preload on Structural Damping
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TECHNICAL ARTICLE—PEER-REVIEWED
Effect of Fastener Preload on Structural Damping Weiwei Xu • Daniel P. Hess
Submitted: 17 July 2013 / Published online: 29 August 2013 ASM International 2013
Abstract This paper presents results from an extensive set of frequency response measurements from a simple plate structure with a single bolt lap joint. The frequency response measurements are obtained from modal impact tests. Both damping ratios and natural frequencies are found to remain fairly constant for medium to high levels of bolt preload. At low levels of preload, damping ratios increase significantly by 25–75%, whereas natural frequencies decrease by only 1–2%. These results identify the monitoring of vibration damping as a means for detecting low preload and fastener loosening. Comparative tests with monolithic plate structures show the introduction of a single bolt lap joint results in an increase in damping and a decrease in frequency for the first three modes of vibration. Similar results are found for steel and aluminum plate materials with grade 5 and 8 fasteners. Keywords Fastener Bolts Joints Structures Damping Friction
Introduction and Background All structures exhibit some level of damping. In most cases, this is desirable because damping limits vibration in structures in buildings, vehicles, bridges, and aerospace systems. Fasteners are often used to connect or build up structures from numerous components. The resulting joints in a structure often provide a significant portion of the total
W. Xu D. P. Hess (&) Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620-5350, USA e-mail: [email protected]
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damping. This is generally true in steel and aluminum structures which inherently have low material damping. The mechanism of damping from a fastened joint results from the friction at the interface of joined components. Micro slip or gross slip at any interface in a joint result in dissipation of energy from friction. Such slip can occur repeatedly as a result of vibration induced from dynamic loads on the structure. It is generally appreciated that frictional slip dissipates energy and adds to structural damping. The effect of joint slip on damping (using logarithmic decrement measurements) and natural frequency of a cantilever beam has been studied [1]. Energy dissipation measurements have been used to model damping from bolted joints in structures [2–4]. Damping has been shown to increase nonlinearly with amplitude of slip [2]. A method of increasing bolted joint damping in structures by adding washers and thereby increasing the number of slip surfaces has been presented [5]. The effects of joint parameters on natural frequencies of vibration have been studied [1, 6]. The result of natural frequencies decreasing with fastener preload is observed, however, the decrease is usually quite small. Both rivets and threaded fasteners are used in structures and there are several excellent design guides or standards available for different t
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