Chapter 17 Experimental Modal Analysis of Structures with High Nonlinear Damping by Using Response-Controlled Stepped-Sine Testing Taylan Karaag˘açlı and H. Nevzat Özgüven Abstrac t In the last decade, various promising nonlinear modal identification techniques have been developed based on the nonlinear normal mode (NNM) concept. Most of these techniques rely on the phase resonance testing approach where the identification of nonlinear modal damping is still an unresolved issue. The response-controlled stepped-sine testing (RCT) framework provides a convenient way of accurately quantifying nonlinear modal damping by applying standard linear modal analysis techniques to frequency response functions (FRFs) measured at constant displacement amplitude levels with standard modal test equipment. Various studies by the authors have shown that these constant-response FRFs come out in quasi-linear form even in the case of a high degree of nonlinearities. The RCT approach has been validated so far on several systems including a real missile structure with moderate damping nonlinearity mostly due to bolted connections and a microelectromechanical device with a stack-type piezo-actuator. This study makes a step further by validating the method on a real control fin actuation mechanism that exhibits very high and nonlinear modal damping; the maximum value of viscous modal damping ratio goes up to 15% and the percentage change of the damping with respect to vibration amplitude is about 70%. Keyword s High nonlinear damping · Nonlinear experimental modal analysis · Response-controlled stepped-sine testing · Control fin actuation mechanism · Unstable branch 17.1 Introduction The ever-growing industrial competition always favors higher speed, lower energy consumption, and longer service life in aircraft and turbomachinery. In the achievement of these high-performance goals, lightweight design becomes an important objective. However, the lightweight design naturally results in more flexible structures that may exhibit large amplitude oscillations under dynamic loads, which may eventually lead to dynamic instabilities such as the aeroelastic flutter [1] or the limit cycle oscillation (LCO) [2]. Structural damping plays a key role in the suppression of aeroelastic instabilities [3, 4]. Therefore, accurate modeling of the damping mechanism is vital for the determination of realistic instability envelopes and for pushing the limits in the design process. However, under large amplitude oscillations as in the case of aeroelastic instabilities, the structural damping exhibits highly nonlinear behavior, which makes its identification a challenging task. Almost all structural damping mechanisms in aircraft and turbomachinery are mainly due to friction. Friction and backlash in the actuation mechanisms of aircraft control surfaces, and special dissipative elements such as under-platform dampers in turbomachinery, are among important sources of structural damping. Another important source of damping common to both aircraft and turbomachinery is friction in mechanical joints. Identification of all these nonlinear damping mechanisms is already a difficult task as mentioned above. Another factor that complicates the identification process even more is the variability of nonlinear dynamics in mechanical joints in repeated testing [5]. The recent work of Al-Habibi et al. [6] gives a comprehensive review of the state-of-the-art techniques that can be used in the identification of nonlinear structural damping. Although it is possible to categorize these techniques in various ways, as is T. Karaag˘açlı ( ) The Scientific and Technological Research Council of Turkey, Defense Industries Research and Development Institute, TÜBI˙TAK-SAGE, Mamak, Ankara, Turkey e-mail: taylan.karaagacli@tubitak.gov.tr H. Nevzat Özgüven Department of Mechanical Engineering, Middle East Technical University, Ankara, Turkey © The Society for Experimental Mechanics, Inc. 2024 M. R. W. Brake et al. (eds.), Nonlinear Structures & Systems, Volume 1 , Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-031-36999-5_17 125
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