Ultra-low frequency vibration isolation of a novel click-beetle-inspired structure with large quasi-zero stiffness region

•We innovatively provide a click-beetle-inspired structure (CBIS) with variable asymmetric stiffness characteristics.•The structure of CBIS is rather simple but the geometric nonlinearity is extremely abundant.•Asymmetric zero or quasi-zero stiffness region is extremely large and can be tuned easily in terms of three different postures of CBIS.•The peak transmissibility of CBIS is extremely low (down to 0 dB) and the resonant frequency is below 2 Hz. Inspired by the three postures of a click beetle, a novel passive click-beetle-inspired structure (CBIS) with variable asymmetric stiffness characteristics is proposed and systematically investigated for stiffness tunning and low-frequency vibration isolation. The main structure of CBIS is consisted of three rods and two springs but the geometrical nonlinearity and stiffness properties are abundant. Based on the Lagrange principle, the stiffness properties of CBIS is analyzed comprehensively for the unbending posture, down-bending posture and up-bending posture, respectively. The corresponding displacement transmissibility derived with the harmonic balance method. The hardening and softening stiffness phenomenon may occur by tuning geometric parameters. The stiffness can be tuned easily among negative, quasi-zero, zero, small positive by the initial assembly angle, length ratio and linear stiffness ratio. The zero stiffness (ZS) or quasi-zero stiffness (QZS) region can be extremely huge under some special parameters, which can broaden the working range of CBIS and benefit to large-deflection vibration isolation. The peak transmissibility can be reduced to 7 dB or even 0 dB and the resonant frequency can be smaller than 2 Hz, which is conductive to ultra-low frequency vibration isolation. The proposed CBIS opens a new route towards tunning asymmetric stiffness for low frequency vibration isolation.