Stiffness-inverting low-profile metamaterial pair for large-stroke quasi-zero-stiffness vibration isolation

Quasi-zero-stiffness (QZS) vibration isolators are encountering growing application challenges such as bulky configurations, small stroke capacity, and significant theory-reality gaps. This study proposes a compact QZS vibration isolator, based on a low-profile, lightweight metamaterial pair comprising a truncated conical shell and a diaphragm spring. Both components support loads and exhibit nonlinear stiffness characteristics that evolve in opposite directions and undergo a stiffness sign inversion during compression. Thus, their resultant force-displacement characteristic features a high load-bearing capacity and a large QZS stroke. A finite element model is developed to verify the stiffness evolution of the proposed vibration isolator and to analyze the influence of geometric parameters on its mechanical response. Static experiments demonstrate the effectiveness of the stiffness inversion strategy and reveal force-displacement hysteresis caused by internal friction. A dynamical analytical model with nonlinear elasticity, internal friction, and fractional derivative damping is developed and solved numerically to obtain displacement transmissibility-frequency response, where the effects of excitation amplitude and damping are investigated. Experiments in various excitations are carried out to evaluate the performance of the proposed vibration isolator. Results from experiments under swept-frequency excitations with different amplitudes demonstrate that an increase in excitation amplitude reduces both the resonance peak (from 14.8 dB to 7.3 dB) and the onset frequency of vibration isolation (from 6.3 Hz at 0.5 mm to 5.5 Hz at 2 mm). Results from the experiment under random excitation further showcase the system's performance, with maximum and RMS accelerations reduced by 80.4 % and 77.8 %, respectively.