High-Static-Low-Dynamic Stiffness and Wideband Vibration Energy Harvesting in a K-Shaped Spring–Rhombus Composite Structure: Modeling, Analysis, and Testing

To address the dual challenges of insufficient low-frequency bandwidth and low energy harvesting efficiency in quasi-zero-stiffness (QZS) isolators, a novel K-shaped spring–rhombus composite structure is proposed. By coupling symmetric oblique K-springs with a rhombus mechanism, this design achieves a flexible tunable nonlinear stiffness, expanding the QZS region by approximately four times compared to conventional rhombus-based isolators while maintaining high static load capacity. Static modeling and nonlinear dynamic responses are systematically investigated using the harmonic balance method, numerical integration, and Floquet theory to identify stability regions and bifurcation behaviors. Experimental validation on a fabricated prototype demonstrates efficient concurrent broadband vibration isolation and sustained piezoelectric energy harvesting. This study provides a validated foundation for designing high-performance, self-powered integrated vibration mitigation systems.