Dynamic characteristics of SMA-based variable-stiffness composite leaf springs

Abstract Composite leaf springs with variable stiffness function (‘shape memory alloy (SMA) hybrid composite leaf springs’) were constructed by embedding three-dimensional SMA actuators into a multi-material composite system. Bench test of dynamic stiffness and fatigue characteristics revealed that epoxy resin matrix leaf springs exhibit the highest initial strength/stiffness but suffer rapid long-term decay due to brittle fracture and Joule thermal softening. Although SMA phase transformation enables temporary crack closure, the interplay between thermal damage and cyclic repair restricts durability. Thermoplastic polyimide (TPI) matrices exhibited outstanding fatigue resistance under actuation, attributed to superior thermal stability and molecular chain reconstruction, while SMA activation effectively inhibited crack propagation. Polydimethylsiloxane (PDMS) matrix leaf springs are suitable for long-term active control scenarios due to hyperelastic interfacial slip and modulus mismatch. However, while the short-term increase in stiffness and strength after energization is high, interfacial delamination triggers catastrophic failure after 5000 cycles, making PDMS matrix leaf springs suitable for short-term deformation requirements only. The TPI-SMA synergistic architecture provides a theoretical foundation for resolving the intrinsic trade-off between active stiffness tunability and fatigue resistance, thereby demonstrating the substantial potential of intelligent structural composites for advanced automotive applications.