Viscoelastic and structural insights into magnetorheological foam fabricated with different volumes of constraint foaming process

• MR foam exhibits frequency-dependent mechanical and rheological responses. • Damping capability of MR foam increases upon higher frequency. • Reduced loss factor indicates superior elastic energy storage efficiency. • Constraint volume foaming process boosts MR foam's storage modulus significantly. • Multi-technique morphological analysis validates structural enhancements. Magnetorheological (MR) foam, a smart magnetic-responsive flexible polyurethane material, has garnered significant attention for its potential in soft robotics. However, limited studies on its dynamic mechanical and damping properties hinder its development for broader applications. This study investigates the influence of constraint volume foaming process (100 %, 75 %, and 50 % volume) on the properties of MR foam containing 75 wt. % carbonyl iron particles (CIPs) under dynamic mechanical compression and rheological shear testing at frequencies ranging from 0.1 to 10 Hz. Results showed that the constrained foaming MR foam with 50 % volume exhibited the most significant enhancement in storage modulus ( E ′ and G ′), attributed to a more compact CIP distribution, resulting in the lowest initial loss factor (tan ẟ ). However, upon increasing frequencies, a notable increase of approximately 20 % in tan ẟ was observed for this MR foam, indicating improved energy dissipation at higher frequencies. In contrast, the free foaming MR foam demonstrated only a 5 % increase in tan δ , highlighting its lower energy dissipation capability. Morphological analysis revealed that constraint volume foaming process produced smaller pores and denser CIP distributions. The compact structure enhanced particle-matrix interactions, increasing friction and contributing to improved damping at elevated frequencies. In summary, the incorporation of constraint volume foaming process significantly enhanced the dynamic mechanical, rheological, and structural properties of MR foam. These advancements broaden its applicability for advanced applications, particularly in soft robotics, where energy dissipation and tunable properties are critical.