Structural integrity assessment of an amphibious spider robot’s flapping fin using FEA method for underwater operating conditions

This study presents a finite element analysis (FEA)-driven design and preliminary experimental validation of a bio-inspired amphibious spider robot’s flapping fin mechanism for hybrid terrestrial–aquatic locomotion. The robot incorporates a six-legged walking system and a passive deployable fin-based swimming mechanism actuated via leg-tip hooks with spring-loaded retraction, enabling automatic transition between land and water operation when triggered by a water contact sensor. Structural performance of the fin under combined hydrostatic and dynamic pressures was evaluated in ANSYS, with dynamic loads derived from fin tip velocity corresponding to a baseline flapping frequency of 1 Hz. Candidate materials, including Nylon (PA12), PETG, TPU (98 A), and 304 L stainless steel foil, were compared through stress–strain–deformation analysis. A multi-criteria decision analysis identified 304 L stainless steel foil as the optimal choice for minimal deformation (0.64 mm) and high fatigue resistance. A functional prototype was fabricated using FDM-based 3D printing, integrating macro and micro servo motors for locomotion and fin deployment. Equipped with TPU fins (0.15 mm thickness) for initial trials, the 1.311 kg prototype achieved a measured flapping speed of 53.4 RPM (0.89 Hz) using a non-contact tachometer, closely matching simulation assumptions. The results confirm the feasibility of the proposed design, validate its actuation performance, and provide a foundation for future in-water propulsion measurements and fluid–structure interaction studies.