Experimental and Numerical Modal Analysis of Composite Sandwich Structures Using Surfboards as Model Systems

Composite sandwich structures derive their mechanical behaviour from the interaction between lightweight cores and stiff fibre‐reinforced skins, making their dynamic response highly sensitive to laminate design and reinforcement strategies. This study investigates the vibrational behaviour of surfboards as representative composite sandwich structures using combined experimental and numerical modal analysis. Boards with wooden stringers, without stringers, and with a composite spine reinforcement were tested using a hammer–accelerometer setup to extract frequency response functions. Poly(urethane) (PU)‐core boards with wooden stringers exhibited two distinctive local ‘stringer modes’ (400–450 Hz), observable only when excitation and sensing were aligned with the stringer. Expanded/extruded poly(styrene) (EPS) boards without reinforcements showed no such modes and displayed only global bending and torsional responses. EPS boards incorporating a composite spine exhibited two high‐frequency local modes (570–632.5 Hz), confirming that reinforcements create characteristic dynamic signatures. Finite element simulations reproduced global modal behaviour with a 2.7% average error. These findings demonstrate modal analysis as an effective tool for characterising flex behaviour and structural integrity in composite sandwich structures.