Development of Experimental Methods for Characterization of Fluid-Filled Sandwich Core Composites

Fluid-filled sandwich core composites (FFSCCs)—inspired by the porous, fluid-filled bone structure of the mammalian skull—show promise for multifunctional aerospace applications combining structural efficiency with rate-dependent energy dissipation. However, no standardized test method exists to characterize these materials because conventional sandwich composite tests cannot retain interstitial fluid during mechanical loading. This study presents the development and validation of a biaxial clamped plate flexure (BCPF) test specifically designed to overcome fluid retention challenges in porous-core composites. The test employs mechanical edge sealing through a bolted flange fixture, a 3D-printed core enclosure with integrated fluid ports, and post-lamination fluid filling to enable characterization across loading rates from quasistatic to dynamic regimes. Analytical plate equations for clamped-edge, cross-ply laminates—extended to calculate layer-wise load contributions through a core foundation model—enable direct extraction of flexural properties from measured load-displacement data. Finite element analysis incorporating shell theory, nonlinear foam constitutive models, and Biot’s poroelasticity provides validation of both the analytical framework and experimental boundary conditions. Quasistatic testing (15 mm/min) of Kevlar/Tyvek/polyurethane-foam FFSCC samples in both dry and water-filled conditions demonstrate successful fluid retention throughout loading to failure, with no observed leakage. Experimental load-displacement curves match FEA predictions within 10%, while digital image correlation of surface strain fields validates the near-ideal clamped boundary condition (RMS error < 8%). At quasistatic rates, fluid inclusion produces a modest 3% increase in flexural modulus accompanied by 52% greater core compression, confirming theoretical predictions that minimal pore pressure develops at low strain rates. This quasistatic baseline—where fluid effects are minimized—provides essential validation of the test method and analytical framework, enabling future investigation of rate-dependent strengthening mechanisms at higher loading rates where fluid viscous resistance becomes significant. The validated BCPF test establishes a systematic methodology for characterizing fluid-structure coupling in porous-core composites, applicable to FFSCCs and related multifunctional material systems.