Inverse Identification of Temperature- and Mode-Dependent Apparent Complex Flexural Properties of Syntactic Foam for Low-Temperature Marine Insulation

Syntactic foams combine low density and thermal-insulation capability, but their dynamic flexural properties at subzero temperatures remain insufficiently characterized. This study applied an Euler–Bernoulli beam-based inverse framework to natural frequencies and damping ratios of four epoxy syntactic-foam specimens representing four formulations at 0, −20, and −40 °C. Apparent storage, loss, and complex flexural moduli and equivalent modal loss factors were identified for the first two bending modes, with intermodal consistency and analytical sensitivity evaluated. The storage flexural modulus increased with decreasing temperature for every specimen and mode, with first-mode increases of approximately 5.8–11.0% from 0 to −40 °C. At equal nominal weight fractions, the S38HS-based specimens exhibited higher storage flexural moduli than the corresponding S28HS-based specimens. First- and second-mode estimates agreed within 5% for most conditions, while S4 showed a 10.93% discrepancy at −40 °C. Equivalent modal loss factors were non-monotonic and more variable in the second mode; the maximum values, η2 = 0.1040 and E2′′ = 0.298 GPa, occurred for S3 at −40 °C. Effective free length showed the greatest normalized influence on the inverse estimate. The identified properties provide temperature- and mode-specific, coupon-level effective inputs for preliminary vibration analyses of moderately low-temperature marine insulation components under comparable conditions.