This study aims to investigate the damping behaviors of unidirectional and laminated flax fiber reinforced composites (FFRCs) under various frequencies and moisture absorption conditions. The damping performances of unidirectional (0°, 45°, 90°), orthotropic and symmetric angle-ply composites were evaluated by the cantilever percussion free-decay method to establish the relationship between frequency, hygroscopicity and damping ratio. To elucidate the frequency- and moisture-dependent damping mechanisms, the glass-transition temperature and modal analysis were examined using dynamic mechanical analysis and a 3D scanning laser Doppler vibrometer respectively. To predict the frequency- and moisture-dependent damping behaviors for hierarchical FFRC laminates, a finite element model subject to the damping test was developed by integrating laminate theory and the complex eigenvalue method in a user-defined material subroutine. The findings indicate that moisture absorption leads to an increase in the damping ratio and changes the frequency-dependent trend. The distinct hierarchical structures of flax yarns result in strong frequency- and moisture-dependent damping performances in FFRC laminates. A significant agreement between the experimental modal frequency, damping ratio, mode of vibration of all composites and those values derived from the established modelling was achieved. It offers a foundational parameter for precisely predicting the damping properties of FFRC laminates with complex stacking sequences and designing safe and reliable FFRC structures integrating load-bearing and damping functionalities.
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