Characterization and prediction of frequency- and moisture-dependent damping behaviors for hierarchical flax fiber reinforced composite laminates

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.