Dynamic Mechanical Response of Flax Roving Under Controlled Thermo‐Hygrometric Conditions

This work investigates the coupled effects of temperature and relative humidity on the mechanical, thermal, damping, and structural responses of flax roving fibers. Tensile tests done between 30% and 90% RH show that humidity strongly governs stiffness through hydroplasticization of amorphous constituents. Tensile strength remains maximal at 50% RH (108 MPa), while Weibull analysis confirms that fiber reliability is also moisture‐dependent, with the highest Weibull modulus ( m = 14.09) obtained at 50% RH, indicating the lowest strength variability among the investigated conditions. Dynamic mechanical analysis (DMA) reveals that both storage modulus and damping are highly sensitive to thermo‐hygrometric conditions. Temperature‐sweep tests (−20°C to 200°C) identify three characteristic regions associated with frozen‐water rearrangements, free‐water evaporation, and thermal softening. Distinct relaxation processes ( α 2 , α ′, α ₁), linked to hemicellulose, interphase domains, and lignin, are observed, and their amplitudes progress with humidity. Time‐dependent tests show that flax roving remains mechanically stable at low temperatures, whereas rapid stiffness loss occurs at temperatures higher than 200°C, due to degradation of pectin, hemicellulose, and the onset of cellulose decomposition. These results are consistent with thermogravimetric analysis, which separates water desorption from early thermal degradation. X‐ray diffraction indicates an increase in crystallinity up to 150°C, followed by a decrease at higher temperatures, evidencing structural destabilization. This study provides a detailed characterization of flax roving using DMA and demonstrates that its thermo‐hygro‐mechanical performance is controlled by a complex interaction between moisture content, temperature, and cellulose microstructure evolution, with direct implications for composite processing and dynamic‐load applications.