Viscoelastic Properties of the Chinese Fir (Cunninghamia lanceolata) during Moisture Sorption Processes Determined by Harmonic Tests

Measured by harmonic tests, the viscoelastic properties of Chinese fir during moisture sorption processes were examined under three relative humidity (RH) modes: RH(ramp), RH(isohume), and RH(step). The stiffness decreased and damping increased as a function of the moisture content (MC), which is presumed to be the effect of plasticization and an unstable state. The increasing damping was associated with the breaking of hydrogen bonds and the formation of free volume within polymer networks. The changes of loss modulus ratio at 1 and 20 Hz, E″(1Hz)/E″(20Hz), proved the changing trend of the unstable state. Higher ramping rates aggravated the unstable state at the RH(ramp) period, and higher constant RH levels provided more recovery of the unstable state at the RH(isohume) period. Changes of viscoelastic properties were associated with RH (varied or remained constant), and the application of Boltzmann’s superposition principle is a good approach to simulate viscoelasticity development.