Orthotropic mechano-sorptive creep behavior of Chinese fir during the moisture adsorption process determined in tensile mode via dynamic mechanical analysis (DMA)

The orthotropic viscoelastic creep (VEC) at a constant moisture content (MC) and mechano-sorptive creep (MSC) during the adsorption process were examined for Chinese fir (Cunninghamia lanceolata) under tension at 20, 40, 60 and 80% relative humidity (RH) (30 degrees C). Free swelling was performed on matched specimens based on the strain partition assumption to better understand the characteristics of the mechano-sorptive (MS) phenomenon. Expansion, elastic and time-dependent creep behaviors of radial (R) and tangential (T) specimens were affected by the MC to a higher degree than those of the longitudinal (L) specimen. A higher proportion of elastic strain in total strain was found in the L specimen as compared with transverse specimens, regardless of VEC and MSC. The RH level had a greater effect on relaxation behavior in the L specimen for MSC. According to the three tests, expansion mainly dominated the creep strain during adsorption, especially for the I, specimen. The MS strain exerted more influence on transverse specimens and had less contribution to the L specimen. Moreover, under all RH isohume (RHI) conditions, the unstable state contributed to MS strain diminishing as MC approached equilibrium moisture content (EMC). A shorter adsorption time to a new equilibrium state was achieved at the expense of intensifying the unstable state of the wood cell wall.