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

Abstract 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°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 L 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.