Orthotropic hygro-mechanical behavior of Chinese fir during cyclical relative humidity variation

Abstract

Abstract The orthotropic free swelling–shrinkage and mechano-sorptive creep (MSC) during cyclical relative humidity (RH) variation (0–80% RH) and time-dependent viscoelastic creep (VEC) in a steady state (0% RH) were examined in Chinese fir (Cunninghamia lanceolata). During RH changing, the occurrences of local maximum or minimum strains of the free swelling–shrinkage and MSC lagged behind the corresponding maximum or minimum RH. The lagged time decreased with increasing cyclical times. Furthermore, the lagged time exhibited an orthotropic behavior: the longer lagged time was found in longitudinal specimen compared to transverse specimens. MSC exhibited a more pronounced anisotropy than VEC. According to the three tests, the performance of free swelling–shrinkage and the mechano-sorptive effect on the orthotropic MSC behavior was addressed. The free swelling–shrinkage mainly dominated the creep strain during cyclic RH variation, especially for longitudinal specimen. The mechano-sorptive effect on the MSC behavior exerted more influence on tangential specimen compared to radial and longitudinal specimens. The mechano-sorptive limit was observed for all specimens. Longitudinal specimen required more cyclic times to approach the mechano-sorptive limit than transverse specimens.

Keywords

• Orthotropic grain orientation
• Mechanical stress
• Cyclic relative humidity
• Free swelling–shrinkage
• Mechano-sorptive creep