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.
Highlights
The first test group consisted of simultaneous measurement of moisture content (MC) changes and the free swelling–shrinkage deformation, being monitored on unloaded specimens subjected to the cycling changes of relative humidity (RH)
Orthotropic free swelling–shrinkage The coefficient of variation in MC among the L, R and T specimens was less than 5%
The wood cell wall is a composite biopolymer consisting of cellulose microfibrils (CMFs) and lignin–hemicellulose matrix [24,25,26]
Summary
Mechanical response of wood to sustained loading (i.e., creep) is affected by the moisture content (MC) state [1,2,3,4,5,6]. A numerical simulation of creep behaviors have to be known. As tensile stress perpendicular to the grain represents the weakest timber point, its knowledge, especially as it relates to load capacity and failure prediction of wooden structures, is highly important in wood engineering. Knowledge of orthotropic MS behavior is essential in the wood industry, in the hygro-mechanical treatment efficiency.
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