Response Mechanism Of Orthotropic Creep Behavior To Temperature And Humidity Change For Chinese Fir Wood

The assessment of creep behavior of wood is the primary consideration for the in-service design of wood used in load-bearing components.Given the distinctive anisotropy of wood,a complete creep characterization requires knowledge of the creep properties related to the three principal axes of anisotropy(longitudinal,radial and tangential).Moreover,the mechano-sorptive(MS)creep under varying moisture conditions remains underexplored.In this study,orthotropic creep behaviors were performed on Chinese fir(Cunninghamia lanceolata [Lamb.] Hook.)under temperature and humidity field via dynamic mechanical analysis(DMA 2980/Q800),while maintaining the pure viscoelastic creep at a constant moisture content(MC)and mechano-sorptive creep during adsorption,desorption and cyclical processes.Swelling/shrinkage during MC changing was also performed on matched specimens based on the strain partition assumption to better understand the characteristics of the MS phenomenon.The major achievements of present study were summarized as follows:1.The validity of time–temperature superposition principle to characterize the orthotropic creep behavior was found in the range of 30 to 150°C.The time–temperature superposition principle were constructed for radial and tangential specimens using horizontal shift factor,while both horizontal shift factor and vertical shift factor were used for longitudinal specimen.Both elastic and viscoelastic behaviors for transverse(radial and tangential)specimens were more affected by temperature than that for longitudinal specimen.When temperature increased from 30°C to 150°C,the elastic strain in longitudinal,radial and tangential direction increased by 45.3%,69.8% and 48.3%,respectively,and the viscoelastic strain in longitudinal,radial and tangential direction increased by 158.1%,410.4% and409.8%,respectively.2.At constant MC conditions,the higher relative humidity or moisture content,the higher the plasticization effect,which aggravated the creep strain.The plasticization effect exhibited a more pronounced influence in tangential direction,followed by radial and longitudinal direction.When MC increased from 0.6% to 14.1%,the elastic strain in longitudinal,radial and tangential direction increased by 230.0%,608.9% and 885.7%,respectively,and the viscoelastic strain in longitudinal,radial and tangential direction increased by 233.0%,553.4% and 571.6%,respectively.3.The MS creep during adsorption process is directed by swelling,plasticization effect and MS effect,while the MS creep during desorption process is controlled by shrinkage,reformed hydrogen bonds(RHB)effect and MS effect.Expansion mainly dominated the creep strain during adsorption and desorption processes,especially for the longitudinal specimen.The MS effect exerted more influence ontangential specimen and had less contribution to the longitudinal specimen.In addition,the MS limit was observed for all specimens during cyclical MC variation.Longitudinal specimen required more cyclic times to approach the MS limit than transverse specimens.4.The plasticization effect and MS effect were less affected by relative humidity level during adsorption process.However,during desorption process,the lower relative humidity,the lower RHB effect contribution and higher MS effect contribution for MS creep was found,especially for transverse specimens.