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In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that
may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation
capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological
model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep.
The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood,
the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time.
The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load
for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever
under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic
element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep
tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore
predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process. 相似文献
2.
Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant
moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological
parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results
difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy
with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under
controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant
difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of
the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents
considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at
constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed
by authors for rheological parameter identification. 相似文献
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