= Solution
A physical mechanics model of wood must at least acknowledge <anisotropy>: stiffness and creep depend on loading direction. Direction-dependent elastic elements can represent different small-strain moduli, and delayed elastic branches can approximate time-dependent <strain>. A linear-solid network can therefore be a local fit under specified direction, moisture and temperature, rather than a universal scalar constitutive law.
An immediate <strain> followed by delayed <material creep> is captured locally by a <spring> plus a <Kelvin-Voigt model> branch. If residual deformation remains after unloading, a <Burgers material> adds a free series <dashpot> $\eta_0$. Under a constant load,
$$
\varepsilon(t)=\frac{\sigma_0}{E_0}+\frac{\sigma_0}{E_1}(1-e^{-t/\tau})+\frac{\sigma_0t}{\eta_0}.
$$
After a hold time $t_h$, the two elastic contributions recover and the dashpot retains
$$
\boxed{\varepsilon(\infty)=\frac{\sigma_0t_h}{\eta_0}}.
$$
Thus immediate recovery, delayed recovery and a residual set identify different roles for the components; an observed permanent set would rule out a purely recoverable linear-solid fit. A viscous dashpot does not itself model a yield threshold or structural damage. Strength loss, splitting, moisture-dependent response and nonlinear compression cannot be represented by one fixed linear <Kelvin-Voigt model>. No direction-specific measured trace is supplied, so selecting numerical parameters requires the practical data; the constitutive predictions and their limitations are the model comparison that can be made from the paper.
Back to article page
