Rheology and the Study of Wood

Rheology is the analysis of the deformation and flow of materials as the result of deforming forces. Phenomenological rheology describes the behavior of the material being subjected to force, and molecular rheology describes the molecular or atomic structure responsible for the change. Creep and relaxation are the most familiar of these phenomena.. Creep occurs when a load is applied at time t0 and then held constant until it is removed at time t1. The curve of deformation plotted versus time indicates that an immediate deformation occurs when the load is applied, which is fully recoverable at t1; that there are also transient vibrations and a delayed elastic response, which takes time to develop, and is fully recoverable, given sufficient time; and that there is also a nonrecoverable deformation, due to flow, which takes time to develop. Relaxation occurs when a material is subjected at time t1 to an instantaneous deformation, which is then kept constant by adjusting the load; the load required decreases with time and ultimately decays to zero for materials which exhibit flow in a creep test, or to some finite value for materials which do not. Time dependent behavior is also produced by vibration; for example, a beam subjected to a sinusoidal force will respond with a sinusoidal deflection. The elastic “constants” for a high polymer material are actually dependent on three variables– time, temperature, and concentration of plasticizers. Recent work has emphasized the usefulness of viscoelastic theory in working with linear high polymers such as wood. For such a material, the equation for creep can be derived from the equation for relaxation and vice versa. An equation based on Boltzmann’s superposition principle indicates that deformation is determined not only by the load acting at the time of observation, but also by the entire loading history of the specimen. Deriving models with behavior approximating that of real viscoelastic materials by connecting several retarded elastic elements in series indicates that in the creep responses, the strain is always proportional to the stress. Thus, if creep tests were made on one specimen at two different stress levels, the ratio of the strains would be the same as that of the stresses at all times. Equations derived from experimental data may describe creep or relaxation quite well for the time scale of the experiment, but yield untrue predictions when extrapolated (e.g., predicting infinite strain at infinite time, or predicting infinite stress at zero time). Cell walls in wood are thought to be comprised largely of highly oriented cellulose, approaching a crystalline configuration, and viscoelastic models theoretically should not be able to predict the behavior of crystalline materials. The explanation for this anomaly may be that the amorphous cell wall material of lignin, which is an amorphous cross-linked polymer, is responsible for the viscoelastic behavior of wood.

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