Linear Viscoelastic Wood Creep Models

The paper focuses on problems of the rheology of wooden beams. The main aim of the theoretical and experimental research was to select a linear viscoelastic rheological model best describing the behavior of wooden beams. The experimental elements were full-size beams made of pine wood. The beams were subjected to a long-term multistage loading program. Simple models of linear viscoelastic materials, i.e., the three-parameter standard model and the four-parameter Burgers' model, reveal essential restrictions and are too poor to describe processes of wood creeping, particularly in the initial period of the phenomenon, as well as in the case of multistage loads. Our research has shown that, under constant heat and moisture conditions, the five-parameter rheological model is sufficient to describe wood creep under constant loading. In contrast, the model deviates significantly from experimental results in the case of a multistage loading program. In view of this, a six-parameter rheological model was used, which fits the experimental data perfectly. Thus, the decisive verification of the assumed rheological model can be ensured only by carrying out experiments with the use of a multistage program of loads (clear separation of the identification of the model's parameters from its verification).