Time-temperature-stress equivalence in compressive creep response of Chinese fir at high-temperature range

Abstract

Predicting long-term viscoelasticity properties of wood is important for designing dimensions for structure-used wood and optimizing thermo-mechanical treatment technique. In this study, compressive creep of Chinese fir wood was tested at a series of temperature (140–220 °C) and stress (0.03–0.15 MPa) conditions. The time-stress superposition principle (TSSP) and time-temperature-stress superposition principle (TTSSP) were applied to predict long-term creep behavior. Changes of chemical components and anatomical features of wood cell wall after creep test were investigated by Raman spectroscopy and scanning electron microscopy. The results showed that quasi-linear change of strain was acquired when temperature at or below 180 °C. TSSP and TTSSP were feasible to predict compressive creep at an up-threshold temperature of 180 °C. When temperature was 200 and 220 °C, obvious non-linear strain was obtained, and TSSP and TTSSP failed to construct master curves. Much significant degradations of carbohydrates was found at 200 °C, which could explain why TSSP failed to construct master curves from a chemical point of view. Crack and rupture on the cell walls and among the cell corners were observed when temperature at or above 200 °C and stress at or above 0.09 MPa. From an anatomical point of view, the destruction of wood cell structures could explain why TSSP failed to construct master curves. The finding in this studies broads the knowledge of the viscoelasticity of wood, and is helpful for further investigations on the thermo-mechanical manufacturing techniques.