Viscoelastic behaviour of green wood across the grain. Part I. Thermally activated creep tests up to 120°C

Abstract

In this work, we present an improved experimental set-up capable of performing creep tests on water-saturated samples up to 120°C. A typical test consists of three phases : a linear increase in temperature up to the desired value, a plateau at this temperature level during 15 h and a cooling period. For each species (spruce and oak) and each direction across the grain (radial and tangential), a whole set of creep tests is available, at different plateau temperatures : 65°C, 85°C, 105°C and 120°C. The modulus of elasticity (MOE) measured at room temperature on green wood confirms expected results: it is almost twice as high in radial direction and more than twice as high for oak: average values of 253 and 119 MPa for Spruce and 687 and 398 MPa for Oak in radial and tangential direction respectively. In the case of oak, the MOE of tension wood can also be distinguished from normal wood: its modulus is smaller in spite of a higher density. The creep tests reveal the importance of the temperature level on the thermal activation: the apparent MOE might loss more than two orders of magnitude after a test at 120°C. This effect is more pronounced and starts at lower temperature values for oak than for spruce while its is almost the same, in relative value, in radial and tangential directions. © INRA, EDP Sciences, 2005.