Tensile creep and recovery of Norway spruce influenced by temperature and moisture

Abstract

The time-dependent mechanical behaviour (TDMB) of wood is important when using the material for structural purposes. Recently, a new method for predicting the TDMB by numerical modelling was established based on the assumption that TDMB is caused by the sliding of the microfi brils past each other. In this study, the TDMB is examined via creep experiments on small specimens of Norway spruce latewood. The results of these are compared with results from numerical modelling. The experiments include results at two levels of moisture content and three levels of temperature, enabling an investigation of these two climatic factors on TDMB of wood. It was found that the mechanical response of wood tissue is the sum of responses from both tracheids and middle lamella, with only the previous being reversible. The effect of moisture and temperature differed in that the latter affected the elastic and time-dependent responses equally. Moisture, on the other hand, reduced both the elastic properties and the activation energy barrier for sliding of the microfi brils, but furthermore changed the microfi bril angle of the sample as a result of swelling. Hereby, moisture had a larger effect on the time-dependent response than the elastic. All of these effects were predicted by numerical modelling.

Creep Behavior of Wood Plasticized by Moisture and Temperature

“…3 and Table 2 it can be concluded that with increasing MC, the IC at the same temperature increased consistently, which was in agreement with the results of previous studies (Tissaoui 1996;Moutee 2006;Engelund et al 2012 Fig. 4.…”

“…3. The IC of samples at six MCs increased continuously as temperature increased, which was consistent with the results of previous studies (Tissaoui 1996;Moutee 2006;Engelund and Salmén 2012). The IC of the samples with higher MCs was more affected by temperature than those at lower MCs.…”

“…In other words, when polymer components of wood are in the glassy state, the mobility of their molecular chain parts and chain segments are completely frozen, and thermal motion only occurs at fixed positions (Yang et al 2004). When the temperature is increased, the amplitude of the thermal motion increases and the molecular chains become loose because of the repulsion between chains (Yang et al 2004;Engelund and Salmén 2012). Secondary relaxation transitions occur in side groups or some individual chain segments.…”

“…Moisture in wood (MC < 30%) could affect the internal hydrogen bonds between wood polymers, thereby influencing the wood's plasticity and deformation. There has been considerable research in creep behavior of wood at both varying MCs (Armstrong andKingston 1960, 1962;Armstrong and Christensen 1961;Armstrong 1972;Ranta-Maunus 1975;Wu 1995;Zhan and Avramidis 2011)), and constant MCs (Youngs 1957;Clouser 1959;Kingston and Budgen 1972;Armstrong 1972;Breese and Bolton 1993;Tissaoui 1996;Jong and Clancy 2002;Kojima and Yamamoto 2005;Moutee 2006;Engelund and Salmén 2012;Hering and Niemz 2012). When creep behavior of wood is investigated at low temperatures, the controlled MC value of the wood during the test is not very difficult to realize.…”

“…When creep behavior of wood is investigated at low temperatures, the controlled MC value of the wood during the test is not very difficult to realize. Creep experiments at low temperature were carried out using a water-saturated cloth (Youngs 1957) and chemical materials such as aqueous NaCl and P2O5 powder (Kojima and Yamamoto 2005), or by simply putting wood specimens in a conditioning room (Clouser 1959;Armstrong 1972), which consisted of a controlled humidity generator with nitrogen as the carrier gas (Engelund and Salmén 2012) and a controlled temperature and humidity chamber (Moutee 2006;Hering and Niemz 2012). With increasing temperature, however, maintaining a constant MC at elevated temperatures is very difficult because changes in MC take place simultaneously with changes in the temperature of the wood.…”