Characteristics of shrinkage stress of wood during drying under high temperature and high pressure steam conditions

Shrinkage stresses and viscoelasticity of wood during drying under superheated steam above 100℃ were studied using a newly developed load cell which is capable of measuring shrinkage stresses of wood inside an autoclave under high temperature and pressure. The shrinkage stresses along radial and tangential directions of a wood sample were continuously measured from its saturated condition until it was completely dried or significant cracks occurred under superheated steam above 100℃. The results showed that during the drying process under high temperature with 0 relative humidity (RH) , the shrinkage stress in the radial direction of the test specimen was about 2 times higher than that in the tangential direction. However, when samples were dried at 60 and 80 RH, the shrinkage stress in the tangential direction was higher than that in the radial direction. The shrinkage stresses were effectively restrained and dramatically decreased with increase of RH during the drying process under superheated steam at 180℃. It was also found that shrinkage stress still existed even under superheated steam at 100 RH.