Original
Paper(Vol.59 No.4 pp.279-284)
Internal Stress in Sugi Square Timber Dried by the
High Temperature Setting Method
-Differences between Indoor and Outdoor
Storage-
Morihiko TOKUMOTO, Makoto KITAHARA, Takashi TAKEDA, Yoshihiro
HOSOO and Takahisa YOSHIDA
Abstract:The high-temperature setting
method, which enables rapid kiln-drying for square timber with pith, has spread
in many saw mills in Japan. It has been supposed that obvious drying tension
set, which is formed at the shell of timber without back-splitting, results in
generation of sufficient compressive stress at the shell, and reducing surface
checks significantly. In the paper, we investigated the differences in internal
stress between indoor and outdoor storage with sugi(Cryptomeria
japonica D.Don) square timber kiln-dried by the method. The compressive
stress was expressed as released strain measured by the slicing technique. Main
results were shown below.
1. Strong compressive stress was observed at the
shell of the timber which was stored indoor for five years.
2. On the other
hand, the compressive stress obviously decreased at the shell of the timber
which was stacked outdoor and covered with a corrugated panel for six years.
3. The compressive stress might be reduced by loss of tension set which was
caused by moisture variation during long outdoor storage.
4. The
hygro-thermal recoverable component of set seemed to be melted, which had been
assumed to be recovered only by boiling.
From the above results, the indoor
storage had little effect on the compressive stress at the shell of timber
kiln-dried by the method, but the outdoor storage decreased the surface stress.
Further research is needed for explanation of the time-dependent change for
practical uses. For practical outdoor uses, the risk of occurrence of surface
checks might increase as time passes.
Key Words:Sugi, Square timber with pith, Slicing technique,
Released strain, Compression stress, Tension set