Effects of Strand Length
Specific mean values and coefficients of
variation of mechanical properties tested in tension,
compression and bending are given in Table 2.
The coefficients of variation, although seeming
relatively high (0% - 35%), are in the same range
of solid wood reported at 14% - 34% (Wood
Handbook, 1999).
For tension parallel to grain, the OSL made
of longer strands exhibited higher sUTS. Statistical
analysis showed that the results are highly
significant (Pr < 0.01). However, for the perpendicular
to grain direction there was no obvious
effect of the strand length on the sUTS. This is
explained by the very weak cohesive force (of
wood itself) in this direction. As a result, the sUTS
parallel to grain is about 35 times higher than that
of perpendicular to grain. The sTMOE parallel to grain is about 6.5 times higher than that of the
other direction. There is no significant evidence of
the role of the strand length in either direction.
It was found that sUCS and sCMOE parallel
to grain increased as the strand length increased
(Pr < 0.05), but were statistically insignificant for
the perpendicular-to-grain direction. The sCMOE
parallel to grain was about 18 times of that perpendicular
to grain. The OSL with longer strand
was significantly stronger than the shorter one in terms of sUBS and sBMOE.
The mechanical properties of rubberwood
(solid wood) and the average properties of OSL
were compared at SG = 0.7 and given in Table 3.
UTS parallel to grain of OSL with strand length
150 mm was slightly higher than that of the solid
wood, while UTS perpendicular to grain was only
50% of the solid wood. Voids have weakened the
tensile properties of the OSL, especially for
direction perpendicular to grain in which the glue
lines were relatively short. UCSs of OSL were
less than that of the solid wood because of the
delamination. Bending properties, UBS and BMOE
of the OSL were not as strong as the solid wood.
However, the properties follow the modified
Hankinson equation. Therefore, it is anticipated
that the mechanical properties of OSL could be
improved if manufactured front longer and thinner
strands. The slenderness ratio (ζ) of 360 could give
OSL strength appoach 90% of the solid wood.
Industrial stranding machine could produce strands
as thin as 0.6 mm (Lowood, 1997), which would
increase the slenderness ratio and, hence, the
properties of the OSL.
Conclusions
1. The longer strands increase UTS UCS
UBS and BMOE parallel to grain of OSL.
Mathematically, OSL properties approach the solid
wood properties as the strand length increases.
OSL made from longer strands will be beneficially
stronger.
2. Hankinson formula can be used to
determine the mechanical properties of OSL
provided that the mechanical properties of the solid
wood and the slenderness ratio are known.