64 taking into account the effect of density on flexural properties Bao et al. 2001;
Bal and Bektaş 2012. As for MOE, the ANOVA Appendix 1 showed that
veneer thickness, poplar cultivars and juvenility had significant effects on SMOE. For SMOR, only veneer thickness did not show any significant effect, while
other factors did comparable to MOR. The Duncan test Appendix 2 also showed that the statistical analyses between MOE and SMOE and between MOR
and SMOR were similar, except for veneer thickness.
The veneer thickness, poplar cultivar and juvenility factors had significant effects on SMOE, independently from density. Anatomical factors such as fibre
length, microfibril angle etc, also probably contributed to this effect. Veneer thickness showed a significant effect for MOR but not for SMOR.
This shows that, in this context, the use of thick veneers is not penalizing for intrinsic LVL mechanical properties.
4.7.5 Structure application
The advantage of using veneers taken from mature part was obvious, since mechanical properties were improved by 15 to 20 for a comparable density
Table 15. This proves that there is an effect due to juvenility for each poplar cultivar. Therefore, users should consider juvenility in estimating LVL
mechanical properties.
Dynamic MOE, static MOE, MOR and density were lower for LVL made of juvenile veneers than for LVL made of mature veneers. This was in agreement
with Kretschmann et al. 1993. A significant difference was found between Southern Pine and Douglas Fir LVL manufactured with mature or juvenile
material. The ratio of juvenile to mature material was approximately 0.8 for strength and stiffness, which was comparable with ours.
According to static MOE values of poplar cultivars and the results of Duncan’s multiple comparison test Appendix 2 for static MOE values, 3
categories were established. ‘Taro’, ‘Lambro’, ‘Soligo’, ‘Brenta’ and ‘Alcinde’ Poplar cultivars could be considered as suitable for structural application blue
coloured in Appendix 2 , whilst ‘Lena’, ‘Trichobel’, ‘Mella’, ‘Koster’ and ‘Dvina’
should be used with careful sample selection red coloured. ‘Polargo’, ‘Triplo’, ‘A4A’ and ‘I214’ should not be selected for such purposes yellow coloured.
Poplar cultivars with static MOE values more than 9000 MPa and according to Duncan analysis had ‘A’ and ‘B’ letters were classified in blue coloured, while
poplar cultivars with values more than 8000 MPa had ‘C’ and ‘D’ letters were classified in red coloured. Poplar cultivars with less than 8000 MPa had ‘E’, ‘F’
and ‘G’ were classified in yellow coloured. The increases on percentage of Douglas-fir LVL bending strength originated
from juvenile and mature veneers were described on Table 16. The advantage of using veneers taken from mature part was the improvement in bending strength by
7 to 22. Dynamic MOE, static MOE, MOR and density were lower for LVL made of juvenile veneers than for LVL made of mature veneers. This was in
agreement with Kretschmann et al. 1993. This proves that there is an effect of juvenility on Douglas-fir LVL. Therefore, producers should consider minimize
utilizing juvenile veneer in LVL Douglas-fir.
The advantage of using douglas-fir 5.25 mm was the improvement in bending strength by 0.6-9 Table 16. It was noted in this study that effect of
65 the veneer thickness on the stiffness or strength is positive. This result was in line
with Echols and Currier 1973. The use of thick veneers appears to be not penalizing to LVL bending strength for this species in the range of parameters in
use. Consequently, the idea of using thicker veneers to reduce the adhesive consumption without altering their mechanical properties has promising prospect.
4.8 The correlation between MFA and fiber length of fast growing wood
species and bending properties
The results in Chapter 2 stated that MFA values tended to decrease from pith to bark. MFA values of sengon near pith and near bark were 71° and 29°,
respectively. While MFA values of jabon were 72° near pith and 28° near bark. MFA values of douglas-fir juvenile and mature wood were 45° and 28°,
respectively. While MFA values of ‘soligo’ poplar cultivars were 59° near pith
and 15° near bark. The decrease of MFA value would lead to the increase of static MOE. The percentage gain of static MOE LVL between juvenile near pith
to mature wood near bark for sengon, jabon, ‘soligo’ and douglas-fir were 12, 36, 13 and 14, respectively. According to Kretschmann 1997, wood
composites properties of loblolly pine depend for a part on MFA of wood fiberparticles in used. In general, small MFA values ensure high mechanical
properties. The longitudinal stiffness of cell wall is determined by MFA. The properties of cell wall material specifically MFA and the amount of cell wall
density both affect the mechanical properties of wood MOE longitudinal. Hence, both MFA and density can be related to wood stiffness, either theoretically
or experimentally Xu et al. 2004.
Moreover, because of its fiber characteristics, juvenile wood has significantly lower strength and stiffness, more longitudinal shrinkage, and less
radial and tangential shrinkage than mature wood Pearson and Gilmore 1971; Bendtsen 1978; Bendtsen and Senft 1986. Results on Chapter 2 regarding fiber
length of sengon near pith and near bark were 770 and 1336 µm, respectively. While jabon fiber lengths were 753 µm near pith and 1642 µm near bark. The
fiber lengths of ‘soligo’ were 1058 µm near pith and 1348 µm near bark. Douglas-fir fiber lengths for juvenile and mature wood were 1200 and 3359 µm,
respectively. The increase of fiber length was followed by the increase of MOR LVL made of juvenile veneers to mature veneers. The percentage gain MOR
LVL between juvenile near pith to mature wood near bark for sengon, jabon,
‘soligo’ and douglas-fir were 10, 27, 22 and 10, respectively. This trend was also observed by Kiaei et al. 2013 who found that there was a relationship
between MOR of Pinus eldarica solid wood and tracheid length R
2
=0.47
4.9 Conclusion
The glue bond strength, MOE and MOR of sengon and jabon LVL increased from pith to bark. The advantage of using veneers from poplar mature wood was
proved with an improvement of 15 to 20 on average for mechanical properties, with almost the same panel weight. Douglas-fir LVL made of mature veneers had
higher bending strength compare to LVL made of juvenile veneers. Utilization of
