63 The average poplar MOR values for 5.25 mm and 3 mm LVL were
statistically different Appendix 2 in line with the results of H’ng et al. 2010 who reported that LVL with thinner veneers 15 plies had better mechanical
performances compared to those of thicker veneers 11 plies. However, the improvement was still limited in that case and could be attributed mainly to the
upgrading of lamination effects and to the reduction in lathe check depth. This improvement was also limited because the material used was almost free of defect
such as knots.
As observed in the literature Daoui et al. 2011; El-Haouzali 2009, the flatwise position gave a higher MOR value than the edgewise position. It was due
to multiple aspects such as the number of veneer layers, the adhesive and the pressure during manufacture had led to higher MOR values in flatwise position.
The ANOVA Appendix 4 shows that all factors significantly influenced douglas-fir MOR LVL p0.01. Average value of MOR are presented in
Appendix 5, MOR 57 + 9 MPa of douglas-fir LVL made of juvenile veneers was significantly lower than MOR 63 + 9 MPa of LVL made of mature. This
corresponds to results from many authors Barrett and Kellogg 1989; Kretschmann et al. 1993; Nazerian et al. 2011.
Figure 41 Douglas-fir LVL failure break on glue line after destructive bending strength test by instron
Douglas-fir LVL MOR 57 Mpa of LVL 3mm was significantly lower than MOR 63 Mpa of LVL 5.25mm. The LVL 3mm suffered severe failure on glue
line after destructive bending strength test by INSRON Figure 41. Although, those samples were excluded from the statistical analysis, we suspected the LVL 3
mm had lower shearing strength compare to LVL 5.25 mm. The results of this work were in line with Echols and Currier 1973 who find that MOR of solid
Douglas-fir is higher compared to 5-ply LVL Douglas-fir and MOR of 5-ply LVL is higher compared to 7-ply LVL Douglas-fir.
Doulgas fir LVL MOR 58 Mpa of flatwise position was significantly lower compare to MOR 62 Mpa of edgewise position. It was due to most of wood
failure on flatwise direction was shear failures. This phenomenon is also noted by Krestchmann et al. 1993 who states flatwise bending strength of Douglas fir
LVL is 11 less compare to edgewise LVL samples.
4.7.4 Specific MOE SMOE and Specific MOR SMOR
Several researchers have used specific modulus of rupture SMOR and specific modulus of elasticity SMOE to evaluate MOE and MOR results by
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
