48
4.5.4 Sample Preparation for Mechanical Properties
Each board was cut into standardized test samples EN 789, parallel to grain with total of 1808 samples for poplar and 140 samples for douglas-fir Figure 25.
Figure 24 Glue machine and gluing process a and cold press b
Figure 25 Preparation samples for non destrustive and destructive test a; and samples for bending properties b
The parameters for mechanical properties were Modulus of Elasticity MOE and Modulus of Rupture MOR. First, dynamic tests BING was
performed then a static 4-point bending test was performed for each sample.
4.5.4.1 Non Destructive Test
Dynamic analysis is a simple and efficient way of characterizing the Module of Elasticity MOE of many materials, including wood Brancheriau and
Bailleres 2002; Bucur 2006. In order to estimate the dynamic MOE from non- destructive test method, bending vibration method BING was used for the 1808
samples poplar cultivars and 140 samples douglas-fir. This is a fully automated system designed by CIRAD-Forêt following work of Bordonné 1989 and Hein
et al
. 2010. It is based on measurement and interpretation of the natural frequencies of vibration from a wood piece subjected to impulse loading. This
method was easy to apply, very quick and practical. The dynamic MOE were obtained through percussion bending perpendicular to the glue joints in two
loading position flatwise FW and edgewise EW. The interpretation of the spectrum of the natural frequencies was based on the fact that the ratio of elastic
modulus to density specific modulus of a material was proportional to the speed of signal propagation in the material.
BING method could be divided into three interrelated steps: 1 begins with a general initialization of the equipment and preparation the samples; 2 the
acquisition and analysis of the digitized signal; and 3 the mathematical and mechanical processing of the signal.
a b
a b
49 After setting the acquisition device, the tested samples were first weighted
and measured in length, width and height. Then the samples were positioned on the two elastic bearings like shown in the Figure 26 and made sure that the
supports were located at a distance of ¼ of the total length of each specimen.
Afterwards the microphone was set up in perpendicular with the length of the samples see Figure 26 with the distance of 1 or 2 cm from the sample. The
samples were hit by percussion bar at one end of the sample and the sound emitted was recorded in another end of the samples by microphone. The emitted sound
from the end of the samples was converted into electrical signal by the microphone. This signal was then amplified and filtered by means of the
acquisition card acting as an analog-digital converter and which delivered to the computer to digitize the signal.
Figure 26 Samples placement on BING bending vibration method After digitizing the signal, then it was recorded and transferred to a users
computer memory. The spectral composition of the recording was given by fast fourier transform, the spectral width of the acquisition depends on fixed
parameters point number and acquisition time. The mathematical calculation of the selected frequency was performed via software from the geometric
characteristics and mass of the sample. It was used to determine the elastic moduli by Bernoulli and Timoshenko models. Timoshenko model was used in several
studies Bordonné 1989; El-Houzali 2009 including this research. Timoshenko had an equation of motion that took into account the bending moment, shear, and
rotational inertia. Bernoullis model did not take into account either the shear or rotation inertia. This model was a simplified model of Timoshenko where we
considered the strain energy due to the negligible shear during bending.
4.5.4.2 Destructive test
Four-point bending tests were performed on an INSTRON universal testing machine Figure 27 to measure MOE static and Modulus of Rupture MOR.
Moisture content values of poplar cultivars and douglas-fir samples were 8.5 ± 0.5 and 13.3 ± 1.6, respectively. The moisture content values were uniform
when the destructive tests were performed. Specific MOE and specific MOR were obtained by dividing static MOE and MOR by the LVL density at those moisture
content values.
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