5.4 Results and discussion
5.5.2 Differences in static and dynamic MOE
When the MOE of the beams that had been used for acoustic testing (resonance method) was determined in static bending it was found that the MOE of the bending test was smaller by approximately 15% (Table 5-8). This discrepancy between static
and dynamic MOE has been observed by Jayne (1959) who found higher values (approximately 5%) for the dynamic method. Divos and Tanaka (2000) discussed the difference between a static bending test and a dynamic test. They did various 3-point bending tests with different velocities of the crosshead. According to Divos and Tanaka (2000), higher E-values were obtained for higher crosshead velocities. This means that, when loading the same beam with the same load but over a longer time (lower crosshead velocities), the MOE will slightly decrease. This is probably caused by creep processes within the wood. Andrews, Ilic and Walker (2003, unpublished) mention that relaxation during the bending test might be caused by rearrangements of the cell wall under stress, for example straightening of a segment of a hemicellulose chain. Divos and Tanaka (2000) established the following formula which takes account of the time factor during a bending test:
)) log( 017 . 0 1 ( 1 2 2 1 t t E E t t = + where:
t1: the characteristic time of Et1 determination
t2: the characteristic time of Et2 determination
TOF versus resonance measurements
A comparison between the TOF and the resonance velocities on various laminated beams showed that the TOF method gave values that were higher by approximately 15% (Table 5-7). This discrepancy between TOF and resonance speed was described by Andrews (2002) and has been discussed in Chapter 2.4.3. It has to be mentioned that the TOF speed in this study was calculated by an electric pulse with a frequency of 54 kHz, while a hammer blow was used to calculate the resonance-based velocity. Table 5-7 shows that the TOF velocities in laminated beams of various configurations were higher by approximately 10% to 20% compared with the resonance velocities
before dowels were introduced. It is obvious that the biggest discrepancy in wave velocity between the two methods was evident in the sandwiched beams. This indicates that the electric pulse of the PUNDIT finds its fastest path in the longitudinal layers of the sandwiched beams. However, the velocity based on the resonance method is an average velocity of the whole system. This was demonstrated in a test conducted by Carter, Chuahan and Walker (2006) in which they tested logs, cants and boards from 24-year-old radiata pine trees using the resonance based tool Director HM200. They found that the acoustic velocity of the diametral cant was very close to the volume-weighted average velocity for all the boards cut from the diametral cant. The velocity measurements on various laminated beams based on the TOF method were far less affected by holes filled with dowels than the velocity measurements based on the resonance method. The resonance velocity decreased by approximately 10% (dependent on the harmonic from which it was calculated, and the configuration of the laminated beam) when a single 32mm centerline dowel was introduced at midspan, while the TOF velocity only dropped between 1-2% (Table 5-7). When various 25mm dowels were introduced to a LVL-L beam, the resonance velocity dropped by approximately 25% and the TOF velocity by 9%. This clearly shows that local defects have a significantly lower impact on the TOF wave speed in wood compared with the resonance wave speed. The reason for this is that the TOF velocity is based on the fastest path that is evident between the active probes. The 9% decrease in TOF velocity for the various hole test (Figure 5-15) of a LVL-L beam seems to be very high compared with a 25% decrease for the resonance-based velocity. This can be explained by the fact that, at the end of the test, there was no straight-through pathway left, since the last two holes were drilled at the edge of either side of the centerline.
5.6
Conclusion
It has been shown that velocity of sound in the longitudinal direction of a veneer sheet is five times higher than in the transverse direction. This indicates that the wood of
radiata pine is about 25 times stiffer parallel to the grain than across the grain. The damping ratio in the transverse direction was four times higher than the damping ratio in the longitudinal direction. The damping ratio for the clearwood beams and the LVL-L beams was between 0.35% and 0.47% and between and 0.45% and 0.6% for cross laminated beams. The increase of the damping ratio (maximum of 1.7%) was strongly affected by the knot volume and the orientation of the knot. It was found that edge knots placed parallel to the layers of the laminated beams had a greater impact on the damping ratio than a knot that was oriented perpendicular to the layers. Another important outcome of this study that the damping ratio can be used to detect defects (knots) in timber. However, it seems that the damping ratio cannot be applied to test engineered wood products that have a high degree of inhomogeneity such as plywood, as the frequency spectrum shows more discontinuities which are enhanced by introducing artificial defects.
It was found that the MOE is more suitable for describing the impact of artificial knots on clearwood and on laminated beams (especially if the orientation of the layers changes). In this context it should be noticed that it is not appropriate to base the calculation of the MOE on only one harmonic. This is because harmonics only detect defects in areas that are close to an anti-node for pressure. It was shown that the accuracy of the MOE prediction can be increased when it is based on the average of an even number of harmonics (half of them odd, half of them even).
When various knots were introduced to a LVL-L beam it was found that the MOE dropped negative exponential while the increase of the damping ratio could be best described as a linear regression line.
Static bending tests on the beams led to MOE values that were lower by 15-20 %, depending on the configuration of the beam, compared with the MOEs derived from acoustic testing.