2 MATERIAL AND METHODS

Oil palm lumber sawn from logs with origin in Kluang (small-sized specimens) and Kulai (Peninsular) (large sized specimen) area (Malaysia) was used:

 112 large-sized specimens with a length of 1950 mm, width 130 – 300 mm, thickness 23 mm and 60 mm (imported as logs).

 120 small-sized specimens (1,150 × 22 × 22 mm³) (dry imports).

The imported logs were sawn in a sawmill into seamed side cuts and square-edged main cuts (large sized timber). This involves taking raw material in green state from 13 trunks at different stem heights (49 specimens from section 0.5 – 3.25 m; 34 from section 3.25 – 6.0 m; 20 from section 6.0 – 8.75 m; 3 from section 8.75 – 11.5 m). Ultrasonic time-of-flight and natural frequency measurements were first carried out on this wet oil palm wood. The material was further kiln dried in a laboratory kiln at the University of Hamburg, where general investigations on the drying process of oil palm wood were examined [5]. On the dry oil palm wood ultrasonic

time-of-flight and natural frequency measurements were carried out again to compare the wet and dry measurements.

To examine the influence of different span-to-depth ratios in the three-point-bending test (with a span-to-depth ratio up to 50x thickness), the small-sized specimens were prepared using preferably even cross-sectional bulk density distribution. The dynamic MOE was determined by ultrasound and natural frequency and a destructive three-point-bending test provided static MOE and MOR values.

Due to the anatomic structure of oil palm wood, larger specimens showed a more distinct bulk density gradient over the cross section compared to the small-sized specimens. Figure 1 illustrates the appearance and the anatomic structure of oil palm wood.

Figure 1: Cross section of an oil palm board with bulk density differences (left) and illustration of the anatomic structure (right).

Bulk density of the small-sized test specimens ranged from 170 kg/m³ to 525 kg/m³. All samples were categorized in bulk density classes. Prior to all measurements, all samples were conditioned to constant weight at 20°C and 65% relative humidity (according to DIN 50014:1985 [6]).

For all test specimens, ultrasonic time-of-flight and natural frequency were measured with subsequent dynamic modulus of elasticity (MOEdyn) computation. Ultrasonic time-of-flight measurements in longitudinal direction were carried out using the ultrasonic device STEINKAMP BP 5, Bremen, Germany, with plane and conical probes for longitudinal waves (50 kHz).

Coupling was performed directly without a coupling agent. On the large-sized specimens, the wet and dry measurements were performed at one marked measuring point within the cross section, because the measurement values may differ over the cross section, resulting in a substantial bulk density gradient. For the time-of-flight measurement on these boards the plane probes were handheld on both faces of the test specimen. Important for the comparison of the wet and dry values is to measure the time-of-flight at the same measuring points. In green condition,

the board is marked to ensure the compliance for the determination of the dry boards.

For the measurement of the small-sized specimens, the samples were fixed free-floating between spring-loaded conical probes in order to keep testing conditions (especially contact pressure) constant. MOE computation was done by the equation according to Steiger [7].

Natural frequency of longitudinal and flexural vibrations were determined using GrindoSonic MK5, J.W. Lemmens N.V., Leuven, Belgium (MOENF, long

and MOENF, flex). Longitudinal vibrations were used to compare the ultrasound results (MOEUS). Flexural vibrations were determined to illustrate the differences between respective vibration modes and for the comparison with the three-point bending test. During vibration measurements, the test specimens were supported on rectangular foam stripes in the region of the nodal points. Impulse was initiated by a small impact hammer. Dynamic MOE for flexural and longitudinal vibrations were calculated using the equations according to Görlacher [8]. Because of higher frequencies, the flexural vibrations on large-sized test specimens were determined by the 3^rd order vibration. The low frequencies resulting from the large dimensions of the boards of the 1^st order vibrations could not be detected by the testing device.

Three-point-bending tests (according to DIN 52186:1987 [9] using a support span of 50 × thickness) were carried out on the small-sized test specimens for comparative purposes.

3 RESULTS

The following results were determined on the small-sized specimens, except the investigations for the comparison between wet and dry measurements in Figure 8.

A compilation of measurement results and evaluated material properties is presented in Table 1 with mean values representing respective bulk density classes. Subject to the bulk density,

 Static MOE ranges between 613…10,500 N/mm²,

 MOR ranges between 3…50 N/mm²,

 Dynamic MOE from longitudinal vibration (GrindoSonic) between 800…12,360 N/mm²,

 Dynamic MOE from flexural vibration (GrindoSonic) between 767…9,570 N/mm², and

 Dynamic MOE from ultrasonic velocity (STEINKAMP) between 774…12,173 N/mm².

According to Table 1, both dynamic MOEs (MOEUS and MOENF, long) show higher values compared to the static MOE (MOEstat). MOE determined by ultrasonic measurement (MOEUS) is 20% higher than MOE of longitudinal vibration measurement (MOENF, long) and 46% above MOEstat, where

MOENF, long is 22% higher than the MOEstat. The computed MOE values from longitudinal vibration (MOENF, long) are around 16% higher compared to those from flexural vibration (MOENF, flex).

Table 1: Results (mean values) according to bulk density.

bulk relation between MOR and bulk density. Determined using a span-to-depth ratio l/h=50, the coefficient of determination for the correlation between the MOR and the MOEstat is R² = 0.9. Likewise, a strong correlation (R² = 0.6) between the MOR and the bulk density is given.

Figure 2: Relation between MOR and MOE from bending test (left) and MOR and bulk density (right).

Figure 3 shows a strong correlation between the ultrasonic velocity and MOR (R² = 0.7) and the frequency from longitudinal/ flexural vibration and the MOR (R² = 0.8/ R² = 0.7).

Figure 3: Relation between ultrasonic velocity respectively natural frequency from longitudinal/flexural vibration and MOR.

Ultrasonic velocity and natural frequency are the basis for calculating the dynamic MOE. To evaluate ultrasonic velocity and natural frequency as an own grading criteria, their relation to bulk density is analyzed in Figure 4.

Both dynamic methods show a strong correlation with R² = 0.6 (ultrasonic velocity / natural frequency with longitudinal vibration) and a moderate correlation with R² = 0.5 for natural frequency with flexural vibration (3^rd order).

Figure 4: Relation between sound velocity by ultrasonic respectively natural frequency with longitudinal/flexural vibration and bulk density.

Divergence of absolute dynamic and static MOE values with increasing bulk density becomes obvious in Figure 5. Furthermore, a strong correlation between all dynamic MOEs, respectively the static MOE and the density, is shown.

Figure 5: Relation between dynamic MOEs by ultrasonic respectively natural frequency with longitudinal vibration and static MOE by bending test.

The close linear relation between both dynamic MOE and static MOE is apparent in Figure 6: The coefficients of determination are given as R² = 0.96 for ultrasonic, R² = 0.99 for natural frequency measurement with longitudinal vibration and R² = 0.98 for natural frequency measurement with flexural vibration.

Given the strong correlation between the MOR and MOE from bending test (Figure 2) and the strong correlation between dynamic MOEs and static MOE (Figure 6), the correlation in Figure 7 can be explained. Based on these linkages, the correlation for the relation between dynamic MOEs by ultrasonic, respectively natural frequency with longitudinal and flexural vibration, and MOR by bending test are strong as well.

Figure 6: Relation between dynamic MOEs by ultrasonic respectively natural frequency with longitudinal/flexural vibration and static MOE.

Figure 7: Relation between dynamic MOEs by ultrasonic respectively natural frequency with longitudinal/flexural vibration and MOR.

The comparison between the measurements of the wet and dry large-sized test specimens in Figure 8 shows a strong correlation with R² = 0.91.

For some of the very wet boards with a high moisture content of about 300 % and a very low density, an ultrasonic velocity below 1000 m/s is determined. Furthermore, the first impulse, measured using the ultrasonic device, cannot be metered precisely. If these low and not precisely measured values (marked in Figure 8 with a circle) are not considered, the coefficient of determination increases to R² = 0.94.

Longitudinal vibrations and 1^st order flexural vibrations are not possible to measure for wet palm wood. 3^rd order vibrations deliver results, but only for the high bulk density or low moisture content with a weak correlation to the dry values. Possible reasons are described in chapter 4.

Figure 8: Relation between sound velocity from dry and wet palm wood.

4 DISCUSSION

In general, the ultrasonic velocity of wet and dry palm wood showed a strong correlation, so ultrasonic pre-grading of green palm wood seems to be possible. In this respect, the non-reproducible results for full size boards (large-sized specimens) with very low densities and high moisture content has to be considered. This can be explained with scattering effects. In dry material sound travels mainly through the fibres of the vascular bundles. In wet conditions the free bounded water in the vessels of the vascular bundles results in higher damping. The transmission of the impulse through water causes lower sound velocities.

Furthermore, investigations with 3^rd order flexural vibrations were carried out on wet and dry boards. Because the width of the seamed side cut boards, were difficult to measure, average values were assumed.

This inaccuracy is associated with a larger potential error since the width is included to the third power in the equation used to calculate the dynamic MOE. Prior measurements on dry large-sized coconut palm lumber have shown that results from boards with precise dimensions are accurate [3]. It was not feasible to measure longitudinal vibration on wet large-sized dynamic tests. The longer the force is applied, the lower the MOE. Another factor influencing the difference between the dynamic MOEs and the static MOE could be the ratio of E/G and G itself that is neglected in the calculation of the MOEs. It is assumed that oil palm wood features lower shear modulus compared to dicotyledonous wood species with similar bulk densities caused by the anatomic structure. At the time of compiling this report, no studies were available on the correlation of shear modulus.

Assuming that the shear modulus correlates with the shear strength and [10], [11] observed a low shear strength of oil palm wood, differences between MOEdyn and MOEstat may be explained. With an expected lower shear modulus for oil palm wood in relation to its MOE, according to [12] the correction factor E/G may have a significantly higher influence in the equation.

Higher values of the dynamic MOE determined with ultrasound compared to the dynamic MOE with natural frequency measurements are caused by bulk density gradients within the test specimens: according to [13] and [14], higher sound velocity can be observed in the area with the highest permeability -in case of oil palm wood the fiber caps of the vascular bundles. Because of the bulk density gradient within the boards, it is necessary to determine values from different measurement points over the cross section. Further studies are necessary to develop a mathematical model for determining the correct mean value for the MOE under consideration of the location in the steam height and cross section. In contrast, the anatomical structure affects only marginally the natural frequencies because the specimen vibrates through its entire dimensions.

The dynamic MOEs show a higher correlation to the MOR (R² = 0.78…0.86, cf. Figure 7) than density (R² = 0.62, cf. Figure 2) or frequency (R² = 0.74 or R² = 0.77, cf. Figure 3) or sound velocity (R² = 0.66, cf. Figure 3). So the dynamic MOE can be used as strength grading criteria.

5 CONCLUSION

In general, both ultrasonic and natural frequency measurements in longitudinal and flexural vibration are suitable for expanding the strength

grading of dry oil palm lumber. Flexural 3^rd order vibration measurements are appropriate especially for tests conducted under laboratory conditions with a large length to thickness ratio. To archive reliable data for large-sized timber, the boards must be square-edged. Longitudinal vibrations seem to be well suited to determine the dynamic MOE of dry large-sized specimens in component dimensions in an industrial production environment. For an industrial application, the in-line implementation of natural frequency measurements is less complicated than for ultrasonic devices. A high speed in-line system for industrial applications needs to be developed.

In cases involving small-sized specimens and an investigated influence of coupling with conical ultrasonic probes on either ground tissue or vascular bundles, ultrasonic coupling can be difficult [3]. Therefore, flat probes, especially for large-sized lumber, have to be examined further in an industrial context.

For green oil palm lumber in construction dimension with a high moisture content and low density, natural frequency measurements are not suitable.

For this reason, the measurement of the ultrasonic velocity provides significantly better results. It is advisable to omit specimens with very low density and or very high moisture content for the wet grading process because of the non-reproducible results.

Considering MOE determination via static bending test, the influence of the shear modulus seems to be higher for oil palm wood compared to dicotyledonous wood species because of its anatomic structure with (high density) vascular bundles embedded in (low density) parenchymatous ground tissue. Therefore, the shear modulus for oil palm wood should be determined to analyze the accuracy of the calculation of the static and dynamic MOE.

ACKNOWLEDGMENTS

The authors thankfully acknowledge the financial support provided by the German Federal Ministry of Education and Research through the “KMU-innovativ” project "Technologies for Lumber Processing from Oil Palm Trunks (02PK2458)", project coordinator: Simon Möhringer Anlagenbau GmbH, Wiesentheid, Germany. Additionally, they are grateful for the GrindoSonic MK5, STEINKAMP BP 5, and oil palm lumber provided for free by the Centre of Wood Sciences, University of Hamburg, Germany, and the additional STEINKAMP BP 5 provided by the Department of Civil Engineering, University of Applied Sciences Ostwestfalen-Lippe, Germany.

For the fundamental research on the span-to-depth ratio in the three-point-bending test, they thank Marvin Götza.