Abstract
Ultrasound was considered as a means for determining mechanical properties of clear wood in six different Acacia mangium provenances from a trial forest planted in Vietnam. A total of 30 trees (5 trees from each provenance) with no major defects were selected, and a 50-cm-long log was obtained at 1.3 m above the ground from each tree for the assessment of mechanical properties. The measured average ultrasound velocities for provenances tested in the longitudinal direction ranged from 4094 m/s to 4271 m/s. The predicted average dynamic modulus of elasticity (Ed) values varied from 7.42 GPa to 8.70 GPa among provenances. The Ed indicated significant positive correlation coefficients with modulus of elasticity (0.64 to 0.96), modulus of rupture (0.44 to 0.87), and compression strength (0.54 to 0.92) for provenances examined in this study. The results indicated that the use of ultrasound was feasible to determine the mechanical properties of A. mangium provenances planted in Vietnam.
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Predicting Mechanical Properties of Clear Wood from Acacia mangium Provenances Using Ultrasound
Doan Van Duong a and Masumi Hasegawa b,*
Ultrasound was considered as a means for determining mechanical properties of clear wood in six different Acacia mangium provenances from a trial forest planted in Vietnam. A total of 30 trees (5 trees from each provenance) with no major defects were selected, and a 50-cm-long log was obtained at 1.3 m above the ground from each tree for the assessment of mechanical properties. The measured average ultrasound velocities for provenances tested in the longitudinal direction ranged from 4094 m/s to 4271 m/s. The predicted average dynamic modulus of elasticity (Ed) values varied from 7.42 GPa to 8.70 GPa among provenances. The Ed indicated significant positive correlation coefficients with modulus of elasticity (0.64 to 0.96), modulus of rupture (0.44 to 0.87), and compression strength (0.54 to 0.92) for provenances examined in this study. The results indicated that the use of ultrasound was feasible to determine the mechanical properties of A. mangium provenances planted in Vietnam.
Keywords: Acacia mangium; Non-destructive evaluation; Ultrasound; Modulus of elasticity; Dynamic MOE; Mechanical properties
Contact information: a: Faculty of Forestry, Thai Nguyen University of Agriculture and Forestry, Thai Nguyen, Vietnam; b: Laboratory of Wood Science, Faculty of Agriculture, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan;
*Corresponding author: kmgtmsm@agr.kyushu-u.ac.jp
INTRODUCTION
Acacia mangium, one of the most important plantation forest tree species in Vietnam, is mainly planted in the Northeast and the North Central regions (Vietnam Ministry of Agriculture and Rural Development 2017). In Vietnam, plantations of A. mangium were originally established for the production of pulp, paper, and particleboard. In recent decades, A. mangium tree breeding programs in Vietnam focused on increasing the quantity and quality of wood products through the appropriate selection of seed provenances within species. However, growth and tree-form properties have been the focus of selection due to the cost of measuring wood properties. One of the main limitations in the breeding programs is the lack of genetic parameters based on wood properties, although wood is the final desired product. Thus, it is necessary to include the characteristics and properties of wood into breeding programs for establishing timber from A. mangium plantations in Vietnam. In addition, to gain the maximum benefits from commercial forestry, it is necessary to improve the capacity for early selection (Schimleck et al. 2019). Therefore, the use of advanced technologies is an efficient way to improve the quality of timber resulting from tree breeding programs.
Non-destructive evaluation (NDE) of wood properties has received much attention during the past few decades because it has contributed considerably toward reducing the limitations of the destructive tests such as expense, time consumption, and damage to experimental material (Wang et al. 2001). Pellerin and Ross (2002) defined NDE as “the science of identifying the physical and mechanical properties of a piece of material without altering its end-use capabilities and then using this information to make decisions regarding appropriate applications”.
There are widespread non-destructive techniques, equipment, and evaluation procedures available today that are used to assess properties of trees (Hasegawa and Sasaki 2000; Schimleck et al. 2006; Guntenkin and Aydin 2016; Duong and Matsumura 2018; Duong and Ridley-Ellis 2021). Among various NDE methods, the acoustic technique using the ultrasonic wave propagation in wood is considered as a good option for the prediction of wood stiffness without modifying its end-use (Karlinasari et al. 2008; Vazquez et al. 2015; Posta et al. 2016). In addition, the benefit of the ultrasonic techniques is the capability to test small specimens and the possibility of testing the same specimens several times due to the nondestructive nature of these measurements (Bucur 2006; Vazquez et al. 2015). For example, Vazquez et al. (2015) reported that the ultrasound technique is a powerful method for determining the elastic constants of small specimens (20 × 20 × 40 mm3) from Castanea sativa wood. The significant relationships between the static and the ultrasound dynamic moduli of elasticity for small clear specimens were also stated in other hardwood species (Baar et al. 2015; De Melo et al. 2020). However, published information on using ultrasonic techniques for assessing mechanical properties of A. mangium – one of the most important woody species in tropical and sub-tropical countries – is currently limited with a few studies (Nugroho et al. 2012; Sharma and Shukla 2012).
The aim of this study was to evaluate the potential of an ultrasonic technique by testing the feasibility of using small clear wood specimens from previous static bending properties of six different A. mangium provenances planted in Vietnam (Duong et al. 2021). The properties of small clear specimens are expected to be usefully representative of these properties for full-sized sawn timber, except for strength. However, in this context, it was hypothesized that ultrasonic measurements will not be better on sawn timber than on small clear wood specimens. The main objective of this study was to determine the strength of relationship between dynamic modulus of elasticity (Ed) using longitudinal ultrasound propagation and mechanical properties (modulus of elasticity – MOE, modulus of rupture – MOR; and compression strength – CS) obtained by destructive methods.
EXPERIMENTAL
Sampling
A total of 30 trees (5 trees from each provenance) were collected from six A. mangium provenances (listed in Table 1) that were established as a trial of testing the growth rate and stem quality of different provenances by the Vietnamese Academy of Forest Science since 2014 (Table 1). The trial site was located in Cam Hieu commune, Cam Lo district, Quang Tri province in center Vietnam (16°46’14″N and 107°01’28″E). A detailed description about the trial forest is described in the authors’ previous paper (Duong et al. 2021).
Table 1. Mean Values and Standard Deviations of Growth and Static Bending Properties in Each Provenance (Duong et al. 2021)
From each tree, four samples [20 (radial) × 20 (tangential) × 300 (longitudinal) mm3] were carefully cut from parts near the pith and near the bark (two samples from each radial position) at 1.3 m height above the ground and dried under laboratory conditions at a constant temperature (20 °C) and relative humidity (60 %) to constant weight, as presented in the authors’ previous study (Duong et al. 2021). Three-point static bending test was conducted using a universal testing machine (Autograph AG-G, Shimazu, Kyoto, Japan) with a bending span of 260 mm. The load was applied at the radial face of center of the specimens at a speed of 5 mm per minute. After static bending tests (MOR and MOE), specimens (20 × 20 × 40 mm3) for ultrasound measurement and compression test were sampled from the ends of the bending samples, if no mechanical damage was observed. Some specimens containing knots, irregular grain, and cracks were rejected. The total number of small clear specimens cut from six provenances was 117. The specimens were continuously conditioned to constant mass at a temperature of 20 °C and a relative humidity of 60% and maintained in this condition until required for testing. The overall mean value of MC in all observed provenances was 9.33%.
Ultrasonic Measurement
Before ultrasonic measurement, the air-dry density (AD) of the specimens was determined by the ratio between mass and volume of the samples. The ultrasonic wave velocities were measured with a setup comprising of a pulser-receiver (JPR-10CK; Japan Probe Co., Ltd., Yokohama, Japan), preamplifier, and monolithic composite transducers (14 mm × 20 mm-type) with a resonant frequency of 200 kHz according to the method described by Duong et al. (2019). Figure 1 illustrates the test and the equipment utilized for the ultrasonic measurement. The propagation time measurement was repeated three times for each specimen, and an average value was used as the experimental value. The longitudinal velocity (Vu) was obtained as a ratio of the length of wood specimen in longitudinal direction to the wave propagation time. The dynamic modulus of elasticity (Ed) was calculated using Eq. 1,
(1)
where Ed is dynamic modulus of elasticity (Pa), AD is air-dry density (kg/m3), and Vu is propagation speed (m/s) of ultrasonic waves.
Fig. 1. Illustration of ultrasonic measurement
Compression Strength
After ultrasonic measurement, CS was assessed for each specimen using an Instron Tester (Autograph AG-G, Shimazu, Kyoto, Japan) in accordance with Japanese industrial standards JIS Z2101:1994 (2000). Compression parallel to the grain was performed using a 100 kN load in the universal testing machine, with 1% load accuracy, and the displacement was measured using the machine cross-head displacement, with a 1% deformation accuracy. After compression test, moisture content (MC) was determined by the oven-drying method for each wood specimen.
Data Analysis
All statistical analyses were performed using R software version 4.0.0. (Version 4.0.0; RStudio, Boston, MA, USA). Mean values for each provenance were obtained using mean values calculated from five sample trees for evaluating the variation in Vu and wood properties among provenances. The data of each measured parameter were analysed using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test with the level of significant differences at P < 0.05.
RESULTS AND DISCUSSION
Table 2 presents the variations in Vu and wood properties among six different A. mangium provenances planted in Vietnam. The ANOVA showed significant differences in Vu among provenances. In all examined provenances, the overall mean of Vu was 4170 m/s. The minimum velocity (4094 m/s) was measured in the provenance BV, and the maximum velocity (4271 m/s) was measured in the provenance BLM (Table 2). These results are in accordance with those obtained by Shamar and Shukla (2012) and Duong et al. (2019) for small clear specimens in the longitudinal direction of A. mangium and Melia azedarach, respectively (Table 3). Using ultrasonic wave velocity, Hasegawa et al. (2015) and Ribeiro et al. (2013) reported a mean Vu of 4500 m/s in A. auriculiformis and 5057 m/s in Eucalyptus grandis, both being hardwoods of higher air-dry density than A. mangium provenances examined in this study (Table 3). It is likely that the ultrasonic velocity is observed to be higher in higher-density wood than in lower-density wood (de Oliveira and Sales 2006; Duong et al. 2019). In contrast, several studies confirmed that the sound propagation velocity is not dependent on wood density (Mishiro 1996; Ilic 2003). The propagation of sound in wood is influenced by factors, such as density, angle of cellulose microfibrils (S2 layer), moisture content, decay, temperature, and geometry, of the specimen (Bucur and Böhnke 1994; Kabir et al. 1997; Baar et al. 2012). Therefore, it is difficult to establish a direct influence of wood density on sound propagation velocity, which explains the varying conclusions of the above reports.
Table 2. Acoustic and Wood Properties of Acacia mangium Trees from Six Provenances (Five Trees for Each Provenance)
Table 2 also shows the among-provenance variation in AD of six A. mangium provenances examined in this study. The overall mean value of AD was 0.47 g/cm3, ranging from 0.44 g/cm3 for provenance BB to 0.50 g/cm3 for provenance LT. The result of ANOVA analysis shows the significant differences in AD among provenances (Table 2). Makino et al. (2012) and Jusoh et al. (2014) reported that the mean values of wood density of A. mangium planted in Indonesia and Malaysia were 0.45 g/cm3 and 0.46 g/cm3, respectively.
Table 3. Ultrasonic Velocity in the Longitudinal Direction in this Study and Previous Studies
In this study, the overall mean values of Ed and CS in six provenances were 8.18 GPa and 44.60 MPa, respectively. Makino et al. (2012) reported that the lower mean CS for 5- and 7-year-old A. mangium trees planted in Indonesia was 30.00 MPa and 32.80 MPa, respectively. There was a significant difference among provenances in Ed, while the variation of CS among provenances was small and without statistical significance (Table 2). It is noteworthy that the lowest values of both Ed and CS were sawn in the provenance BB (respective values for Ed and CS were 7.42 GPa and 41.91 MPa) and the highest values of Ed and CS were sawn in the provenance LT (respective values for Ed and CS were 8.70 GPa and 47.30 MPa) (Table 2). The findings of the present study are in agreement with the authors’ previous study, wherein the highest average value of MOE and MOR measured by destructive method were observed in the provenance LT (Duong et al. 2021). Therefore, this result once again indicates that LT might be more appropriate provenance than the others examined for breeding programs focused on timber or saw log of A. mangium in the north central region of Vietnam.
Table 4 summarizes the results of the relations of Vu and Ed with mechanical properties (MOE, MOR, and CS) for six different A. mangium provenances examined in this study. Correlation coefficients for Vu and mechanical properties were different among the six provenances. In provenances BLM, HY, and combined provenances, significant positive correlation coefficients were found between Vu and mechanical properties (Table 4 and Figs. 2A, C, and E). However, in the other four provenances, there were no statistically significant correlations between Vu and mechanical properties, except for the relationship between Vu and MOE in provenance BB (r = 0.49; P < 0.05) (Table 4).
There is a contradiction in the literature on whether Vu is correlated with mechanical properties or not. Sharma and Shukla (2012) obtained good relationships (r2 = 0.95 to 0.98) for small clear specimens between ultrasonic velocity along the longitudinal direction and MOE in air-dry condition of Acacia mangium, Grevillea robusta, and Mangifera indica. Duong et al. (2019) also reported a strong positive correlation existed between Vu and CS (r = 0.70) of Melia azedarach. In contrast, Mascarenhas et al. (2021) showed that there was a significant (P < 0.001) but weak correlation (r2 = 0.29) between ultrasonic propagation speed and bending strength of tropical wood species. In addition, Duong and Ridley-Ellis (2021) reported a poor relationship between stress wave velocity and MOE (r2 = 0.23) and no significant relationship between stress wave velocity and MOR for Melia azedarach. Based on the present results and previous reports, the relationship between acoustic velocity and static bending properties depends on tree species.
Table 4. Correlation of Vu and Ed with Mechanical Properties at Provenance Level
Fig. 2. Relationships between longitudinal ultrasonic velocity (Vu), dynamic modulus of elasticity (Ed), and mechanical properties (MOE, MOR, and CS)
Significant relationships were found between Ed and mechanical properties measured by destructive method in each provenance as well as in combined provenances (Table 4). The correlation coefficient between Ed and MOE in combined provenances was 0.83 (P < 0.001) that ranged from 0.64 for provenance HY to 0.96 for provenance BLM (Table 3 and Fig. 2B). In general, the results from other studies pointed out that NDE methods based on propagation speed of ultrasonic waves are suitable for measurement of the Ed and have a good relationship with the destructive static bending test (de Oliveira et al. 2002; Karlinasari et al. 2008; Baar et al. 2015). In this study, the relationships between Ed and CS were observed to be moderately good to very good correlations (r = 0.54 to 0.92) (Table 4). When all provenances were considered together, the correlation coefficient between Ed and CS was 0.84 (P < 0.001) (Fig. 2F).
The correlation coefficient between Ed determined using ultrasound and bending strength from destructive test was 0.75 (P < 0.001) when all samples of the observed provenances were combined (Fig. 2D). The correlation coefficients ranged between 0.44 and 0.87 for individual provenances (Table 4). Considering the relationship between Ed and MOR in this study, the results were similar to those from tropical wood species (Baar et al. 2015; Mascarenhas et al. 2021). Using ultrasound, de Oliveira et al. (2002) reported coefficients of determination between Ed and MOR for Goupia glabra and Hymenaea sp. were 0.36 and 0.55, respectively. The lower accuracy of the ultrasound method for prediction of MOR than MOE and CS is probably caused by its property in the measurement path. The acoustic velocity is directly related to elasticity (stiffness) not the failure point of the material; therefore, Ed only reflects the properties in the measurement path of the sample (Daniels and Clark 2006).
CONCLUSIONS
- Coupled with the higher modulus and elasticity (MOE) and modulus of rupture (MOR), provenance LT also had higher dynamic modulus of elasticity (Ed) and compression strength (CS) than the other provenances examined in this study. Therefore, LT might be selected for A. mangium tree breeding programs focused on improving wood quality specifically for lumber productions. However, as the present study was based on a single site, further research must be done to examine its properties across a range of locations.
- There were significant positive correlations between Ed and mechanical properties for small clear specimens at MC approximately 9.33%. Therefore, it is highly promising that the non-destructive ultrasonic method can be used to evaluate mechanical properties of A. mangium wood in situations where a static bending test is not feasible to undertake. In the future, the authors will measure the properties of standing trees with ultrasound method and report the prediction of stiffness in lumber production.
ACKNOWLEDGMENTS
This research was funded by Vietnam Ministry of Education and Training, Grant No. B2020-TNA-05.
REFERENCES CITED
Baar, J., Tippner, J., and Gryc, V. (2012). “The influence of wood density on longitudinal wave velocity determined by the ultrasound method in comparison to the resonance longitudinal method,” Eur. J. Wood Wood Prod. 70(5), 767-769. DOI: 10.1007/s00107-011-0550-2
Baar, J., Tippner, J., and Rademacher, P. (2015). “Prediction of mechanical properties – modulus of rupture and modulus of elasticity – of five tropical species by nondestructive methods,” Maderas. Cienc. Tecnol. 17(2), 239-252. DOI: 10.4067/S0718-221X2015005000023
Bucur, V. (2006). Acoustics of Wood, 2nd Ed., Springer Series in Wood Science, Springer, Berlin, Heidelberg.
Bucur, V., and Böhnke, I. (1994). “Factors affecting ultrasonic measurements in solid wood,” Ultrasonics 32(5), 385-390. DOI: 10.1016/0041-624X(94)90109-0
Daniels, R. F., and Clark, A. (2006). Quantifying and Predicting Wood Quality of Loblolly and Slash Pine Under Intensive Forest Management (Report No. DOE/FR/1
TRN: US200716%%165), University of Georgia, Athens, GA, USA.
De Melo, R. R., Barbosa, K. T., Beltrame, R., Acosta, A. P., Pimenta, A. S., and Mascarenhas, A. R. P. (2020). “Ultrasound to determine physical-mechanical properties of Eucalyptus camaldulensis wood,” Wood Mater. Sci. Eng. 1(1), 1-7. DOI: 10.1080/17480272.2020.1830435
De Oliveira, F. G. R., and Sales, A. (2006). “Relationship between density and ultrasonic velocity in Brazilian tropical woods,” Bioresource Technol. 97(18), 2443-2446. DOI: 10.1016/j.biortech.2005.04.050
De Oliveira, F. G. R., de Campos, J. A. O., and Sales, A. (2002). “Ultrasonic measurements in Brazilian hardwood,” Mater. Res. 5(1), 51-55. DOI: 10.1590/S1516-14392002000100009
Duong, D. V., and Matsumura, J. (2018). “Within-stem variations in mechanical properties of Melia azedarach planted in northern Vietnam,” J. Wood Sci. 64(1), 329-337. DOI: 10.1007/s10086-018-1725-9
Duong, D. V., and Ridley-Ellis, D. (2021). “Estimating mechanical properties of clear wood from ten-year-old Melia azedarach trees using the stress wave method,” Eur. J. Wood Wood Prod. 79(4), 941-949. DOI: 10.1007/s00107-021-01664-8
Duong, D. V., Hasegawa, M., and Matsumura, J. (2019). “The relations of fiber length, wood density, and compressive strength to ultrasonic wave velocity within stem of Melia azedarach,” J. Ind. Acad. Wood Sci. 16(1), 1-8. DOI: 10.1007/s13196-018-0227-0
Duong, D. V., Schimleck, L., and Tran, D. L. (2021). “Variation in wood density and mechanical properties of Acacia mangium provenances planted in Vietnam,” Eur. J. Wood Wood Prod. (Submitted).
Guntenkin, E., and Aydin, T. Y. (2016). “Prediction of bending properties for some softwood species grown in Turkey using ultrasound,” Wood Res. 61(6), 993-1002.
Hasegawa, M., and Sasaki, Y. (2000). “Acoustoelastic effect of wood III: Effect of applied stresses on the velocities of ultrasonic waves propagating normal to the direction of the applied stress,” J. Wood Sci. 46(2), 102-108. DOI: 10.1007/BF00777355
Hasegawa, M., Mori, M., and Matsumura, J. (2015). “Relations of fiber length to within-tree variation of ultrasonic wave velocity in fast-growing trees,” Wood Fiber Sci. 47(3), 313-318.
Ilic, J. (2003). “Dynamic MOE of 55 species using small wood beams,” Holz. Roh. Werkst. 61(3), 167-172. DOI: 10.1007/s00107-003-0367-8
JIS Z2101:1994 (2000). “Methods of test for woods,” Japanese Standard Association, Tokyo, Japan. (In Japanese)
Jusoh, I., Zaharin, F. A., and Adam, N. S. (2014). “Wood quality of Acacia hybrid and second-generation Acacia mangium,” BioResources 9(1), 150-160. DOI: 10.15376/biores.9.1.150-160
Kabir, M. F., Sidek, H. A. A., Daud, W. M., and Khalid, K. (1997). “Effect of moisture content and grain angle on the ultrasonic properties of rubber wood,” Holzforchung 51(3), 263-267. DOI: 10.1515/hfsg.1997.51.3.263
Karlinasari, L., Wabyuna, M. E., and Nugroho, N. (2008). “Non-destructive ultrasonic testing method for determining bending strength properties of Gmelina wood (Gmelia arborea),” J. Trop. For. Sci. 20(2), 99-104.
Makino, K., Ishiguri, F., Wahyudi, I., Takashima, Y., Iizuka, K., Yokota, S., and Yoshizawa, N. (2012). “Wood properties of young Acacia mangium trees planted in Indonesia,” Forest Prod. J. 62(2), 102-106. DOI: 10.13073/0015-7473-62.2.102
Mascarenhas, A. R. P., de Melo, R. R., Pimenta, A. S., Stangerlin, D. M., de Oliveira Correa, F. L., Sccoti, M. S. V., and de Oliveira Paula, E. A. (2021). “Ultrasound to estimate the physical-mechanical properties of tropical wood species grown in an agroforestry system,” Holzforschung 75(10), 1-13. DOI: 10.1515/hf-2020-0249
Mishiro, A. (1996). “Effect of density on ultrasonic velocity in wood,” Mokuzai Gakkaishi 42, 887-894. (In Japanese)
Nugroho, W. D., Marsoem, S. N., Yasue, K., Fujiwara, T., Nakajima, T., Hayakawa, M., Nakaba, S., Yamahishi, Y., Jin, H. O., Kobo, T., and Funada, R. (2012). “Radial variations in the anatomical characteristics and density of Acacia mangium of five different provenances in Indonesia,” J. Wood Sci. 58(3), 185-194. DOI: 10.1007/s10086-011-1236-4
Pellerin, R. F., and Ross, R. J. (2002). Nondestructive Evaluation of Wood (Report No. FPL-GTR-238), Forest Product Society, Madison, WI, USA.
Posta, J., Ptacek, P., Jara, R., Terebesyova, M., Kuklik, P., and Dolejs, J. (2016). “Correlations and differences between methods for non-destructive evaluation of timber elements,” Wood Res. 61(1), 129-140.
Ribeiro, P. G., Goncalez, J. C., Goncalves, R., Teles, R. F., and de Souza, F. (2013). “Ultrasound waves for assessing the technological properties of Pinus caribaea var. hondurensis and Eucalyptus grandis wood,” Maderas. Cienc. Tecnol. 15(2), 195-204. DOI: 10.4067/S0718-221X2013005000016
Schimleck, L. R., Downes, G. M., and Evans, R. (2006). “Estimation of Eucalyptus nitens wood properties by near infrared spectroscopy,” Appita J. 59, 136-141.
Schimleck, L., Dahlen, J., Apiolaza, L. A., Downes, G., Emms, G., Evans, R., Moore, J., Paques, L., Van den Bulcke, J., and Wang, X. (2019). “Non-destructive evaluation techniques and what they tell us about wood property variation,” Forests 10(9), Article Number 728. DOI: 10.3390/f10090728
Sharma, S. K., and Shukla, S. R. (2012). “Properties evaluation and defects detection in timbers by ultrasonic non-destructive technique,” J. Ind. Acad. Wood Sci. 9(1), 66-71. DOI: 10.1007/s13196-012-0064-5
Vazquez, C., Goncalves, R., Bertoldo, C., Bano, V., Vega, A., Crespo, J., and Guaita, M. (2015). “Determination of the mechanical properties of Castanea sativa Mill. using ultrasonic wave propagation and comparison with static compression and bending methods,” Wood Sci. Technol. 49(3), 607-622. DOI: 10.1007/s00226-015-0719-7
Vietnam Administration of Forestry (2017). “Decision Approving: Planning on Conversion of Forest Plant Species to Serve the Project on Restructuring the Forestry Sector, No. 23/QD-BNN-TCLN,” Vietnam Ministry of Agriculture and Rural Development, Ha Noi, Vietnam.
Wang, X., Ross, R. J., McClellan, M., Barbour, R. J., Erickson, J. R., Forsman, J. W., and McGinnis, G. D. (2001). “Nondestructive evaluation of standing trees with a stress wave method,” Wood Fiber Sci. 33(4), 522-533.
Article submitted: July 23, 2021; Peer review completed: October 5, 2021; Revised version received and accepted: October 25, 2021; Published: October 28, 2021.
DOI: 10.15376/biores.16.4.8309-8319