S-Space College of Agriculture and Life Sciences (농업생명과학대학) Dept. of Forest Sciences (산림과학부) Theses (Master's Degree_산림과학부)
Detection of the presence of heart rots in Teak trees by stress wave method
- Jun-Jae Lee
- 농업생명과학대학 산림과학부
- Issue Date
- 서울대학교 대학원
- Environmental Materials Science
- 학위논문 (석사)-- 서울대학교 대학원 : 산림과학부, 2014. 8. Jun-Jae Lee.
- Teak tree is famous for its strength, attractive grain pattern, resistance to termites, its durability and pleasant color. Over the past decades, teak natural forests in Myanmar have been degraded because of extraction of timber to export to foreign countries, illegal logging and extensive requirement of timber because of population pressures. Myanmar Government has planned and implemented to establish teak plantations throughout the country to get sustainable supply of desirable amount and quality of timber from teak plantation without increasing pressure on natural teak forests.
Thinning operation is necessary for successful teak plantation. By eliminating some trees from a plantation, the remaining trees can get more sunlight, nutrient and reducing competition among them. Wrongly choosing sound trees to fell and leaving defective trees will lead to a great economic loss. Visual inspection cannot detect the defects located inside tree trunks like heart rots. Although the existing stress wave CT instruments provide a tree cross-section with tomograph, a deep experience is necessary to correctly decide tree condition by interpreting the images. To avoid wrong decision-making, additional criteria were required to decide the presence of defect. The purpose of this study was to develop criteria to determine the presence of heart rots in teak trees.
When stress wave CT imaging instrument is used to inspect trees, transducers are mounted around the circumference of tree stem. Transducer numbers 8, 10 and 12 were used to measure the time of flight among transducers. The angles among transducers were measured on the basis of a straight baseline (180˚) in a cross-section. The angles among 8, 10 and 12 transducers were (45˚, 90˚, 135˚ and 180˚), (36˚, 72˚, 108˚, 144˚, and 180˚) and (30˚, 60˚, 90˚, 120˚, 150˚ and 180˚) respectively. To develop reference velocity for each transducer angle, 25 sound discs were used. As heart rot forms near the centre of a tree stem, the cross-section was distinguished into two parts
outer zone and inner zone based on transducers angles. Outer zone includes angles smaller and equal to 90˚ and inner zone includes angles larger than 90˚.
The result defines that the velocity of outer zone is slower than that of inner zone in 25 sound discs. This is because the smaller angles (≤90˚) are closely oriented with the annual rings in tangential direction and the larger angles are nearer to radial direction. Firstly, analysis of the average velocities of inner zone with outer zone was carried out on referenced 25 sound discs. ADV (Average Difference Velocity %) between these two zones of each disc with each transducer number was calculated. The reference mean ADV indices of 25 sound discs were determined (-6.26%, -9.53% and -8.35%) for 8, 10 and 12 transducers relatively.
Same experimental measurements on 10 sound discs and 9 heart-rot discs were done. The ADVs of 10 sound discs range from (-8.61% ~ -15.25%), (-5.7% ~ -11.62%) and (-6.97% ~ -15.94%) for 8, 10 and 12 transducers. The ADVs of 9 heart-rot discs range from (10.68% ~ -8.38%), (14.2% ~ -5.26%) and (9.93% ~ -7.4%) for 8, 10 and 12 transducers. Since heart rots locate in the inner zone of cross-section, the average velocity of outer zone is not much slower or even faster than that of inner zones average velocity in defective discs. The resulted ADVs of 19 tested discs were compared with reference ADV indices. The result indicated that ADV indices could successfully distinguish sound and heart-rot discs with high accuracy (89.5 ~ 100%).
Secondly, the measured velocities of 10 sound and 9 heart-rot discs were compared with reference velocity at each transducer angle. The DIVs (Dissimilarity in Velocity) were calculated for each disc. Based on average ± DIVs of all angles and of only inner zones angles of 19 tested discs, calibration was performed to distinguish sound and heart-rot discs. Although the accuracy (59.1 ~ 63.6%) of DIV indices did not reach satisfactory level, a linear relationship between heart-rot area ratios of defective discs and their calculated DIVs was found. This study concludes that the developed indices can be used as decision-making criteria to detect heart rots and to predict their area ratios while using the stress wave CT imaging instruments.