Detection of Internal Holes in Swietenia mahagoni Disks Using a Stress Wave Device

The objectives of this study were to evaluate the feasibility of detecting different sizes and positions of hidden defects in trees using a 2-sensor stress wave device. In addition, we attempted to evaluate the detection resolution of the stress wave method which was rarely discussed previously. Artificial holes were created in Swietenia mahagoni disks to imitate natural decay in a trunk and were then detected using stress waves. The results revealed that as the diameter of the artificial hole increased, the radial transit time of the disk increased. This increasing tendency was repeated in the relationship between the radial relative transit time (RT) and the relative hole diameter (RD) and was discordant in the tangential direction. The tangential RT was greatly influenced by the position of the hole: it was highly related to the RD when the hole was near the 2 sensors; while it is not related to the RD when the hole was far from the 2 sensors. We also found that detection resolutions of stress waves, defined as the diameter or area ratio when the radial velocity had decreased by 10%, were 0.29 and 8.4%, respectively. A tree was considered to be decayed when the relative velocity (RV) was ＜ 0.9 and was considered hazardous when the RV was ＜ 0.66 with a central hole. On the basis of the results obtained, it appears that a 2-sensor stress wave device allows the nondestructive inspection of the presence, size, and location of defects in trees and could be a cost-effective way to perform tree risk assessments.