Effect of wood anisotropy in ultrasonic wave propagation: A ray-tracing approach

Ultrasonic nondestructive imaging methods allow analyzing the inner structures of trees, without altering their condition. In this study, we are interested in evaluating the influence of anisotropy condition in the wood on the ultrasonic waves time-of-flight (TOF) estimation, by means of a raytracing approach. This technique is used particularly in the field of exploration seismography to simulate wavefronts in elastic media. Wood sections from two species were tested. Defects in the wood were simulated by drilling holes. Defects were tested in two positions, centric and eccentric, and three different defect diameters were used for each position. First, experiments with healthy wood showed that the orthotropic behavior resulted in curved rays from the transmitter to every receiver, compared to the straight-line paths for the isotropic case, considering that the radial direction presents a higher wave velocity. Defects inside the wood resulted in low velocity propagation areas, that modified the trajectories compared to the healthy case. Centric defects resulted in larger TOF variations than eccentric defects. A combination of centric position and bigger size corresponded to a higher probability of decay detection using a tomographic image. To increase the tomographic image quality, curved rays should be considered when performing the image reconstruction.