Detection of wetwood by ultrasonics
Wetwood, or bacterially infected wood, is a severe processing problem and causes serious drying defects in lumber. The detection of wetwood is, therefore, important for proper processing and quality wood products. An investigation has been carried out to detect wetwood of cherrybark oak (Quercus pagoda Raf.), water oak (Quercus nigra, L.), hickory (Carya spp.), and eastern cottonwood (Populus deltoids, Bartr. ex Marsh.) using ultrasound signals. The ultrasound measurements were carried out at different moisture contents (MCs) in three different grain directions: longitudinal (L), radial (R), and tangential (T). Each ultrasonic waveform was characterized using eight ultrasonic variables: three involving time of flight, two involving ultrasound pulse energy, one using ultrasound pulse duration, and one for peak frequency. Linear positive correlations were found for most of the time of flight (TOF) measurements with MC. A significant strong correlation was found for TOF-energy. Wetwood exhibited higher MC than healthy wood for all species and higher specific gravity for hickory. Results also showed that wetwood has a higher TOF and greater loss of energy compared to healthy wood. The grain orientation has a significant effect on ultrasound signal propagation with the lowest TOF and energy loss in the L direction. This study suggests that wetwood in living trees as well as in lumber can be identified using ultrasound-based systems.