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 deltoides, 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. The presence of wetwood in lumber is a severe processing problem for the lumber industry. Wetwood, which is also known as bacterially infected wood, causes serious drying defects such as excessive and deep surface checking, honeycomb, collapse, and ring separation. The wetwood-related drying defects in oak lumber alone are as much as 500 million board feet, which costs the lumber industry about $25 million per year (Murdoch 1992). The losses due to honeycomb and ring separation during kiln-drying of oak lumber are in the order of 10 to 25 percent of the dry lumber volume. In addition to the drying defects, other problems associated with machining, finishing, gluing, and odor have been reported (Murdoch 1992). Bacterial infection in living oak trees has been associated with changes in land use practices, including logging, flooding, and impeded drainage of clay soils; growth on bottomland or heavy clay soils; over maturity of the stand; and root injuries (Murdoch 1992). Wetwood is caused by aerobic and anaerobic bacterial infection in the heartwood of living trees. It can be differentiated from healthy wood by its visibly darkened color, vinegar-like odor, higher moisture content (MC), decreased concentration of gaseous nitrogen and oxygen, higher pH, and lower electrical resistance. Bacteria in wetwood produce acetic acid and fatty acids that are often rancid and have characteristics like vinegar (Schink and Ward 1984). It is difficult to identify live trees infected with bacteria unless some external indicators such as fluxing of bacterial metabolic products occur through wounds in the bark. Wetwood has a considerable effect on the physical and mechanical properties of wood. Xu et al. (2001b) reported that wetwood has a greater MC, abnormally high radial and tangential shrinkage, and lower tension strength perpendicular to the grain for red oaks. A higher specific gravity (SG) of wetwood of western hemlock was reported by Schroeder and Kozlic (1972). Enzymes produced by the bacteria degrade hemicellulose and pectins present in the middle lamella of the cell wall, which develops abnormal checking and honeycombing in the wood. For the last few years, several methods have been employed for detecting wetwood in living trees and lumber using stress The authors are, respectively, Corporate Quality Manager, American Woodmark Corp., Winchester, VA ([email protected]); Project Leader, USDA Forest Serv., Southern Res. Sta., Stoneville, MS ([email protected]); Project Leader and Forestry Technician, USDA Forest Serv., Southern Res. Sta., Blacksburg, VA (paraman@ vt.edu; [email protected]). This paper was received for publication in July 2004. Article No. 9906. ✳Forest Products Society Member. ©Forest Products Society 2006. Forest Prod. J. 56(3):70-74.