Detecting Sinker-Stock Lumber with Ultrasound Measurements

Bacterial infestation of the sapwood tissue resulting from wet storage of logs prior to processing destroys ray parenchyma. Damaged rays act as mass transfer pathways. Lumber with bacterial damaged rays, referred to as sinker-stock, results in excessive uptake of preservatives during treatment that requires further processing, such as drying. A window and door manufacturer requested an investigation to develop a reliable method for inline detection of sinker-stock lumber. An ultrasonic technique was used in this study based on a hypothesis that damaged rays would reduce intensity of sound waves transmitted through the wood in the radial direction. Intensity of sound waves through the thickness of sample sections was measured across the width including at least one through the radial direction. These measurements were done with a pair of 500 kHz transducers and with a pair of 120 kHz air-coupled transducers to obtain intensity imaging of the entire section. Based on these measurements, samples were classified into sinker, intermediate, and non-sinker groups. Microscopic examinations of samples verified that samples with low intensity measurements had nearly complete damage of the rays, samples with intermediate intensity measurements contained sporadic ray damage, and samples with little ray damage corresponded to samples with high intensity measurements. It is concluded that it is feasible to use multiple pairs of air-coupled transducers for inline detection of sinker-stock lumber