Acousto-Ultrasonic Assessment of Internal Decay in Glulam Beams

Markku Tiitta, Frank C. Beall, Jacek M. Biernacki

Abstract


An acousto-ultrasonic (AU) through-transmission technique was evaluated for assessing brown-rot decay in Douglas-fir glulam beams that had been removed from service. The effect of decay on different AU signal features was compared to that from normal variations in wood, such as growth ring angle, knots, and moisture gradient. The analysis was based on measurement of velocity, attenuation, shape, and frequency content of the received signals. All of the studied signal features were correlated with the degree of decay; however, they were affected by natural characteristics of wood. Attenuation and signal shape were more affected by the growth ring angle variations and knots than were velocity and frequency features. The effect of knots depended upon size, type, orientation, and distance from the surface. A steep moisture gradient obscured the detection of small degrees of decay, with the greatest effect on signal shape and frequency parameters. This study suggests that multiple signal feature analysis can be used to distinguish decay from certain types of natural wood characteristics such as growth ring angle variations and knots.

Keywords


Acousto-ultrasonics;brown-rot decay;glulam;growth ring angle;moisture gradient;knot;wood

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