Ultrasonic and thermal testing to non-destructively identify internal defects in plantation eucalypts

Effective non-destructive methods for identifying poor quality and knotty timber in standing trees can contribute to higher quality timber reaching the appropriate processing mills and to a reduction in transport costs of harvested timber. Eucalyptus nitens is a major temperate plantation hardwood species primarily used in paper production. More recently it has also been considered as a potentially suitable source to produce high quality sawlogs. One important aspect of the economic viability of this sawlog business is being able to differentiate between pruned (higher quality) and unpruned (lower quality) plantation eucalyptus. Low quality stems contain higher percentages of features such as knots and/or branch traces that lessen the quality and value of logs that can be produced. Unfortunately, it is not possible to visually discriminate between pruned and unpruned standing trees and while wood processors do use large x-ray image machines during processing, finding ways to reduce transportation costs by enhancing timber quality remains a major challenge. This research presents results from ultrasonic and thermal testing to non-destructively detect internal defects in 17 year old E. nitens grown in Tasmania, Australia. For the investigation, 12 samples (billets) from different parts of the trunk of an E. nitens tree were selected and conditioned to levels of forest moisture content of 120% (70% water content). The samples were scanned by two ultrasonic techniques through-transmission and pulse-echo. In the through-transmission method, the waves were propagated at 10 cm intervals in the longitudinal direction and at 45 degree angle spacing in the circumferential direction. In the pulse-eco method, the surface of the billets were scanned in 10cms intervals around the billets. Unpruned billets were also evaluated for moisture content using a thermal camera to examine the effect on ultrasonic wave propagation. The ultrasound revealed significant differences between recorded ultrasonic waveforms propagated through unpruned billets and pruned ones. Unpruned billets produce a much larger effect on ultrasonic waves when compared with the waves propagated by the pruned billets tested. The thermal camera highlighted that wet knotty wood tends to have higher moisture content than clear wood except when the timber is air dried which produces a slightly lower moisture content being detected in knotty timber over clear wood. Results provide evidence that in-field non-destructive techniques for standing trees internal structural assessment are viable and could be operationalised within Tasmanian E. nitens plantations.