Non-destructive testing of knot-free timber in a Tasmanian eucalyptus nitens plot

In plantation forestry, tree stems are usually pruned under a silviculture regime. While regimes vary, often pruning will occur in the 3rd year and 5th year to support strong tree growth and high quality wood production. Silviculture science has advanced considerably over the last twenty years, optimising management of stems to prevent viral, bacterial and fungal infections, and to mitigate against insects and other factors damaging stems. Commonly as part of the management regimes, tree stems in a plantation plot that have not been pruned are removed during a thinning operation at about the 10th year of the plantation. This optimises harvest quality and allows sufficient volumes of stems to be recovered to mitigate any thinning costs.

Frequently the interval between pruning and thinning causes self-pruned branch stubs to develop in many stands, and it is a problem for harvesters to be able to discriminate between pruned and unpruned stems within the plantation plot. While wood processors use large x-ray image machines during processing to optimise wood recovery, large costs are incurred from transporting poor quality, knotty timber following harvest. This study aims to investigate the potential of ultrasound waves to detect defects in 17 years old eucalyptus nitens planted in Tasmania, Australia. In this work, 4 samples (billets) from different part of the trunk were used. The trunk parts were conditioned to natural forest moisture content (120%). The samples were scanned with ultrasound waves at 10 cm intervals in the longitudinal direction and every 45 degrees around the circumference. Results show that it is possible to identify cracks and knots in billets that are not pruned. The unpruned billets displayed highly variable ultrasound traces, while pruned billets showed far more consistent traces.