Acoustic Tomography for Tension Wood Detection in Eucalypts

The detection and the location of reaction wood (compression or tension wood) in trees and logs is a major interest for wood industry. In tension wood which has an important thick gelatinous layer (or G - layer) in the cell wall, radial delaminations have been observed in Eucalyptus spp. by Chafe (1977) as local disruptions to the microfibril orientation. On the other hand, the tension wood has lower properties than normal wood because of morphological difference between the lignified secondary wall S2 and the unlignified G -layer (Donaldson 2001). These structural particularities as well as the disruption of microfibril orientation in tension wood are evident with ultrasonic techniques such as acoustic tomography and ultrasonic velocity method. The detection of tension wood in L direction was performed using a direct transmission ultrasonic technique and measuring the time of flight with 1 MHz probes and calculating the corresponding velocity noted VLL. The corresponding values are higher in tension wood (average 3847 m/s), than in opposite wood (average 3187 m/s), or lateral wood considered as normal wood (average 3544 m/s). The location of tension wood was possible using a stress wave method (frequency 40 kHz) and a linear filtered back projection technique for image reconstruction. A hammer blow was used to produce stress waves and the corresponding signal was strongly amplified. Acoustic waves are emitted sequentially from the source probe position and recorded at receiver source position. Maps were obtained with longitudinal bulk waves when the transducers were inserted in the radial direction to the disc periphery and with surface waves when the transducers were inserted in T direction. The tangential stress wave velocity maps were obtained at different distances from the discs periphery (ex.: 20 mm, 140 mm).