Effect of decay on ultrasonic velocity and attenuation measurements in wood

The percentage mass loss of loblolly pine (Pinus taeda) wood cube specimens exposed to Gloeophyllum fungus (Gloeophyllum trabeum) for increasing periods of time ranging from 1 to 12 weeks was recorded after being subjected to controlled decay following ASTM International standard ASTM D1413-99. The specimens' corresponding volume loss due to decay and corresponding densities were calculated using X-ray computed tomography. Blocks decayed for 12 weeks experienced, on the average, the greatest loss of mass (≈40%), volume (≈30%) and density (≈37%). For each of the three principal material directions of these specimens with controlled decay, ultrasonic longitudinal and shear velocity values, along with the corresponding attenuation values, were measured using longitudinal and shear ultrasonic transducers with a center frequency of 100 kHz. Because of the relatively small size of the wooden specimens, a steel delay line was used, along with waveform averaging and the phase-comparison technique, to measure velocities. It was observed that the velocities increased with increasing frequency and decreased with increasing amount of decay, while the corresponding attenuation values increased with increasing frequency and amount of decay. Towards estimation of velocity and attenuation values, polynomial expressions fitted to the experimentally obtained data are presented for the frequency band of 4.5 to 200 kHz and up to a mass loss of 40%.