Vibrational properties of wood in frequency ranges including ultrasonic waves: Temperature dependences of dynamic young's modulus and loss tangent

The dynamic Young's modulus and loss tangent for untreated, heat-treated, and formaldehyde-treated woods were measured by the free-free vibration method. The temperature range was -95° C to -20°C. The resonance frequencies were 300 Hz to 30 kHz. The moisture contents were almost 0% to 16%. Three or four tan δ peaks at -60°C, (-50°C and/or -40°C), and -20°C were observed for the untreated and heat-treated specimens at 1 kHz. The peaks for the untreated specimen were more apparent at lower temperature. These results are probably due to the difference in the moisture contents of specimens. The tan δ peaks for the formaldehyde-treated specimens were small and not so evident, although the peak positions did not seem to be different from those for the untreated and heat-treated specimens. The peaks were due to the moisture absorbed by the wood because the peaks were not apparent for the oven-dried specimen. Furthermore, another peak at -80°C became apparent for the specimens with greater moisture contents. The temperature spectrum of tan δ were very smooth at a lower frequency of 300 Hz. However, those at higher frequencies had several different peaks which have not been measured and reported.