Effects of fungal decay on elasticity and damping of small-diameter silver fir logs assessed by the transverse vibration resonant method

Abstract

A number of studies have shown the ability of the vibration resonant method (VRM) to measure the modulus of elasticity (MOE) and the damping ratio ( \(\xi\) ) of calibrated wooden samples of various dimensions. This method based on free transverse vibrations was also studied to assess the decay extent in small stakes. Nevertheless, a need for investigations was identified in order to apply the method to decayed logs. Both decay and geometrical singularities brought heterogeneity not taken into account by the VRM. The aim of this study was to examine the effects of fungal decay on these two mechanical properties of small-diameter silver fir logs using the VRM. Fifty-five small-diameter logs were monitored over a decay process taking place in a greenhouse located near Grenoble (France). The MOE and \(\xi\) were measured in intact and decayed states. The results showed the reliability of the measurements of both properties. \(\xi\) was found to be independent of moisture content for wood above the fibre saturation point. The results validated the loss in MOE and the gain in \(\xi\) due to fungal activity. Thus, indicators based on the MOE and \(\xi\) were proposed and compared to quantify the decay extent. Indicators calculated from the first mode of vibration appeared to be relevant for that purpose. Future work is needed to validate indicators on structural size logs and to compare this quantitative assessment to those obtained by the standard EN 152 that currently serves as a benchmark.