Internal friction measurement of tropical species by various acoustic methods
2010
Brancheriau, Loïc | Kouchade, Clément | Brémaud, Iris
A batch of 54 tropical species was analyzed using free-vibration and forced-released vibration tests. The free-vibration tests were conducted by bending and compression using nylon thread or elastic thread as supports. The wood species used cover a broad spectrum of density values and were obtained from the CIRAD wood collection. Samples were stabilized at a mean moisture content of 11.1%. The goals of the study were (a) to observe the effects of nylon or elastic supports on the measurement of vibration damping, (b) to compare the damping measurements obtained through free vibration in bending and in compression, (c) to understand the relationship between temporal damping and internal friction based on free-vibration and forced-vibration bending tests, and (d) to observe the effect of frequency on bending free-vibration damping on a rosewood specimen (Dalbergia sp., Madagascar). In this study we were able to demonstrate that (a) the type of support has a significant influence on the measurement of the temporal damping, (b) the temporal damping measurements obtained during bending free vibration are linearly linked to those obtained during compression vibration, (c) the expression of internal friction η V according to temporal damping α was identical during compression and bending free vibration: η V = α/(πf), and (d) changes in temporal damping α according to frequency f can be modeled in the form α = β ₁ f ² - β ₂ f ⁴. This form is theoretically justified as the first-order form obtained from the generalized differential equation of linear viscoelasticity.
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