Impulse excitation, also known as impact excitation and resonance technique, is a commonly used method to assess the stiffness of wood in both scientific and industrial settings. One or more modes of vibration are excited by a mechanical impulse and their resonant frequencies measured. When combined with density and dimensions, these frequencies can be used to calculate elastic modulii – most commonly modulus of elasticity parallel to grain, which is an important property for construction timber. The technique may also be used to assess damping, which is an important property for wood used in musical instruments. Although the technique is simple and commonly used, the principle and analysis are not always well understood by users. This paper briefly explains the physical basis of the technique, and some finer details of the signal analysis that can cause errors when attempting to make very accurate measurements. It explains how to use the free Open Source statistical computing software R to analyse the recorded waves and illustrates its use with data from a series of experiments aimed at tracking small changes in modulus of elasticity, shear modulus and damping on Scots pine and Douglas-fir clear wood specimens. Mathematically generated waves are also used to test the precision and robustness of the analysis. The analysis in R is built on the packages Seewave (created for bioacoustics analysis) and tuneR (created for analysis of music and speech), and new functions specifically tailored to impact excitation. Frequency domain interpolation methods are introduced that enhance the potential precision of the technique to a very high degree, even when using very inexpensive audio equipment. With multiple measurements, resonant frequencies can be assessed to a precision of 0.1 Hz and better, allowing for tracking of very small changes due to wood treatments or moisture content. Other researchers are invited to improve and build on this R code and its algorithms.
Keywords: Non-destructive testing, acoustics, heat treatment, open source
Authors
Ridley-Ellis D.
Centre for Wood Science & Technology, Edinburgh Napier University, Unit 1, Edinburgh, UK
Libeau M.
Bordeaux Sciences Agro, Gradignan Cedex, France
Mignerat D.
Institut polytechnique de Grenoble, Grenoble Cedex, France
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