Farm Forestry New ZealandPredicting Strain Levels in Air-dried Wood using near Infrared Spectroscopy
By Fei Guo and Clemens Altaner, June 2017.
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Executive summary
To investigate the possibility of measuring strain using near infrared (NIR) spectroscopy, spectra of air-dried radiata wood samples were collected while under different levels of tension stress. The spectral changes were analysed by data manipulations including normalization and calculating second derivative spectra. Band shifts were calculated by curve fitting. The effects of stress relaxation, modulus of elasticity (MoE) and microfibril angle (MFA) on band shifts were studied as well.
In this study, consistent spectral changes caused by mechanical strain were observed. Peaks at 6286 cm-1 and 6465 cm-1 in the second derivative spectra shifted to higher wavenumbers with the increase of strain levels. There was stress relaxation during the spectra collecting process but the effect was negligible. The standard deviation of peak position measurement was less than 0.162 cm-1. Band shifts at 6286 cm-1 of 10 samples all showed good linear relationship with strain. On average, 1000 microstrains (a value exceeded in eucalypt logs) can induce a band shift of 1 cm-1 at 6286 cm-1.This correlation indicated that it is possible to predict strain or stress in logs using NIR spectroscopy. But further studies are needed. MOE and MFA were not found to be correlated with NIR band shift rates at 6286 cm-1 but more samples are required for a definitive conclusion.
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