Image Based local strain measurement of wood

Abstract

A new method for local strain measurement of soft materials like wood is proposed. Norway spruce samples were subjected to radial compression in an encapsulated split-Hopkinson device (ESHD). High speed photography was used at two magnifications for image based analysis. The strain estimation was made from high magnification images showing compression on local, fiber level for 1-2 growth rings and from low magnification images showing compression on sample level, for 5-8 growth rings. Strain gauges on the ESHD bars give stress and average strain for comparison. Image analysis based on PIV technique gives local and average strain propagation as a function of time. Wood is an inhomogeneous material and thus, local strain is a more proper measure of the response of the material. The high magnification captures differences between earlywood and latewood while the low magnification gives the strain distribution over the whole sample. Both magnifications are important in order to understand the response of the wood material to the sudden compression. A way to estimate the stress field was developed. The results showed similarity to the strain gauge measurement results. © The Society for Experimental Mechanics, Inc. 2014.