Experimental Determination of Stiffness Variation Across Growth Rings in Picea abies

Abstract

Summary The radial and tangential deformations in a single growth ring of Norway spruce, which was subjected to moderate tensile loading in its radial direction, were measured by use of digital speckle photography. From the measured displacement field, the transverse coefficients of elasticity ER , ET , GRT and νTR were determined with respect to radial position in the growth ring. As a result of the abrupt change in stiffness from latewood to earlywood in the growth ring, it was possible to establish all four coefficients solely from the radial tension test. The elastic modulus ET was found to be a factor 15 larger in latewood than in earlywood, whereas the corresponding variation for ER was merely a factor 3. The measured variations were in reasonable agreement with predictions from simple two-dimensional honeycomb models of the growth ring cellular structure. The average elastic moduli of the growth ring were found to be twice as large as ER and ET measured in conventional tests on macroscale specimens, whereas the reverse was true for the shear modulus. An explanation to these differences is sought by analysing the loading conditions and deformation behaviour of macroscale specimens used in transverse stiffness testing of wood.