Experimental determination of stiffness variation across growth rings in Picea abies

The radial and tangential deformations in a single growth ring of Norway sp ruce, which was subjected to moderate tensile loading in its radial directi on, were measured by use of digital speckle photography. From the measured displacement field, the transverse coefficients of elasticity E-R, E-T, G(R T) and v(TR) were determined with respect to radial position in the growth ring. As a result of the abrupt change in stiffness from latewood to earlyw ood in the growth ring, it was possible to establish all four coefficients solely from the radial tension test. The elastic modulus E-T was found to b e a factor 15 larger in latewood than in earlywood, whereas the correspondi ng variation for E-R was merely a factor 3. The measured variations were in reasonable agreement with predictions from simple two-dimensional honeycom b models of the growth ring cellular structure. The average elastic moduli of the growth ring were found to be twice as lar ge as E-R and E-T measured in conventional tests on macroscale specimens, w hereas the reverse was true for the shear modulus. An explanation to these differences is sought by analysing the loading conditions and deformation b ehaviour of macroscale specimens used in transverse stiffness testing of wo od.