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 E_R, E_T, G_RT and ν_TR 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 E_T was found to be a factor 15 larger in latewood than in earlywood, whereas the corresponding variation for E_R 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 E_R and E_T 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.