Experimental Determination of Stiffness Variation Across Growth Rings in Picea abies

Abstract

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 n 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.

The Virtual Fields Method for Extracting Constitutive Parameters From Full-Field Measurements: a Review

“…The main problem here is that at that scale, the test specimen consists of only a few growth rings and the stiffness of early and late wood is very different [62]. Therefore, the average moduli thus identified might vary depending on the constitution of the test specimen (number of growth rings, percentage of early wood).…”

A close-up view of wood structure and properties across a growth ring of Norway spruce (Picea abies [L] Karst.)

“…Jernkvist and Thuvander (2001) analyzed stiffness variation across a growth ring of Norway spruce by applying loads in the transverse direction and measuring strain fields in the radial-tangential-plane. They found that in the tangential direction, latewood is much stiffer than earlywood and that the stiffness of latewood in the radial direction (E R ) exceeded that of earlywood by a factor of 3.…”

High strain rate radial compression of Norway spruce earlywood and latewood

“…Bergander and Salmén (2000) studied Young's modulus of the cell wall of Norway spruce fibres and found the tangential modulus to be 1.4-3.0 GPa, which is significantly higher than the results of Farruggia et al for the wood sample. Jernkvist and Thuvander (2001) studied radial and tangential deformation caused by tensile loading in the radial direction. They found that the strain amplitudes in earlywood were more than twice as large as the strain amplitudes in latewood.…”