Elastic deformation mechanisms of softwoods in radial tension – Cell wall bending or stretching?

Radial softwood modulus E R is typically twice as high as the tangential modulus E T . The reason for this is unclear, although cell geometry is likely to contribute. The established hexagonal honeycomb model for prediction of E R is based on a cell wall bending mechanism only. If cell wall stretching also takes place, the dependence of E R on relative density will be different. If experimental data for E R as a function of relative density show deviations from cell wall bending predictions, this may indicate the presence of cell wall stretching. A SilviScan apparatus is used to measure density distribution. A procedure by means of digital speckle photography is then developed for measurements of local E R within the annual rings of spruce. Comparison is made between experimental data and the two expected density dependencies from cell wall bending and from stretching. The hypothesis of cell wall stretching as a contributing mechanism is supported based on the observed linear dependence of E R over a wide density range.