Compression mechanisms and strength properties of semi- isostatically densified wood

Abstract: SAMMANFATTNING Since most strength properties increase with increased wood density, densifying wood through compression will increase strength and wear resistance. After densification, wood with originally low density can substitute denser wood and originally dense wood can be used for purposes where wood is considered too soft, e.g. in flooring and stairs in public environments. In this thesis compression was done semi-isostatically in a Quintus press at pressures up to 140 MPa on wood with moisture contents ranging between 5 and 15% and at 20-25°C. The wood was placed on a rigid steel plate and the pressure is mediated through a flexible rubber diaphragm filled with oil. This causes denser parts of the wood to deform less than wood with lower density. The shape of the densified wood will then be irregular. The objective of this work was to evaluate wood properties prior, during and after compression. Properties of particular interest were shape, density and strength. Small quarter-sawn and flat-sawn clear specimens of Scots pine were compressed at different pressures up to 140 MPa. The specimens were compressed with different orientations of the annual rings relative to the press table. Plastic, elastic and delayed elastic strains were measured. The influence of original wood properties on plastic strains at different parts of the crosscut area was evaluated using multivariate statistic analysis. The results showed that wood was compressed without major checks in a Quintus press. The sawing pattern and orientation of the specimens in the press had great influence on the shape. Most regular shape with least buckled annual rings were obtained when plain-sawn wood was oriented with the inside face (pith side) to the press table. At 140 MPa the density almost reaches 1500 kg/m3, i.e. compact density, but at release of the pressure the density decreases to about 1000 kg/m3 due to elastic springback. The delayed elastic strain was very small during five years of indoor storage and should not be a problem in long-term indoor use of densified wood. X-ray computerised tomography scanning (CT) before and after compression was used to analyse the density increment over the crosscut area and the influence of resinous wood and knots. An algorithm was developed for transforming CT-images of the crosscuts to the same sizes to enable pixel wise comparison of density prior and after compression. Heartwood in resinous boards prevented compression as well as knots. These types of wood should thus be avoided when high and homogenous density is requested. Strength to density relationships was compared between non-densified and densified wood of eight species. Strength increased with increased density. Most compression at densification is in radial direction and nearly no in axial. In axial compression the ratio between strength and density was similar for non-densified and densified wood, whereas densified wood became rubbery in radial direction with very low modulus of elasticity and no limit of proportionality.