Étude de l'effet de la densification THM du peuplier sur son comportement au vissage

Details

Supervisors

Pardoen, Thomas

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil électromécanicien, à finalité spécialisée: mécatronique

Abstract

enIn the current climate context, it is necessary to diversify forest ecosystems that are under pressure. To make them more resilient, fast-growing and low-density tree species, such as poplar, are gaining interest and are emerging as a sustainable alternative to traditionally used woods. Thermo-hygro-mechanical (THM) densification improves their mechanical properties and promotes their use in comparison with naturally denser species. Whilst the effects of densification on properties such as hardness, bending strength or modulus of elasticity are relatively well documented, its influence on screw-in behaviour remains poorly studied. This study explores the effect of THM densification on poplar’s behaviour during screwing. Particular attention is given to insertion torque, internal damage and pull-out resistance. An instrumented screwing device was specially designed to measure torque during insertion. Tomographic analysis was used to characterise the internal cracks created during screwing, and pull-out tests were carried out to assess pull-out resistance. These tests were performed on non-densified poplar, densified poplar and birch of comparable natural density. Comparing these three types of wood allows the effects specific to densification to be distinguished from those related to density alone. The results show that densification significantly increases the insertion torque and that the screw diameter is the most decisive factor. Tomographic analysis reveals a significant risk of internal cracking in the densified wood, attributed to the residual stresses introduced during the densification process. Despite this damage, densification considerably improves the resistance to extraction. An increase in the pull-out parameter of approximately 55 \% and in the energy parameter of over 300 \% is observed. This seemingly paradoxical result can be explained by the energy-dissipating effect of the cracks created by the screw during pull-out, and by a higher apparent tenacity. Furthermore, comparison with birch shows that density alone does not govern screw-in behaviour. The changes in internal structures induced by the THM process also play a decisive role.