Stress distribution and microstructural response of radiata pine under high-intensity microwave (HIMW) treatment

In this study, the stress distribution within the cell wall layers and the microstructural response under high-intensity microwave (HIMW) treatment were investigated. Finite-element models were developed for both single tracheid and multicellular structures, considering the influence of the saturated vapour pressure. The findings showed uniform vapour pressure caused expansion and deformation of the tracheid layers, with the radial wall deforming slightly more than the tangential wall due to differences in microfibril angle (MFA) and elastic properties. Despite its thinness, the S3 layer, with a wide MFA, played a pivotal role in supporting the saturated vapour pressure within the cell lumen under HIMW treatment, effectively constraining lateral cell-wall deformation. In contrast, the S2 layer, being the thickest, exhibited lower lateral elasticity and sustained damage at the interfaces with the S3 and S1 layers during the expansion and recovery stages. The multicellular simulation demonstrated that earlywood cells experienced larger deformations than latewood cells and revealed a negative correlation between the cell-wall thickness and deformation. The simulation results were validated through the microscopic analysis of the wood topography after HIMW treatment. The findings of this study provide significant insights into understanding the microstructural deformation of wood under HIMW treatment.