Influence of drying methods before thermal modification on the mechanical properties of oak sawn timber

This study investigated the influence of three drying strategies – conventional kiln, radio-frequency/vacuum with mechanical press (RF/VP), and hot-air/vacuum drying – on the mechanical properties of oak sawn timber after vacuum heat treatment at 190°C and subsequent superheated steam conditioning. Mechanical performance was evaluated through bending, compressive, and tensile tests across different processing stages. Hot-air/vacuum drying showed the highest strength after heat treatment (bending: 131 MPa, compressive: 86 MPa, tensile: 113 MPa) and after conditioning (bending: 104 MPa, compressive: 83 MPa, tensile: 100 MPa), outperforming conventional kiln drying (74/71 MPa, 81/78 MPa, and 57/53 MPa), highlighting its potential as the most effective strategy. Although RF/VP drying increased density, it did not translate into proportional strength benefits, which is likely attributable to microstructural deformation and residual stress induced by high compressive loading during RF/VP drying, indicating that density alone is not a reliable indicator of structural integrity in thermally modified wood. These results demonstrate that drying history critically shapes the mechanical outcomes of heat-treated oak. Hot-air/vacuum drying, in particular, enables improved strength retention and recovery, offering practical guidelines for producing durable, high-performance hardwood products. The findings provide insights for optimising drying and post-treatment protocols to balance efficiency and mechanical performance in modern wood processing.