Study on the macro-microscopic deformation characteristics and moisture response mechanism of Eucalyptus urophylla × E. grandis during conventional drying

Eucalyptus urophylla × E. grandis is prone to collapse and cracking during drying. This study aimed to clarify how drying temperature affects the coupled evolution of moisture-content (MC) gradients, macro- and microscopic deformation, and residual stress. Specimens were dried at 40°C and 75°C. Increasing temperature shortened drying time from 501 h to 135 h and increased the average drying rate from 0.22%/h to 0.65%/h. Using 30% MC as a nominal fibre-saturation point (FSP) reference, temperature had a stronger effect above than below this reference. Average MC was the dominant factor affecting the interlayer MC gradient; at 50% MC, the maximum tangential difference increased from 31% to 40% as temperature increased. Final tangential deformation reached 9.6% and 11.4% at 40 and 75°C, respectively. At 15% MC, macroscopic tangential deformation exceeded fibre deformation at 40°C, whereas the opposite occurred at 75°C. The residual-stress index remained higher at 75°C. These results show that temperature-amplified moisture heterogeneity links faster drying to greater cross-scale strain incompatibility and stress accumulation. Under the tested laboratory conditions, 40°C provided better dimensional stability and stress control, but validation using broader material sources and full-size timber is required before industrial application.