Analysis of the mechanical properties and short-term creep and stress relaxation of bamboo under hydrothermal treatment

Bamboo’s potential as a construction material is limited by its natural straight form. Hydrothermal bending treatments enable curved shapes but often drastically reduce mechanical strength and cause elastic recovery. This study investigates the impact of hot-water treatment. At 50 °C, 75 °C, and 90 °C for 15 min, the mechanical properties and short-term viscoelastic behavior of Moso bamboo (Phyllostachys edulis) were studied. Mechanical characterization (three-point bending) and rheological analysis (creep and relaxation) were conducted, accounting for the material’s functional gradient under two loading modes. Differential Scanning Calorimetry revealed that liquid water significantly lowers the glass transition temperature to approximately 38 °C, acting as a plasticizer. Results show that the modulus of elasticity and modulus of rupture decrease with increasing treatment temperature, while the ductility factor increases significantly. A fractional Maxwell model effectively described the creep and relaxation behavior, showing a reduction in viscosity with higher temperatures. The study identifies a treatment at 70 °C for 15 min as an acceptable compromise for bamboo bending processes, preserving 93% of the modulus of rupture and 77% of the modulus of elasticity, increasing the ductility factor by 12%, and minimizing delayed deformation from relaxation.