Viscoelasticity of wood cell walls with different moisture content as measured by nanoindentation
Instrumental indentation was performed on wood cell walls to measure the cell walls’ mechanical properties and creep behaviour over a range of various steady moisture conditions. The wood cell walls’ Young’s modulus and hardness were found to negatively correlate with moisture content. Theoretical modeling with Burgers model and finite element analysis with the Generalized Maxwell model were used to simulate the process of nanoindentation separately and to gain insight into the response of viscoelastic materials. The Young’s modulus extracted using Burgers model was lower than the values obtained using the Generalized Maxwell model and Oliver–Pharr method, indicating that creep happened during the indentation loading process. The extracted parameters from both models exhibited negative trends with increasing moisture content. Both models confirmed an intuitive understanding of creep in viscoelastic wood cell walls, which reflects previous basic information about the composite structure of wood cell walls and their rheological properties.