Quasistatic and Dynamic Nanoindentation Measurements of Pinus radiata D. Don S2 and CCML Cell Wall Layers

Quasistatic nanoindentation is a proven tool that provides information on the micromechanical behavior of wood cell walls. However, quasistatic tests cannot probe the time-dependent mechanical behavior shown by wood. Nanoindentation dynamic mechanical analysis (nanoDMA) can measure the viscoelastic properties of wood cell walls. This research aimed to study the quasistatic and viscoelastic properties of individual radiata pine wood (Pinus radiata D. Don) cell wall layers. To minimize variability and retrieve both properties at the same locations, a load function composed of a multiload-quasistatic function followed by dynamic reference frequency segments was developed. Nanoindentations were then performed on the S2 layer and compound corner middle lamella (CCML) of unembedded latewood cells. Because the S2 layer is anisotropic, both transverse and longitudinal–tangential wood planes were studied. In the transverse plane, the average results of the quasistatic elastic moduli $\left(E_s\right)$ for the S2 layer and CCML were 15.7 GPa and 4.6 GPa, respectively. In the longitudinal–tangential plane, the $E_s$ was 3.9 GPa. In the transverse section, the hardness $\left(H\right)$ of the S2 layer and CCML were 331 MPa and 277 MPa, respectively, and in the longitudinal–tangential section H was 244 MPa. To acquire the viscoelastic properties, measurements were made over more than three decades of frequency. An increase of the storage modulus $\left(E^{\prime }\right),$ and a reduction of the loss modulus $\left(E^{″}\right)$ and loss factor $\left(\tan \delta \right)$ as frequency increased were found in both wood orientations. The quasi-static and dynamic indentations equivalent at 0.1 Hz showed similar values for $E_s$ and E′. This study contributes to our knowledge of wood cell wall micromechanical properties.