Investigating Nano-Mechanical Properties of the Wood and Its Composites by Continuous Nanoindentation

Nanoindentation testing is a technique that determines the mechanical properties of a material in the sub-micron scale. The test involves penetrating a sample material using an indenter, while the penetration depth and load are recorded so that the stiffness and hardness of the indented location can be subsequently calculated. The continuous stiffness measurement technique offers a tool to probe stiffness as a function of indentation depth in one single experiment. Commonly, a discrete indentation loading, with a targeted penetration load or depth, provides only one value of hardness or stiffness. In the continuous measurement technique, cycles of indentation, each of which consists of incremental loading and partial unloading, are performed until a final desired depth is attained. Each loading-and-partial unloading cycle provides a value of hardness and stiffness; hence as the penetration progresses, various hardness or stiffness values were determined as a function of the indentation depth. The objective of this study was to measure the mechanical properties of wood cell wall and the interphase in a cellulose fiberreinforced polypropylene composite using nanoindentation and the continuous measurement technique. The study also examined the effects of microfibril angle on the stiffness values determined from nanoindentation experiments.