Characterizing strength and fracture of wood cell wall through uniaxial micro-compression test

As the potential of using natural wood derivatives in the fabrication of composites is explored, it is important to gain further understanding of the structure and properties of wood cells. Past research has focused on estimating and measuring mechanical properties of wood cell walls such as hardness and modulus of elasticity by means of nano-indentation tests. However, to date, the mechanical properties of wood cell walls have not been fully understood or documented in the literature. The research described in this paper focuses, for the first time, on investigating the strength and fracture behavior of wood cell walls through an innovative approach, the uniaxial micro-compression test. Specimens of Keranji (Dialium ssp.), a dense Asian hardwood, and loblolly pine (Pinus taeda), an American softwood, were chosen as hardwood and softwood representatives for the micro-compression test. After the initial preparation by microtoming, the samples were further prepared following a novel approach, in which 37 cylindrical-shaped micro-pillars were fabricated using a Focused Ion Beam (FIB) with a voltage of 30 kV, while each micropillar was milled inside a single wood cell wall. After the dimensions of each micropillar were measured by analysis of the SEM images using ImageJ software, a micro-compression test was conducted on the micropillar at a loading rate of 20 nm per second using a Nano II Indenter system. The load-displacement curves were plotted, and the yield stress and compressive strength obtained for the Keranji cell wall were 136.5 MPa and 160 MPa, respectively; the yield stress and compressive strength of the loblolly pine cell wall were 111.3 MPa and 125 MPa, respectively. The fracture behavior of the wood micro-pillars confirmed the brittleness of the wood cell walls.