Characterizing Strength and Fracture of Wood Micropillars Under Uniaxial Compression

The structure of the actual wood cell wall is very complex and it consists of several layers. Some research has been done to measure the mechanical properties of wood cell wall. For example, the hardness and modulus of wood cell wall was estimated using a nanoindentation test. But the mechanical contribution of wood cell wall is not fully understood and documented in the literature. Understanding the micro mechanical properties of the wood cell wall are paramount because of the potential for applications in cellulose nano-composites research and development.

The focus of this research was to investigate the essential of the strength and fracture of wood cell walls by uniaxial micro-compression test. Keranji and loblolly pine were chosen to perform the micro-compression tests. After initial sample preparation by microtoming, a novel method for sample preparation was adopted. The cylindrical shaped micro pillars were milled using a Focused Ion Beam (FIB) while each pillar was milled in a single wood cell wall. The beam voltage of this FIB system was 30 KV. After measuring the dimension of the micropillar through analyzing the SEM images by ImageJ software, the uniaxial compression test on the micro pillar was conducted using a Nano II Indenter system with a 10 micrometers diameter flat tip. The loading rate of 20 nm/s was used to obtain the load-displacement curves. As a result, the yield stress of keranji cell wall was 136.5 MPa and the compression strength was 160 MPa. The yield stress of loblolly pine cell wall was 111.3 MPa and the compression strength was 125 iv MPa. The fracture behavior of wood micropillar confirmed that wood cell wall also is a brittle type of material.

KEY WORDS: wood, cell wall, loblolly pine, keranji, focused ion beam (FIB), scanning electron microscopy (SEM), micropillar, uniaxial micro-compression test, fracture behavior.