Multi-scale analysis of the structure and mechanical performance of woody hemp core and the dependence on the sampling location
Résumé
A multi-scale length and multi-feature exploration of woody hemp core (WHC) using 6 internodes (INs) along the height of the stem was conducted, and then, the putative IN location dependence of composites made of the WHC was evaluated. At the nanoscale level, we observed a good inter-stem reproducibility of cell wall properties using nanoindentation tests as well as moderate increases in the values of the indentation modulus and hardness from the bottom to top of the stem (8.5-10.5 GPa and 200-350 MPa, respectively). Biochemical analysis revealed that the lignin content decreased significantly (by approximately 1.5% in terms of the WHC dry matter mass) from the bottom to the top of the stem. Individual stem carbohydrate trends differed when compared with the average values of a group of 10 stems. Along the height of the stem, no variation in the total carbohydrate content was observed in the average of 10 stems, but the monosaccharide details exhibited mixed behaviors. The arabinose and galactose contents increased from the bottom to top of the stem and were strongly correlated with structural properties, i.e., relative density, Young's modulus, and toughness, as well as with lignin content. At the microscale level, from the bottom to top IN of the stems, the relative density of the WHC exhibits a 2.8-fold reduction. At the mesoscale level, bending tests showed huge variations between IN locations; Young's modulus, strength at break and fracture energy exhibit 4,2 and 5-fold variations, respectively. Despite all of these structural and biochemical variations along the WHC stem, composites made of particles from the bottom, median, or top WHC locations did not show any variations in Young's modulus, stress or elongation at break. The influence of the extrusion and injection process steps of making wood-plastic composites is discussed in connection with the preservation of the pre-processing intrinsic potential of the raw fiber materials.