Effects of fiber geometry and orientation distribution on the anisotropy of mechanical properties, creep behavior, and thermal expansion of natural fiber/HDPE composites

Natural fiber/high-density polyethylene (HDPE) composites (NFPCs) were fabricated via extrusion using three types of natural fibers from poplar wood (PW), radiata pine (RP), and rice husk (RH), respectively. The specimens were cut from the extruded samples at various off-axis angles (from 0° to 90°). The effect of fiber geometry and orientation distribution on the mechanical properties, creep behavior, and thermal expansion of the NFPCs was investigated. The natural fibers in the composites showed a preferential orientation along the extrusion direction, as evidenced by the optics micrographs. The flexural properties and impact strength of the NFPCs were the highest at zero angle and decreased considerably with increasing orientation angle, which was further validated by the finite element analysis. At zero angle, the PW/HDPE composites had the highest flexural and impact strength and the smallest creep strain and thermal expansion, but a small difference in mechanical properties was observed at 90° among the NFPCs. The significant anisotropy of properties at various angles of the PW/HDPE composites was due to the high aspect ratio (L/D) of poplar wood fiber. These results indicate that high fiber L/D and orientation distribution had a beneficial effect on the properties of NFPCs.