Effect of cellulose and lignin content on the mechanical properties and drop-weight impact damage of injection-molded polypropylene-flax and -pine fiber composites

Designing bio-composites for structural applications requires a thorough understanding of their mechanical behavior. In this study, we examined the differences in the tensile strength and drop-weight impact response between polypropylene reinforced with flax fibers and that reinforced with pinewood short fibers, as both fibers differ in composition (cellulose, hemicellulose, and lignin) and length-to-diameter ratio. We found that flax fibers, which have higher cellulose content and are twice as long as pine fibers, increased the stiffness and shock resistance of bio-composite materials. However, pine fibers, which contain more lignin, showed increased material ductility and energy absorption. Impulse excitation, acoustic emission and micro-CT techniques were used to evaluate the post-impact mechanical properties and the contribution of each damage mechanism to the final material failure (tearing). The experimental results were used to validate a model based on finite elements. Our results revealed that the experimental and finite-element analyses were in good agreement.