Experimental and Numerical Study of the Critical Length of Short Kenaf Fiber Reinforced Polypropylene Composites

Widespread attention to make use of biodegradable resources as a replacement for petroleum products leads to the exploitation of natural fiber reinforced composites. Natural fiber reinforced polymer composites usually exhibit lower mechanical properties than synthetic fiber ones. Thus, understanding key factors affecting the overall mechanical properties in order to increase them is crucial. One underlying factor is the length of fiber highly contributing to the extent of matrix/fiber interfacial load transfer at the interface. However, the concurrent examination of the load transfer mechanism at the interface of fiber/matrix in terms of fiber length has not been well performed using computational, analytical and experimental approaches. This work is aimed at the determination of the critical fiber length associated with a full load transfer condition using various methods to better understand their accuracy and the interfacial load transfer mechanism. For this purpose, specimens of neat polypropylene (PP) and 20 wt% kenaf/PP composites were fabricated using extrusion injection molding. Tensile testing, scanning electron microscopy and density measurements were conducted to incorporate the obtained results into the models and to verify the results predicted by the models. A three dimensional representative volume element (RVE) representing the filler content of fabricated specimens was assumed. A micromechanical model was employed to make the results of analysis independent of the RVE dimensions. The critical fiber length for a full load transfer was determined by evaluating the stored elastic strain energy changes against the fiber length. The results showed that the kenaf fiber length is critical to both the load transfer efficiency and stiffening of composites. The results further revealed that to obtain the full interfacial load transfer, the length of kenaf fibers needs to be greater than the critical length being similar to 2.4 mm provided that perfect kenaf/PP interfacial interaction exists.