Understanding Fiber-Matrix Integrity in Fiber-Reinforced Polymer Composites from Strain Rate Sensitivity Concept

Abstract

The understanding of fiber-matrix integrity in fiber-reinforced polymer composites from strain rate sensitivity concept was considered from interfacial energetics point-of-view. The methodology involved the preparation of plantain fibers, treatment of fibers with nine different liquids to render their surfaces hydrophobic, molding of fiber reinforced composites, measurement of contact angles on fibers and composites, composites tensile strength determination, and fiber pullout tests. The results showed that Methyl Ethyl Ketone Peroxide (MEKP) and mercerization (NaOH) rendered the fibers most hydrophobic. MEKP and NaOH treated fibers gave the lowest surface energies, maximum works of adhesion and hence better fiber-matrix bonding (increased fiber-matrix integrity) when compared with other treatments. The strain-rate sensitivity index, m, obtained ranged from 0.2264 for phosphoric acid-treated fiber in the composite to 0.2385 MEKP treatment fiber with an overall average value of m of 0.2341, while the value of m, for untreated fiber reinforced composite was 0.2321. MEKP and NaOH treated fibers were most hydrophobic gave the highest m values and therefore most desired for treatment of fibers for composite formation. The pullout tests result showed that increase in work of adhesion led to an increase in the tensile energy, showing that stronger bonding will guarantee fiber-matrix composite stability and integrity. MEKP and NaOH with maximum free energies of adhesion also exhibit maximum pullout forces which implied that the bonding between fiber and matrix was stronger for these treatments and hence higher fiber-matrix integrity in the composite. It was also shown that the pull-out force increased with the strain rate sensitivity index confirming that one would need composites with higher m-values for stronger fiber/matrix composites. The results of this work are very important in structures that are made with fiber reinforced composite materials, such as, airplane and car bodies. This work shows that the use of the adhesive energy-stress concept to determine the strain rate is valid since the values of strain rate sensitivity indices calculated are comparable with values reported in the literature and that fiber-matrix integrity can be understood from interfacial free energies and strain rate sensitivity concepts.