Fracture Toughness and Fatigue Behavior of Nanoreinforced Carbon/Epoxy Composites

Abstract

In this study, the objective was to develop, manufacture, and test hybrid nano/microcomposites with a nanoparticle reinforced matrix and demonstrate enhancements to damage tolerance properties in the form of fracture toughness and fatigue life. The material employed was a woven carbon fiber/epoxy composite, with multi-wall carbon nanotubes (CNT) as a nano-scale reinforcement to the epoxy matrix. A direct-mixing process, aided by a block copolymer dispersant and sonication, was employed to produce the nanoparticle-filled epoxy matrix. Initial tests were performed on cast epoxy sheets (neat and with nanotubes) to determine effects of nanotubes on the matrix alone. Specimens were tested in Mode I three point bend, showing a 20 % increase in critical stress intensity factor K for nanotube-filled epoxy over neat resin. Woven carbon fiber performs were then infused with epoxy (neat and with nanotubes) by a wet layup process to produce flat composite plates. Composite specimens cut from these plates were subjected to Mode I double cantilever beam (DCB) tests (straight and tapered) showing nearly a 200 % increase in Mode-I fracture toughness G for nano-reinforced composite over reference composite. Fatigue tests were then performed on the woven carbon fiber composite in the form of cyclic short-beam three point bend to produce interlaminar shear fatigue. Stress-life curves obtained from cyclic short-bearm three point bend showed an increase of more than an order of magnitude in cyclic life at a given cyclic load between reference and nano-reinforced composite. Fatigue-fracture tests were performed on interlaminar Mode-I tapered double cantilever beams to produce Mode-I interlaminar fatigue-crack growth. The results of cyclic interlaminar Mode-I testing showed a much lower crack growth rate for nano-reinforced composite than for reference material. SEM micrographs of failed specimens also showed significant differences in fracture surface morphology between nano-reinforced and reference composite.