Comparing the fracture toughness of 3-D braided preform composites with z-fiber-reinforced laminar composites

Organic polymer engineering composite materials based on layered fabrics have many advantageous properties and processing features. However, performance properties of the layered OPECs, especially impact strength, delamination resistance, and fracture toughness are poor. Three-dimensional preforms fabricated with 3-D weaving, knitting and braiding techniques, are employed to improve the shortcomings of 2-D layered fabric reinforcements. z-directional microfiber-reinforced laminar composites were recently developed to improve impact strength and fracture toughness without reducing in-plane performance properties at minimal added cost and at higher productivity in the Flock Materials Laboratory at the University of Massachusetts Dartmouth. The effectiveness of laminar composite z-directional microfiber reinforcement (by a flocking process) in improving fracture toughness was compared with that of a 3-D braided 8-layer glass fiber preform/epoxy composite plate. The results show that the Mode I fracture toughness (G I ) of the 3-D braided preform reinforced composites are about 10 times of the 2-D layered glass fabric laminar composites (control) as expected. This is comparable to the results of z-directional microfiber-reinforced composites; up to 9 times increase in G I over that of 2-D glass fabric/epoxy laminar composite (control). The presence of through-thickness microfiber (flock fiber) reinforcements in the matrix resin between reinforcement layers is found not to reduce the in-plane mechanical properties; the fracture toughness however, increases significantly.