(A) study on delamination toughness of carbon fiber/epoxy composite with chopped kevlar fiber and nylon-6 powder reinforcement

Abstract

Advanced composite materials have been broadly used for both military and aerospace application. Their high specific modulus and specific strength make them attractive candidates for structural materials. However, as a result of poor out-of-plane strength, brittle composites such as carbon-fiber-reinforced epoxy, exhibit restricted impact strength, low damage tolerance, and poor delamination resistance. Several approaches have been developed to improve the impact toughness and interlaminar fracture toughness of composites. In this study, short Kevlar fiber (SKF) and/or Nylon-6 powder (N6P) was spread in the mid-plane of the continuous-carbon-fiber/epoxy laminate before processing where the initial crack is introduced. The extreme case of SKF/epoxy interface. was obtained by $O_2$-plasma SKF (P-SKF) or silicon release agent SKF (S-SKF). Mode I ($G_{IC}$) and Mode II ($G_{IIC}$) interlaminar fracture toughness of carbon-fiber/epoxy composites have been investigated as a function of amount of SKF and/or N6P between continuous fiber layers with different surface condition of SKF. The fracture surface was studied using scanning electron microscopy (SEM) and optical microscopy. Interfacial shear strength of SKF/epoxy interface was determined by Kevlar fiber pull-out test. Surface chemistry of SKF, P-SKF and S-SKF was examined by ESCA and FT-IR. $G_{IIC}$ increased with increasing crack length as a consequence of the presence of SKF``s bridging in the wake of propagating crack. $G_{IIC}$ of SKF alone could reach the maximum at intermediate amount of SKF. $G_{IIC}$ of SKF and N6P was lower than that of SKF alone because N6P prevented the orientation of SKF to out-of-plane. The extent of SKF``s bridging phenomenon may be influenced by the amount and orientation of SKF. $G_{IC}$ showed no significant effect with SKF and uniform irrespective of crack length. Scanning electron microscopy (SEM) after $G_{IIC}$ test showed that new surfaces were created by extensive fi...