Composites have been used in many structural applications throughout the aerospace industry. This popularity is due to the combination of lightweight and high-strength properties possessed by these materials. A common composite material system is unidirectional graphite-reinforced epoxy. Unidirectional composites possess a high fiber volume fraction and thus maximize strength and stiffness. However, laminates fabricated with unidirectional fiber layers are susceptible to delamination when subjected to impact testing or other out-of-plane loads.
In this study, AS4/3501-6 composites were interleaved with nonwoven carbon fiber mats. The mats increased composite delamination resistance as measured with edge notch flexure (ENF) testing. The Mode II fracture energy (GIIc) of a composite without an interleaf was 359 J/m2. The use of a 12 g/m2 aerial weight mat increased GIIc by 60% (574 J/m2). A mat having an aerial weight of 6.8 g/m2 increased GIIc by 130% (834 J/m2). The ENF failure surfaces were investigated to determine how the mat affects failure surface topography. The mat fibers close to prepreg fibers acted as obstacles in the crack path. The failure topography was similar for both mats. The 6.8 g/m2 mat was thinner when consolidated and possessed a higher fiber volume than the 12 g/m2 mat. These factors were responsible for the greater increase in delamination resistance achieved with the 6.8 g/m2 mat.
This study also examined the effect of the mat interleaf on composite tensile and compressive strength. Both tensile and compressive strength were unaffected by the mat interleaf.