Carbon nanotube shear-pressed sheet interleaves for Mode I interlaminar fracture toughness enhancement

Failure of composite laminates is often the result of “secondary” transverse stresses causing delamination. One well known approach to prevent such failure is to incorporate a distinct interleaf material into the interlaminar region in order to increase its fracture toughness and, consequently, its resistance to delamination. In the recent years various carbon nanotube (CNT) interleaves gained much attention. This work presents experimental study of the Mode I progressive fracture of carbon/epoxy composite laminates modified with high volume fraction, aligned, non-functionalized and functionalized CNT interleaves. The interleaves used here are thin solid sheets produced from vertically grown multiwalled CNT arrays by shear pressing method. A dry or resin infused sheet is integrated between prepreg plies prior to the laminate cure. The obtained results show that both dry and pre-infused CNT interleaves significantly, up to two times, increase the critical strain energy release rate of the baseline non-interleaved laminate. Two methods of functionalizing CNTs within the preform are explored: O2/CF4 plasma and H2SO4/KnO4 wet chemical treatments. Both methods maintain the high alignment and aspect ratio of the CNTs. Although, functionalization results in no additional GIC toughening compared to the non-functionalized interleaves, the characteristics of the fracture surfaces are dramatically different.