Effects of loading rates on mode I interlaminar fracture toughness of carbon/epoxy composite toughened by carbon nanotube films

Carbon fiber reinforced resin matrix composites were toughened by the continuous and randomly distributed carbon nanotubes (CNTs), which were fabricated by floating catalytic chemical vapor deposition (FCCVD) method. Toughening was achieved by deploying CNT film of different a real density as interleaves in the carbon fiber reinforced composites. Double cantilever beam (DCB) specimens were adopted in both quasi-static and dynamic fracture tests. The electromagnetic Hopkinson bar was applied to achieve pure type I fracture mode at high loading speeds of 15 m/s and 24 m/s. The effects of CNTs and loading rates on the Mode I fracture toughness of the composites are investigated. It is shown that fracture toughness approaches a maximum value of 626 J/m(2) under quasi-static loading, only if a 2-layered CNT film is being applied as an interleave, which is twice that of the neat composite (without CNT interleaves). Under dynamic loading, all the specimens either with or without CNTs show increases in the fracture toughness with the increase in loading rate. Different from the results of quasi-static tests, under dynamic loading conditions, the fracture toughness increases monotonously and approaches to a maximum value of 1380 J/m(2) when 40-layered CNT film was applied, which is a remarkable increase of 140% if compared with that of the neat specimen. A detailed analysis of mechanisms responsible for the ameliorating effects of CNTs interleaves is carried out. It is believed that strengthening of interfaces between carbon fibers and matrix, deformation of resin and detachment of CNTs from the matrix are the main factors responsible for this effect.