Damage Evolution of CNT Interleaves Under Mode I and Mode II Fractures of Laminates: Experimental and Numerical Investigation

This work reveals the interlaminar fracture behavior and failure modes of carbon nanotube (CNT) film toughening composite laminates under Mode I and Mode II fractures. Experiment results display that the Mode I fracture toughness increases to its maximum value when a 2-layer CNT film is added, then it decreases with the increase in CNT layers. However, the trend changes with the number of CNT layers under Mode II fracture, that is, the fracture toughness gradually increases with the increase in CNT layers. This result indicates that compared to a Mode II fracture, the toughening effect of multi-layer CNT under a Mode I fracture has not been effectively produced. A novel micro-mechanical model, based on a Voronoi diagram, is established to identify the failure mode within the CNT toughening region. It is shown that the crack propagation paths of the two kinds of fracture modes are different: cracks propagate along the CNT/resin interface for Mode I fracture, while propagating simultaneously at both the interface and resin for Mode II fracture. The change in failure mode of the CNT toughening region is the reason for the various effects under the two-fracture loading. This work innovatively utilizes finite element simulation and cross-sectional micro characterization methods to reveal the differences in interlayer failure modes of CNT film interlayer toughening materials under different fracture modes, aiming to provide guidance for the application of CNT films in the field of interlayer toughening.