Loading rate dependence of mode II fracture toughness in laminated composites reinforced by carbon nanotube films

In this study, the loading rate sensitivity of the mode II fracture behaviour of composites toughened by carbon nanotubes (CNTs) was investigated. CNTs were inserted into laminates in the form of films. The specimens for the mode II fracture test were subjected to loading rates varying from the quasi-static 8 × 10⁻⁶ m/s) to dynamic (15 m/s) states using an Instron machine and electromagnetic Hopkinson bar, respectively. Under the quasi-static loading conditions, the fracture toughness of the composites increased monotonously with an increase in the number of CNT layers and approached a maximum value when 30 layers of the CNT film were added, which is twice the value of the virgin specimen (without CNTs). However, under the dynamic loading conditions, the fracture toughness of the composites improved only by 25% compared with that of the virgin specimen, and the change in the number of CNT layers had little effect on the fracture toughness of the composites. This indicated that under dynamic loading, the failure mode of the interlaminar region changed. Microscope observation results illustrate that a large amount of in-plane (x-y plane) shear failure occurs in the CNT-interleaved zone under the dynamic loading conditions.