Limiting Mechanisms of Mode I Interlaminar Toughening of Composites Reinforced with Aligned Carbon Nanotubes

Abstract

Analytical models are presented for the Mode I interlaminar fracture of laminated composites reinforced with aligned carbon nanotubes (CNTs). The models are based on the crack-closure technique for fiber bridging, where the aligned CNTs enhance toughness mechanistically through either pullout (frictional sliding) from the matrix or sword-in-sheath sliding. The models are independent of the scale of reinforcement and demonstrate significant enhanced toughening for nanoscale reinforcement (CNTs) as opposed to typical mm-scale reinforcements (stitches and Z-pins). Complete analytical expressions for crack-growth resistance (GR(Δa)) are obtained including normalized closed-form expressions for steady-state toughness for any scale of z-direction fiber reinforcement. The model is verified by comparison to previous experimental results for Z-pins and also aligned CNTs, and is used to define regimes where the competing mechanisms of toughening are operative. CNT strength is a key parameter limiting toughness enhancement in the frictional pullout mechanism.

Aligned Carbon Nanotube Reinforcement of Aerospace Carbon Fiber Composites: Substructural Strength Evaluation for Aerostructure Applications

“…In past work, vertically aligned carbon nanotubes (VACNTs) placed at the interface (so-termed 'nanostitching' 5 ) of prepreg plies can improve the critical energy release rate 2.5X in Mode I over the baseline composites. This improvement is greater than the results reported for standard z-pinning reinforcement 2,5,13 . Apart from interlaminar toughness, there are other mechanical properties than can benefit from interlaminar reinforcement.…”

The effect of carbon nanotubes on the fracture toughness and fatigue performance of a thermosetting epoxy polymer

“…Further, quantitative modelling of this CNT pull-out mechanism during the fracture of these nanocomposites confirmed that such a mechanism may indeed account for the observed increases in toughness [29,30]. However, pull-out requires the CNTs to debond from the matrix, which may also absorb energy.…”

“…Further tests [59] gave a range of values between 15 and 86 MPa. Blanco et al [29] suggested that the value of the interfacial shear strength would lie between 1 and 100 MPa, so 47 MPa is a good estimate for use in this study. Substituting Eq.…”

“…Fibre pull-out is considered to be the main toughening mechanism in short-fibre-reinforced composites. For MWCNT-modified polymers, Blanco et al [29] identified (i) nanotube pull-out from the matrix and (ii) sword-insheath pull-out as the toughening mechanisms. For the sword-in-sheath mechanism, the outer shell of the MWCNT fractures in tension and the inner shells pull-out from within it.…”

“…The main mechanism by which the addition of CNTs increases the observed toughness of the thermosetting epoxy polymers has been identified [22][23][24]29] as pull-out of the CNTs from the epoxy polymer when the crack faces open as crack propagation occurs. Further, quantitative modelling of this CNT pull-out mechanism during the fracture of these nanocomposites confirmed that such a mechanism may indeed account for the observed increases in toughness [29,30].…”