Characterization of Interlaminar Static and Fatigue Delamination Growth in Carbon/Epoxy Composites Reinforced with Carbon Nanotubes

The present study focuses on the effect of CNT nanofillers on the interlaminar static and fatigue crack propagation in carbon fiber reinforced composite laminates. Multi-walled carbon nanotubes (MWCNTs) were dispersed over the laminate interface through solvent spraying technique. The mode I fracture toughness and R curve behavior were determined first from DCB specimens. Then, the fatigue tests were performed at different stress ratios for laminates containing different contents of CNTs to determine the delamination growth rate da/dN from fatigue crack growth (FCG) curves. When FCG curves are expressed as a function of G, where G is the energy release rate, the growth curves are dependent on the R-ratio. It was found that the addition of CNTs enhances the delamination resistance in the initial part of FCG curves, i.e. low cyclic region. As the test progresses, the effect gradually diminishes making nanofillers ineffective. It is then shown that the FCG curves can be characterized when crack growth rates are expressed as a function of the crack-driving force (Delta kappa) over bar used in the Hartman-Schijve equation. Therefore, the present paper presents a methodology to account for the stress ratio effect to evaluate the crack growth rate for any given R-ratio and to obtain a valid, upper-bound FCG rate curves in CNT reinforced laminates that exhibit high degree of scatter.