Mixed mode (I + II) fracture toughness of polymer nanoclay nanocomposites

•The mixed mode fracture behaviour of an epoxy/nanoclay nanocomposite system is analysed.•Nanomodification enhances the toughness of the resin on the entire range of loadings.•Experimental results are compared to theoretical predictions based on different criteria.

In this work the mixed mode fracture behaviour (I + II) of an epoxy/nanoclay nanocomposite system is analysed, discussing the results from Single Edge Notch Bending tests. It is found that nanomodification significantly enhances the fracture toughness of the epoxy resin on the entire range of mixed mode loadings (from pure mode I to pure mode II), improvements depend however on the mode mixity.Experimental results are compared to theoretical predictions based on different criteria for mixed mode fracture in brittle homogeneous materials. As expected, it is found that, while the data from pure epoxy are satisfactorily predicted almost independently of the adopted approach, the agreement is much worse in the case of nanomodified materials. Explanations of this behaviour can be found in the emergence of additional microscale and nanoscale toughening mechanisms due to nanomodification not properly described by conventional models.