| Summary: | The addition of thermoplastic particles in the interlaminar region of a carbon-epoxy composite is known to generally improve mode I and mode II fracture toughnesses and also improve damage tolerance. However, the mechanisms of toughening are poorly understood. Most studies so far have selected one interlaminar toughening particle (ILTP) and studied the effect of particle size and/or spatial distribution. A missing link in the continued development of interlaminar toughened systems is study into the effect of the particle material and interface. Whilst matrix mode I toughness is a good indication of composite mode I toughness, no such relationship has previously been established or investigated for mode II. This work focuses on measuring fracture parameters in bulk, particulate toughened epoxy resins using an experimental approach. Digital image correlation tools were used to determine displacement fields around the crack tip at a small scale, in both standard, pure mode I specimens and mixedmode I/II specimens for five resin formulations, four with ILTP and one without. Mixed-mode stress intensity factors and the non-singular T-stress were extracted from the displacement fields using the Williams' crack tip stress solutions. The Tstress term governs crack path stability and it was found that this term can be used successfully to differentiate between the crack path behaviour at fracture of the different materials studied. A new methodology was developed to determine an apparent mode II toughness for resins and this parameter was found to be directly proportional to the composite mode II toughness. This is believed to be the first time a relationship has been established between the mode II performance of particulate toughened resins and their composites. The novel parameters developed here allow inference of mode II composite behaviour from resin tests. Therefore this work is a significant boost to the continued development of interlaminar toughened composites.
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