A comprehensive study was undertaken on the specific role of rubber on toughening when other rigid polymer or non-polymer phases were present. Nylon 6,6/SAN blends of various SAN concentrations ranging from pure SAN to pure nylon 6,6 were investigated with and without fibre reinforcements. These results could be compared with the toughness values of unreinforced and fibre-reinforced nylon 6,6/ABS alloys from a previous study in order to elucidate the role of rubber. Fracture behaviour was investigated rigorously by characterizing the fracture initiation toughness, JIC, and the steady-state fracture toughness, Jss. These were then related to the microstructure and failure modes determined by microscopy and fractography methods. It was found that rubber increased both fracture initiation and propagation toughness in the presence of the rigid phase, while the rigid phase toughened the alloy only when the rigid phase/matrix interface was strong enough. The role played by glass fibres was found to be critically related to the fibre/matrix interfacial strength. Toughening was generally observed, both in the presence and absence of rubber, when the interface was strong. In all cases toughening could be related to the enhancement of plasticity in the crack tip by the presence of the rubber phase or the reinforcing glass phase.
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