Fracture toughness characterization nanocomposites

It is well known that one of the major requirements of polymer nanocomposites is to optimize the balance between the strength/stifihess and the toughness as much as possible. Therefore it is usually necessary to characterize the mechanical properties of nanocomposites from different viewpoints. Several criteria, including tensile strength, impact strength, flexural strength, hardness, fracture toughness, and so forth, have been used to evaluate the nanocomposites. 

Nanoindentation is nowadays one of the most used methods to measure the mechanical properties of polymers, attracting great attention as a technique to mechanically characterize polymer nanocomposites [137-142]. This technique uses the same principle as microindentation, but with much smaller probe areas and very low loads (on the order of nanonewtons), so as to produce indentations from less than a hundred nanometers to a few micrometers in size and depth [143]. Although it has been vastly used to characterize the mechanical properties, particularly hardness, elastic modulus, yield stress, and fracture toughness, of several polymers [144—152] and shown to be mainly influenced by the testing procedure, penetration depths, and holding time, limited work has been dedicated to the characterization of the mechanical behavior of polymer nanocomposites using this technique. 

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