Mechanical Characterization by Means of Indentation Techniques

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. 

Conventionally, the compliance method proposed by Oliver and Pharr [153] has been used to determine the hardness (H) and elastic modulus ( ) by means of nanoindentation from the analysis of the load-displacement curve. In this method, the unloading segment of the curve is fitted to a power law function to obtain the contact depth and thus the contact area (A) at the peak load (Pmax) required to determine H H= A typical load-unload displacement curve for a 

Other nanoindentation-based techniques have been developed in recent years that, instead of applying a static load and determining material stiffness from the unloading curve, employ an oscillating force [157]. Such is the case of the continuous stiffness module (GSM), which enables us to determine the contact stiffness throughout the whole experiment during loading, and thus has proved to be a useful technique in the study of polymer s plastic and elastic properties [158]. 

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