Indentation creep of polymeric materials: Experimental and analysis

Abstract

This paper presents studies on time-dependent properties, namely creep behavior, of polymeric materials by indentation techniques. A semi-empirical model in which includes the elastic-viscoelastic-viscous properties (EVEV model) by using a generalized Kelvin model to analyze indentation creep has been established. It is shown that this EVEV model can fit well the experimental creep data and to give the exact changes of creep behavior during nanoindentation. The creep deformations of different polymeric materials, including amorphous thermoplastic, thermosetting and polymer-clay nanocomposites have been studied in details. It has also proposed a method to determine the elastic modulus and viscosity coefficient in which is independent of the holding and unloading process of the indentations, again, the elastic modulus derived based on this new analysis has good agreement with that from other methods. The creep deformation is further used to derive creep compliance and retardation spectrum, which are very useful to predict other mechanical properties of polymeric materials. The indentation creep behavior of polymers have been also associated with their molecular structures, such as molecular weight, and cross-linking density. This model and analysis is useful to understand the mechanical properties of the polymeric materials. Theoretical analysis by using generalized viscoelastic models has been also performed to verify the EVEV model proposed in this study. The viscoelastic properties of several polymers are extracted by fitting the experimental results based on this theoretical analysis and compared with that from the EVEV model. The creep constitutive models for thermoplastic and thermosetting polymers are suggested. A good agreement is obtained between the experiments and theoretical model. The experimental and theoretical works together provide a new scheme for analyzing and application of the indentation creep experiments. This experiments and analysis are further extended to the indentation creep experiments at elevated temperature for thermoplastic and thermosetting polymers, and the results again show good agreement between the experiments and theoretical analysis.