Spherical Indentation Testing of Polymers at Various Temperatures
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Spherical Indentation Testing of Polymers at Various Temperatures. Vincent D. Jardret*, Pierre Morel**, Nicolas Conté**, *MTS Nano Instruments Innovation Center, 1001 Larson Drive, Oak Ridge, Tennessee **University of Tennessee, Knoxville, Tennessee.
ABSTRACT Contact mechanics for indentation testing with spherical indenter is very attractive. Numerous projects have established equations to define strain and stress distribution in order to obtain stress-strain relationship from a single indentation experiment. Also a large number of studies have focused on metallic materials with the objective of estimating the yield point. The subject of this work is to analyze the behavior of various polymeric materials during spherical indentation testing at various temperature in order to observe the relationship between the indentation behavior and compression stress-strain behavior of the same materials as a function of temperature. Thermal effects on the indentation data are used to understand the actual effects of the mechanical properties on the indentation behavior. In addition to the load, displacement, and frequency specific stiffness information, topographic analysis of the residual indentation print is used to accurately estimate the contact area, therefore, validate the indentation models for contact depth calculations using spherical indentation. Results presented in this article include spherical indentation data obtained on PMMA and Polycarbonate over a range of temperature from 5oC to l00oC.
INTRODUCTION The objective of this work is to compare various existing models for spherical indentation applied on polymer materials at different temperature. These models are compared to the residual morphology of the indent, in order to validate the estimation of the contact area from the LoadPenetration-Stiffness curves. Using the understanding developed at room temperature on Polycarbonate and PMMA, the evolution of the spherical indentation behavior at various temperature was compared to the evolution of the Elastic modulus and hardness of the two materials over a range of temperature from 5oC to 100oC.
THEORY AND EXPERIMENTS The specificity of spherical indentation compared to other types of indentation experiments using cones or pyramidal indenter geometries, is the fact that the strain applied by the indenter is not a constant and increases with the penetration depth. This renders the modeling of contact more complex, and various analytical models need to be used to reduce the Load-Displacement data into stress and strain results [1]. For this study, indentation tests with a sphere of 65microns of radius were performed. The residual morphology of the indents was measured thanks to profiles across the residual indentation print.
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Elastic contact Upon the first loading, the contact should comply with the hertzian hypothesis: the width of the contact, a, is very small compared to the radius of the sphere, R, and the deformation are very small and we can assume a frictionless contact. Using the results from the
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