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Identifying Material Properties from Mechanical Tests
Material behavior is quantified in terms of a set of “material properties” with a material property being a value, function or functional that can be used to predict responses under a variety of conditions. What constitutes a material property depends on the theory employed. For example, for isotropic linear elasticity, Young’s modulus and Poisson’s ratio are properties. In the simplest cases, mechanical properties are obtained from tests, such as uniaxial tension and compression, that give rise to homogeneous deformations. However, for a variety of materials, particularly “soft” materials, instabilities limit the range of homogeneous deformations to a more limited range than that over which properties are needed for applications. In other situations preparation of tension/compression specimens is precluded and indentation, where the deformations are inherently inhomogeneous, is used to characterize the mechanical response. In both these circumstances, the overall response measured experimentally reflects a structural rather than a material property and extracting a material property is not straightforward. In this talk, I will focus on the mechanical instabilities that can be encountered in tension and compression and on the load-deformation response in indentation, with a focus on implications for obtaining properties characterizing the nonlinear mechanical response of polymers and vertically aligned carbon nanotubes (VACNTs).