Indentation of freestanding circular elastomer films using spherical indenters

Abstract

We describe indentation testing of freestanding elastomeric films with thickness ranging from 0.1 to 3 mm. The relationship between load–deflection measurements and mechanical properties is discussed in the context of constitutive behavior, test variables (such as span length, film thickness and indenter size) and analytical indentation models. Regimes are identified wherein elastic properties can be obtained from either linear or non-linear load–deflection response, using point-load plate models and finite-contact membrane models, respectively. Testing of cast specimens of a poly(dimethyl siloxane)-based elastomer illustrates that elastic moduli determined via indentation is consistent with uni-axial tests. The models for point loads on plates include the effects of both indenter penetration and plate deflection; both are important for soft materials, such as elastomers and biological materials. The results demonstrate measurement approaches to characterize soft materials and ultra-thin freestanding films with load–deflection stiffness on the order of 0.01 N/m. The macroscale experiments and theoretical framework outlined here can be extended to films with sub-micron thickness and moduli greater than ∼1 GPa, or samples with thickness on the order of 100 μm and moduli less than ∼1 MPa.