Constitutive characterization of polymers by depth sensing indentation technique

L. A. FASCE; L. SANCHEZ FELLAY; P. M. FRONTINI

Congreso; POLY-CHAR 2020; 2020

Depht sensing indentation tests have been widely used to characterize materials since 70?s. However, due to the complex deformation field developed beneath the indenter tip, the analysis of indentation data is not straightforward for polymeric materials. Problem complexity relies on the different mechanical behavior characteristics that polymers exhibit such as hiperelasticity, time and hydrostatic pressure yielding dependency, post-yield stress softening and strain hardening at large strains. A standard method to obtain constitutive parameter of polymers from depth sensing indentation tests has not been yet developed. The main objective of this presentation is to enhance the understanding about polymers load-depth response and analyze the feasibility of identifying constitutive parameters from load-depth indentation data. To these aims, simulation tools are constructed to model the indentation response of hyperelastic and elastic visco-plastic materials under several indentation configurations (flat-ended cylindrical, spherical and Berkovich tips). Constitutive parameters identification methodologies are developed combining physical indentation data, the developed simulation tools, the inverse approach and numerical optimization methods. Several materials systems are considered.In many papers, depth sensing indentation tests are used to analyze the strain rate dependence of polymer behavior. Pseudo-plastic approach is adopted, in which reduced elastic modulus and hardness are determined by means of the Oliver- Pharr method at different loading rates. Even the Oliver-Pharr approach is widely used to characterize materials its application to polymer is discussed. To contribute to the understanding of indentation response of polymers and limitations of Oliver-Pharr method, nano-indentation experiments were conducted on an epoxy resin using a Berkovich tip and different constant strain rate loading cycles. Elastic modulus and hardness values determined by the Oliver-Pharr method showed an anomalous trend with loading rate. The causes of such tendency were explored using the indentation simulation tool for and by a deep analysis of drift displacement correction. It was found that the elastic visco-plastic nature of the epoxy resin generates the anomalous trend in hardness values and that the uncertainty in drift displacement correction in very long time experiments greatly affects the obtained load-depth curves.To develop hyperelastic constitutive parameters identification method, first order Ogden constitutive model and flat ended cylindrical configuration were taken into account. The method was successfully constructed and applied to characterize several different polymeric gels. The identified parameters were compared with those simply extracted from uniaxial compression tests. A sensitive analysis was carried out and it was found that the feasibility of extracting hyperelastic parameters depends on theparameter itself and its range of values.To develop elastic visco-plastic constitutive parameters identification methodology a nine parameter model called EVP-9 was designed and used. Each model parameter represents a polymer mechanical behavior characteristic. Experiments were carried out on PC and spherical and Berkovich indentations were considered.