On the Feasibility of Identifying First Order Ogden Constitutive Parameters of Gelatin Gels from Flat Punch Indentation Tests

SANCHEZ FELLAY, LUCAS; FASCE, LAURA ALEJANDRA; CZERNER, MARINA; FRONTINI, PATRICIA MARIA

SOFT MATERIALS

TAYLOR & FRANCIS INC

Lugar: Londres; Año: 2015

1539-445X

In this work, the feasibility of extracting First Order Ogden constitutive parameters of gelatin gels from experimental flat punch indentation curves is considered. The adopted indentation configuration is that used in the technological Bloom test for gelatins. Eight gelatin gel samples of different formulations are prepared and evaluated. Identification of constitutive parameters ( μ and α ) is performed by an inverse method which combines finite element modeling simulations andnumerical optimization. Parameters are compared with those obtained from uniaxial compression using the analytical expression for the stress-stretch ratio relationship. Under uniaxial compression, gelatin gels display strain hardening over the Neo-Hookean model prediction. Multiple set of Ogden parameters are identified depending on the initial guesses. A single set of Ogden parameters is identified from flat punch indentation curves, which not always matches with those obtained inuniaxial compression. The origin of such discrepancy is investigated through a parametric study of the influence of μ and α on the shape of simulated flat punch indentation and uniaxial compression curves. Also, a sensitivity analysis of parameters re-identification is performed for two different ranges of α ( α < 2 and α > 2). Flat punch indentation response is insensitive to α in the α > 2 range.For such cases, α parameter can not be accurately re-identified. For the tested gelatin gels, α values extracted from flat punch indentation curves appear to be in the insensitive range. However, the uniaxial compression responses can be well described using the parameters identified from experimental flat punch indentation tests at least up to a stretch ratio of about 0.65. For gels that display appreciable strain hardening at low deformations, ie. α < 2, first order Ogden constitutiveparameters can be accurately identified from flat punch indentation tests. This test configuration is appealing since, opposite to spherical indentation configuration, the non-linearity of the load-depth curve is solely due to strain hardening of the material unlike that displayed by a Neo-Hookean solid.