Patricia Maria Frontini

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

The feasibility of extracting First Order Ogden constitutive parameters of gelatin gels from experimental flat punch indentation curves is analyzed. Eight gelatin gel samples of different formulations are evaluated. Identification of constitutive parameters is carried out by an inverse method that combines finite element modeling simulations and numerical optimization. Parameters are compared with those obtained from uniaxial compression. A parametric study of the influence of model parameters on the shape of curves and a sensitivity analysis of parameter re-identification is performed. Accurate extraction of parameters is possible if the domain in which α parameter stands are lower than 2.

Wire Cutting Method to Assess Fracture Toughness of Gelatin Gels: Phenomenological Analysis and Limitations of Methodology

In this work the performance of the wire cutting method for determining the fracture toughness, Gc, of gelatin hydrogels is assessed. In this method, wires of different diameters are pushed into a sample while the force and displacement are continuously recorded. The cutting action reaches a steady state, in which fracture propagation, deformation, and friction occur simultaneously. The method implies a linear relationship between the steady-state cutting force per unit sample width and the wire diameter, of which the y-intercept is Gc. Several gel samples differing in gelatin concentration, source (bovine or porcine), solvent (water or water–glycerol mixture), and crosslink type (physical or chemical induced by glutaraldehyde) were tested at different rates. Post-mortem fracture surfaces examined via optical microscopy displayed four different morphologies depending on the gel formulation, cutting rate, and wire diameter: I, striated; II, with one or two oblique straight lines; III, with rhombus-like figures; and IV, with material pull-out. A direct relationship between the developed fracture surface morphology and the method performance existed. One necessary condition for obtaining the linear relationship is a unique fracture surface morphology remaining for all of the wires utilized in the determination. The method is invalid if the fracture surface morphology changes with changing wire diameter, abnormal crack path deflection takes place, or material pull-out occurs as a result of adhesion effects. The applicability of the method seems to be not constrained to physical gels. An appropriate cutting rate and wire diameter have to be selected in order for a unique fracture surface morphology to be achieved. In such cases, reasonable Gc values were obtained from the y-intercept of the best linear fit of experimental data. Gc increased with increasing gelatin concentration, Bloom number, and solvent viscosity. Moreover, Gc was greater when a rhombus-like pattern was induced rather than other morphology due to greater crack path tortuosity.

Assessment of multiaxial mechanical response of rigid polyurethane foams

Multiaxial deformation behavior and failure surface of rigid polyurethane foams were determined using standard experimental facilities. Two commercial foams of different densities were assayed under uniaxial, biaxial, and triaxial stress states. These different stress states were reached in a uniaxial universal testing machine using suitable testing configurations which imply the use of special grips and lateral restricted samples. Actual strains were monitored with a video extensometer. Polyurethane foams exhibited typical isotropic brittle behavior, except under compressive loads where the response turned out to be ductile. A general failure surface in the stress space which accounts for density effects could be successfully generated. All of failure data, determined at the loss of linear elasticity point, collapsed in a single locus defined as the combination of a brittle crushing of closed-cell cellular materials criterion capped by an elastic buckling criterion.