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

MARINA CZENER; LAURA FASCE; PATRICIA FRONTINI

Materials Performance and Characterization

astm

Año: 2014 p. 1 - 10

2165-3992

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