Hard Problems with Soft Materials: Fracture Toughness by the cutting method

F. RUEDA; L. A. FASCE; P. M. FRONTINI

Encuentro; III Encuentro Nacional de Materia Blanda - III MAB; 2010

The basis of fracture mechanics relies on the fact that bodies contain flaws or cracks and that failure occurs at the largest of these cracks. When the crack grows in size, the potential energy in the body is released to form new surfaces of the growing crack. The minimum energy per unit area necessary to produce the crack is called the fracture toughness, Gc. The term Gc includes the intrinsic surface-separation work as well as other local dissipative work at the crack tip. When materials exhibit a high degree of non-linearity, visco-elastic effects and low stiffness, conventional fracture mechanics tests such as SENB (single-edge notched bend) and SENT (single edge notched tension), that are widely used to determine Gc of engineering materials are very difficult to apply. This is the case of soft polymers, wood, hydrogels, food and some biological materials. Instead, alternative tests in the form of wire and blade cutting tests have been proposed. Since the cutting process is accompanied by material deformation and surface friction effects, it is, in principle possible to quantify these material parameters from the cutting test. However, the separation of total cutting energy into individual components is still an open question. The force–displacement relationship depends on a combination of fracture, plastic/viscous deformation and surface friction effects in a complex way. The wire cutting test involves pushing wires of known diameters through specimens from an initial indentation to a steady-state cutting stage. The steady state cutting force increases with increasing wire diameter because of the increase in energy loss through dissipative process in the bulk material. By extrapolating the steady state cutting energy to zero wire diameter, Gc is obtained since the work input is consumed solely by the propagating crack. In the blade cutting test, a microtome or blade makes an offcut of known thickness at a fixed cutting angle (a). The steady state cutting force decreases with the offcut thickness because of the reduction in the dissipated energy due to plastic work and/or viscous losses as well as surface friction effects. By extrapolating to zero offcut thickness, the dissipative losses are removed and Gc can be determined. The presence of secondary cracks as well as material adherence with the instruments (wire or blade) makes the cutting process less predictable. Indeed, the variation of the strain rate when cutting at different conditions-necessary for Gc extrapolation- has to be taken into account. Despite the unsolved theoretical and experimental questions, and due to the inherent advantages of cutting tests like definition of fracture initiation point, this method has recently been extended to evaluate simultaneously the fracture toughness and flow strength of a wide range of materials. Our aim is to apply both cutting tests methodologies to evaluate the fracture toughness of model soft materials based on bovine gelatin (hydrogels). Advantages and drawbacks of each approach will be evaluated in the next future.