Loading Rate Effect on Tensile Failure Behavior of Gelatins under Mode I

For decades, ballistic gelatin has been used as a tissue surrogate to test and evaluate bullets and firearms due to its similar viscosity to natural tissue. However, the high water content in ballistic gelatin makes it unstable at room temperature, and therefore causes it to have a poor shelf life. The development of polymer-based gels has shown promise as an alternative tissue surrogate. Polymer gels such as Perma-Gel are stable at room temperature and can be stored for long periods of time. Gels often fail due to tensile stresses during penetration. The failure behavior in tension is highly influenced by the presence of defects, such as cracks and voids, in the bulk material. A mode I experimental method was developed to obtain tensile failure criteria for the initiation and propagation behavior of these types of soft materials. Digital image correlation is used to determine the full-field surface strains around the crack tip to obtain a quantitative measure of the critical strain-field required for initiation and propagation of failure due to a defect. This systematic study utilizes these experimental techniques to determine the critical criteria for crack growth initiation and crack propagation of ballistic gelatins and a polymer gel as a function of loading rate. This paper presents experimental methodologies and results from Mode I fracture experiments including measured critical energy and strain-based criteria for failure initiation and growth, as well as their dependence on the rate of loading.