Rheological behavior of gelatin at high shear rates

Gelatin is used as a model material or as a surrogate in biomedical investigation due to its properties being similar to human organ tissue. For successful utilization of this material in many biomedical applications, its properties must be determined at a range of loading conditions. In recent years, gelatin is being used to model the shock response of brain for studies related to traumatic brain injury (TBI) in soldiers deployed in Iraq and Afghanistan. The short time durations associated with shock loading causes high strain rate response in gelatin material. Therefore, the current investigation is focused on determining the high strain rate deformation behavior of gelatin. A coordinated modeling and experimental effort to investigate the uniaxial compressive stress-strain and high rate shear response of gelatin is presented in this dissertation. For compressive behavior, it was found that the compressive strength increased from 3 kPa at a strain rate of around 0.0013/s to 6 MPa at a strain rate of around 3,200/s. This dramatic increase in strength of gelatin at high rates is attributed to its shear-thickening behavior and is argued on the basis of hydrocluster formation mechanism and differences in internal energy absorption mechanism under static and dynamic loading. In addition, the stress relaxation response of gelatin was also analyzed and its elastic stiffness and time constant were determined. For shear response, on the other hand, a power law constitutive model that captures non-Newtonian shear-thickening behavior, the evolution of viscosity, and the momentum diffusion at high shear rates in the range of 2,000/s-8,000/s, is proposed. The model has been applied to experimental observations on double lap-shear test fixture with gelatin specimen subjected to high velocity input on the inner surface. This test fixture allows visualization of momentum diffusion through gelatin (when imaged by a high-speed camera) and measurement of shear stress on the outer surface.;The parameters in the power-law model can be extracted using two methods: (i) the measured velocity of striking plate and the transferred stress through gelatin to the stationary plate, and (ii) the time resolved images of unsteady deformation of gelatin and the self-similar solution. The power in this model was found to be 2.2 from first method and 2.25 from the second method. The results from both methods show a good agreement. This power value implies that gelatin is highly rate sensitive material and its behavior can be modeled as a strong shear-thickening fluid.;The above results are useful in computational models of human tissue deformation when subjected to high velocity or high strain rate loading. Determination of properties and constitutive behavior of surrogate materials facilitates progress in computational model development relating to TBI and other biomedical applications.;The suggested future work includes development of a constitutive model for intermediate shear rate regime and the determination for viscoelastic contribution under high shear rate loading. In addition, it is proposed that a similar technique can be utilized on other rheological fluids to determine their properties.