|M.Sc Student||Richler Daniel|
|Subject||Static and Dynamic Mechanical Properties of Gels|
|Department||Department of Mechanical Engineering||Supervisor||Professor Daniel Rittel|
|Full Thesis text|
This research focuses on the measurement of the static and dynamic mechanical properties of ballistic gelatin. A brief summary of the elastic and viscoelastic governing equations of gels and other soft materials is reviewed and summarized. This analytic review gives an understanding of gel behavior at low strain rates and is an introduction for the static and dynamic tests that are presented in the following chapters.
A series of quasi-static tests was performed on three separate concentrations of ballistic gelatin, and strain rate dependence is seen already at these low strain rates for all concentrations.
There are several diverse, acceptable methods for determining these properties while the most popular is the Split Hopkinson Pressure Bar (SHPB). This method is highly effective for testing materials at high strain rates while enabling the researcher to implement a relatively uniform strain rate throughout most of the experiment. Therefore, this method is fully presented here in order to observe the difficulties in implementing it upon soft materials.
These difficulties include the fact that the SHPB technique requires the specimen to be in dynamic equilibrium throughout the duration of the experiment, a requirement that is difficult to fulfill for soft biological materials.
Two methods for the testing and measurement of ballistic gelatin behavior at high strain rates have been reviewed, and both have implemeted the use of the Polymeric Split Hopkinson Pressure Bar (PSHPB) technique in which the incident and transmission bars are polymeric as opposed to the metallic bars traditionally used.
Due to the complex nature of these two methods, we propose a novel, simple experimental setup that provides accurate results, while eliminating unnecessary elements that distort the true values that we seek to obtain. This experimental setup includes direct force sensing of an SHPB setup of aluminum bars, with the additional use of a single strain gauge. This method is presented, validated mathematically, and tested, while providing stress-strain curves that enhance the strain rate dependence seen in the quasi-static tests.
Finally, these stress strain relationships are combined with the quasi-static results in order to produce an empirical constitutive law for the ballistic gelatins presented in this research that is of the form σ=K•εm•(dε/dt)n and is consistent with constitutive models of other strain rate dependent materials.