Algorithmic and experimental contributions to the study of ballistic penetration of tissue-like materials [electronic resource]
Keywords: Mechanical Engineering

In this work we present a series of scientific contributions made to the study of the impact of projectiles into tissue-like materials, specifically the synthetic artificial tissue simulant Perma-Gel. These contributions consist of a combination of experimental observations, algorithmic ideas and numerical tools which demonstrate a series of problems and solutions to trying to simulate nearly incompressible soft tissues using finite elements. A number of experiments were performed by taking high-speed footage of the firing of spherical steel bullets at different speeds into Perma-Gel, a new thermoplastic material used as a proxy to human muscle tissue. This work appears to be the first publicly released experimental work using Perma-Gel and is part of the small amount of non-classified work looking at ballistic gelatin behavior. A number of experimental observations were made regarding the material behavior, elastic and plastic deformation around the projectile, and the possibility of cavitation. This work introduces an explicit dynamic contact algorithm that takes advantage of the asynchronous time stepping nature of Asynchronous Variational Integrators (AVI) to improve performance when simulating elastic-body rigid-wall contact. We demonstrate a number of desirable properties over traditional one-time-step methods for the simulation of solid dynamics and provide a number of examples highlighting the advantages of this method. The explicit contact algorithm and AVI was used to simulate the impact of a projectile into a simulated block of gelatin, but was hindered by difficulties using the realistic material parameters. Using a parallelized version of the algorithm, large-scale simulations were performed for progressively smaller shear moduli. As the simulations approached realistic values for the shear modulus, unstable element configurations formed which required infeasibly small time steps to successfully resolve. The behavior observed for the shear moduli we could numerically simulate with did not resemble the experimental results. To simulate with smaller values, we had to go to an axisymmetric setting. The axisymmetric setting increased the range of shear moduli which could be simulated and demonstrated the same dynamic behavior, though the issue of unstable element configurations continued to occur in extreme cases. To deal with the issue of unstable elements, we created an axisymmetric remeshing strategy to compensate for the unstable element configurations and insufficient spatial resolution. This strategy consists of periodically applying a remeshing and transfer algorithm that updates highly deformed finite element meshes with configurations formed with elements having uniform aspect ratios and local refinement in important areas. The axisymmetric setting with remeshing increased the range of potential shear modulus values that could be simulated. This allowed for the identification of qualitative similarities in the transient behavior between the numerical results and the experimental footage.