Doctor of Philosophy (PhD)
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Space structures are subjected to micro-meteorite impact at extremely high velocities of several kilometers per second. Similarly, design of military equipment requires understanding of material behavior under extremely high pressure and temperature. Study of material behavior under hypervelocity impact (HVI) poses many challenges since few researchers so far have approached this problem. Material models, equations of the state, and fracture mechanics are not well understood under these conditions.
The objective of this research is to present an approach for studying plastic deformation of metallic plates under HVI conditions. A two-stage light gas gun can be used to simulate these conditions under controlled experimental conditions. In all gas gun experiments, projectiles are driven by light gas propellants (typically Hydrogen or Helium) to impact target plates. These experiments, which typically last few microseconds, are associated with large localized deformation, which makes capturing data challenging. Additionally, the resulting shock waves create several failure modes simultaneously such as formations of spalls, petals, discs, and plugs in target plates. These failure modes are mostly controlled by several factors such as impact velocity, material properties, projectile and target geometry, etc.
In this work, plastic deformation is related to the velocities at multiple points on the back surface of the impacted plate. These velocities are measured using a multi-channel Multiplexed Photonic Doppler Velocimetry (MPDV) diagnostic system, which is an interferometric fiber-optic technique that can determine the velocity of a point by measuring the Doppler shift of light reflected from a moving surface.
These experiments are simulated using the Lagrangian-based smooth particle hydrodynamics (SPH) solver in LS-DYNA. Simulation models use Johnson-Cook material model with Mie-Grüneisen equation of state (EOS).
To assess the proposed approach, preliminary testing was conducted using 12.7 mm (0.5 inch) thick ASTM A36 steel plates, which were impacted by cylindrical Lexan projectiles at velocities ranging from 4.5 to 6 km/s. Impact typically created a small crater in the front of the plate and a bulge on the back surface which usually led to fracture near the crate and spallation of material inside the plate. In addition to recording impact velocity, back surface velocities were recorded, and crater details were measured.
Several simulation models were developed with the objective of producing simulation results that are close to experimental ones. Simulation results were assessed based on comparison physical damage and velocity profiles. Adjustment of the simulation model included varying the parameters of the Johnson-Cook material model, EOS, and bulk viscosity, choosing appropriate spall model, and tuning particle spacing.
Initial experiments with A36 steel showed reasonable success to use the MPDV system. Further experiments were planned to design with better MPDV arrangement. Two other different steels ware studied in: 304L and HY100. Simulation of these experiments were also compared with MPDV results to better understand material model behavior.
Hypervelocity; Impact; Shock; Smoothed Particle Hydrodynamics
Engineering Science and Materials | Materials Science and Engineering | Mechanical Engineering
University of Nevada, Las Vegas
Roy, Shawoon Kumar, "An Approach to Model Plastic Deformation of Metallic Plates in Hypervelocity Impact Experiments" (2015). UNLV Theses, Dissertations, Professional Papers, and Capstones. 2575.
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