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PhD Thesis Defense - Jack Weeks

Friday, September 9, 2022
9:00am to 10:00am
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Online and In-Person Event
Mechanical Response of Lattice Structures under High Strain-Rate and Shock Loading
Jack Weeks, Graduate Student, Mechanical Engineering,


Lattice structures are a class of architected cellular materials composed of periodic unit cells with structural components of rods, plates, or sheets. Current additive manufacturing techniques allow control and tunability of unit cell geometries, which enable lattice structures to demonstrate exceptional mechanical properties such as high stiffness/strength-to-mass ratios and energy absorption. Lattice structures exist on two length scales corresponding to the unit cell and continuum material, and therefore demonstrate mechanical behavior dependent on structural geometry and base material. Experimental investigation of the dynamic and shock compression behaviors of lattice structures remains largely unstudied and is the central focus of this thesis.

The first part of this thesis investigates the high strain-rate behavior of lattice structures via polymeric Kelvin lattices with rod- and plate-based geometries with relative densities of 15-30%. High strain-rate experiments (1000/s) are performed using a viscoelastic polycarbonate split-Hopkinson (Kolsky) pressure bar system with high-speed imaging. Both low and high strain-rate experiments show the formation of a localized deformation band which initiates in the middle of the specimen. Strain-rate effects of lattice specimens are observed to correlate with effects of the base polymer material and mechanical properties exhibit distinct scaling between relative density and geometry type and loading rate. Explicit finite element simulations with a tensile failure material model are then performed to validate deformation modes and scaling/property trends.

The second part of this thesis explores the transient dynamic and transition to shock compression behavior of lattice structures using polymeric lattices with cubic, Kelvin, and octet-truss topologies with relative densities of about 8%. Dynamic testing is conducted through gas gun direct impact experiments (25 – 70 m/s) with high-speed imaging coupled with digital image correlation (DIC) and a polycarbonate Hopkinson pressure bar. At lower impact velocities, a transient dynamic response is characterized by a compaction wave initiating at the impact surface and additional deformation bands with modes similar to low strain-rate behavior. At higher impact velocities, shock compression behavior is characterized by a sole compaction wave initiating and propagating from the impact surface. 1D continuum shock theory with Eulerian forms of the Rankine-Hugoniot jump conditions is used with full-field DIC measurements to quantify a non-steady shock response and the effect of topology on mechanical behavior.

The final part of this thesis examines the steady-state shock compression behavior of lattice structures through stainless steel 316L octet-truss lattices with relative densities of 10-30%. Powder gun plate impact experiments (270 – 390 m/s) with high-speed imaging and DIC are conducted and reveal a two-wave structure consisting of an elastic precursor wave and a planar compaction (shock) wave. Local shock parameters of lattice structures are defined using full-field DIC measurements and 1D continuum shock analysis is again used to extract relevant mechanical quantities. Explicit finite element simulations using the Johnson-Cook constitutive model are performed and exhibit similar shock behavior to experiments. Notably, 1D shock theory is applied to simulations without resorting to a shock velocity – particle velocity relation for the base material, which characterizes this deformation regime as a `structural shock'.

Please attend this thesis defense: 135 Gates-Thomas (Hall Auditorium) or Zoom Link:

For more information, please contact Stacie Takase by email at [email protected].