Energy dissipation mechanisms during impact in jammed ductile granular media

Fonseka, Rannulu Devanjith Janek Indrajith

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https://hdl.handle.net/2142/127389 Copy

Description

Title

Energy dissipation mechanisms during impact in jammed ductile granular media

Author(s)

Fonseka, Rannulu Devanjith Janek Indrajith

Issue Date

2024-12-06

Director of Research (if dissertation) or Advisor (if thesis)

• Geubelle, Philippe
• Lambros, John

Doctoral Committee Chair(s)

• Geubelle, Philippe
• Lambros, John

Committee Member(s)

• Chew, Huck Beng
• Hashash, Youssef

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Aerospace Engineering
• Granular Materials
• Granular Jamming
• Plasticity
• Shockwaves
• Ductility
• Contact Mechanics

Abstract

The discrete nature of granular materials makes them effective for impact mitigation applications, as energy dissipates at contact points and impact energy can be redirected based on the connectivity of the contact network. A key controllable variable that alters the mechanical behavior of granular materials is the confining pressure, which can jam the granules and increase the rigidity of the packing. In elastic granular packings that follow Hertzian contact conditions, the confining pressure governs the transition between linear and nonlinear wave propagation when subjected to impact loading. However, because of stress concentrations at the contact points, ductile granules readily yield and deviate from the Hertzian contact law, following an elastoplastic contact law that dissipates both plastic and frictional energy. While elastoplastic wave propagation has been studied extensively in ordered, unconfined ductile granular crystals, the effects of confinement and randomness in ductile packings remain largely unexplored. This thesis investigates the physics behind the effect of confinement on the impact response of elastoplastic granular systems across various configurations and length scales. The first portion of this thesis investigates the effect confinement has on ordered granular crystals by impact loading a vacuum-packed, 2D, hexagonal array consisting of brass spheres. Both experimental and discrete element method simulations (DEM) results indicate that a non-zero confining pressure enhances plastic dissipation when compared with a loose, unconfined packing. However, no further increase in dissipation is observed as confining pressure increases within the range of impact velocities and pressures studied. The second portion of this work investigates elastoplastic shock propagation in 2D, frictionless, random packings using DEM simulations. The wave propagation characteristics resemble those in an equivalent elastic Hertzian system, namely the existence of a weak shock (confining pressure dependent wave propagation realized for low velocity impacts) and strong shock (independent of confining pressure for high velocity impacts) regimes, with the transition between the two controlled by the confining pressure and loading intensity. Unlike ordered hexagonal arrays, random packings exhibit a distinct increase in dissipation with confining pressure during weak shocks; however, this distinction disappears in the strong shock regime. One limitation of conventional elastoplastic contact laws (i.e., those derived from the study of a single ductile contact) used in the first two portions of this thesis is that they break down under large dynamic confining pressures due to greatly increased triaxiality, where distinct contact points on a granule begin interacting with each other. Therefore, in the third portion of this thesis, a truncated sphere contact model, originally used to study quasi-static compression of granules, is appropriately modified to accurately capture multiaxial loading of brass spheres through finite element simulations. The new model is implemented in the LAMMPS molecular dynamics code and is validated by simulating impacts in laterally confined 1D granular chains under different initial conditions. The final portion of this thesis uses the truncated sphere model described above to study piston-driven (constant velocity impact) shock propagation in confined, 3D, random granular columns containing both plastic and frictional dissipation mechanisms. For weak shocks, increasing confining pressure leads to greater plastic dissipation at the expense of frictional dissipation. Since dissipation at the macroscale is a consequence of the forces and displacements at the contact level (microscale), we establish relationships between the two scales and understand how the loading conditions affect them. Another important finding is that the transition between weak and strong shock regimes occurs when the non-dimensional inertial number (scales with the ratio of the relaxation time to the shear time) reaches 0.01, which also corresponds to the transition between quasi-static and dense flow regimes. This observation highlights the utility of the inertial number - commonly used to classify granular shear flow regimes - in characterizing compressive shock regimes.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127389

Copyright and License Information

Copyright 2024 Rannulu Devanjith Janek Indrajith Fonseka

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