Full-Field Quantitative Visualization of Shock-Driven Pore Collapse in Solids: Mechanics of Deformation, Failure, and Interaction

Citation

Lawlor, Barry Patrick (2025) Full-Field Quantitative Visualization of Shock-Driven Pore Collapse in Solids: Mechanics of Deformation, Failure, and Interaction. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p8hs-da41. https://resolver.caltech.edu/CaltechTHESIS:05302025-081050551

Abstract

Porosity in solids is ubiquitous throughout engineering applications: inherent in energetic materials and exaggerated upon degradation, incorporated into shock-absorbing structures via materials such as metallic foams and metamaterials, and arising through manufacturing defects---especially in metal additive manufacturing methods. In these applications, many phenomena at both the macro- and meso-scale are critical to the operation under dynamic compression. Macroscopic shock wave structure, including shock attenuation and disruption are important for engineered structures like metallic foams, while mesoscopic localized shear deformation near porous defects can be a cause of failure in structures and is thought to be a mechanism for mechanically-induced hot spots in energetic materials which can dictate their ignition behavior. While the macroscopic shock response of porous materials has been well studied, the mesoscopic response has received less attention. Recent studies have improved the understanding through sophisticated numerical simulations and pore collapse experiments leveraging innovative high-speed imaging technologies, but many details of the mesoscopic response remain unclear.

This thesis is focused on the mesoscopic domain, with an overarching goal of characterizing local details of pore collapse, such as the rate of collapse, pore geometry (asymmetry) evolution, deformation induced in the material surrounding the pore, localization/failure mechanisms, and interactions between pores. Fundamental understanding of these mesoscopic phenomena is a critical step toward unraveling the physics which couple the mesoscale and macroscale responses, enabling predictive modeling for the dynamic response of porous materials/structures, and developing innovative engineering designs with porous materials.

The first part of this thesis develops a novel internal digital image correlation (DIC) technique for use in full-scale dynamic laboratory experiments, which enables investigation of phenomena which occur under confinement or are sensitive to boundary effects. The technique consists of manufacturing transparent specimens with an internally embedded speckle pattern, which is then dynamically deformed via the experiment of choice. During dynamic loading, the internal speckle pattern is visualized with a high-speed camera, after which DIC software is used to process the images and compute the displacement, velocity, and strain fields. The technique is implemented and validated using polymethyl methacrylate (PMMA) specimens under compression with split-Hopkinson (Kolsky) pressure bar and plate impact experiments---providing validation under both uniaxial stress and uniaxial strain conditions, at strain rates of 10³-10⁶ s⁻¹ and impact stresses up to 0.65 GPa.

The second part of the thesis implements the internal DIC technique to investigate the mechanics of a single spherical pore during collapse induced by weak shock loading up to 1 GPa impact stress in PMMA. The first of its kind internal strain measurements reveal concentrations around the collapsing pore, which are approximately consistent with elastostatic theory. Equivalent shear strain measurements uncover a transition from classical strain concentrations to the development of shear bands at 0.6 GPa, and raw deformation images show the development of fracture at 0.8 GPa---representing two distinct failure mechanisms arising within a small range of impact stresses. The shear bands arise due to large stress concentrations near the pore, which leads to plastic deformation and heating. Thermal softening generates local material instabilities, which can grow into regions of large, localized deformation. These bands are captured via explicit finite element analysis through a thermo-viscoplastic material model. The numerical simulations further indicate the crack to be a shear crack propagating through the weakened material of an adiabatic shear band. Finally, theoretical approaches elucidate the mechanics which govern the initiation of, spacing between, and preferred paths for these failure modes.

The third part of the thesis follows a natural extension toward real porous media, investigating the collapse of pore arrays in PMMA with a focus on the role of interactions between pores on the localization and failure response. Experiments are conducted on pairs of pores in vertical and horizontal configurations. By utilizing internal DIC and shadowgraphy, the evolution of shear bands and cracks is visualized and measured. Further, apparent interactions between pores are identified through shifts in impact stress thresholds for failure initiation and through delayed crack growth. Baroclinicity, and accompanying baroclinic torque, is identified as the driving mechanism for crack propagation in these experiments. Finally, shear diffraction waves initiate upon plane wave interaction with pores and propagate toward neighboring pores. This is considered as a possible interaction mechanism between pores which alters the failure response.

The work presented in this thesis enabled the first in-situ observation of adiabatic shear banding during pore collapse in addition to a much-improved spatiotemporal characterization of crack propagation compared to previous works. Analysis of the experimental results revealed the ability of theoretical and numerical (FEA) models to capture many details of shear localization in pore collapse. Further analysis unraveled mechanisms governing pore collapse and associated failure modes, including the importance of pore asymmetry during collapse as well as planar shock interaction with the pore and the resultant baroclinicity and diffracted shear waves.

Thesis Files