Session: 12-04-02: Drucker Medal Symposium

Paper Number: 149855

149855 - Full-Field Quantitative Visualization of Shock-Driven Pore Collapse and Associated Failure Modes in Pmma 

Porous materials feature in many engineering applications, including energetic materials, in which pores have been linked to detonation under dynamic loading, and in structural materials, where localized material and structural failure are often driven by stress-concentrators such as pores. Porosity in structural materials spans many length scales from microns to centimeters, with examples such as additive manufacturing defects, engineered porous media (e.g., foams), and metamaterials. When these materials are loaded dynamically via shock compression, a phenomenon known as pore collapse occurs, in which the pore rapidly collapses on itself, leading to large plastic deformation and, at sufficiently high stresses, the development of jetting. Computational research has also suggested that shear localization via adiabatic shear banding can occur due to pore collapse.

While shock compression experiments typically leverage continuum velocimetry techniques to study the macroscopic material response, the localized nature of pore collapse requires an alternative approach. High-speed imaging has primarily answered that call, leading to the observations of collapsing cylindrical and spherical pores. However, the existing literature is limited to time-resolved, qualitative observations of the shape of the pore and surrounding material during loading and post-mortem analysis of localized deformation and failure.

In this study, an experimental technique is developed to enable internal, full-field strain measurements during shock compression of transparent materials. After validation on specimens without pores, the method is applied to study the pore collapse phenomenon at shock stresses ranging from 0.4-1.0 GPa. Static, elastic theory, and dynamic/explicit finite element analysis are utilized to understand the physics of the observed material and structural response.

The technique is based on normal plate impact in which a flyer plate impacts a target plate, generating a controlled planar shock. However, rather than using velocimetry methods to measure free surface velocity, high-speed imaging is used to visualize the in-plane deformation as the shock propagates through the target and compresses the material. This is accomplished by manufacturing target plates from PMMA, a material that remains optically transparent under shock loading at the stresses of interest for this work. A spherical pore and an internally embedded speckle pattern are generated inside the plate for use with digital image correlation. Thus, the captured images contain information about the geometry of the pore collapse itself, and the digital image correlation of the speckle pattern enables quantitative measurement of the deformation field surrounding the pore during the collapse.

In these experiments, a critical stress is determined at which adiabatic shear bands (ASBs) initiate at the surface of the pore, providing the first in-situ observation of adiabatic shear banding during pore collapse. At higher stresses, the localized shear response transitions from the initiation of many self-organized ASBs to the emergence of one dominant ASB that weakens the material and gives way to dynamic shear fracture.

Presenting Author: Guruswami Ravichandran California Institute of Technology

Presenting Author Biography: Guruswami (Ravi) Ravichandran is the John E. Goode, Jr. Professor of Aerospace and Mechanical Engineering at the California Institute of Technology. He received his Ph.D. in Solid Mechanics and Structures from Brown University. He is a member of the National Academy of Engineering and Academia Europaea. He is a Fellow of AAM, ASME, and SEM. His awards include the Timoshenko Medal (ASME), Eringen Medal (SES), and Murray Award (SEM). His research interests are in mechanics of materials, active materials, cell mechanics, and experimental methods.

Authors:

Barry Lawlor California Institute of Technology
Guruswami Ravichandran California Institute of Technology