A Unique Method to Dynamically Characterize Powders and Granular Materials, 18-R9621

Principal Investigators
Alexander B. Bernardo
Gary L. Burkhardt

Inclusive Dates:  04/01/06 – Current

Background - The goal of this research is to develop a method to precisely measure radial strain of powders and granular materials subject to quasistatic and dynamic loading while confined in a hydraulic fluid pressure vessel at 50,000 psia. Despite the pressure, powders and granulars still undergo large radial strains (up to 50 percent), thus precluding strain gages as measurement devices. Obtaining this measurement will be particularly challenging. Perfection of the new technique will lend to the quantification of the fundamental response of powder/granular materials; provide essential data for the development of constitutive models for numerical simulations; and result in enhanced understanding and increased fidelity of simulations. A wider variety of problems can thus be solved through numerical simulations and analytical modeling in the following areas: armor development, asteroid impact research, deep-earth penetration research, and WMD dispersion research.

Approach - The technical approach is focused on conducting investigations to select one of two promising techniques in measuring radial strain in specimens: 1) an optical technique, and 2) Eddy Current Testing (ECT). These investigations will concentrate on the feasibility of implementing each technique in a hydraulic fluid environment subject to the impulse dynamics of an impact event. The optical approach is based on measuring the change in diameter of the specimen by sensing the proportion of a light beam that is interrupted by the specimen. The second technique, ECT, is based on the use of a wire coil, energized with alternating current, which induces the flow of eddy currents in the test piece via transformer action. The change in impedance of the electrical circuit is related to the change in diameter (i.e., radial strain) of the specimen.

Accomplishments - The system-level risks identified at the beginning of the effort were investigated and resolved through engineering analysis. Regardless of which measurement technique was used, fluid and mechanical shock propagation were thought to affect measurement during a dynamic experimental event. CFD and analytical calculations were made that revealed hydraulic fluid is not adequately accelerated by the impact event to cause shock propagation in the vessel. Analytical calculations also showed that it is entirely feasible to install windows on a vessel containing 50,000 psia of hydraulic fluid. For the optical method, windows are required, through which diagnostic light energy can be passed to measure radial strain. Investigations to select one of the two measurement techniques have been completed, and the optical method was chosen. While both the ECT and the optical methods were found to be feasible for implementation, the optical method was thought to be less complicated to set up during an experiment. The engineering design of the new pressure vessel has begun and will feature one-inch- thick sapphire windows.

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