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

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Principal Investigators
Alexander B. Bernardo
Gary L. Burkhardt

Inclusive Dates:  04/01/06 – 09/31/07

Background - The goal of this research was 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. Perfection of the new technique will lead to the quantification of the fundamental response of powder and 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 (U.S. Army), asteroid impact research (NASA), deep-earth penetration research (U.S. Air Force), and weapons of mass destruction dispersion research (USAF/DTRA).

Approach - The technical approach focused on investigations to select and demonstrate one of two promising techniques for measuring radial strain in specimens: optical or eddy current. These investigations were concentrated 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 interrupted by the specimen. The eddy current approach 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 - Investigations were conducted to select and demonstrate proof of concept of the optical or eddy current techniques to measure radial strain in specimens. The environment was a hydraulic fluid subjected to the impulse dynamics of a Split Hopkinson Pressure Bar (SHPB) impact event.

Each method met the required measurement time resolution of 0.5 microns per second over an interval of 50 microns per second with an accuracy of 1 percent. Potential risks were associated with fluid and mechanical shock propagation when testing under high strain rate with the SPHB. Calculations showed that the effect of shock on either measurement technique was negligible.

The optical method was viewed to be less complicated to implement than the eddy current method. The eddy current method required mounting an induction device on the specimen and running wiring from within the pressurized chamber to the outside. No physical contact of the specimen would be required with the optical method. Thus, the optical method was chosen over eddy current, with the challenge that a chamber with optical windows could be developed to withstand the required pressures.

Using finite element analysis, a chamber with optical windows was engineered that could contain 50,000 psi pressurized hydraulic fluid. The pressure vessel fabricated was such that it could be mated with a mechanical loading device and the SPHB test fixture at SwRI. In particular, the design allowed translation of load bars on either end of the pressure vessel while maintaining a seal as the vessel and specimen were pressurized.

An optical assembly was designed, fabricated and integrated with the pressure vessel. Static measurements were taken with the detection system on metal rods of varying diameters for calibration. The signal response of the detection system was measured for the extreme case of a 50 percent strain and accounted for the possibility of compression during pressurization of the chamber.

Using the optical measuring device and the pressure vessel, radial strain measurements were taken of a specimen of SiC-N powder subject to quasistatic load and confinement pressure of 100 MPa (14,503.7 psia). The loading device provided axial loading to an excess of 400 MPa (58,015.1 psia). Radial strain response was successfully recorded as a function of increasing axial compression. Based on these measurements, the optical method was successfully demonstrated.

As a result of this project, two patent applications were submitted: SwRI ID No. 3282, "Measurement of Constitutive Properties of a Powder Subject to Compressive Axial and Radial Loading, using Optical Sensing" and SwRI ID No. 3281, "Measurement of Constitutive Properties of a Powder Subject to Compressive Axial and Radial Loading, using Eddy Current Sensing."

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