Abullet cannot penetrate through a sandbag while an arrow can, so understanding the behavior of granular materials such as sand and its energy absorption in a blast is critical in the design of armor. To this end, as part of the “Soil Blast Modeling and Simulations” project sponsored by the Office of Naval Research’s Multidisciplinary University Research Initiative (MURI), experiments were performed using a portable data recorder to determine the mechanical behavior of sand grains under confined compression.

A Sample of Sand that had fractured during the experiments

One of the most effective ways to understand this mechanical behavior is by studying sand grains’ acoustic emissions (AE) under impact. AEs are high-frequency, short-duration elastic waves generated by the rapid release of stored energy. In this case, sand grain sliding friction and fracture are the sources of AEs. Acoustic emissions carry real-time information about the locations, intensities, and deformation mechanisms occurring in a granular material.

In these experiments, AE data was acquired using a Genesis high-speed data recorder from HBM, Inc. First, sand was put into a hollow cylinder, and acoustic emission sensors (piezoelectricbased devices that transform the pressure wave into an electrical signal) were placed at the boundary of a confining cylinder. The AE sensor was attached to a charge amplifier that output a voltage fed to the Genesis. To replicate blast forces, a cylindrical rod was used to push on the sand and apply compression. The applied pressure caused the grains to deform until some grains fractured. The AE events that resulted were associated with fractures in sub-micron to millimeter in size, giving rise to signals with frequencies in the range of 100 kHz to several MHz.

A confined compression of sand test was conducted using a strain gauge attached to the outer surface of the cylinder, and the data recorder was used to measure lateral deformation in addition to acquiring the AE data. The strain gauge was connected to the Genesis using a Wheatstone bridge powered by 15V DC. The strain signal was magnified by a signal amplifier, and connected to the Genesis. Even at a fast data sampling rate of 10 MS/s, the data recorder readings were accurate and of a high resolution. The recorder’s built-in 10x amplifier/ conditioner signal conditioners also helped ensure high-quality data.

The AE spikes were correlated in frequency shown on the Genesis, with the deformation and fracture of sand grains and the number of sand grains that fractured. The events proved to have entirely different signatures. Sand grains sliding against each other caused a low frequency wave and low amplitude of sound, while grain fracture caused a high frequency wave and high-amplitude acoustic wave.

Signal processing, such as computing the energy of the acoustic emission source, provided information about the processes and mechanisms of energy absorption. A tremendous amount of energy is dissipated when grains fracture to create new fracture surfaces. In this case, code was written to capture the Genesis data, the data was saved onto a USB drive, and then it ran on a computer to count the number of AE events. This information helped infer the magnitude of energy absorbed. Basically, the data about AE signal frequencies and sand grain fracture rates indicated what kind of blasts sand can forestall.

The team supplied information on the mechanical behavior of sand to other collaborating teams. The other teams will use information collected from the experiments to develop materials models, and simulate explosions on a computer to better understand how blast waves interact with sand and how they further propagate to potentially hit vehicles. The upshot is that the processed information from the Genesis proves useful to design engineers developing armor for military vehicles.

This work was done by Hongbing Lu, Professor & Associate Department Head for Graduate Program, and Louis A. Beecherl Jr., Chair, Department of Mechanical Engineering, at the University of Texas at Dallas. For more information on the HBM products used in this work, visit http://info.hotims.com/45608-505.

Defense Tech Briefs Magazine

This article first appeared in the October, 2013 issue of Defense Tech Briefs Magazine.

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