Author of this article

Hongbing Lu
Professor & Associate Department Head for Graduate Program
Louis A. Beecherl Jr. Chair
Department of Mechanical Engineering
The University of Texas at Dallas

The Customer

University of Texas in Dallas

Department of Mechanical Engineering

Problem:

A project sponsored by the Office of Naval Research wanted to understand the behavior of sand and its energy absorption in a blast to improve the design of armor.

Solution:

Engineers used a Genesis HighSpeed DAQ equipped with an acoustic emission sensor to gather data which they correlated with the deformation of sand grains and the number of sand grains that fractured.

Benefit:

This information helps engineers design better armor for military vehicles.

Portable Data Recorder Measures Sand Grain Fracture after Explosions

Bullets cannot penetrate sandbags, so understanding the behavior of granular material such as sand and its energy absorption in a blast is critical in the design of body armor for soldiers. 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), our lab performed hundreds of experiments using a portable data recorder to determine the mechanical behavior of grains of sand sliding against each other after explosions.

The most effective way to understand this mechanical behavior is by studying sand’s acoustic emissions (AE) during a blast event. AEs are short lived, high frequency, elastic waves generated by the rapid release of stored energy. In this case, sliding friction and rolling between sand grains are the sources of AEs. Acoustic emissions carry real time information about the location, intensity, and deformation mechanisms occurring in a material.

Images above are acquired by Cordin 550-62 camera on sand with an initial mass density of 1.58 g/cm3. The image on the left shows the sand sample before impact, and the image on the right shows the sand sample under impact.

The figure above shows the input signal from the incident bar, the output signal from transmission bar and the circumferential strain on the sapphire tube.

Correlating AE Spikes with Deformations of Sand Grains

In our experiments, we acquired AE data using a Genesis HighSpeed portable data recorder from HBM. The company provides solutions for test and measurement applications such as sensors, transducers, strain gages, amplifiers, data recorders and data acquisition systems (DAQ), as well as software for structural durability analysis.

We first put sand into a hollow cylinder and placed an acoustic emission sensor (a piezoelectric-based device that transforms the pressure wave into an electrical signal) at the boundary of the material. The AE sensor was attached to a charge amplifier, which output a voltage that was fed to the Genesis.

To replicate blast forces, we used a cylindrical rod to push on the sand and apply compression. The applied pressure caused the grains to deform until some grains actually fractured. The AE events that resulted were sub-micron to mm in size, giving rise to signals with frequencies in the range of 10 kHz to several MHz.

We also compressed the outside of the cylinder and used a strain gauge attached to the data recorder to measure lateral deformation for additional AE data. We connected the strain gauge to the Genesis using a Wheatstone bridge that was powered by 10 volts DC. The signal was magnified by a signal amplifier. The voltage from the signal amplifier was converted through the Genesis via another circuit. 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.

Next, we correlated the AE spikes in frequency shown on the Genesis with the deformation of sand grains and the number of sand grains that fractured. The events proved to have two 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

Signal processing, such as computing the energy of the acoustic emission source, provided information about the processes and mechanisms of deformation. In this case, we wrote special code to capture the data, saved the data onto a USB drive and then then ran it on a computer in mathematical software to count the number of grains that fractured. This information helped us infer the magnitude of the blast. Basically, the data about AE signal frequencies and sand particle fracture rates told us what kind of blasts sand can forestall.

The Genesis HighSpeed data recorder, unlike other systems, is an integrated electronic device that does not require the use of a separate computer. The entire package weighs only 25 pounds, which makes it easier to carry from one location to another than other data recorders. And although the unit is small, it provides up to 96 fully configurable input ports. A large, easy-to-read and intuitive user interface makes it easy to select the desired data recording mode — continuous recording or triggered sweeps.

Our team supplied the information we gathered from our experiments to another project team. The team then used the data to simulate explosions on a computer to better understand how blast-waves interact with sand and how they further propagate to potentially hit soldiers and their vehicles. The upshot is the processed information from the Genesis proves useful to design engineers developing body armor for military personnel.