"Ground Testing," Feature Article, Aug. 2002

ground testing

From lab samples to computer models, research uncovers secrets of blast waves and how to use—or resist—them.

This article was prepared by staff writers in collaboration with outside contributors.

A researcher at the Colorado School of Mines is taking a close look at shock waves—how they move through mixed ground, and how mathematical models can predict the effects of blasts. According to the researcher, Vilem Petr, insight derived from the studies one day may save energy in mining operations and also may help walls withstand bombings.

He and his associates at the school are developing a resilient concrete for which they plan to file a patent application. Petr is framing separate proposals to the U.S. Department of Energy and the Department of Defense for funding to put his investigative work to practical use.

Petr, a research assistant professor in the Department of Mining Engineering at the school in Golden, has studied the pattern of blasts through combinations of materials simulating the mixed earth encountered in nature. His aim was to get a better understanding of how shock waves behave as they cross joints or fissures in a rock formation, or pass between materials of different density and elasticity.

He said that observing shock waves and modeling them mathematically will improve the ability to forecast fracture phenomena. If that knowledge can be applied to control fragmentation of material in a mine, it can save money and energy, which is the DOE connection. He believes that a concrete with superior resistance to blasts may be something the DOD will want to study.

Petr's data formed the basis of computer models, including some developed with the pro bono help of a Houston-based consultant, Keith Orgeron of Integra Engineering Inc. The models recreate the patterns of shock waves moving through the test specimens and analyze their effects. Petr conducted the first phase of the work, which was completed a little more than a year ago, in pursuit of his Ph.D. under the direction of a faculty advisory committee led by Tibor G. Rozgonyi, head of the school's Department of Mining Engineering.

"We still do not understand fracture phenomena completely," Petr said. "A more complete understanding could lead to refinement of blasting techniques, which would lower mine operation costs."

Petr, who is from the Czech Republic, comes from a long line of miners. His grandfather was a coal miner and his father was an open-pit miner. In 1992, he graduated from the Institute of Mining Engineering at the Technical University of Ostrava in the Czech Republic and then worked as a mining engineer for several years before deciding to continue his education in the United States.

Images show the stages of a physical blast test (top); simulations recreate the event in Algor software.

Physical experiments at the School of Mines included the use of strain gauges and photoelastic material to determine the velocity of shock waves and the stress fields they create. In order to study how shock waves spread through various materials, Petr built test specimens in which flat sections of photoelastic material represented the matter in the Earth's crust. A casting resin cemented the sections, which were sandwiched between clear glass for observation.

Some tests were run on a single material between the glass plates and others on specimens made of two sections cemented together. Still other test specimens had one material distributed within the other. Experiments used PSM 1 and 9 from Vishay Measurements Group Inc. of Raleigh, N.C. PSM 1 has a higher modulus than PSM 9, according to Tom Rummage, an applications engineer at Vishay. A third material used in specimens was Homalite, a plastic sold by the Homalite division of Brandywine Investment Group Inc. of Wilmington, Del.

The shock wave generator was a cone covered by a thin film of plastic explosive. Detonation began at the peak. The geometry of the structure was calculated so that, as the combustion of the explosion proceeded down the slope of the cone, the shock waves produced remained in a single plane.

Photoelastic materials under stress change the speed of polarized light passing through them, creating patterns of light and dark that track the forces through the specimen. A high-speed camera, shooting at a rate of a million frames per second, captured 16 images of each test, recording the stress patterns moving through the samples.

By the Numbers

Researchers analyzed data from the physical experiments using discrete element method software developed at the School of Mines and Mechanical Event Simulation software from Algor Inc. of Pittsburgh. "Each method provided distinct advantages," Petr said.

According to Petr, the primary advantage of the discrete element modeling software, which was developed by Graham Mustoe of the School of Mines' Engineering Division, was that it modeled the geomedium as a system of several hundred rigid particles joined together elastically. "Discrete particles were well suited to simulating the grain lattice," Petr said. Mustoe worked with Petr to create the models.

Petr said the Algor software analysis produced a record of conditions inside the material, including displacement, stress, and strain.

In the finite element model, Orgeron and Petr decided to represent the initial blast by a block hitting the top surface of the specimen at the same velocity as measured in the physical experiment.

"The challenge was to make an FEA model of an explosion, which is a chemical reaction," Petr said. "Using the impactor block was a simple way to create a shock wave similar to an explosion."

A simulation used nonlinear material models to include the effects of large deformation and large stress. Built-in result-monitoring tools were used to track the velocity of selected nodes, which enabled calculation of stress history curves. The original models were two-dimensional, focusing on the surface of the specimens.

Analysis results showed, with more detail than could be captured in experimental studies, exactly how shock wave velocity is affected by different material densities and different packing arrangements within a sample.

"The packing of the material and the grain boundary can play a very important role in rock fragmentation," Petr said. "The shock wave can lose a lot of energy as it passes across joints and through materials of different densities."

Data from the physical experiments and the Algor models were published in February in a paper listing Petr, Mustoe, Orgeron, and Rozgonyi as authors. They presented it at the Annual Conference on Explosives and Blasting Technique in Las Vegas.

Additional work by Orgeron, an ASME member, has extruded the 2-D models into three dimensions.

According to Petr, understanding how matter responds to impact will lead to better calculations of how much energy to send through rock to break it into workable bits. A blast with insufficient energy, for example, can create oversize rocks, which may require a second blast or the use of a primary crusher before they can be processed. In either case, there is a cost in time and energy.

Petr said that one of the next steps in his research will be to test his models against blasts in actual mines.

Keith Orgeron developed 3-D simulations from the original 2-D models.

An industrial application of his research involves concrete to support caverns, the underground openings in mines. "We are thinking that if we can improve the elastic-brittle behavior of regular concrete to elastic-plastic, that this underground concrete support will improve the safety and stability of the underground tunnels," he said.

Another goal is to create a database predicting the amount of energy to use for different types of rock—sandstone, limestone, granite, and so forth. But that still lies in the future. "There are lots of unknowns," Petr said.

Knowledge of how material densities and patterns of particle grains can alter a shock wave can be applied to the development of shock-resistant materials. Petr said he has tested a composite concrete that stands up to shocks better than conventional concrete. He is reluctant to say much about it because the school hopes to patent the idea.

The researchers want to apply for funding to cover further study. For instance, they need to establish the appropriate distribution of composite materials and the bonding material to achieve a required strength.

Meanwhile, Petr plans to ask the Department of Energy for $1.5 million over three years to fund research into using the concept from his models to make better use of energy to break rock in mines.

According to Petr, "This work has shown the importance of the interaction of strain waves with discontinuities in fragmentation. We must know shock wave propagation characteristics in order to be able to determine the effect of blast parameters on fragmentation."