Research done at the Colorado School of Mines on shock-wave propagation through rock and stone could help in the war against terrorism.
FEA software from Algor shows how a shock wave from a blast travels through a pillar constructed of nonhomogeneous rock.
An experiment in which a shock wave is initiated and travels through a block of photoelastic discs of different densities is compared to a simulation done on Algor's Mechanical Event Simulation software.
Working with the Depts. of Defense and Transportation and the Defense Threat Reduction Agency, the school is developing "smart" construction materials, such as explosive-resistant concrete, to protect military personnel and equipment and build safer mines.
"We still do not understand fracture phenomena completely," says Vilem Petr, Research Assistant Professor at the school's Mining Engineering Dept. "So it is important to develop a theoretical model validated with experimental data for a better understanding of how shock-wave energy, due to dynamic loading, travels and bounces through nonhomogeneous rock and stone."
Over the past three years, researchers have looked at shock waves using experiments and numerical studies. Experiments determine the stresses inside specimens and this data was used in numerical analyses, including a discrete-element method program developed at CSM and Mechanical Event Simulation (MES) software from Algor Inc. to examine shock wave effects more precisely.
Researchers hypothesized that shock-wave velocities traveling inside nonhomogeneous rock are affected by the geometrical arrangement of particles and differences in material properties. "Shock waves can lose a lot of energy as they pass across joints and through materials of different densities," explained Petr.
To study shock waves propagating through various materials, specimens were constructed out of photoelastic discs, which represented grains within a rock specimen. Casting resin held the discs together. The relative densities of the discs and resin were varied in different specimens to test the effects of material density on shock-wave velocity. The discs' packing arrangement was also varied to see how different grain patterns might slow down shock waves.
"Since computer simulations came of age, photoelastic experimentation is not used much because it is time consuming, expensive, and provides inconsistent results," said Petr. "Still, it is a good tool for studying stress-wave propagation inside a specimen because it yields experimental data you can compare to numerical analysis."
In the finite-element model, initiation of the shock wave was simulated by hitting the top surface of the specimen with a block moving at the same velocity as measured in the physical experiment. "The challenge was to model an explosion, which is a chemical reaction," says Petr. "An impactor block was a simple way to create a shock wave similar to an explosion."
Petr plans to continue modeling different disc patterns to test the effects of grain lattice on shock-wave velocity. "We must understand shock-wave propagation to determine the effects of blasts on fragmentation."