Test Methods for Explosives (Shock Wave and High Pressure Phenomena)

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Contents

  1. Effect of blast pressure on discrete models using a shock tube - IEEE Conference Publication
  2. Test Methods for Explosives (Shock Wave and High Pressure Phenomena)
  3. ISBN 13: 9781461269045
  4. Bibliographic Information

Detonation characteristics of non-ideal explosive depend strongly on confinement, and JWL parameters determined by cylinder test do not represent the state of detonation products in many applications. We developed a method to determine JWL parameters from the underwater explosion test. JWL parameters were determined through a method of characteristics applied to the configuration of the underwater shock waves of cylindrical explosives.

The numerical results obtained using JWL parameters determined by the underwater explosion test and those obtained using JWL parameters determined by cylinder test were compared with experimental results for typical non-ideal explosive; emulsion explosive. This speed depends on air temperature, but a is typically about meters per second in "standard" air. Shock waves, on the other hand, travel faster than a , being supersonic wave phenomena. They're also stronger and more energetic than sound waves, are highly nonlinear and cause significant jumps in temperature, pressure and density of the air over their wave thickness of only nanometers.

The passage of a strong shock wave through the human body, for example, causes severe damage owing to the large instantaneous pressure change. Normal conversation, with a sound intensity in the to decibel dB range, involves minuscule air-pressure fluctuations of less than one millionth of an atmosphere. Painfully loud "noises," such as those from a jet engine in the dB range, are actually very weak shock waves.

Effect of blast pressure on discrete models using a shock tube - IEEE Conference Publication

One can see them using the optical methods described here, but they travel barely faster than sound waves, with pressure peaks of only some hundred-thousandths of an atmosphere. On the other hand, a strong shock wave in air, such as one traveling at Mach 2, produces an overpressure peak of 4. However, this phenomenon can be controlled for medically beneficial purposes as well: A method called shock wave lithotripsy focuses shock-wave energy at a point inside the body to break up kidney stones without significantly damaging the surrounding tissue. Spherical shock waves from explosions decrease quickly in strength with distance from the explosion center, rapidly leveling out to Mach 1.

This rate of speed decrease can be extracted from a high-speed shadowgraph video. As Harald Kleine of the Australian Defence Force Academy and his colleagues outlined in their paper in the journal Shock Waves, the shape of the curve produced by graphing this speed-decrease data can be used to find an explosive's equivalent mass, as compared with the standard of trinitrotoluene TNT. Close to an explosion, a shock wave can travel at several times the speed of sound and reach pressures of ten or more atmospheres, producing devastating effects. Also, the "wind" that immediately follows a strong shock wave is brief but very intense.

Test Methods for Explosives (Shock Wave and High Pressure Phenomena)

In an explosion, the fireball expands very quickly and pushes air ahead of it. As the shock wave ripples out from the explosion center, the speed of its following wind is the same as the speed of expansion of the initial fireball. A shock wave at a mere Mach 1. Footage of pre aboveground nuclear tests shows the shock wave smashing whole buildings, whose debris is then swept downrange by the following wind. What causes such a strong shock wave?

Since a stereo system makes sound waves, can one turn the volume up to maximum and make shock waves? No, stereo speakers are only designed to vibrate in order to reproduce sound. Shock waves are made by a rapid, continuous "push," or by an object traveling at supersonic speed. Cracking a whip creates weak shock waves, because the whip tip moves faster than the speed of sound. Figure 6.


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When a toy balloon bursts, a schlieren photograph shows that the balloon skin shreds very rapidly, revealing a balloon-shaped bubble of compressed air inside. A spherical shock wave is formed despite the non-spherical initial shape of the balloon. The colors in this image were introduced by color filters that take the place of a knife-edge.

But the best way to generate a strong shock wave in the air is suddenly to release a lot of energy stored in a small space. Pressurized gas is an example: On release, the gas expands very quickly and pushes the atmosphere out of the way, forming a shock wave.

Explosive Effects and Applications Shock Wave and High Pressure Phenomena

Even popping a balloon is enough to generate a very weak shock wave from the gas released when the balloon skin ruptures. In the laboratory, shock waves are best studied in a facility known as a shock tube , where they are generated by the rupture of a thin diaphragm separating high- and low-pressure gases. Explosives are another good way to produce shock waves.

In this case, the energy is stored in an unstable chemical form—often in nitrates—and can be released in about a microsecond. Ironically, most chemical explosives contain less energy per unit mass than ordinary table butter, but fortunately the butter is too stable to explode. Figure 7. Shadowgrams of two small explosive charges show the dangers of fragmentation. The charges are 1 gram each of triacetone triperoxide TATP encased in solid containers.

Ignited electrically, they produce spherical shock waves that were captured here by 1-microsecond exposures when each shock was about 1 meter in diameter. At left, the container fragments into large pieces that are hurled at near the speed of sound behind the shock wave. In the image at right, the fragments are much smaller and travel at supersonic speeds ahead of the main shock.

In full-scale explosions, fragments like these are as deadly as a hail of bullets. Photographs courtesy of Gary S. The loss of life caused by an explosion is often due to fragmentation rather than the overpressure or the following wind of the shock wave itself. Shrapnel behaves like a hail of supersonic bullets, being accelerated along radial lines in all directions from the explosion center by the aerodynamic drag force exerted by the rapidly expanding gas.

But strong shock waves are also devastating to structures. In the terrorist bombing of the Murrah Federal Building in Oklahoma City, a huge truck bomb was detonated only a few meters from the building.

ISBN 13: 9781461269045

The resulting strong shock wave and its many concomitant effects destroyed the columns supporting the north face of the building, whence it collapsed. As a result, lives were lost and there were many more injuries. Both experiments and computational blast simulations now help inform building designers on how to mitigate such lethal effects and how to prevent building collapse and improve survivability.

Figure 8. Detonation of a small, milligram silver nitrate charge three centimeters above a surface produces primary and secondary spherical shock waves that are irregularly reflected by the ground. Although shock waves from explosions have spherical symmetry in the open air, reflections off objects can make the shock-wave pattern very complicated.

The pastel colors of this photograph are introduced by color filters and coded to indicate the direction in which light has been refracted.

The experiments can sometimes be dangerous and costly, however, when done at full scale. A recent trend is toward cheaper, safer, quicker simulations of blast effects using gram-range explosive charges, scale models and optical shock-wave imaging. By applying known scaling laws to small explosions in the laboratory, investigators can simulate shock-wave and fragmentation effects on planned buildings or transportation vehicles, for example, using scale models. The high-speed digital video cameras my colleagues and I use record shock position over time by schlieren or shadowgraphy, from which we can determine all post-shock fluid properties.

Figure 9. Schlieren images that show shock wave reflections may help aircraft designers harden planes against explosions. Even after several costly full-scale blast experiments involving real airplanes, the gas-dynamics of explosions onboard commercial aircraft remains poorly understood. Better understanding is needed if aircraft are ever to be hardened against catastrophic in-flight failure resulting from explosions, whether deliberate or accidental.

Interior explosions in aircraft as in buildings are complicated by shock-wave reverberation from interior surfaces. In , the wreckage of Pan Am Flight in Lockerbie, Scotland, at first seemed to show the effects of multiple simultaneous blasts at various fuselage locations. As investigations progressed, it was realized that shock waves had traveled the length and breadth of the fuselage, sometimes reflecting and thus causing local blowouts remote from the actual terrorist bomb located in the forward cargo hold.

Optical shock-wave imaging can help explain the complicated consequences of such onboard explosions. In addition to simulations, the U. Transportation Security Administration recently did tests on actual air-cargo containers filled with luggage, which were blown up by planted terrorist-scale explosives.

Bibliographic Information

For the first time, high-speed videography captured shock-wave motion in these experiments. To do this, a retroreflective shadowgraphy method pioneered by Harold E. Retroreflective screens return to the camera lens orders of magnitude greater illumination than does the simple diffuse white screen that is often used for shadowgraphy. The screen functions like a spherical reflector, returning much of the light striking it to its point of origin.

For high-speed video shadowgraphy, a retroreflective screen is a necessity for creating a bright image. A flaw in Edgerton's original method is that the camera axis had to be slightly offset from that of the light source. This creates a confusing double image in the resulting video. A beamsplitter could be used to correct this, but with a large loss in illumination intensity. Instead, we affixed a small mirror at a degree angle to the center of a filter over the camera lens and reflected the beam off of this surface before sending it to the screen. This arrangement provides perfect alignment between light source and camera axes, and there is no noticeable loss of shadowgram quality as a result of the small area of camera lens occluded by the mirror.