Shock-Compression Events

Under what circumstances are shock-compression events encountered, and what is shock-compression science  

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

Some wave phenomena, familiar to many people from the human senses, include the easy undulation of water waves from a dropped stone or the sharp shock of the sonic boom from high-speed aircraft. The great power and energy of shock events is apparent to the human observer as he stands on the rim of the Meteor Crater of Arizona. Human senses provide little insight into the transition from these directly sensed phenomena to the high-pressure, shock-compression effects in solids. This transition must come from development of the science of shock compression, based on the usual methods of scientific experimentation, theoretical modeling, and numerical simulation. 

Introduction to High-Pressure Shock Compression of Solids 3 

The challenge is to understand how the laws of mechanics, physics, and chemistry, and how materials behaviors, control the processes. 

---

Investigations in the field of shoek eompression of solid materials were originally performed for military purposes. Speeimens sueh as armor were subjected to either projectile impact or explosive detonation, and the severity and character of the resulting damage constituted the experimental data (see, e.g., Helie, 1840). Investigations of this type continue today, and although they certainly have their place, they are now considered more as engineering experiments than scientific research, inasmuch as they do little to illuminate the basic physics and material properties which determine the results of shock-compression events. 

Shock-compression science originated during and after World War II when experimental facilities for creating planar shock waves were developed, along with prompt instrumentation techniques enabling shock velocity and particle velocity measurements to be made. The main thrust of shock-compression science is to understand the physics and to measure the material properties which govern the outcome of shock-compression events. Experiments involving planar shock waves are the most useful in shock-compression science. 

The diagnostics applied to shock experiments can be characterized as either prompt or delayed. Prompt instrumentation measures shock velocity, particle velocity, stress history, or temperature during the initial few shock transits of the specimen, and leads to the basic equation of state information on the specimen material. Delayed instrumentation includes optical photography and flash X-rays of shock-compression events, as well as post-mortem examinations of shock-produced craters and soft-recovered debris material. 

Fig. 1.1. The pressure range over which shock-compression events are of interest is very broad. Quite different and distinctive behaviors are to be expected at the various pressures. The figure shows pressures produced by impact and detonation as well as physical (p), mechanical (m), and chemical (c) events at selected pressures. The indicated impact pressures are those for impactor and target materials which are the same.

The shock-compression events are so extreme in intensity and duration, and remote from direct evaluation and from other environments, that experiment plays a crucial role in verifying and grounding the various theoretical descriptions. Indeed, the material models developed and advances in realistic numerical simulation are a direct result of advances in experimental methods. Furthermore, the experimental capabilities available to a particular scientist strongly control the problems pursued and the resulting descriptions of shock-compressed matter. Given the decisive role that experimental methods play, it is essential that careful consideration be given to their characteristics. 

Fig. 3.5. The experimental arrangement used for a typical compressed gas gun is shown. The apparatus is designed to impact a selected impactor upon a target material with precision on the alignment of the impacting surfaces. Velocity at the impact surface can be measured to an accuracy and precision of 0.1%. This loading produces the most precisely known condition of all shock-compression events.

The figure characterizes materials in some initial configuration, which is altered in time as a loading pulse sweeps over it. The shock-compression event is characterized by a transition zone in which significant changes are occurring. After the transition, the material is in a substantially different state, and, finally, the pressure is released. 

In this chapter the scope of the subject the fluidlike deformation of shock-compressed solids modeling the shock as benign or catastrophic the origins of shock-compression science the pressure scale of events the plan of the present work. 

When the pressures to induce shock-induced transformations are compared to those of static high pressure, the values are sufficiently close to indicate that they are the same events. In spite of this first-order agreement, differences between the values observed between static and shock compression are usually significant and reveal effects controlled by the physical and chemical nature of the imposed deformation. Improved time resolution of wave profile measurements has not led to more accurate shock values rather. 

Melting, a major physical event, has small, subtle effects on shock-compression wave profiles. The relatively small volume changes and limited mixed-phase regions result in modest, localized changes in loading wave speed. Consequently, shock-induced melting and freezing remains an area with little data and virtually no information on the influence of solid properties and defects on its kinetics. 

Solid state chemistry was vigorously pursued in the Soviet Union from their earliest work, but other shock-eompression groups showed little interest in the area. Within a benign shock compression pieture, such chemical effects could not occur in the mieroseeond duration of the shoek pulse. Observations of chemieal changes must therefore be interpreted to be the result of poor experimental eontrol or proeesses that occurred long after the shock event. 

This volume is concerned primarily with the chemical and physical effects of shock waves on typical Earth and planetary solid materials. The emphasis is on comparing naturally occurring materials with similar materials produced by shock compression in the laboratory. Such comparisons can provide clues about the environment and events that produced the natural materials. 

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

In a hydroelectric plant it is necessary to decrease the flow of water to a turbine very rapidly in the event of a sudden drop in the load. In such a case a relief valve may be opened so as to bypass water around the machine, and then this relief valve may be slowly closed. Alternatively, an air chamber may be used to absorb the shock. Some excess water may flow into this device and compress the air to a higher pressure. One disadvantage is that water under pressure absorbs air, so it is necessary to renew the air supply periodically by the operation of a small air compressor. Also, an air chamber has distinct size limitations. 

Many causes of acute spinal cord infarction (of arterial and venous origin) have been reported (Table 17.2). They include diseases of the aorta and aortic surgery, thromboembolic events and cartilaginous disc embolism, vasculitis, coagulopathy, radiation-induced vasculopathy, toxic effects of contrast medium, epidural anesthesia, periradicu-lar nerve root therapy with crystalline corticoids, decompression illness, shock or cardiac arrest, lumbar artery compression and other etiologies... 

Assume next that the compression occurs so rapidly that the material has no time to respond before it is transformed from the undisturbed state to the steady state behind the shock front. In this event the transformation occurs adiabatically. 

---