Pressure shearing

In most flat-plate impact experiments, the direction of motion of the impacting plate is normal to its surface, such that only a planar compressive shock is introduced into the specimen. Within the last fifteen years, however, techniques have been developed for dynamic pressure-shear loading of specimens (Abou-Sayed et al., 1976 Chhabildas and Swegle, 1980). These involve an oblique impact, as illustrated in Fig. 3.6, in which the impact surface on the... 

Figure 3.12. Experimental configuration and velocity profiles demonstrating the use of VISAR interferometric techniques in pressure-shear instrumentation to determine in-plane shear motion as well as longitudinal (P-wave) motion (Chhabildas and Swegle, 1980).

Samples are most frequently shock deformed under laboratory conditions utilizing either explosive or gun-launched flyer (driver) plates. Given sufficient lateral extent and assembly thickness, a sample may be shocked in a onedimensional strain manner such that the sample experiences concurrently uniaxial-strain loading and unloading. Based on the reproducibility of projectile launch velocity and impact planarity, convenience of use, and ability to perform controlled oblique impact (such as for pressure-shear studies) guns have become the method of choice for many material equation-of-state and shock-recovery studies [21], [22]. 

A pressure sore is also called a decubitus ulcer and bed sore. A classification system for pressure sores is presented in Table 47-5. Many factors are thought to predispose patients to the formation of pressure ulcers paralysis, paresis, immobilization, malnutrition, anemia, infection, and advanced age. Four factors thought to be most critical to their formation are pressure, shearing forces, friction, and moisture however, there is still debate as to the exact pathophysiology of pressure sore formation. The areas of highest pressure are generated over the bony prominences. 

Pressure of the overburden does not cause chemical reactions which lead to a higher rank. Experiments have shown that static pressure even retards coalification processes. By contrast, pressure affects the physical properties, notably the porosity and moisture content in low rank coals. Further, the optical anisotropy of vitrinites (which is a tension anisotropy) is caused by pressure. Shearing movements have influenced the chemical coalification only occasionally and locally in the foredeeps that we have studied (for instance in the immediate vicinity of overthrusts). In such cases the tectonic movements probably were so quick that the friction heat and the shearing could operate. Shearing in no way can account for the gradual increase in coal rank with depth. 

S Ratio of specific heats of particle to gas at constant pressure Shear strain on particle surface... 

Cell lysis Mechanical methods pressure shearing, ultrasonic disintegration, bead-mill homogenizers Nonmechanical methods enzymatic lysis, osmotic lysis, freezing and thawing, detergent-based lysis and electroporation... 

Pressure shearing Required, moderate cost Harsh Shear forces... 

Pressure shearing is a widely used mechanical means of cell breakage. A sample of bacterial suspension (5 to 40 mL, up to 30% cells by volume) is placed in a steel cylinder fitted with a piston and a small relief valve connected to an outlet tube. The entire assembly is placed in a 10-ton hydraulic press. When the piston is forced down, high shear forces are generated as the... 

Pressure Pressure Shearing force on ab force on dc force on ad... 

Once the machine is operating, the processor methodically makes one change at a time, to determine the result for each change. It provides a range of processing conditions such as melt temperature, pressure, shear rate, etc. within which a specific plastic can be fabricated with... 

R. J. Clifton and R. W. Klopp, Pressure-Shear Plate Impact Testing, in Metal Handbook, Vol. 8, 9th edn., American Society for Metals, Metals Park, OH, 1985, pp. 230-239. 

There is a further generation of extrusion products now emerging. This relates to the fact that extruders can apply an enormous variety of external variables such as pressure, shear and temperature to complex food materials. Extruders are now being used as chemical reactors, to activate or destroy enzymes (Cheftel et al. 1992). Also, their ability to transfer energy into preformed structures is being used to transform... 

Although relatively unexplored, an alternative approach is to determine frictional yield properties by high-pressure shear and triaxial cells, and to incorporate these properties into soils or plasticity models for finite element simulations of flow within the extruder body, as has been done for compaction (cf. Fig. 21-141). 

Conclusive experimental evidence is still lacking in support of any of the models. The complexity of the process under shock or impact for solid materials is at best a multiscale event in time and space. Current atomistic computational studies, first principle approaches, are thus limited by the size of the system of interest, and/or by the duration of the process. These limitations usually restrict the investigation to individual effects, such as pressure, shear, and vacancies. 

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