Hydrostatic pressure, plastics mechanical

Ainbinder, S.B., Laka, M.G. and Maiors, I.Y. (1965) Effect of hydrostatic pressure on mechanical properties of plastics. Mech. Comp. Mater, 1, 50. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

A fuze is a complicated mechanical device, metallic or plastic, housing combustible and/or explosive components in a row known as "train". It is designed to initiate an item of ammunition on being subjected to one of the following actions stab, percussion, friction, mechanical time, chemical, electrical or hydrostatic pressure... 

The influence of temperature and strain rate can be well represented by Eyring s law physical aging leads to an increase of the yield stress and a decrease of ductility the yield stress increases with hydrostatic pressure, and decreases with plasticization effect. Furthermore, it has been demonstrated that constant strain rate. Structure-property relationships display similar trends e.g., chain stiffness through a Tg increase and yielding is favored by the existence of mechanically active relaxations due to local molecular motions (fi relaxation). 

Tensile and shear forces are not the only types of loads that can result in deformation. Compressive forces may as well. For example, if a body is subjected to hydrostatic pressure, which exists at any place in a body of fluid (e.g. air, water) owing to the weight of the fluid above, the elastic response of the body would be a change in volume, but not shape. This behavior is quantified by the bulk modulus, B, which is the resistance to volume change, or the specific incompressibihty, of a material. A related, but not identical property, is hardness, H, which is defined as the resistance offered by a material to external mechanical action (plastic deformation). A material may have a high bulk modulus but low hardness (tungsten carbide, B = 439 GPa, hardness = 30 GPa). 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

An increase in pressure results in an increase in T. When the polymer chains are subjected to pressure, more therma energy is required to activate the energy loss mechanisms. Heydemann and Guicking (23) determined the specific volume of unplasticized and DOP plasticized PVC by dilatometry over the temperature range of -80 to 150 °C at hydrostatic pressures of 1-1000 atm. Figure 19 shows... 

An example of a material model based on the physics of material behavior is classical metals plasticity theory. This theory, often referred to as /2-flow theory, is based on a Mises yield surface with an associated flow rule, followed by rate-independent isotropic hardening (Khan and Huang 1995). Physically, plastic flow in metals is a result of dislocation motion, a mechanism known to be driven by shear stresses and to be insensitive to hydrostatic pressure. 

The mathematical description of the powder billet SSE cannot rely on the above conditions of plasticity, as they do not take the change in density of the extruded sample into account. Nor the conditions of plasticity used to model the compaction of metal powders are suitable for this case (48). They imply that with the increase in hydrostatic pressure the relative density of powder material tends to unity. However, in the case of hydrostatic compaction of polymer powders, the relative density of compacts tends to a value less than unity, which is typical of the polymer. The reason is in difference of compaction mechanisms for the metal and polymer powders. 

Indentation techniques were widely used for a long time to investigate the plasticity of silicon at low temperature. However, a difficulty arises because silicon shows a phase transition from diamond-cubic structure to p-Sn metallic Si at a pressure of about 12GPa [49]. As the shear stress increases during an indentation test, the hydrostatic component simultaneously increases and the critical pressure for the phase transition can be reached. In such conditions, the hardness values and dislocation microstructures cannot be representative of dislocation-induced plasticity mechanisms. Indeed, the occurrence of the phase transition under indentation was found to explain the observed saturation of hardness values below 400 °C ([50,51] see also Fig. 8). Dislocations were nevertheless observed after indentation at temperatures lower than 400 °C, but their possible connection with the phase transition was not investigated. 

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