Hydrostatic compressibility

Foam Density lb./ft.3 Glass Microballoons Epoxy Macroballoons Uniaxial Compressive Yield Strength, psi Hydrostatic Compressive Strength, psi Method of Preparation Resin System... 

Fig. 3a-e. Supermolecular structures of polymers crystallized in various force fields a structure of the shish-kebab type, b structure formed during crystallization in a capillary with a conical inlet and c structure of a polymer crystallized at hydrostatic compression at 4 x 108 Pa... 

The crystallization of polymer melts at high pressure (several thousand of atmospheres), just as the crystallization in flowing melts and stirred solutions, leads to the formation of ECC in which the molecules assume almost completely extended conformations. This suprising feet (the crystallization was carried out at hydrostatic compression) was first extablished experimentally by Wunderlich30 and then confirmed by other authors31 33. However, diverse opinions exist about the mechanism of ECC formation. 

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

In spite of the presence of ECC, the sample exhibiting a domain structure remains unoriented on the macroscopic level. Figure 3 c shows a great difference in the structures obtained, if molecular orientation exists and if hydrostatic compression is applied. Although the method of hydrostatic compression of the melt is of paramount importance from the scientific view point just for samples crystallized under pressure it was possible to prove unequivocally the existence of ECC), it does not allow a direct preparation of oriented samples of high strength (they are brittle and readily crumble to powder under minimum strain). However, the material obtained in this way can probably serve as a semi-finished product for further technological treatment that would improve its mechanical properties. 

At the instant of contact between a sphere and a flat specimen there is no strain in the specimen, but the sphere then becomes flattened by the surface tractions which creates forces of reaction which produce strain in the specimen as well as the sphere. The strain consists of both hydrostatic compression and shear. The maximum shear strain is at a point along the axis of contact, lying a distance equal to about half of the radius of the area of contact (both solids having the same elastic properties with Poisson s ratio = 1/3). When this maximum shear strain reaches a critical value, plastic flow begins, or twinning occurs, or a phase transformation begins. Note that the critical value may be very small (e.g., in pure simple metals it is zero) or it may be quite large (e.g., in diamond). 

In relatively recent years, it has been found that that indentations made in covalent crystals at temperatures below their Debye temperatures often result from crystal structure changes, as well as from plastic deformation via dislocation activity. Thus, indentation hardness numbers may provide better critical parameters for structural stability than pressure cell studies because indentation involves a combination of shear and hydrostatic compression and a phase transformation involves both of these quantities. 

Sato Y, Akaogi M., and Akimoto S. (1978). Hydrostatic compression of the synthetic garnets pyrope and almandine. J. Geophys. Res., 83 335-338. 

Mehldahl (65) depicts several failure surfaces by photographs of various three-dimensional models. Figure 23 illustrates three such surfaces taken from Ref. 110, which shows geometries which are symmetrical about the space diagonal, oi = triaxial compression octant should be open ( because hydrostatic compression cannot lead to failure in the ordinary sense ). 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

Fluxes of chemical components may arise from several different types of driving forces. For example, a charged species tends to flow in response to an applied electrostatic field a solute atom induces a local volume dilation and tends to flow toward regions of lower hydrostatic compression. Chemical components tend to flow toward regions with lower chemical potential. The last case—flux in response to a chemical potential gradient—leads to Fick s first law, which is an empirical relation between the flux of a chemical species, J, and its concentration gradient, Vcj in the form J, = —DVcj, where the quantity D is termed the mass diffusivity. 

In experiments with cultured cells it has been shown that osteocytes, but not periosteal fibroblasts, are extremely sensitive to fluid flow, resulting in increased prostaglandin as well as nitric oxide production [104, 105], Three different cell populations, namely osteocytes, osteoblasts, and periosteal fibroblasts, were subjected to two stress regimes, pulsatile fluid flow and intermittent hydrostatic compression [104], Intermittent hydrostatic compression was applied at 0.3 Hz with a 13-kPa peak pressure. The pulsatile fluid flow was a fluid flow with a mean shear stress of 0.5 Pa with cyclic variations of 0.02 Pa at 5 Hz. The maximal hydrostatic pressure rate was 130 kPa/sec and the maximal fluid shear stress rate was 12 Pa/sec. Under both stress regimes, osteocytes appeared more sensitive than osteoblasts, and osteoblasts more sensitive than periosteal fibroblasts. However, despite the large difference in peak stress and peak stress rate, pulsatile fluid flow was more effective than intermittent hydrostatic compression. Osteocytes, but not the other cell types, responded to 1 hour pulsatile fluid flow treatment with a sustained prostaglandin E2 upregula-... 

D.C. Sorescu, B.M. Rice, D.L. Thompson, Molecular Packing and NPT-Molecular Dynamics Investigation of the Transferability of the RDX Intermolecular Potential to 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane, J. Phys Chem B, 102 (1998) 948-952. ibid A Transferable Intermolecular Potential for Nitramine Crystals J. Phys Chem A, 102 (1998) 8386-8392. ibid Theoretical Studies of the Hydrostatic Compression of RDX, HMX, HNIW, and PETN Crystals, J. Phys Chem B, 103 (1999) 6783-6790. 

A final set of tests of this series of investigations focused on the analysis of hydrostatic compression of some representative energetic materials. Sorescu et al. [117] have considered the case of nitramine RDX (a-phase), HMX ((3-phase), and HNIW (e-phase) and the non-nitramine PETN crystal. These studies have been performed based on NPT-MD simulations at room temperature over the pressure ranges 0-4 GPa for RDX, 0-7.5 GPa for HMX, 0-3.5 GPa for HNIW, and 0-9 GPa for PETN. In the case of the RDX, HMX, and HNIW crystals, the results indicate that the proposed potential model is able to reproduce accurately the changes in the... 

The isotropic moduli, particularly the initial bulk modulus and its pressure derivative, are key ingredients in specifying the mechanical equation of state. As noted above, determination of these properties from experimental hydrostatic compression data is difficult due to issues with acquisition of high precision at low pressures and particular sensitivity in the choice of equation of state fitting form to data below about one GPa. Alternative routes to this information at low pressures included impulsive stimulated light scattering (ISLS) and resonant ultrasound spectroscopy (RUS), which can in principle provide the complete elastic tensor (ISLS) and isotropic bulk and shear moduli (RUS). 

Studies involving fluid shear, hydrostatic compression, biaxial and uniaxial stretch, or a combination of two or more of these factors indicate that fluid shear is a major factor affecting bone cell metabolism and cells subjected to mechanical stress reshape and align themselves with their long axis perpendicular to the axis of force. Cells also exhibited remodeling of the actin cytoskeleton and increases in PKC levels, processes thought to be involved in the early phase of mechanochemical transduction. 

Isotropic (hydrostatic) compression Bulk modulus Ka Hydrostatic pressure p pV() Volume change per unit volume AV/Va AV (13.4)... 

Hydroplastics, 29 Hydrostatic compression, 385 Hydrostatic extrusion, 739 Hydrostatic pressure, 384 Hyper-polarisability, 351 tensor, 351... 

---