High pressure torsion

High pressure torsion (HPT) (Fig. la) and equal-channel angular (ECA) pressing (Fig. lb) refer to the techniques which were used in pioneer works devoted to UFG structures formation in metals and alloys [11,12] by severe plastic deformation. These methods have been further developed lately. 

Figure 1. Principles of severe plastic deformation techniques a - high pressure torsion, b - ECA pressing.

Samples processed under high pressure torsion are disk-shaped (Fig. la). In this process the sample is being put between anvils and compressed under the applied pressure (P) of several GPa. The lower anvil turns and friction forces result in shear straining of the sample. As a result, the deforming sample does not break notwithstanding high strains of deformation [7],... 

Faghihi S., Azari F, Zhilyaev A. P, Szpunar J. A., Vali H. et al (2007), Cellular and molecular interactions between MC3T3-E1 pre-osteoblasts and nanostructured titanium produced by high-pressure torsion , Biomaterials, 28, 3887-95. (doi 10.1016/j. biomaterials.2007.05.010)... 

Very High Pressures Use balanced o-ring cartridge seals up to about 500 psi. Above 500 psi use the tandem double seal with the barrier fluid pressurized at / the seal chamber pressure. Remember as pressure goes up the o-rings will extrude and metal parts will distort. Use a torsion balanced seal. 

Several studies confirmed the high-pressure response of sulfur crystals by Raman [109, 119, 120, 135-137] and IR spectroscopy [109,138]. Accordingly, external and torsional vibrations have values for the mode Griineisen parameter around 2, and the parameters of the bending and stretching vibra-... 

Because of their lower level of crystallinity these copolymeis soften at lower temperatures than polyethylene. Table I compares the torsional stiffness at various temperatures of an 18% ethyl acrylate-82 % ethylene copolymer with that of a 21,000 molecular weight, 0.918 density polyethylene made by high pressure polymerization (Bakelite DYNH, Union Carbide Corp.). 

In the case of high pressures, different types of viscometer have been employed owing to the need to reduce the volume of fluid required. The most popular have been falling-body viscometers and torsional-crystal viscometers. Neither of these have, however, completely developed theories so that their accuracy is intrinsically limited. On the other hand, the newly developed vibrating-wire viscometer that makes use of the damping of a transverse oscillation of a thin wire enjoys a complete theory. 

Two fundamentally different types of failure may occur in vessels operated under vacuum (as opposed to high pressure). The problem in vacuum operation is the elastic stability of the vessel shell when it is under an external pressure loading. In general, elastic instability is a problem that must be considered in all structures having limited rigidity when subjected to bending, torsion, compression or a combination of these loadings. In failure by elastic instability, the structure buckles or collapses like an evacuated thin-shelled vessel. 

Mullin N, Hobbs JK (2011) Direct imaging of polyethylene films at single-chain resolution with torsional tapping atomic force microscopy. Phys Rev Lett 107(19) 197801 Muthukumar M (2005) Modeling polymer crystallization. Adv Polym Sci 191 241-274 Nie Y, Gao H, Ma Y, Hu Z, Reiter G, Hu W (2013) Competition of crystal nucleation to fabricate the oriented semi-crystalline polymers. Polymer 54(13) 3402-3407 Olley R, Bassett D (1977) Molecular conformations in polyethylene after recrystallization or annealing at high pressures. J Polym Sci Polym Phys Ed 15(6) 1011-1027 Olmsted PD, Poon WC, Mcleish T, Terrill N, Ryan A (1998) Spinodal-assisted crystallization in polymer melts. Phys Rev Lett 81(2) 373-376... 

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