Hydrostatic Behavior Research Articles

Bearing misalignment effects on the hydrostatic and hydrodynamic behaviour of gears in fixed clearance end plates

Lubrication and sealing mechanisms of fixed clearance end plates in high-pressure pumps have been analysed theoretically and experimentally. Bearing misalignment was found to be the main lubrication mechanism, and it was effective in determining the gear position between two end plates. The minimum film thickness between the gear end and end plate has been found to depend on the magnitude of the relative tilt of the surfaces and the position of the maximum clearance. The theory developed can predict the film thickness between the end plate and gear end face, and this corresponds very closely to the clearances measured experimentally under a variety of operating conditions.

Study of the hydrostatic behaviour of the lumbar nucleus pulposus in dynamic by the measurement of the intranuclear pressures in cadavers and in vitro

Study of the hydrostatic behaviour of the lumbar nucleus pulposus in dynamic by the measurement of the intranuclear pressures in cadavers and in vitro

Measuring Hydro-Static Dependent Constitutive Behaviour of Adhesives Using a Bend Specimen

Abstract A simple bend test and associated equations that determine, simultaneously, both the tensile and compressive uniaxial stress-strain behaviour of a bulk adhesive are presented. From this, the ratio of the flow stress in compression to tension (S) can be found. Such data are required if a meaningful elastoplastic stress analysis of an adhesive joint is to be carried out. Experimental results for both the tensile and compressive stress-strain data are obtained for an epoxy specimen using this technique. The tensile data are compared with the results from uniaxial tensile tests on flat dumbell specimens of the same material and the good correlation that is found indicates that this is a reliable technique. Values of the ratio (S) for this adhesive are calculated as a function of both strain hardening and work hardening parameters. These results indicate that this technique complements standard test techniques and provides an elegant method not only of obtaining the ratio (S) but also of deriving uniaxial stress-strain curves.

Micromechanics of pressure‐induced grain crushing in porous rocks

The hydrostatic compaction behavior of a suite of porous sandstones was investigated at confining pressures up to 600 MPa and constant pore pressures ranging up to 50 MPa. These five sandstones (Boise, Kayenta, St. Peter, Berea, and Weber) were selected because of their wide range of porosity (5–35%) and grain size (60–460 μm). We tested the law of effective stress for the porosity change as a function of pressure. Except for Weber sandstone (which has the lowest porosity and smallest grain size), the hydrostat of each sandstone shows an inflection point corresponding to a critical effective pressure beyond which an accelerated, irrecoverable compaction occurs. Our microstructural observations show that brittle grain crushing initiates at this critical pressure. We also observed distributed cleavage cracking in calcite and intensive kinking in mica. The critical pressures for grain crushing in our sandstones range from 75 to 380 MPa. In general, a sandstone with higher porosity and larger grain size has a critical pressure which is lower than that of a sandstone with lower porosity and smaller grain size. We formulate a Hertzian fracture model to analyze the micromechanics of grain crushing. Assuming that the solid grains have preexisting microcracks with dimensions which scale with grain size, we derive an expression for the critical pressure which depends on the porosity, grain size, and fracture toughness of the solid matrix. The theoretical prediction is in reasonable agreement with our experimental data as well as other data from soil and rock mechanics studies for which the critical pressures range over 3 orders of magnitude.

Cyclic stress-strain response of porous aluminum

A model of endochronic constitutive theory is applied to the analysis of stress-strain behavior of sintered high purity aluminum powder-based material (P/M) under uniaxial strain-controlled cyclic test conditions. Different strain amplitudes were used on specimens which during fabrication were subjected to cold isostatic pressure of 25 ksi. It was found that mean strain does not significantly affect the fatigue life. It was also discovered that Poisson's ratio at the peak tensile strain decreases gradually with the number of loading cycles, while it reaches a constant value at the peak compressive strain as the number of cycles increases. Results provide a comparison between theory and experiment for both hydrostatic and deviatoric stress-strain behavior for several cases of peak strains in the strain-controlled test. Reasonable agreement has been achieved. The changing trends of the peak volumetric strain, deviatoric stress, hydrostatic stress and Poisson's ratio versus loading cycles are also reported for each case.

On the isotope effect ofTcin squaric acid (H2C4O4)

Abstract The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on Tc is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H2SQ) is stretched to 2R(D2SQ) the phase transition temperature 7H c of H2SQ coincides with TD c of D2SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.

The effect of postural changes on the inferred pressures within the nucleus pulposus during lumbar discography.

By observing the variation of intradiscal pressure occurring at different body postures, it is possible to infer a functional hydrostatic behavior of a lumbar disc. Results from such observations on normal discs are already available. However, observations on degenerate discs are largely restricted to in vitro studies. The authors are now able to report a series of recordings taken from discographically degenerate lumbar discs in patients presenting with low-back pain. In this study of twenty patients, pressure observations were made on 21 normal and 19 abnormal discs. From the results that the authors have obtained, they can reaffirm that normal discs behave predictably and as previously described. The abnormal discs, however, did not behave as a single group. They showed patterns of pressure changes in different postures often dissimilar from that shown by the normal discs both in the absolute values recorded and the sequential changes that occur during the postural change. Unfortunately, the authors were unable to correlate either the extent or character of disc degeneration with the observed variation in pressure changes.

The hydrostatic extrusion of polymethylmethacrylate

The hydrostatic extrusion behaviour of two grades of polymethylmethacrylate (PMMA) is reported and an analysis of the mechanics of the extrusion process is presented. Although the maximum degree of deformation achieved is considerably lower than that obtained for crystalline polymers, the process mechanics are controlled by the same factors (i.e. the billet-die friction, and the effects of strain rate and pressure on the material flow stress). A method for equating the effects of pressure and friction is described, following the work of Tabor on the adhesive mechanism of friction in polymers. This method gives a friction coefficient in the range 0.1 to 0.2 for the hydrostatic extrusion of PMMA, compared to values in the region 0.03 to 0.08 for crystalline polymers, suggesting conditions of boundary lubrication. The relatively high values of friction for PMMA are consistent with the requirement for careful preparation of the billet surface prior to extrusion, and the observation of the ″stick-slip″ phenomenon during extrusion. The differences between the grades of PMMA in both extrusion behaviour and tensile drawing behaviour are explicable in terms of their different glass transition temperatures.

Manufacture of high stiffness solid rods by the hydrostatic extrusion of linear polyethylene. Part II: Effect of polymer grade

AbstractThe solid phase hydrostatic extrusion behavior of a range of linear polyethylenes, including methyl and butyl copolymer grades, has been compared in terms of processing behavior and product properties, for large diameter extrusions in the adiabatic thermal regime. The most useful improvements in both stiffness and long term creep behavior were obtained from medium melt flow index homopolymers, although such grades required higher forming pressures than the copolymers. This represents a reversal of the creep behavior trends observed for small diameter isothermal extrusions, where copolymer grades were superior. The engineering potential of hydrostatically extruded linear polyethylene is discussed.

Hydrostatic extrusion of linear polyethylene: Effects of extrusion temperature and polymer grade

AbstractThe hydrostatic extrusion behavior of linear polyethylene has been examined for two homopolymers of very different molecular weight characteristics and for a copolymer. Good unflawed extrudates could be obtained in all cases, and the extrusion behavior at a fixed temperature correlated well with the melt flow index. Although the maximum values of axial Young's modulus obtainable from the higher molecular weight homopolymer and the copolymer were lower than those possible for the lower molecular weight homopolymer, such materials do show improvements in creep behavior which could be advantageous. The effect of temperature on the extrusion behavior is discussed; the results suggest that for each grade of polymer there is an optimum temperature for effective extrusion, i.e., extrusion which gives optimum modulus enhancement. Finally, the melting behavior and the temperature dependence of the axial Young's moduli of the extrudates are considered in terms of our present knowledge of the structure of these high modulus materials.

Hydrostatic extrusion behaviour of high density polyethylene

The solid state processing of a high density polyethylene by hydrostatic extrusion at room temperature has been investigated. The extrusion pressure for a given extrusion ratio is found to depend on the pressure fluid used, the die angle and the velocity of extrusion. The strain hardening behaviour of the resulting extrudates is found to be independent of the extrusion ratio. An effective flow stress equation which accounts for the dependence of the flow stress of high density polyethylene on strain, strain rate and hydrostatic pressure is developed and used in an analysis of the extrusion process based on the upper bound approach. The calculated values of the extrusion pressure as a function of the various extrusion variables are in reasonable agreement with the experimental results.

Hydrostatic extrusion of polyoxymethylene

AbstractThe hydrostatic extrusion behavior of polyoxymethylene (POM) is described. Extrusions were performed at 164°C for a range of different molecular weight grades. Excellent unflawed lengths of extrudate were obtained with axial Young's moduli, measured at room temperature, reaching values as high as 24 GPa. The extrusion characteristics are discussed in terms of the very strong dependence of the flow stress of POM on strain and strain rate and pressure. In addition to measurements of Young's modulus over a wide temperature range, data for shear modulus and transvers modulus are also presented. A limited amount of other structural measurements is presented.

Hydrostatic pressure behavior of the exchange fields in MnF 2 : Fe 2+

We have determined the pressure dependence of both impurity and host exchange fields in MnF 2 : Fe 2+ by measuring the frequency shifts of far infrared active modes in MnF 2 : Fe 2+, MnF 2, and ZnF 2 : Fe 2+ at pressure up to 7.1 kbar.