High Hydrostatic Stress Research Articles

Plastic flow between Bridgman anvils under high pressures

The shear strength of materials after extensive plastic flow under a superimposed high hydrostatic stress component is most conveniently studied by means of Bridgman opposed anvils between which thin disk-shaped samples are sheared through relative rotation. A newly designed apparatus of this type permits, for the first time ever, the monitoring of sample thickness during shearing and thus the obtaining of averaged shear stress/shear strain curves. Such relations are needed for the better understanding of geological processes, behavior under shock or explosive impact, and of the surface layers during friction and wear, among others. A semiquantitative analysis shows that nonuniform pressure distribution in the samples cannot significantly falsify the results. It is concluded that distributed shearing is always accompanied by sample thinning and that, conversely, slippage between anvils and specimens is indicated whenever the sample stops thinning during rotation. Such slippage can be greatly reduced by raising the friction coefficient through etching of samples and/or anvils, but it apparently occurred undiscovered in previous studies, partly after initial distributed shearing. Further, in previous results slippage between directly contacting anvils was frequently mistaken for sample shearing. Additionally, the previously neglected sample material being extruded during shearing can falsify results, as can “turbulent flow” initiated at sample perforation. Correspondingly all prior data gained with Bridgman apparatuses are suspect. Present best results indicate (i) that in metals ordinary dislocation glide but apparently with strongly increased Peierls–Nabarro stresses continues to the highest pressures studied, (ii) that independent of thermal activation workhardening may cease at high strains, and (iii) that “turbulent flow” resulting through sample perforation or when the sample thickness decreases below a critical value, can give rise to mechanical alloying. The majority if not all of the data by Bridgman as well as Vereshchagin et al. probably involved such mechanical alloying. When plotted versus pressure in units of shear moduli, the apparent coefficients of friction of the five cubic metals examined so far follow nearly the same curve but they are lower for metals of lower symmetry.

Mechanical characterization and modeling of non-linear deformation and fracture of a fiber reinforced metal matrix composite

The non-linear anisotropic mechanical behavior of an aluminum alloy metal matrix composite reinforced with continuous alumina fibers has been determined experimentally. The mechanical behavior of the composite has been modeled by assuming that the composite has a periodical microstructure. The resulting unit cell problem has been solved with the finite element method. Excellent agreement was found between theoretically predicted and measured stress-strain responses for various tensile and shear loadings. The stress-strain responses for transverse and in-plane shear were found to be identical and this will provide a simplification of the constitutive equations for the composite. The composite has a very low ductility in transverse tension and a limited ductility in transverse shear that has been correlated to high hydrostatic stresses that develop in the matrix. The shape of the initial yield surface has been calculated and good agreement was found between the calculated shape and the experimentally determined shape.

The influence of second-phase dispersions on shear instability and fracture toughness of ultrahigh strength AISI 4340 steel

The resistance to shear instability and subsequent flow localization in ultrahigh strength (UHS) steels is dependent upon second-phase particle dispersions and the matrix strain hardening. The effect of the interparticle spacing λ to the geometric mean particle radius R ratio on the shear instability strain of UHS AISI 4340 steel is discussed. Experimental results indicate that a linear relationship exists between shear instability strain and this λ/ R ratio. Microvoid nucleation softening associated with second-phase particles appears to be the dominant destabilizing event leading to fracture. The effect of the hydrostatic stress is also discussed. Experimental results of mode I and II fracture toughness testing are compared. A high hydrostatic tension field was found to be the cause for the lower mode I critical stress intensity factor ( K Ic ) than mode II ( K IIc ). The high hydrostatic tensile stress field induced early microvoid nucleation which promoted flow localization leading to fracture. However, both mode I ( K Ic ) and mode II ( K IIc ) critical stress intensity factors directly relate to the critical particle parameter λ/R 1 2 .

Elastic-Plastic Analysis of Cracks on Bimaterial Interfaces: Part III—Large-Scale Yielding

In Parts I and II, the structure of small-scale yielding fields of interface cracks were described in the context of small strain plasticity and J2 deformation theory. These fields are members of a family parameterized by the plastic phase angle ξ which also determines the shape or phase of the plastic zone. Through full-field analysis, we showed the resemblance between the plane-strain interface crack-tip fields and mixed-mode HRR fields in homogeneous material. This connection was exploited, to the extent possible, inasmuch as the interface fields do not appear to have a separable form. The present investigation is focused on “opening” dominated load states (| ξ | ≤ π/6) and the scope is broadened to include finite ligament plasticity and finite deformation effects on near-tip fields. We adopt a geometrically rigorous formulation of J2 flow theory taking full account of crack-tip blunting. Our results reveal several surprising effects, that have important implications for fracture, associated with finite ligament plasticity and finite strains. For one thing the fields that develop near bimaterial interfaces are more intense than those in homogeneous material when compared at the same value of J or remote load. For example, the plastic zones, plastic strains, and the crack-tip openings, δt, that evolve near bimaterial interfaces are considerably larger than those that develop in homogeneous materials. The stresses within the finite strain zone are also higher. In addition, a localized zone of high hydrostatic stresses develops near the crack tip but then expands rapidly within the weaker material as the plasticity spreads across the ligament. These stresses can be as much as 30 percent higher than those in homogeneous materials. Thus, the weaker material is subjected to large stresses as well as strains—states which promote ductile fracture processes. At the same time, the accompanying high interfacial stresses can promote interfacial fracture.

Theoretical stress analysis of organ culture osteogenesis

In a series of in vitro organ culture experiments by Klein-Nulend et al., intermittent ambient hydrostatic pressure was found to significantly increase the rate of calcification in the diaphysis of mouse rudiments while proteoglycan production was increased in the noncalcified epiphyses. In this study, we have conducted finite element stress analyses of this organ culture system in an effort to better understand the distribution of local tissue stresses which were created. Furthermore, we sought to clarify how these experimental results relate to our theory for skeletal morphogenesis which postulates that endochondral ossification is accelerated in regions of octahedral shear (deviatoric) stress and inhibited in areas of compressive hydrostatic (dilatational) stress. The results of the stress analyses show that externally applied hydrostatic pressure produces significant shear stresses at the cartilage/calcified cartilage interface and pure hydrostatic pressure at the rudiment ends. The accelerated osteogenesis in regions of high shear and the increased synthesis of proteoglycans in regions of high compressive hydrostatic stress are therefore consistent with our theory which predicts the regulation of cartilage maintenance and endochrondral ossification by intermittent tissue stresses.

Clastic Dikes in Eutaw Formation (Upper Cretaceous) of Southwestern Georgia Coastal Plain: ABSTRACT

The Eutaw Formation is the second oldest stratigraphic unit of the southwestern Georgia coastal plain, and it introduced marine and paralic deposits into the eastern Gulf coastal province during the Late Cretaceous. Penecontemporaneous deformation structures are common in middle and upper sections of the Eutaw, notably large, well-exposed clastic dikes. This report describes these clastic dikes and speculates on the mechanics of their emplacement. Eutaw clastic dikes range up to tens of meters long and up to 1-5 m wide. Their vertical extent is indeterminate because their bases have never been observed; maximum observed vertical extent is 7.5 m. Dikes invariably occur in sets and intersect each other at 30/sup 0/ and 150/sup 0/ angles, suggesting that the dikes formed in conjugate sets. Sediment within dikes is well to very well-sorted, silty to very fine, micaceous, quartzarenitic sand. Micas tend to be oriented parallel to dike walls, and indistinct, wispy, laminar structures within dike sand also parallel the walls. Both observations are interpreted to imply that sand emplacement occurred while sand was in a fully fluidized state. Eutaw dikes typically occur near the top of the unit, in dark-gray, fissile, clayey shale. These shales have a restricted fauna, represented principallymore » by Lingula; they are interpreted as restricted lagoonal and poorly drained marsh deposits. Dike margins are extremely sharp and often show brecciation and sand injections into minor apophyses within the shale. Organic-rich muds of the wall rock are interpreted to be well compacted and to have behaved in a brittle manner during strain. Therefore, sand injection occurred at some time considerably after the mud was deposited, and the sands contained interstitial water under relatively high hydrostatic stress.« less

The effect of geometry on the crack opening displacement of a low carbon steel

The standard specimen fur measuring the crack opening displacement uses a deep crack that provides a high hydrostatic stress. If a V notch, which produces a lower hydrostatic stress is substituted for the deep crack, then the initiation crack opening displacement is increased. Using both V notch bend and tension specimens, it is shown that the initiation crack opening displacement is a function of the hydrostatic stress.

Development of New Processes for Control of Internal Deformation and Internal Stress in Hot Free Forging of Heavy Ingots

For developing an optimum free forging process to close and consolidate internal cavities in heavy ingots by direct forging alone, internal deformation and internal stress were investigated with Plasticine models. First, a method to simulate the material behavior with the existence of a large temperature gradient was established. The forging processes used were the followings;1) the conventional process which uses symmetrical upper and lower anvils2) a process called the FM process in which the upper anvil is of the usual type but the lower anvil is flat and larger than the dimension of the material to be forged3) a process called the FML process-where the lower anvil is the same as that of the FM process and the upper anvil is smaller than the dimension of the material to be forged4) the so-called JTS forging process which uses the same combination of anvils as that of the FML process and is carried out with a large temperature gradient in the material.Among these processes, the conventional process can effectively cause a large deformation rate at the center and the use of wide anvils in any process is also effective for a large deformation even without a temperature gradient in the material. For attaining compressive stress, that is, a higher hydrostatic stress at the center, it is effective even without a temperature gradient to raise the value of the parameter w/h (w: width of anvil, h: height of, forging material) or to carry out unsymmetrical reductions such as that in the FM process and the FML process. A tri-axial compressive stress state is considered to be effective for closing and consolidating the central cavities.

The Effect of Water Chemistry on the Laboratory Compression and Permeability Characteristics of Some North Sea Chalks

Summary Chalks or chalklike carbonates have been reported to fail when returned to or near in-situ reservoir stress in the laboratory. Failure of chalks during reservoir production tests has also been reported. This paper presents laboratory results showing that the compaction behavior of the chalks is dependent on the water chemistry of the saturating fluid. The chalks were found to be strong and stable under hydrostatic stress when saturated with oil. Injection of water into the oil-saturated chalks, at stress, caused immediate compaction. The weakening of chalks by water increased the compressibility values four to eight times the oil-saturated values. The chalks resisted failure when stabilized with synthesized formation water containing calcium/bicarbonate ratios equal to the calcium/ bicarbonate ratio present in the formation-water analysis. Permeabilities of the chalks saturated with oil and brines are also presented. Introduction The failure or yielding of high porosity >25 %) chalks or chalklike carbonates during the application of stress in the laboratory has been extensively reported in the literature. Meigh and Early and Carter and Mallard found that the strength of air- or oven-dried outcrop chalks was two to three times the strength of water-saturated chalks. Blanton studied both dry and oil-saturated outcrop chalks from the North Sea area and from the U.S. gulf coast. He found no significant difference in strength between dry and oil-saturated chalks. However, he found that, although the ultimate strength increased initially with increasing hydrostatic stress, the chalks eventually collapsed at higher hydrostatic stress. Marek used a simulated reservoir brine to obtain similar results on chalklike carbonates. Owen reported that brine-saturated chalks from the Ekofisk field in the Norwegian North Sea failed when subjected to only a few hundred pounds (- 1500 kPa) of pressure in a compressibility apparatus. The weakening of calcite rocks by water has also been described by Rebinder and Likhtman, Boozer et al., and Rutter. The reported chalk failure has not been restricted to laboratory tests. Voll and Holman reported that chalk flow and casing collapse had been observed in the Valhall field, Norwegian North Sea, and cited one example where 1,500 ft (457.2 m) of produced chalk was found inside a test string. This paper presents a summary of laboratory results obtained on the compaction and permeability characteristics of some chalk samples from the Danish North Sea. The studies were conducted on core material from wells in the Goren, Cora (renamed Tyra), and Dan fields. The investigation was done to confirm the reported failure of the chalks following their return to simulated reservoir stress in the laboratory. It was postulated that, even if the chalks were weak, they should be able to withstand the stresses they had been subjected to in the reservoir before removal, unless they were weakened by the recovery or testing process. Compressibility tests conducted by a service company on a suite of samples from the Gorm field showed that most of the samples failed at stresses much less than the anticipated reservoir stress. Laboratory Tests Core-Material Preparation Core plugs 2 1/8 in. (5.40 cm) in diameter and about 3 in. (7.62 cm) long were cut from water-base-mud core material from Well DNN-3, Gorm field; Well DNE-4, Cora field; and Well DNM-8X, Dan field. The samples were cut either parallel with (vertical), or perpendicular to (horizontal), the coring axis of the submitted full-diameter core. Kerosene was used as drilling fluid to cut the samples, which were extracted in toluene to remove hydrocarbons and water, vacuum-dried, and encased in a flexible Teflons sleeve. The air permeability of each sample was measured before the samples were saturated with kerosene to obtain the PV. JPT P. 976^

Dynamic plasticity: A comparison between results from mechanical testing and machining

The data from several previously published mechanical tests on steel and copper at high strains and strain rates are reviewed. These are presented in the form of true stress-true strain rate curves at various true strain levels and are corrected for the effects of adiabatic heating where appropriate. It is found that thermal softening cannot always satisfactorily account for the shape of the curves, and hence a dual deformation—heating model, in which the scale or intensity of the deformation is carefully considered, is proposed. Further data from machining tests are then examined and compared with those generated by mechanical testing. It is found that the evidence for the dual model is substantiated. It is concluded that, when strain and heating effects are operating in the bulk sense, adiabatic corrections to the stress—strain rate curves are possible. However, catastrophic shear, which may be induced under certain conditions of high strain, strain rate, hydrostatic stress and microstructure, cannot be accounted for in such a simple fashion.

Nonlinear pressure dependence of the elastic moduli of fused quartz up to 3 GPa

The elastic moduli of fused quartz have been determined by ultrasonic measurements under high hydrostatic compressive stress and showed a nonlinear decrease to a minimum value at 2.1–2.5 GPa, and thereafter an increase with increasing pressure. Changes in the Debye temperature and the Grüneisen constant are determined using the present results. Assuming a contribution of the fourth-order elastic constants to the nonlinear dependence, C1111, C1112, C1122, and C1123 have been tentatively evaluated to be 11.0±1.0, 10.8±3.6, 22.7±11.1, and 8.6±2.0 TPa, respectively. However, the possibilities of a complicated potential curve or a higher-order phase transition are also discussed.

The role of lubricants in machining

Friction between the rake face of a tool and the freshly formed chip surface plays a vital role in influencing the ease of the metal cutting process. The existence of clean surfaces and high hydrostatic stresses favours the formation of strong adhesive friction junctions; the extent of these can be limited by the provision of a suitable lubricant. Experiments using a novel planing machine, which enables the orthogonal cutting operation to be carried out in carefully controlled atmospheres, are described; it is suggested that processes of molecular transport within a network of interfacial capillaries are dominant in controlling the effectiveness of cutting fluids provided they are capable of reaction with, or adsorption by, the metal surfaces.

Laboratory Study of Effect of Overburden, Formation and Mud Column Pressures on Drilling Rate of Permeable Formations

Published in Petroleum Transactions, AIME, Volume 217, 1959, pages 9–17. Abstract One phase has been completed of a laboratory investigation of formations with relatively high permeability under conditions of overburden, formation and mud column pressures. The following statements are based on these tests within the limitations described. Drilling rate decreased when mud column pressure was greater than formation pressure. The decrease was primarily due to a layer of cuttings and mud particles held to the hole bottom by the difference in pressure. Much of the force of the bit was wasted in this layer and was unavailable for penetrating virgin formation. Adequate jet velocities helped clean the filter cake and chips away and resulted in increased drilling rate the higher the jet velocity the faster the drilling rate. Drilling rate increased slightly when formation pressure was greater than mud column pressure. The increase resulted from cleaning the hole bottom as formation fluid flowed into the borehole. Overburden pressure had practically no effect on drilling rate. Introduction Some formations are difficult to drill in a wellbore hole but are easily drilled under atmospheric conditions in the laboratory. The reasons for this apparent difference in drill ability are not fully understood. An increase in formation compressive strength causes a reduction in drilling rate. It has been shown by Griggs, Handin, and others that formation strength increases under conditions of high hydrostatic stress. Based on these results, a decrease in drilling rate would be expected with increased drilling fluid pressure in formations of low permeability having low formation fluid pressure. Such a decrease has been shown by Murray, et al and Eckel. This effect holds even when using nitrogen under high pressure as the drilling fluid (unreported tests from the Hughes laboratory). Poor bottom-hole cleaning may account for decreased drilling rates. Even though it may not be easily explained, evidence of poor cleaning can easily be seen.