Action Of Normal Stresses Research Articles

Strength and fracture mechanism during torsion of ultrafine-grained austenitic steel for medical applications

The article considers evaluation of torsional strength and fracture of austenitic corrosion-resistant steel 08Kh18N9 with an ultrafine-grained (UFG) and coarse-grained (CG) structure, widely used in medicine for the production of plates, screws, rods for bone osteosynthesis and other medical products. The structure of the CG steel was studied using an Axiovert 40 MAT metallographic microscope, and the fine structure of the UFG steel was investigated with a JEM-2100 transmission electron microscope. Torsion tests of the cylindrical samples with a diameter of 10 mm were carried out at a temperature of 20 °C on MK-50 installation. JEOL JCM-6000 scanning electron microscope was used for the microfractographic studies of fracture surfaces. The analysis of the “Torque - torsion angle” diagrams showed that the torsional ultimate strength (τt) and yield strength (τ0.3) of UFG steel increase by 1.3 - 3.8 times, and the relative shear (g) decreases by 2.4 times in comparison with CG steel. High values of torsional strength properties of UFG steel make it possible to provide high torque without destroying the product. Consequently UFG steel 08Kh18N9 in comparison with CG steel is a more promising material for the manufacture of medical screws and other medical products that experience significant loads during the torsion process. Three areas were identified on the surface of all fractures: fibrous central part, transitional (middle) part, and a relatively smooth peripheral part. Fracture begins with the formation of shear pits in the middle and peripheral parts, which, with further rotation of the sample, are completely rubbed out (in case CG steel), or remain (in case of UFG steel). Final failure occurs under the action of normal stresses in the central part of the sample.

Strength and Fracture Mechanism during Torsion of Ultrafine-Grained Austenitic Steel for Medical Applications

The article considers evaluation of torsional strength and fracture of austenitic corrosion-resistant steel 08Kh18N9 with an ultrafine-grained (UFG) and coarse-grained (CG) structure, widely used in medicine for the production of plates, screws, rods for bone osteosynthesis and other medical products. The structure of the CG steel was studied using an Axiovert 40 MAT metallographic microscope, and the fine structure of the UFG steel was investigated with a JEM-2100 transmission electron microscope. Torsion tests of the cylindrical samples with a diameter of 10 mm were carried out at a temperature of 20°C on MK-50 installation. JEOL JCM-6000 scanning electron microscope was used for the microfractographic studies of fracture surfaces. The analysis of the torque–torsion angle diagrams showed that the torsional ultimate strength (τt) and yield strength (τ0.3) of UFG steel increase by the factor of 1.3–3.8, and the relative shear (g) decreases by the factor of 2.4 in comparison with CG steel. High values of torsional strength properties of UFG steel make it possible to provide high torque without destroying the product. Consequently, UFG steel 08Kh18N9 in comparison with CG steel is a more promising material for the manufacturing of medical screws and other medical products that experience significant loads during the torsion process. Three areas were identified on the surface of all fractures: fibrous central part, transitional (middle) part, and a relatively smooth peripheral part. Fracture begins with the formation of shear pits in the middle and peripheral parts, which, with further rotation of the sample, are completely rubbed out (in case CG steel), or remain (in case of UFG steel). Final failure occurs under the action of normal stresses in the central part of the sample.

Study on Overburden Rock Movement and Stress Distribution Characteristics under the Influence of a Normal Fault

Fault activation triggers local deformation and dislocation, releasing a large amount of energy that can easily cause mining disasters, such as rock bursts and roadway instability. To study the changing characteristics of overburden structures and the evolution law of mining‐induced stress as panel advances towards a fault from a footwall, two similar models were established, namely, a simulation experimental model and a numerical simulation model. In addition, the relationship among mining, mining stress, and rock bursts induced by fault activation was investigated. The results of this study reveal that when the working face is 30 m away from the fault, the high‐position rock mass near the fault turns to the goaf where the fault is activated, and the two walls display relatively obvious dislocation. During the process of footwall panel mining to the fault, the abutment stress of the coal pillar tends to increase initially, followed by a decrease. When the working face is 20 m away from the fault, the abutment stress ahead of the working face reaches its maximum. When the width of the coal pillar is within the range of 10–40 m, the coal pillar accumulates a large amount of energy, and the working face affected by the fault easily induces a rock burst. Before fault activation, disturbances arising from the mining activities destroy the equilibrium stress environment of the rock system surrounding the fault, and the fault continuously accumulates energy. When the accumulated energy reaches a certain threshold, under the action of normal stress or shear stress, the fault will be activated, and a large amount of energy will be released, which can easily induce a rock burst. The research results in this paper provide a scientific basis for the classification, prediction, and prevention of rock bursts under similar geological conditions.

Вплив водню на якість сталевих виробів

In order to improve the quality of steel products, patterns of hydrogen dissolution in molten ironcarbon alloys and isolation during their crystallization and phase transformations are analyzed. The mechanism of hydrogen accumulation on the defects of the crystal structure of metals and alloys, the formation and propagation of cracks under the action of normal stress with the participation of a hydrogen cluster and dislocations, as well as the directions of preventing the negative effect of hydrogen are given. It has been established that the main reason for the formation of flocs is the presence of increased amounts of hydrogen in steel in tensile stress zones that occur during steel structural transformations, plastic deformation, uneven cooling, stress concentration sites, lattice defects, grain boundaries, non-metallic inclusions and segregation non homogeneities. Based on the established regularities, the modes of cooling and holding of blanks from carbon and low-alloyed steels after hot working by pressure, as well as preliminary and final anti-flocking heat treatment of steel blanks containing hydrogen, which allow to obtain high-quality steel products without flocs, are developed.

The influence of elastic deformations on the hysteresis properties of a two-layer ferromagnet composed of components with magnetostrictions of opposite signs

The influence of an elastic deformation that is produced by an uniaxial extension (compression) on the behavior of several magnetic characteristics of Armco iron and nickel, whose magnetostrictions have opposite signs, and a two-layer material composed of them was studied. The magnetic characteristics were measured both under the conditions of a closed magnetic circuit and using attachable transducers along and across the direction of an applied load. The results are explained from the standpoint of the formation of magnetic anisotropy, which is induced by the action of normal stresses. The results of measurements of the hysteresis and magnetostriction properties of the investigated materials are compared. The magnetic parameters of the investigated materials that monotonically change with an increase in the applied stresses are determined.

Contact interaction in the cutting of high-temperature alloys

1293 It is often assumed that the physicomechanical parameters of the blank’s surface layer and the wear rate of the cutting tool are closely related to the frictional characteristics of the tool‐blank contact [1]. However, no models have been presented for the determination of this relationship. A mathematical model of the formation of the machined blank surface may be derived from research based on the classical theory of elasticity and plasticity, taking account of the plastic deformation of the metal, its hardening and destruction, and current concepts regarding the mechanism by which residual stress is formed [2]. The validity of this model is verified by experiments on the turning of high-temperature nickel alloys by tools made of hard alloys based on tungsten carbide and high-speed steel. A functional relation has been established between the normal p r and tangential τ n stresses at the tool’s working surface and the contacting surface of the blank during mutual slip, on the one hand, and the output parameters of machining (tool wear, surface roughness, depth and degree of surface hardening, residual stress in the surface layer), on the other. It is found that the minimum stress in the tool‐blank contact zone due to the normal and tangential contact stresses, which is characterized by the maximum tangential stress is observed in machining with the optimal temperature in the cutting zone [1]. Consequently, the minimum stress in the tool‐blank contact zone is associated with the formation of a surface layer more suitable for subsequent operation of the part. Calculations by the proposed mathematical model call for numerical values of the normal stress p r and tangential stress τ n . If we adopt the method and equipment developed at Ufa State Aviation-Engineering University for laboratory study of the functional interaction of tool and blank materials—by simulation of the contact between microprojections of the harder material and a soft counterbody (the tool tip and the blank)—we may determine those characteristics over the temperature and stress range observed in machining [3]. This is the best experimental method of determining the contact characteristics of the tool and blank, since it provides reliable and convenient data for calculation of surface quality of the blank on the basis of the proposed mathematical models. τmax In the experiments, we study the turning of KhN60VT, KhN78T, and ZhS6UVI high-temperature alloys, 40 i and 30 iEe A steel, and VT9 titanium alloy by tools made of VK8, VK6M, VK60M, R18, R12F5M, R9K5, R12F2K8M3, and other alloys. The frictional characteristics for the contact of hightemperature KhN60VT nickel alloy and VK6M hard alloy are shown in Fig. 1, where f a is the adhesive component of the frictional coefficient; and τ 0 is the tangential stress in the absence of normal stress p r . As is evident from Fig. 1, the normal stress p r declines with increase in the temperature θ , on account of softening of the blank. The increase in tangential stress τ n with increase in θ to values at which f a is a maximum is associated with the accumulation of excess energy in the course of plastic contact deformation, under the action of normal stress p r .

Stretching of a confined ferrofluid: Influence of viscous stresses and magnetic field

An analytical investigation is presented for the stretch flow of a viscous Newtonian ferrofluid highly confined between parallel plates. We focus on the development of interfacial instabilities when the upper plate is lifted at a described rate, under the action of an applied magnetic field. We derive the mode-coupling differential equation for the interface perturbation amplitudes and study both linear and nonlinear flow regimes. In contrast to the great majority of works in stretch flow we take into account stresses originated from velocity gradients normal to the ferrofluid interface. The impact of such normal stresses is accounted for through a modified Young-Laplace pressure jump interfacial boundary condition, which also includes the contribution from magnetic normal traction. We study how the stability properties of the interface and the shape of the emerging patterns respond to the combined action of normal stresses and magnetic field, both in the presence and absence of surface tension. We show that the inclusion of normal viscous stresses introduces a pertinent dependence on the initial aspect ratio, indicating that the number of fingers formed would be overestimated if such stresses are not taken into account. At early linear stages it is found that such stresses regularize the system, acting as an effective interfacial tension. At weakly nonlinear stages we verified that normal stresses reduce finger competition, which can be completely suppressed with the assistance of an azimuthal magnetic field. We have also found that the magnetic normal traction introduces a purely nonlinear contribution to the problem, revealing the key role played by the magnetic susceptibility in the control of finger competition.

Quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope

AbstractTwo‐dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows. The results suggest that in natural flows, turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser‐grained beds in the bottomsets of Gilbert‐type fan deltas. Experimental flows were generated on 0°, 3°, 6° and 9° slopes of equal length which terminated abruptly on a horizontal bed. Two‐component velocities were measured on the slope, at the slope break and downstream of the slope break. Flows were depositional and non‐uniform, visibly slowing and thickening with distance downstream. One‐dimensional continuous wavelet transforms of velocity time series were used to produce time‐period variance maps. Peaks in variance were tested against a background red‐noise spectrum at the 95% level; a significant period banding occurs in the cross‐wavelet transform at the slope break, attributed to increased formation of coherent flow structures (Kelvin–Helmholtz billows). Variance becomes distributed at progressively longer periods and the shape of the bed‐normal‐velocity spectral energy distribution changes with distance downstream. This is attributed to a shift towards larger turbulent structures caused by wake stretching. Mean velocity, Reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents. A turbulent kinetic energy transfer balance shows that flow non‐uniformity arises through the transfer of mean streamwise slowing to mean bed‐normal motion through the action of Reynolds normal stresses. Net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed‐normal motion is limited. At the slope break, an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free‐shear boundaries at the bottom and top of the flows, respectively. A rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope. The increased turbulent kinetic energy, limited bed‐normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period. Thus, mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break. The internal effects of flow non‐uniformity therefore can be separated from the external influence of the slope break.

Dynamics and Rheology of Fouling Cakes Formed During Ultrafil Tration

The solute cake which forms on a membrane surface during ultrafiltration processes is well known for its fouling characteristics. The dynamics and rheology of the cake are investigated and observed under the action of cross-flow shear. Experiments with slurries having 300 nm diameter particles of titanium dioxide indicate average volume concentrations of 0.56 or 0.57 and show that this cake indeed ‘flows’ with a creeping velocity under applied shear. The cake thickness reaches a steady state when the solute advection towards the membrane balances the solute mass carried away at the trailing edge by the creeping cake. The viscosityshear rate dependence of this layer is determined experimentally and the ‘Creeping velocity’ of the cake is calculated assuming the transverse drag force is determined from the Kozeny-Carman equation. Upon instantaneous compression the cake compresses while maintaining its mass distribution. The change in cake resistance allows interpetation of the pressure modified concentration. The volume concentration, determined from the mathematical modeling, is shown to lie between 0.54 to 0.65. Observations show that the top few layers of this cake move freely with the shearing flow due to the lifting action of normal stresses in the cake under external shear. Volume concentrations up to 0.65 are indicated from the analysis.

Extrudate Swelling: Physics, Models, and Computations

Viscous, viscoelastic, or elastic normal stresses are superimposed to pressure within flowing fluids. These stresses act normal to the boundaries of the flow that may deform depending on their modulus or viscosity. At absolutely rigid boundaries of infinite modulus of elasticity any boundary deformation and therefore any fluid expansion or swelling is surpressed (eg, flow in rigid pipes, annuli, channels). Elastic boundaries (eg, flow in veins and arteries, flow by membranes, around inflating/deflating balloons) deform under the action of normal stresses, allowing expansion or swelling of fluid. The same mechanism prevails in lubrication, where pressure and superimposed normal viscoelastic stresses keep surfaces in relative motion apart, with simultaneous increase in load capacity. Viscous boundaries (eg, liquid jet in air or in immiscible liquid, slow extrusion of viscoelastic liquids from dies, expanding/collapsing air-bubbles or liquid-droplets) are displaced by flowing adjacent immiscible fluids, allowing swelling or imposing contraction depending on relative rheological characteristics. Thus, the kind of swelling examined here is independent of density, ie, incompressible, and is due to the action of normal stresses against the boundary that is imposed either by adjacent deformable obstacles or else by surface tension. The resulting swelling is dynamic (ie, it initiates, changes and ceases with the flow) and can be made permanent by solidification, crystallization or glassification. The most profound form of incompressible swelling is the extrude swelling that controls the ultimate shape of extruded parts. Incompressible swelling is enhanced by the ability of macromolecules to deform and recover (eg, viscoelastic) and by the design of flow conduits to impose sharp transitions of deformation modes (eg, singular exit flows). The same swelling is reduced by the ability of molecules (or fibers in fiber-suspensions) to align with the flow streamines, as well as any tendency of solid-like structure formulation (eg, viscoplastic).

Complex characteristics of the mechanical properties of metals

The method of computation of o n is based on two limiting states of the material: a completely brittle material for which ~ = 0, and a completely viscous material for which 9 = 1 (100%). In the first case, which can be called "decohesion," the specimen fails by tearing under the action of normal stresses. In this case, the decohesion indicator K 4 = 1 -~, where } is reckoned in fractions of a unit. In the second case, the specimen is failed by shearing under the action of shear stresses. This process can be called "cohesion": The cohesion indicator K = 9. The elastoplastic state of real structural alloys is intermediate between these two ~imiting states, and can be characterized by a certain parameter relating both indicators -K d and K c. This parameter, however, cannot assume the form of the ratios Kj/K~, or K /K., since they are meaningless when ~ = 0, and ~ = 1 Neither can the indicator Q ~ e Q ° (K d + Kc)/2 be used, since it is 0.5 for any value of 4. It is therefore proposed that the product Kel a = KdK c = 4(1 -~), which varies as is shown in Fig. 1 as ~ increases from 0 to i, be employed as this parameter. Let us call it the elasticity indicator.

Effect of Hydrostatic Pressure on Geometry of Slip in Silicon Iron Single Crystals

ABSTRACTThe effect of the pressure 550MPa on the choice of the acting slip plane in the single crystals of Fe−2,9%Si was studied. It was established that the presence of pressure resulted in the increase of the slip asymmetry. The effect of pressure is attributed to the action of normal stresses and to the special structure of screw dislocations.

Some comments on post extrusion swelling phenomena: Die entrance or die passage

AbstractThe mechanistic origins of the post extrusion swelling phenomenon have been the subject of considerable debate. Conflicting theories have postulated that it is caused either by (1) deformations at the die entrance which are recovered upon die exit as a consequence of viscoelastic memory or (2) elastic enorgy, which is stored during capillary flow by the action of normal stress, and subsequently released upon die exit or by both (1) and (2). We have described these situations by mathematical models and show that the resulting equations are similar despite different origins. The difficulties in interpreting the results mechanistically are discussed. Empirically, the results can be described as being exponential in capillary residence irrespective of cause.

Exact solution for the stress concentration in a slot

A mixed boundary value problem for the stress concentration m an elastic layer weakened by a plane circular slot, which is under the action of normal stresses, is analyzed by reducing it to an initial value problem using the theory of invariant imbedding.

Normal stresses, non‐Newtonian flow, and dynamic mechanical behavior of polymer solutions

AbstractComplex dynamic shear moduli, shear and normal stresses in steady shearing flow, and complex moduli with superposed steady shear are investigated in both theoretical and experimental aspects. Firs) it summary of phenomenological and molecular theories is given. Experimental tests of phenomenological theory are made by comparing dynamic (linear viscoelastic) data and steady shearing flow (nonlinear viscoelastic) data. It is found that the recent theory of memory fluids is quite satisfactory and also the results show that the action of normal stress in steady shearing flow is mainly pull in the direction of the flow line. The behavior of dilute polymer solutions (near A condition are best approximated by the molecular theory of Zimm, particularly at very low shear rates. At higher shear rates, the Zimm theory fails in predicting the appearance of non‐Newtonian viscosities and nonquadratic normal stresses. Various proposed modifications of the theory are examined experimentally. The behavior of concentrated polymer solution does not seem to fit any of the existing molecular theories; therefore, some typical features of the experimentally observed behavior of concentrated solutions are simply summarized. The results of the measurements of complex moduli with superposed steady shear show that in the low‐frequency region the effect of superposed shear rate results in a much larger decrease in the real part of the complex modulus that in the imaginary part, while in the higher‐frequency region the real part is again decreased but the imaginary part is increased with the increase of superposed shear rate.

Note on the Normal Stress Effect in the Solution of Rodlike Macromolecules

Three components of normal stresses in a solution of rodlike macromolecules are calculated on the basis of the general theory of irreversible processes of macromolecular solutions developed by Kirkwood. The results are expressed in expansion series of even powers of the rate of shear and are explicitly given to their fourth powers. When the hydrodynamic interaction is not taken into account, two components of the normal stresses perpendicular to the direction of flow identically vanish, while, by the introduction of this interaction, they appear. The action of normal stresses is still mainly ``pull'' in the direction of flow as in the free-draining case.

High strength austenitic steels

‘Transitional’ steels have an unusual combination of mechanical properties. They may have a low yield stress (20–40 kg/sq. mm=28, 400–56, 900 psi) and a high tensile strength (100–200 kg/sq mm=142, 000–284, 000 psi). The mechanical properties of these steels depend mainly on the extent to which austenite decomposes into martensite in deformation, and on the fracture strength [resistance to failure under the action of normal stresses], of the martensite formed. When martensite produced by quenching is present in the initial steel, its rupture resistance also determines the mechanical properties of the steel. In tensile tests on transitional nickel steels containing 0. 26% carbon, initially purely austenitic, there was a considerable plastic deformation (elongation up to 60%); fracture occurs without necking, and the elongation exceeds the reduction of area. In spite of considerable preliminary plastic deformation, the failure is of the brittle type. 3. It has been confirmed that in steels containing unstable austenite, a combination of strength and ductility can be achieved, unattainable in steels of other structural categories.