Yield Stress Parameters Research Articles

Plastic anisotropy of straight and cross rolled molybdenum sheets

The microstructure, texture and mechanical properties of molybdenum sheets produced by different rolling processes were investigated by orientation imaging in the scanning electron microscope, X-ray diffraction and tensile tests, respectively. For comparable recrystallization degree of the sheets investigated, straight rolling with low reduction ratio produces α-fiber textures with a maximum at {100} 〈110〉. At higher rolling degrees the maximum shifts to {112} 〈110〉. Cross rolling increases the rotated cube component {100} 〈110〉. The strong differences in the texture measured are reflected in the plastic anisotropy characterized by differences in the yield stress and Lankford parameter which were measured along directions in the rolling plane at angles of 0°, 45° and 90° with the rolling direction. The Taylor–Bishop–Hill theory is used successfully to qualitatively explain the plastic anisotropy.

Effect of solid-solution softening of crystalline materials: Review

The phenomenon of softening of crystalline materials by alloying, which is promising for controlling mechanical properties of materials, is due to the occurrence of an additional channel of facilitated formation of dislocation kinks at impurity centers. This effect is related to the kink mechanism of dislocation motion. Hence, a range of materials that are capable of softening can be singled out: metals with a bcc structure, semiconductors, ceramic materials, intermetallic compounds, etc. A unified basis for description of softening regularities is given by the phenomenological theory. This theory predicts many properties that are general for all materials under consideration: the range of strength characteristics of impurities capable of softening crystals, limiting possibilities of softening at the optimal choice of the components, etc. The theory, supplemented with the knowledge of some material constants (determined from microscopic calculations or, even better, from experiment), makes it possible to calculate the temperature and concentration dependences of the yield stress and other parameters measured in mechanical tests.

Effect of thermal and hygrothermal aging on the plane stress fracture toughness of poly(ethylene terephthalate) sheets

The in-plane (plane stress) fracture toughness of two polyester (PET, PETG) sheets were assessed using the essential work of fracture (EWF) method after thermal and hygrothermal aging performed just below glass temperature. This ensured that physical aging takes place. On the aged sheets the yield stress (σy), enthalpy relaxation (ΔH) and EWF parameters were determined. It was observed that the essential work of fracture component related to the specific yielding (we,y) is suitable for assessing physical aging - independently of the humidity content. A good linear correlations exists between we,y, σy and ΔH.

Prediction of Cleavage Fracture in the Ductile-to-Brittle Transition Region of Pressure Vessel Steels: A Probabilistic Model

This paper presents a modified Weibull stress model, which accounts for the effects of plastic strain and stress triaxiality at the crack tip region. The proposed model is applied to predict cleavage fracture in a modified A508 pressure vessel steel. It is demonstrated that the Weibull modulus (m) remains constant in the temperature range considered, while the threshold Weibull stress (σw-min) decreases with an increase in temperature due to reduction of the yield stress and the scale parameter of the Weibull model (σu) increases with temperature reflecting the influences of temperature on both material flow properties and toughness. The proposed model accurately predicts the scatter of the measured fracture toughness data and the strong effects of constraint and temperature on cleavage fracture toughness.

The Effect of Dietary Probiotic Supplementation on Tibial Bone Characteristics and Strength in Broilers

A 6-wk study with 50 birds was conducted to investigate the effects of a dietary supplemental probiotic on morphometric parameters and yield stress of the tibia. Twenty-five 1-d-old broiler chicks were assigned to a control or an experimental diet containing Bacillus licheniformis and Bacillus subtilis (BioPlus 2B, CHR Hansen BioSystems, Denmark, Ugur Ecza Deposu, Turkiye Distributoru, Adapazari 41400, Turkey; each containing 2.3 × 108 cfu/g of spores) supplemented to the starter and finisher diets at 500 g/1,000 kg of feed. Each diet was replicated 5 times with 5 birds in each replicate. Tibiotarsi weight, length, and weight/length index, robusticity index, diaphysis diameter, modulus of elasticity, yield stress parameters, and percentage Ca content were not affected by the dietary supplementation of probiotic, whereas thickness of the medial and lateral wall of the tibia, tibiotarsal index, percentage ash, and P content were significantly improved by the probiotic. Medullary canal diameter of the tibia of the birds fed the control diet was significantly greater than that of birds fed the probiotic diet. There was no treatment impact on live performance of the birds throughout the 6-wk feeding trial.

Rheology of concentrated xanthan gum solutions: Steady shear flow behavior

Using a strain-controlled rheometer, the steady shear flow properties of aqueous xanthan gum solutions of different concentrations were measured over a wide range of shear rates. In this article, both the shear rate and concentration dependencies of steady shear flow behavior are reported from the experimentally obtained data. The viscous behavior is quantitatively discussed using a well-known power law type flow equation with a special emphasis on its importance in industrial processing and actual usage. In addition, several inelastic-viscoplastic flow models including a yield stress parameter are employed to make a quantitative evaluation of the steady shear flow behavior, and then the applicability of these models is also examined in detail. Finally, the elastic nature is explained with a brief comment on its practical significance. Main results obtained from this study can be summarized as follows: (1) Concentrated xanthan gum solutions exhibit a finite magnitude of yield stress. This may come from the fact that a large number of hydrogen bonds in the helix structure result in a stable configuration that can show a resistance to flow. (2) Concentrated xanthan gum solutions show a marked non-Newtonian shear-thinning behavior which is well described by a power law flow equation and may be interpreted in terms of the conformational status of the polymer molecules under the influence of shear flow. This rheological feature enhances sensory qualities in food, pharmaceutical, and cosmetic products and guarantees a high degree of mixability, pumpability, and pourability during their processing and/or actual use. (3) The Herschel-Bulkley, Mizrahi-Berk, and Heinz-Casson models are all applicable and have equivalent ability to describe the steady shear flow behavior of concentrated xanthan gum solutions, whereas both the Bingham and Casson models do not give a good applicability. (4) Concentrated xanthan gum solutions exhibit a quite important elastic flow behavior which acts as a significant factor for many industrial applications such as food, pharmaceutical, and cosmetic manufacturing processes.

Effect of Ammonium Salt of Styrene‐Maleate Copolymer on the Rheology of Quinacridone Red Pigment Dispersion

The rheological properties of quinacridone pigment dispersion with ammonium salt of styrene‐maleate (SMA) copolymer (SMA‐NH4) as a dispersant were measured at 25°C. The effects of the SMA‐NH4 concentration, pH, and ionic strength of the system on the rheology of concentrated quinacridone pigment dispersion (in aqueous medium) were studied. All of the experimental results were correlated with the Herschel–Bulkley model. The estimated values of yield stress, τy, and model parameters such as the consistency coefficient, K, and the flow characteristic exponential, n, could reflect the variation of the surface charges of pigment particles, interactions between particles and the structure of dispersion. The flow curve of the quinacridone pigment dispersion without SMA‐NH4 showed a pseudoplastic character, while the flow curve of the quinacridone pigment dispersion with SMA‐NH4 copolymer showed a nearly Newtonian behavior. The rheological properties of aqueous quinacridone pigment dispersion with SMA‐NH4 were strongly affected by both pH and ionic strength.

Laminar fully developed flow and heat transfer for Robertson-stiff fluids in rectangular ducts with temperature defendent viscosity

Laminar fully developed flow and heat transfer for Robertson-Stiff fluids in rectangular ducts with temperature dependent viscosity is studied numerically. The viscosity is taken to vary exponentially with temperature. The effect of the temperature dependent viscosity on the behavior of the flow and the heat transfer is investigated. The coupled momentum and energy equations are solved iteratively using finite difference method to obtain the velocity, fanning friction factor and temperature distributions. The average Nusslet numbers are obtained for H 1 thermal boundary condition. Computations are obtained for different values of yield stress, flow index, aspect ratio and viscosity parameters

Nanoindentation stress–strain curves as a method for thin-film complete mechanical characterization: application to nanometric CrN/Cr multilayer coatings

The mechanical behavior of CrN/Cr multilayer coatings deposited by rf magnetron sputtering has been investigated by nanoindentation measurements performed with indenters of different geometries. Nanoindentation stress–strain curves generated from these measurements allow us to characterize the complete mechanical behavior of these coatings in the elastic, elastoplastic, plastic and fracture deformation regimes. In particular, indentation measurements carried out with a 100-μm-radius spherical indenter allowed us to study the elastic deformation regime and estimate the yield stress parameter through the initial indentation yielding point. The elastoplastic deformation regime has been studied using a 5-μm-radius spherical indenter and the stationary yielding regime (fully plastic regime) has been investigated with a pyramidal indenter of Berkovich geometry. The use of a pyramidal cube-corner indenter allowed us to study coating fracture characteristics. Nanometric CrN/Cr multilayer structures as well as single CrN and Cr coatings have been characterized. The study has shown that multilayered coatings with period thicknesses less than 46 nm present values of yield stress, Young’s modulus, hardness and toughness higher than those for single-layer CrN and Cr coatings.

Natural convection of non-Newtonian fluids through permeable axisymmetric and two-dimensional bodies in a porous medium

The present work concerns the natural convection of non-Newtonian power-law fluids with or without yield stress over the permeable two-dimensional or axisymmetric bodies of arbitrary shape in a fluid-saturated porous medium. Using the fourth-order Runge–Kutta scheme method and shooting method we obtain the local non-similarity solutions. The parameters that include the dimensionless yield stress Ω, permeable constant c and power index n are studied, and the heat flux and the wall temperature are taken into consideration as variables. The local non-similarity solutions are found to be in excellent agreement with the exact solution. It is found that the results depend strongly on the values of the yield stress parameter, the wall temperature distributions, the lateral mass flux rate, and the heat flux at the boundary.

Effect of rheological properties of zircon-alumina suspensions on density of green casts

Rheological properties of aqueous suspensions (48 vol%) of zircon-alumina mixtures containing different amounts of polyelectrolyte as dispersant were studied. Slip casting in a plaster mold and pressure filtration at 8 MPa experiments were performed to correlate the rheological properties of the suspensions with the relative density of green casts. Flow curves fitted satisfactorily to the Casson model. The Casson viscosity values remained nearly constant whereas Casson yield stress parameter decreased to a minimum and then increased with increasing the amount of dispersant added. The influence of particle size distribution (PSD) of the powder mixtures on Casson yield stress parameter and on the minimum viscosity was also examined. Green densities increased as yield stress Casson parameter decreased. The mixture of alumina and zircon as fine and coarse powders produced bimodal PSD which maximized the green density of the compacts. Some weakly flocculated suspensions having low apparent viscosity and small yield stress produced casts with relatively high densification. This is attributed to a slight higher viscosity at low shear rates of these suspensions in which settling of particles and /or segregation of components can not occur.

An Elasto-Plastic Model for Unsaturated Soil in Three-Dimensional Stresses

A three-dimensional elasto-plastic model for unsaturated soil is presented, using a transformed stress tensor based on the Extended SMP criterion. The influences of suction on the mechanical behavior of unsaturated soil are taken into account in the model by using a parameter named suction stress σ0(s), which is added to net stress to form an effective stress, and associating a yield stress and model parameters with it. The proposed model needs few material parameters, which can be easily determined from the results of ordinary tests, for example, oedometer and direct shear tests or triaxial test. It can predict the stress-strain-strength behavior of unsaturated soil under the conditions of three-dimensional stresses and constant or variable suction. A considerable number of triaxial compression and extension tests on unsaturated compacted kaolin were performed by using a suction-controlled triaxial apparatus, to measure the stress-strain behavior under different stress paths with constant and decreasing suction. The prediction of the proposed model shows good agreement with the measured behavior of unsaturated soil not only in triaxial compression but also in triaxial extension under the conditions of constant suction and reduction in suction, which may induce an irrecoverable volumetric compression (collapse).

The Effects of Yield Stress Variation on Uniaxial Exchange Flows of Two Bingham Fluids in a Pipe

Buoyancy driven flows of two Bingham fluids in an inclined pipe are considered, providing a simplified model for flows occurring in the oilfield process of plug cementing. The flows studied are near-uniaxial and stratified, with the heavy fluid moving down the incline, displacing the lighter fluid upwards. Continuity and monotonicity results are given describing parametric dependence of the solutions on the yield stresses. Solutions are found for two specific fluid-fluid interface configurations that are of practical interest. For these flows there exists a single curve in the dimensionless yield stress parameter space, which separates regions of zero and nonzero flow. For the example interface configurations, this curve is determined exactly. For arbitrary interface configurations a rigorous but conservative general result is proven. Heuristic methods are then used to improve this estimate for arbitrary interfaces.

A Brief History of the Yield Stress

Abstract This short article is a summary of a recently published, lengthy review [1] in which the author challenges the often-accepted view that materials have yield stresses, below which no flow takes place. However, following the introduction of the new generation of controlled-stress rheometers it is shown that when careful measurements are made below the supposed ’yield stress‘, flow does actually take place. The argument for the nonexistence of the yield stress as a physical entity now seems insuperable. However, in spite of this, it is nevertheless accepted that an apparent yield stress is a useful mathematical abbreviated description of limited data over a given range of flow conditions. This yield stress parameter can be used effectively for predicting flow, but only within the region of the original measurement that furnished the yield stress.

Rheological properties of guar galactomannan and rice starch mixtures II. Creep measurements

The effect of particulate inclusions, or ‘fillers’, on the rheological properties of a typical polysaccharide entanglement solution (guar galactomannan/water) have been studied. Earlier steady shear experiments have shown an apparent upswing of the viscosity-shear rate profiles at low shear rates, with higher filler concentrations (Rayment et al., 1995). In creep experiments, a low constant stress is applied to the sample. In the present paper, the creep experimental data have been compared to the steady shear flow behaviour fitted by the yield stress modified Cross equation (introduced in Part I of this series). The apparent zero-shear viscosity and yield stress parameters have been constrained in a number of ways in the model to establish the effect of these parameters on the modified Cross model. Although the creep data appears to alter the precision of the modified Cross equation, due to the low shear rates accessed, the apparent upswing of the steady shear data at low shear rates appear to be supported by the data described in this paper. This lends further credibility to the scaled filler models introduced previously.

Physically based temperature dependence of elastic-viscoplastic constitutive equations for copper between 20 and 500°C

Abstract An anisothermal material model has been established for cyclic mechanical and thermal loading in chill-cast pure copper undergoing creep and cyclic plasticity. The temperature dependences in the model are physically based. The material parameters arising in the model have been determined from a programme of tests carried out on pure copper, enabling the model to be validated over the temperature range 20–500°C, strain range 0-±1·% and strain rate range 0·006–0·6%s−1. The variations, with inverse temperature, in the viscoplastic strain rate parameter, the cyclically hardened yield stress, and hardening and softening parameters, when considered with respect to logarithmic scales have been shown to be bilinear in nature, over the temperature range considered. The transition temperature, in all cases, was found to be 209°C. For the viscoplastic strain rate parameter, an activation energy of 200 kJ mol−1 was determined for temperatures in excess of 209°C, and 33kJ mol−1 for temperatures less than 209°C. Deformation mechanisms of bulk diffusion, and dislocation glide and climb, are thought to predominate for the two temperature ranges respectively. However, in the case of the cyclically hardened yield stress, and the hardening and recovery parameters, activation energies of 34kJ mol−1 and 11 kJ mol−1 were determined for temperatures greater than and less than 209°C respectively. The deformation mechanisms predominating are thought to be dislocation glide and climb for the case of the higher temperature range, and glide only for the lower temperature range, because of the dependence of dislocation climb on core diffusion which is thought not to occur significantly in copper below about 209°C. The constitutive equations developed have been generalized to multiaxial stress states, and a methodology for the determination of the material parameters given, which ensures that they are physically meaningful.

Free convection heat transfer of non-Newtonian fluids over axisymmetric and two-dimensional bodies of arbitrary shape embedded in a fluid-saturated porous medium

The problem of natural convection of a non-Newtonian power-law fluid with or without yield stress about a two-dimensional or axisymmetric body of arbitrary shape in a fluid-saturated porous medium is analyzed on the basis of boundary layer approximation. For a high modified Rayleigh number, similarity solutions are obtained by using the fourth-order Runge—Kutta scheme and shooting method for two-dimensional bodies without yield stress and a cone with yield stress. The effects of the surface heat transfer rate q W( x), the local Nusselt number Nu x , the overall heat transfer rate Q* and the power indices n of fluids with the yield stresses on the free convection heat transfer characteristics are discussed. It is found that the results depend strongly on the high values of the yield stress parameter Ω/ a at the boundary.

Predicting erosion resistance of muds

When measured or apparent yield stress of cohesive muds is assumed to reflect average particle-particle bond strength in network structures, critical bed shear stress required for grain-by-grain entrainment can be predicted from an analysis of the forces acting on component grains of geometrically-flat mud beds. Erosion resistance is then defined in terms of cohesive yield stress as well as nominal particle size, shape, relative density, and packing geometry. Predicted values of critical bed shear stress compare favorably with existing data from selected studies in which appropriate yield stress and hydrodynamic parameters have been reported.

Viscoplastic Response of Clamped Beams under Impact Loading

A three‐stage analytical model for predicting the final midpoint deflections of viscoplastic, clamped beams under impact loading conditions is presented. The model includes traveling plastic hinges and accounts for strain rate sensitivity by continuously varying the dynamic yield stress parameter during the impact event. This new model is compared both with another impact model and with two different sets of experimental results for steel and aluminum beams. In addition, the finite element program, ABAQUS, is employed to provide a detailed history of the dynamic response of the system. The finite element results are used to assess the appropriateness of the simplifying assumptions contained in the three‐stage model. The model predictions are shown to be in good agreement with both sets of experimental results and the assumptions made in developing the model are in general agreement with the ABAQUS predictions.

Mechanical properties and dislocation structures of TiAl single crystals deformed at 4.2–293 K

Mechanical properties and dislocation structures of Ti-56 at.% Al single crystals have been studied in specimens with [010], [001], [ 110 ], A; [ 132 ], B; [ 245 ] and C; [ 452 ] orientations at 4.2, 77, and 293 K in compression. The yield stress and strain rate sensitivity parameter, S, increased rapidly with decreasing temperature. The steep increase of the yield stress with decreasing temperature would be caused by dislocation reactions and dipoles as well as Peierls stress. Slip markings on deformed specimens were straight and rather coarse and slip planes were {111} at all test temperatures. Superdislocations was curved at temperatures below 196 K and seemed not to lie along a given direction. The 1 2 < 112] and < 101] type superdislocations have been observed at all test temperatures. Some of the <101] type superdislocations dissociated into superpartials with the Burgers vector 1 2 < 101] bound by an antiphase boundary (APB) at 77 K. The 1 6 < 112] type Shockley partials bound by a stacking fault (SF) were also observed in the [001] orientation at 4.2 K and the B orientation at 77 K. However, 1 2 < 110] type ordinary dislocations have not been observed below 196 K, which can react very easily with the same and other types of dislocations and change into stair rod dislocations resulting in obstacles to the subsequent dislocations. Therefore, it would be difficult to observe free and reacted portions of the ordinary dislocations because of a small number of the dislocations comparing to the other type dislocations. The serrated flow occurred at 4.2 K due to adiabatic deformation. A new type strengthening mechanism due to dislocation dipoles was proposed.