Variation Of Yield Stress Research Articles

Thermo-Mechanically Coupled Modeling of Cooling Temperature History Effects on Precipitation Hardening in Hot Strip Coiled Products

When producing hot strip, HSLA(High Strength Low Alloyed) products the coiling temperature and the subsequent coil cooling is of great importance for the final mechanical properties. A thermo-mechanically coupled model has been developed where the anisotropic stress dependent thermal properties caused by the layered structure and the asymmetric cooling are included. Additionally the precipitation hardening effect on the yield strength, influenced by the thermal history during cooling was compared with mechanical tensile testing along the strip length at SSAB EMEA works in Borlänge, Sweden. Good agreement between measured and predicted yield stress variations in head and tail was obtained.

Finite Element Implementation of a Bonding Model and Application to Roll Bonding of Aluminum Sheets of Largely Different Yield Strength

Roll bonding is a joining-by-forming operation, in which two or more metallic strips or plates are bonded permanently through the pressure and plastic deformation in the roll gap. Although roll bonding has been successfully used in industrial production over many years, difficulties occur especially when materials of largely different yield strength are roll-bonded, e.g. when hard aluminum alloys are clad with soft commercially pure aluminum. Examples are AA2024 sheets used in wing and fuselage structures of aircrafts, which are clad with AA1050 to improve the corrosion resistance. Likewise, aluminum sheets for heat exchangers consist of a hard base material that is clad with a soft solderable aluminum alloy. In these cases, the strength difference may influence the bonding behavior since the softer face sheet has to transmit the deformation to the harder core material. To analyze and optimize such cases, a bonding model integrated into a numerical framework for the simulation of the roll bonding process is required. In this paper, a finite element model is presented, in which the development of bond strength is simulated using a cohesive contact formulation. The model is used to study the bonding behavior of laboratory-scale roll bonding trials of two aluminum alloys with a large difference in yield strength. It is found that shear stresses are generated towards the end of the roll gap that may exceed the shear bond strength created earlier in the roll gap such that no firm bond is obtained. The conditions under which bonding is successful are analyzed using a finite element simulation study with varying yield stress differences and pass reductions and summarized in a map.

CFD MODEL OF A MAGNETORHEOLOGICAL FLUID IN SQUEEZE MODE

Abstract The study briefly outlines a CFD model of a magnetorheological (MR) fluid operated in squeeze mode with a constant interface area using the CFD (Computational Fluid Dynamics) approach. The underlying assumption is that the MR fluid is placed between two surfaces of which at least one can be subject to a prescribed displacement or a force input. The widely employed Bingham model, which fails to take into account the yield stress variations depending on the height of the gap, has been modified. Computation data obtained in the ANSYS CFX environment are compared with experimental results.

Viscosity and water demand of limestone- and fly ash-blended cement pastes in the presence of superplasticisers

The rheological behaviour of fresh cement has a direct effect on the microstructural development of mortar and concrete. Inasmuch as the presence of mineral additions impact cement paste rheology and consequently its permanent microstructure and strength, a full understanding of blended cement behaviour should be pursued. The present study addresses the joint effect of mineral additions (limestone and fly ash) and superplasticisers admixtures on the viscosity and water demand of cement pastes.Cement pastes were prepared with 10, 30 or 50 wt% limestone or fly ash as mineral admixtures. Melamine-, naphthalene- and polycarboxylate-based superplasticisers were used. Paste rheology was studied in terms of variations in yield stress and viscosity with the solids content and amount of mineral additions added. The strength and microstructure of the blended cement pastes were determined at viscosity values of 1.5 Pa·s. in the presence of superplasticisers.The findings showed that the Krieger–Dougherty equation could be used to determine the effect of solids content on the apparent viscosity of limestone- and fly ash-blended cement suspensions, as well as the effect of superplasticisers. Adding less than 30% limestone to cement had no effect on paste rheology: i.e., the w/c ratios for minimum and optimal workability were similar to the ratios for ordinary cement. However, adding fly ash did lower the minimum water demand, and the optimal amount of water needed for suitable fluidity. The inclusion of 10% of either addition raised paste strength, while higher proportions 30 or 50%) had the opposite effect. The use of mineral additions reduced the effectiveness of cement superplasticisers.

Effect of Plasticizer and Superplasticizer on Rheology of Fly-Ash-Based Geopolymer Concrete

An attempt has been made in this investigation to study the variation of yield stress, plastic viscosity, and slump of fly-ash-based geopolymer concrete with the variation of lignin-based plasticizer dosage and polycarboxylic-ether-based superplasticizer dosage. It has been observed that a critical value of molar strength of sodium hydroxide exists that is equal to 4 M. Beyond this critical molar strength, superplasticizer and plasticizer have an adverse effect on yield stress, plastic viscosity, and slump of fly-ash-based geopolymer concrete. Below the critical molar strength of sodium hydroxide, there is a decrease in both the rheological parameters and an increase in slump. Lignin-based first-generation plasticizer shows better performance in terms of workability over third-generation superplasticizer below the critical value of molar strength. It was also observed that there is a good correlation between the rheological parameters and slump for fly-ash-based geopolymer concrete incorporating plasticizer and superplasticizer.

Hot-Tensile Data and Creep Properties Derived There-from for 316L(N) Stainless Steel with Various Nitrogen Contents

Many researchers have proposed equivalence between hot-tensile data and creep rupture properties based on Larson-Miller or similar time-temperature parameter approaches. This is attractive from the point of view economy of time and cost of testing as compared to the long-time creep tests. Basic idea behind such a comparison is that the plastic deformation behavior controlling both the hot-tensile and creep properties are the same, provided the microstructure does not ndergo significant changes for the time periods under consideration. Particularly, such a procedure will be useful in alloy development and preliminary screening. Such a time-temperature parameter approach is adopted in the following for predicting the stress-rupture behavior of 316 L(N) stainless steel from reported hot-tensile properties and the results are compared with actual creep test data for almost a similar steel at two temperatures. It is found that the predicted data follow the ASME Code Case minimum stress-rupture data for 316 SS (stainless steel) and thus are very conservative. Predicted creep rates are also much larger than the actual ones for the one case where comparison has been made. The conclusion drawn is that considering the microstructural stability of the low-carbon nitrogen bearing SSs (at least for lower nitrogen contents), the method of equivalence of hot-tensile and stress-rupture data seems feasible. But test results at different strain rates, rather than the single value considered here, along with the more accurate method suggested would yield better results. In addition, using some old results from the literature, relation for variation of yield stress with temperature, nitrogen content and grain-size for the present heats of steel (with various nitrogen contents) has been derived.

The yield stress of jammed magnetofluidized beds

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during initialization of the bed in the bubbling regime and particles are free to move and restructure in chains. Measurements of the yield stress are presented when the applied magnetic field is oriented either in the vertical or horizontal direction (B co-flow and B cross-flow modes, respectively). The variation of the magnetic yield stress with particle size was found to be dependent on the field direction. This can be justified by the dependence of the interparticle magnetic force on the chain average angle with the field, which is affected by particle size as the stabilized bed is subjected to small consolidations.

On the structural dependence of the stress–strain diagram parameters and fracture toughness of metastable austenitic steels

The results of the investigation into the effect of structure on the variation of yield stresses and fracture toughness of a number of austenitic steels at the temperatures of suppression and evolution of the martensitic transformation are generalized taking into account their chemical composition and mode of loading. The correlations are established between the structural characteristics of steels of the class under consideration and the parameters of the analytic expressions for their stress–strain diagrams that determine the conditions for the competitive influence of the austenite strength, the intensity of the formation process of deformation-induced martensite, its amount and the strength of the strain hardening of the material. The specific character of creating its fracture toughness indices is justified structurally.

Analyzing batch-to-batch and part-to-part springback variation of DP600 steels using a double-loop Monte Carlo simulation

Many automobile companies are actively exploring the use of high-strength dual-phase steels as an alternative to aluminum and magnesium alloys owing to their light weight, low cost and durability. However, dual-phase steels have a tendency to springback more than other structural steels in a forming operation owing to their high tensile strength. In addition, variations in manufacturing process parameters and material properties cause springback variation over different manufactured parts. Therefore, it is an important task to reduce the magnitude of springback, as well as its variation within, to produce robust and cost-effective parts. This article investigates minimization of the magnitude and variation of springback of DP600 steels in U-channel forming within a robust optimization framework. The computational cost was reduced by utilizing metamodels for prediction of the springback and its variation during optimization. Three different allowable sheet thinning levels were considered in solving the robust optimization problem, and it was found that, as the allowable thinning increased, the die radius decreased, thereby the magnitude and variation of springback reduced. A simple sensitivity analysis was performed and the yield stress was found to be the most important random variable. Finally, a double-loop Monte Carlo simulation method was proposed to calculate part-to-part and batch-to-batch springback variations. It was found that, as the batch-to-batch variation of yield stress increased, the batch-to-batch springback variation increased, while the part-to-part springback variation remained unchanged.

Study of dynamic strain aging in dual phase steel

Abstract The susceptibility to dynamic strain aging of a dual phase steel was evaluated by the variation of mechanical properties in tension with the temperature and the strain rate. The tensile tests were performed at temperatures varying between 25 °C and 600 °C and at strain rates ranging from 10−2 to 5 × 10−4 s−1. The studied steel presented typical manifestations related to dynamic strain aging: serrated flow (the Portevin–Le Chatelier effect) for certain combinations of temperature and strain rates; the presence of a plateau in the variation of yield stress with temperature; a maximum in the curves of tensile strength, flow stress, and work hardening exponent as a function of temperature; and a minimum in the variation of total elongation with temperature. The determined apparent activation energy values, associated with the beginning of the Portevin–Le Chatelier effect and the maximum in the variation of flow stress with temperature, were 83 kJ/mol and 156 kJ/mol, respectively. These values suggest that the mechanism responsible for dynamic strain aging in the dual phase steel is the locking of dislocations by carbon atoms in ferrite and that the formation of clusters and/or transition carbides and carbide precipitation in martensite do not interfere with the dynamic strain aging process.

Experimental and numerical study on fragmentation of steel projectiles

A previous experimental study on penetration and perforation of circular Weldox 460E target plates with varying thicknesses struck by blunt-nose projectiles revealed that fragmentation of the projectile occurred if the target thickness or impact velocity exceeded a certain value. Thus, numerical simulations that do not account for fragmentation during impact can underestimate the perforation resistance of protective structures. Previous numerical studies have focused primarily on the target plate behaviour. This study considers the behaviour of the projectile and its possible fragmentation during impact. Hardened steel projectiles were launched at varying velocities in a series of Taylor tests. The impact events were captured using a high-speed camera. Fractography of the fragmented projectiles showed that there are several fracture mechanisms present during the fragmentation process. Tensile tests of the projectile material revealed that the hardened material has considerable variations in yield stress and fracture stress and strain. In the finite element model, the stress-strain behaviour from tensile tests was used to model the projectile material with solid elements and the modified Johnson-Cook constitutive relation. Numerical simulations incorporating the variations in material properties are capable of reproducing the experimental fracture patterns, albeit the predicted fragmentation velocities are too low.

Influence of Humidity on Yield Stress Determination by Slump Test of Slip-Prone Clayey Soils and Their Relation with the Chemical Properties

In this work, the yield stress evaluation as a function of water content for slip-prone clayey soils is studied in order to understand how yield stress decreases as water content increases, and their relation with the chemical properties. The clayey soil samples were taken from the region of Teziutlan-Puebla-Mexico. Yield stress was calculated using the slump test in cylindrical geometry. Results show three zones. The first one shows an exponential decrement on yield stress due to lower water content in accord with clayey soils with high content of illita, followed by a second region where yield stress decreases dramatically at a certain critical water concentration, and the third one where yield stress dependence is not well-defined since the clayey soil flow is seen. Finally, it is discussed how yield stress variation due to the water increment influences the landslide risk increment.

Deburring microparts using a magnetorheological fluid

It is difficult to deburr a micro-machined surface because the micro-features are easily damaged during the deburring processes. This paper proposes a new deburring process utilizing a magnetorheological fluid and applies it to the production of micromolds. A magnetorheological fluid is a functional fluid with a variable yield stress that is controlled by an external magnetic field. The proposed process utilizes two material removal mechanisms induced by the magnetorheological fluid flow. Extensive yielding of material is effective for deburring sheet-shaped burrs, and abrasive wear is effective for deburring protrusion-shaped burrs. A process model was developed to describe the effectiveness of each mechanism for specific burr geometries. The performance of the proposed method was verified experimentally. Metal burrs with a height of 200 μm and thickness of 1 μm were removed successfully with extensive yielding and abrasive wear. Burrs shorter than a few micrometers could be removed only by abrasive wear. The material removal behaviors of sheet-shaped and protrusion shaped burrs matched the proposed process model well.

Microstructure and hardness of Al2O3 nanoparticle reinforced Al–Mg composites fabricated by reactive wetting and stir mixing

The use of reactive wetting and stir mixing to disperse nano-sized ceramic particles in molten metal was studied using Al2O3 nanoparticle reinforced Al–Cu–Mg composites that were synthesized using gravity casting in a permanent mold and a single sample synthesized by squeeze casting. These experiments have shown that reactive wetting of nanoparticles of Al2O3 in Al–Cu–Mg alloys, combined with stir mixing, can result in significant improvements in the hardness of gravity cast specimens provided that a reaction between the reinforcement and the matrix was either on-going or recently completed at the time of solidification. The results indicate that microstructure and strengthening mechanisms depend on the degree of clustering after reaction completion. It is hypothesized that a small amount of clustering resulted in particle engulfment and Orowan strengthening and increased clustering resulted in particle pushing and grain refinement strengthening described by the Hall–Petch relation. TEM analysis of a squeeze cast sample showed that individual particles and small flocculi were present within the grains rather than at grain boundaries, suggesting that the higher solidification rate of squeeze casting should follow particle pushing models and result in engulfment of larger particles than gravity casting. A comparison of yield stress or hardness variation with the inverse of the square root of grain size or dendrite arm spacing clearly shows the dominant strengthening mechanism as a result of the different process parameters. This method is also used to compare the results of this study to those of other studies.

Local Elasto-Plastic Identification of the Behaviour of Friction Stir Welds with the Virtual Fields Method

Welding is one of the most popular joining technologies in industry. Depending on the materials to be joined, the geometry of the parts and the number of parts to be joined, there is a wide variety of methods that can be used. These joining techniques share a common feature: the material in the weld zone experiences different thermo-mechanical history, resulting in significant variations in material microstructure and spatial heterogeneity in mechanical properties. To optimize the joining process, or to refine the design of welded structures, it is necessary to identify the local mechanical properties within the different regions of the weld. The development of full-field kinematic measurements (digital image correlation, speckle interferometry, etc.) helps to shed a new light on this problem. The large amount of experimental information attained with these methods makes it possible to visualize the spatial distribution of strain on the specimen surface. Full-field kinematic measurements provide more information regarding the spatial variations in material behaviour. As a consequence, it is now possible to quantify the spatial variations in mechanical properties within the weld region through a properly constructed inverse analysis procedure. High speed tensile tests have been performed on FSW aluminium welds. The test was performed on an MTS machine at a cross-head speed of up to 76 mm/s. Displacement fields were measured across the specimen by coupling digital image correlation with a high-speed camera (Phantom V7.1) taking 1000 frames per second. Then, through the use of the virtual fields method it is possible to retrieve the mechanical parameters of the different areas of the weld from the strain field and the loading. The elastic parameters (Young’s modulus and Poisson’s ratio) are supposed to be constant through the weld. Their identification was carried out using the virtual fields method in elasticity using the data of the early stage of the experiment. Assuming that the mechanical properties (elastic and plastic) of the weld are constant through the thickness, the plastic parameters were identified on small sections through the specimen, using a simple linear hardening model. This method leads to a discrete identification of the evolution of the mechanical properties through the weld. It allows the understanding of the slight variations of yield stress and hardening due to the complexity of the welding process.

Microstructural characterization and mechanical properties of nanostructured AA1070 aluminum after equal channel angular extrusion

In the present research, equal channel angular extrusion (ECAE) of commercial purity aluminum (1070) was conducted using route B C. For ECAE processing a proper die set was designed and constructed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) analyses were used to evaluate the microstructure and texture of the extruded materials. The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to a rather equiaxed array of ultrafine grains and from 4 to 8 passes there is no strict change in the average grain size. The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 8 passes. Also, the variation of hardness and yield stress with number of extrusion was documented up to 8 passes. The present results showed that first ECAE pass has resulted in enhancement of mechanical properties more than four times over the annealed condition. Further ECAE processing has resulted in slight improvement. Based on two strengthening mechanisms, variations of the strength as a function of the pass numbers were related to the calculated dislocation densities and the average boundary spacing.

An investigation to the microstructural evolution of Fe–29Mn–5Al dual-phase twinning-induced plasticity steel through annealing

The potential effects of twinning induced plasticity have been taken into consideration to further improve the mechanical properties of advanced high-strength steels. Accordingly the high-Mn twinning-induced plasticity (TWIP) steels with austenite–ferrite dual-phase microstructure have been developed. In the present study, the influence of cold rolling and post-annealing treatments on the microstructural evolution and mechanical behavior of a group of dual-phase TWIP steel as a function of annealing time have been investigated. The mechanical behavior of processed materials has been examined through applying a set of low strain rate (0.001 s −1) compression tests at room temperature. The austenite recrystallization characteristics during various annealing conditions are explained through proper microstructural examinations. The results indicate that as the recrystallization proceeds with annealing time the related yield stress deceases. The rapid drop of yield stress in short annealing periods is related to the onset of recrystallization. The yield stress variation diminishes as the annealing duration increases. This is attributed to the formation of austenite side-plates which may balance the softening effects of restoration processes.

A DNS Investigation of the Effect of Yield Stress for Turbulent Non-Newtonian Suspension Flow in Open Channels

The use of fixed-shape open channels in industrial processes is common in the mineral processing industry. With lack of fundamental understanding about the mechanisms involved in how a turbulent flow of a non-Newtonian carrier fluid transports suspension particles, direct numerical simulation may come into the research as a validation tool. Direct numerical simulation (DNS) of the turbulent flow of non-Newtonian fluids in an open channel is modeled using a spectral element-Fourier method. The simulation of a yield-pseudoplastic fluid using the Herschel-Bulkley model agrees qualitatively with experimental results from field measurements of mineral tailing slurries. The simulation results over-predict the flow velocity by approximately 40% for the cases considered, however, the source of the discrepancy is difficult to ascertain. The effect of variation in yield stress, flow behavior index, and assumed flow depth are investigated and used to assess the sensitivity of the flow to these physical parameters. This methodology is seen to be useful in designing and optimizing the transport of slurries in open channels.

MICROSCOPIC MECHANISM OF THE GIANT ELECTRORHEOLOGICAL EFFECT

Electrorheological (ER) fluids are a type of colloids whose rheological characteristics can be varied reversibly with the application of an electric field. The traditional ER mechanism is based on induced polarization arising from the dielectric constant contrast between the suspended solid particles and the fluid. However, for induced polarization there is a maximum value for the dimensionless electric susceptibility χ ∼ 0.24. That implies an upper bound for the traditional ER effect. For permanent molecular dipoles, χ can be as high as 4–50. Thus, one to two orders of magnitude higher electric energy can be gained by harnessing the molecular dipoles. Indeed, the recent discovery of the giant electrorheological (GER) effect, in nanoparticles of urea-coated barium titanate oxalate suspended in silicone oil, has shown that the theoretical upper bound of the traditional ER effect is no longer applicable to this new type of electrorheological fluid. A phenomenological model of the GER mechanism, based on aligned molecular dipoles of urea molecules in the contact regions of the nanoparticles, yields an adequate account of the observed effect but without a microscopic picture on how this can occur. More recently, by using molecular dynamics (MD) to simulate the urea-silicone oil mixture confined between two bounding surfaces of a nanocontact, we find that the urea molecules, which has a molecular dipole moment of 4.6 Debye, can form aligned dipolar filaments that penetrate the oil film to bridge the two substrates, with the attendant lowering of the aligning field for the urea dipoles. This phenomenon is explainable on the basis of a 3D to 1D crossover in urea molecules' microgeometry, realized through the confinement effect provided by the oil chains. The resulting electrical energy density is shown to give an excellent account of the observed yield stress variation as a function of the electric field.

Yield Stress of 21-6-9 Stainless Steel Over Very Wide Ranges of Strain Rates and Temperatures

The yield strength of solution-annealed 21-6-9 austenitic stainless steel was determined over a wider temperature range (−195 to 1100 °C) and strain rate (10−4 to 104 s−1) than has been previously reported. The most noteworthy characteristic of the variation of yield stress with temperature was the dramatic decrease in yield strength from −195 to 300 °C. The strain-rate sensitivity exponent, m, was determined using strain-rate change tests. m dramatically increases at about 850 °C with increasing temperature and m is approximately independent of strain (structure). Hopkinson split-bar tests from ambient temperature to 750 °C indicate that the strain-rate sensitivity of 21-6-9 is not strongly influenced by the over eight orders of magnitude change in strain rate. This suggests that the mechanism(s) of plastic flow at the higher rates is similar to that at lower rates. This contention was corroborated by transmission electron microscopy. The yield stress shows grain-size dependency.