Variation Of Yield Stress Research Articles

The effect of defects on the mechanical response of Ti-6Al-4V cubic lattice structures fabricated by electron beam melting

Electron Beam Melting (EBM) as a means of Additive Manufacturing (AM), is of interest for the fabrication of intricate geometries for cellular materials in areas where complex architectures are needed, e.g. biomedical implants. Most studies have focused on specific geometries and so the effect of the structure on mechanical performance is not well understood. Many kinds of micro- and macro-scale defects can arise in additively manufactured components, so assessment of their influence on properties is needed. In this work, lattices of Ti-6Al-4V having a cubic structure have been manufactured by EBM, and the effect of heat treatments above and below the β-transus temperature on microstructure and compression response have been investigated. The former modifies only slightly the α+β structure and mechanical performance whereas the latter leads to coarse alternating α and β lamellae packets and α at the prior grain boundaries with a 10% loss in yield strength. The variation in the compressive yield stress with strut diameter is in good accord with simple models based on compressive deformation rather than shearing or buckling. Internal pores for struts aligned with the build direction are found around the edges of the solid form, in regions which seem to be associated with the EB scan pattern. Struts normal to the build direction show more significant defects but their redundancy means that they do not compromise the compressive performance in the build direction. Using a particle size in the range 45–100 μm minimum weld-track sizes were experimentally and numerically identified to be 176 and 148 μm in depth respectively with a depth-to-width ratio of 0.55. This produced a beam pass of the order of 300 μm oversizing small features (struts of 0.4 and 0.6 mm nominal diameter) when a contour around the strut periphery was applied.

Disposing waste latex paints in cement-based materials – Effect on flow and rheological properties

The effect of waste latex paints (WLPs) on fresh and hardened properties of cement-based materials is not well understood. In this project, WLPs were not randomly collected from waste collection sites; rather produced to ensure full traceability of composition, then stored in sealed or opened pail conditions to get expired. Test results have shown that rheological properties of cement pastes are directly affected by WLP constituents, substitution rates, and storage conditions. A series of charts were proposed to predict the variations in flow, yield stress, and plastic viscosity of pastes containing 4%, 8%, 12%, and 16% substitution rates of mixing water by WLPs. The indicative measures used for prediction included the gloss unit of paints and their pigment volume concentrations. The compressive strength slightly increased at relatively low WLP rates, but then dropped sharply at higher substitutions.

Effect of particle morphological parameters on sand grains packing properties and rheology of model mortars

In this paper, we study the rheological behavior of mixtures of various sand particles suspended in model laboratory yield stress fluids. Using image analysis, we assess the morphology of the studied sand particles. We then measure the packing properties of these particles and show that, as a first approximation, the overall shape of the particles (i.e. the aspect ratio) is the dominant morphological parameter conditioning packing. We finally use magnetic resonance imaging (MRI) to assess the rheological behavior of suspensions of these sands in a water-in-oil emulsion and show that both the yield stress and consistency diverge for the same critical volume fraction, which seems to be fully correlated to both the random dense packing fraction and loose packing fraction of the grains. We finally suggest that there exists a correlation between a variation in yield stress and a variation in viscosity due to a change in the particle morphology.

Water filling and electric field-induced enhancement in the mechanical property of carbon nanotubes.

The effects of water filling and electric field on the mechanical property of carbon nanotubes (CNTs) are investigated with molecular dynamics simulations. The simulation results indicate that the water filling and electric field could enhance the elastic modulus but reduce the Poisson’s ratio of the CNTs. As for the buckling behaviors, a significant enhancement could be observed in the yield stress and average post-buckling stress of the CNTs. In particular, the enhancement in the yield stress induced by the water filling and electric field could be even higher than that resulted from the solid filling. Moreover, a transition mechanism from the rod instability to shell buckling is shown to explain the nonmonotonic variation of yield stress, and the critical diameter can be tuned through filling the water molecules and applying the electric field. The present findings provide a valuable route for the optimized design and application of the nanoscale functional devices based on the water-filled CNTs.

Modeling of structural hardening in oxide dispersion-strengthened (ODS) ferritic alloys

Based on a rather simple macroscopic and statistical model, experimentally observed variations of yield stress at room temperature in various ODS alloys were theoretically reproduced. For the first time, yield stress values of ODS steels were calculated by taking into account: (1) two interaction mechanisms between dislocations and nanoprecipitates (shearing or bypassing, simultaneously, depending on the particle size); and (2) the whole, possibly multimodal, nanoparticle distributions experimentally determined by SANS. The relative importances of the various strengthening mechanisms can be easily deduced from these calculations.

Yield strength development from high strength steel plate to UOE pipe

The yield strength of the UOE pipe is different from raw steel plate and inhomogeneously distributed on pipe body, because of the Bauschinger effect and work hardening during UOE forming process. The yield strength development of high-strength pipeline steels from raw plate to UOE pipe is investigated by using a modified Chaboche hardening model that incorporates the Bauschinger effect and yield plateau of X70, X80 and X90 steels. The final yield strength of pipe is evaluated by finite element simulation of uniaxial tension of a pipe specimen and an empirical equation. The influences of UOE process parameters on yield strength development are discussed. The results show the modified hardening model can predict the yield stress with good accuracy. The circumferential variation of yield stresses for X80 and X90 pipes are about two times more than that for X70 pipe due to their stronger Bauschinger effect. In UOE forming, C-, U- and O-forming reduce tensile yield stress, and expansion increases and homogenizes yield stress. Fattening operation reduces the measured yield stress, which should be considered in expansion ratio design especially for high strength steels. The yield strength increases logarithmically with t/D ratio and linearly with expansion ratio.

Critical shear stress for mass erosion of organic-rich fine sediments

In shallow lakes of Florida laden with low-strength organic-rich sediments, wind-induced water movement is believed to actuate bed surface erosion as well as mass erosion. Experiments in hydraulic flumes to measure the critical shear stress for mass erosion tend to be lengthy and require large quantities of sediment. For bottom sediment from Lake Okeechobee at naturally occurring values of the floc volume fraction, a comparison of the viscoplastic yield stress, readily obtained from rheometry, with the mass erosion critical stress from flume tests indicates that it may be permissible to consider the yield stress as a surrogate for the critical stress. This inference appears to be supported by ancillary observations from Lake Apopka and Newnans Lake. Interestingly enough, the variation of yield stress with the floc volume fraction of the organic-rich bed is found to conform to fractal characterization commonly invoked for mineral sediment flocs, consistent with a representative constant value of 2.55 of the fractal dimension. Pending fuller investigations with a wide range of organic-rich sediments, recourse to rheometry in lieu of flume experiments holds promise as a means to simplify testing requirements for estimating the mass erosion critical stress.

Implementation of bond-slip effects on behaviour of slabs in structures

Employing discrete elements for considering bond-slip effects in reinforced concrete structures is very time consuming. In this study, a new modified embedded element method is used to consider the bond-slip phenomenon in structural behavior of reinforced concrete structures. A comprehensive parametric study of RC slabs is performed to determine influence of different variables on structural behavior. The parametric study includes a set of simple models accompanied with complex models such as multi-storey buildings. The procedure includes the decrease in the effective stiffness of steel bar in the layered model. Validation of the proposed model with existing experimental results demonstrates that the model is capable of considering the bond-slip effects in embedded elements. Results demonstrate the significant effect of bond-slip on total behavior of structural members. Concrete characteristic strengths, steel yield stress, bar diameter, concrete coverage and reinforcement ratios are the parameters considered in the parametric study. Results revealed that the overall behavior of slab is significantly affected by bar diameter compared with other parameters. Variation of steel yield stress has insignificant impact in static response of RC slabs; however, its effect in cyclic behavior is important.

Determination of high-temperature elastoplastic properties of welded joints by indentation test

The stress and strain fields formed in the heat-affected zone (HAZ) of welds in pipes in thermal power plants has recently attracted attention as a key plant management issue. The microstructure in the HAZ changes locally as a result of the thermal history imposed by the welding procedure; these changes are manifested in the form of a stress–strain response different from that of the base metal. In the present study, a method based on indentation tests is proposed to estimate the variation of yield stress over the HAZ. The well-known indentation tests to infer the stress–strain curve from indentation load and penetration depth measurements are familiar. However, there are difficulties in applying this approach at high temperatures. An alternative procedure based on impression size is proposed and applied to evaluate the variation of mechanical properties of Grade 122 steel over the HAZ in a welded part at 873 K.

Using the flume test for yield stress measurement of thickened tailings

In thickened tailings disposal operations, the yield stress, which is a unique physical property of thickened tailings, is a key design parameter as it has a controlling influence on the final slope of deposited tailings. A quick but rough estimate of the yield stress is typically obtained using a mini-slump test in industry. This paper explores the feasibility of using a laboratory flume test to measure the yield stress of thickened tailings.The model of slow sheet flow (SSF) which has previously been used to model flume tests and the Fourie and Gawu (FG) model, which was developed for interpretation of flume tests on thickened tailings, are compared. The SSF model, derived within the framework of long-wave approximation, is shown to only hold for flumes with frictionless sidewalls (or very wide flumes), whereas the FG model is valid for flumes of finite width and nonslip sidewalls. These findings were confirmed using CFD simulations of laboratory flume tests with nonslip and free-slip sidewalls on materials with yield stresses ranging from 20 to 60Pa. Simulations to investigate the sensitivity of the final beach profile in the flume test to variations of yield stress and viscosity were performed. The results suggest that the final profile is very sensitive to yield stress variation but relatively insensitive to viscosity variation. This relative insensitivity to viscosity further justifies the use of the FG model for evaluation of yield stress from flume test data, as this model ignores the effect of viscosity.Simulations of mini-slump tests were conducted to demonstrate that different mould lifting velocities may introduce different inertial effects, thereby impacting the final profile and hence the yield stress extrapolated from slump tests. Moreover, comparison between the profiles predicted by several theoretical models for slump tests and CFD simulation results revealed that the existing models are not capable of capturing the final shape of the slumped material, which is invariably distorted by the mould friction to some extent. Consequently, the accuracy of the yield stress extrapolated from mini-slump tests is not high. The small errors in yield stresses calculated from the CFD simulation results using the FG model suggest that yield stresses may be determined from flume tests with very high accuracy using the FG model.

Temperature-Induced Collapse, and Arrested Collapse, of Anisotropic Endoskeleton Droplets.

Micron-scale rod-shaped droplets with a range of aspect ratios are produced using extrusion of oil containing a soft wax crystal network to permit shape customization. A physical model of the droplet shape stability is developed based on balancing interfacial stresses with the internal crystal network yield stress. The model predicts the mechanical properties required for particular droplet size stability, in a given physicochemical environment, and is tested by microscopic observations of droplets over a range of relevant applied temperatures. The time-dependent response to temperature of individual rods is monitored and used to identify the collapse temperature based on structural yielding. Precise temperature control allows variation of the droplet endoskeleton yield stress and direct determination of the droplet stability as a function of size, by observing the onset of collapse by interfacial compression, and enables validation of the model predictions. Mapping the regions of droplet stability and instability for various-sized droplets yields a basis for designing droplet shapes for multiple applications using easily measured physical variables. The phenomenon of arrested collapse is also explored as a means of transforming simple rod-shaped starting materials into more complex shapes and enhancing adhesion to targeted solid surfaces, enabling exploitation of the hybrid solid-liquid nature of these droplets.

The Effect of Consistency and Freeness on the Yield Stress of Chemical Pulp Fibre Suspensions

To study the influence of mechanical treatments on the yield stress of chemical pulp suspensions, a traditional rheometer, coupled with local velocity measurements (ultrasonic Doppler velocimetry), was used to measure the yield stress of two types of commercial chemical pulp suspensions with different freeness values at mass concentrations (consistencies) ranging from 0.5 to 1.5%. Over the range of consistencies tested, the yield stress was found to depend on the consistency through a power law relationship for all tested samples. Moreover, the results showed that as the freeness decreased, the yield stress of hardwood suspensions increased to a maximum value then decreased. This variation in yield stress was also observed in softwood suspensions with mass concentrations above 1%. However, when the consistency was lower than 0.75%, the yield stress of softwood suspensions increased with decreasing freeness.This behaviour can be understood based on the underlying fibre properties of fibrillation, curl, and stiffness, suggesting that fibre morphology plays a significant role on the yield stress of pulp suspensions over the concentration range studied.

An interpolation-type orthotropic yield function and its application under biaxial tension

Most previous work in anisotropic yield criteria has focused on describing the yield loci using a continuous function, which is ideal when applying to materials with limited anisotropy. But in practice, many widely used metals such as aluminum alloys and magnesium alloys show strong features in different crystallographic directions, which cause increasing undetermined coefficients and more complex functional forms. Furthermore, with development of new materials, existing yield criteria shall meet challenges in feasibility because a continuous function is expected to predict only a few kinds of yield surface shapes. To extend the anisotropic yield criteria family, an interpolation-type orthotropic yield function for plane stress is proposed in this paper, in which the physical meaning of coefficients is directly defined. The proposed interpolation-type orthotropic yield function reproduces with great accuracy the anisotropic behaviors under biaxial tension and uniaxial tension/compression, including but not limited to the varying yield stress and R-coefficients with loading direction and tension/compression asymmetry. Moreover, comparing the predictions of the yield loci by the interpolation-type orthotropic yield function with those by classical yield functions, it is shown that the proposed function gives the best agreement with experimental data of FCC, BCC and HCP sheet metal samples.

Concentration dependence of yield stress and dynamic moduli of kaolinite suspensions.

The concentration dependence of yield stress and dynamic moduli of kaolinite suspensions is studied. Complex electrostatic interactions between kaolinite platelets promote a more liquid-like behavior, with clay particles changing from attractive interactions to a face-face repulsive interaction as the mass fraction of clay particles increases. A yield stress model is developed based on the repulsive interaction between disk-shaped particles, which yields a good prediction of experimental observations when repulsive face-face interaction is dominant. The critical concentration when attractive interaction changes completely to face-face repulsive interaction is estimated from the theory. Four regions are identified in the variation of yield stress as a function of the concentration.

Assessment of Rheology Variations of Cement Pastes Containing Clinker Grinding Aids Compliant With ASTM C465

Abstract ASTM C465 (Standard Specification for Processing Additions for Use in the Manufacture of Hydraulic Cements, ASTM International, West Conshohocken, PA, 2010,www.astm.org) is widely used in the cement industry to evaluate variability of cement properties as a result of the addition of grinding aids (GAs). The standard specifies a series of physical and chemical tests to determine whether such additions detrimentally affect cement properties; however, it refrains from considering their effects on rheological properties. This paper seeks to assess the variations in yield stress (τ0) and viscosity (η) of cement pastes containing GAs compliant with ASTM C465. Three series of cement mixtures were ground for different Blaine fineness of 2880, 3460, and 4420 ± 100 cm2/g using various concentrations and types of GAs. Regardless of Blaine and GA type, the addition of increased GA concentrations leads to improved flowability together with reduced τ0 and η responses. For dosage rates satisfactory fulfilling ASTM C465, the variations of such responses with respect to control values obtained using cement ground without GA were found to reach −25 %, −30 %, and −40 % when clinker is ground with amine-, glycol-, and phenol-based GAs, respectively. The variations in τ0 and η are independent of the water-to-cement ratio, but tend to increase for higher GA concentration and increased cement fineness. Four threshold regions corresponding to −10 %, −20 %, −30 %, and −40 % were suggested to evaluate adaptability of ASTM C465 to reflect changes in rheological properties.

Characterization of Aging Behavior of AA6061 Aluminum Alloy Through Destructive and Ultrasonic Non-destructive Testing Techniques

An experiment was conducted to characterize microstructural and mechanical behavior of AA6061 alloy during an artificial aging treatment through destructive and ultrasonic nondestructive techniques. Identical aluminum plates of AA6061-T6 aluminum alloy were solutionized at ≈800 K, followed by quenching and artificial aging at 493 K up to 8 h. The microstructural evolution was analyzed using scanning electron microscopy. The variations in hardness, yield stress, and ultimate tensile strength with respect to aging time were discussed. Besides, ultrasonic parameters velocity and attenuation were measured and correlated with the microstructural evolution as well as certain mechanical properties. The ultrasonic measurements showed a good correlation with the microstructural features and mechanical properties of the alloy during aging, whereby an excellent correlation was found between the longitudinal wave velocity and strength properties of the aged specimens. The longitudinal wave velocity was found to be directly related to the hardness with a correlation coefficient of 0.99.

Preparation, Property Tests, and Limited Chain Model of Magnetorheological Fluid

AbstractMagnetorheological (MR) fluids are a new type of smart material whose performance can be evaluated by considering three important factors: stability, viscosity, and shear yield stress of the fluids. In this study, to produce fine, antisettling, and well-dispersed MR fluids, appropriate components and preparation techniques were adopted and a surface treatment method was developed involving different surfactants. Property tests of the MR fluids were conducted, and the results demonstrated that the proposed preparation method decelerated the sedimentation of MR fluids, that the viscosity of the MR fluids varied with temperature and shear rate, and that liquid paraffin can reduce the viscosity of the MR fluids more gradually than oleic acid. In addition, variations in shear yield stress with magnetic induction intensity and volume fraction were determined. To accurately describe the shear yield stress of the MR fluids, a limited chain model that considers the spatial distribution of carbonyl iron (CI...

Development of new cyclic plasticity model for 304LN stainless steel through simulation and experimental investigation

Cyclic plastic deformation characteristics of 304LN stainless steel material have been studied with two proposed cyclic plasticity models. Model MM-I has been proposed to improve the simulation of ratcheting phenomenon and model MM-II has the capability to simulate both cyclic hardening and softening characteristics of the material at various strain ranges. In the present paper, strain controlled simulations are performed with constant, increasing and decreasing strain amplitudes to verify the influences of loading schemes on cyclic plasticity behaviors through simulations and experiments. It is observed that the material 304LN exhibits non Masing characteristics under cyclic plastic deformation. The measured deviation from Masing is well established from the simulation as well as from experiment. Simulation result shows that the assumption of only isotropic hardening is unable to explain the hardening or softening characteristics of the material in low cycle fatigue test. The introduction of memory stress based cyclic hardening coefficient and an exponentially varying ratcheting parameter in the recall term of kinematic hardening rule, have resulted in exceptional improvement in the ratcheting simulation with the proposed model, MM-II. Plastic energy, shape and size of the hysteresis loops are additionally used to verify the nature of cyclic plasticity deformations. Ratcheting test and simulation have been performed to estimate the accumulated plastic strain with different mean and amplitude stresses. In the proposed model MM-I, a new proposition is incorporated for yield stress variation based on the memory stress of loading history along with the evolution of ratcheting parameter with an exponential function of plastic strain. These formulations lead to better realization of ratcheting rate in the transient cycles for all loading schemes. Effect of mean stress on the plastic energy is examined by the simulation model, MM-I. Finally, the micro structural investigation from transmission electronic microscopy is used to correlate the macroscopic and microscopic non Masing behavior of the material.

Zone wise local characterization of welds using digital image correlation technique

The process of welding is associated with high and varying thermal gradients across the weld, resulting in inhomogeneous material properties surrounding the weldment. A proper understanding of the varying mechanical properties of the weld and surrounding materials is important in designing and modelling of components with weld. In the present study the characterization of different zones such as fusion zone, heat affected zones and unaffected base material of a deposited weld is carried out using digital image correlation (DIC) technique. A methodology using the micrographic observation and image processing is proposed for accurate identification of various weld zones. The response of welded samples in the elastic and plastic region is compared with the virgin sample. Full range stress–strain curves are obtained for each zone using the whole field strain measurement involving DIC. The parameters investigated are Young׳s modulus, Poisson׳s ratio, yield stress, strain hardening exponent and strength coefficient. A study regarding the variation of properties with respect to varying weld currents of 100A, 130A and 150A is carried out. The Vickers microhardness measurement is also conducted to obtain the variation in hardness across weldment. Fusion zone of all the welded samples have reported lower Young׳s modulus and higher yield strength compared to virgin samples. The Vickers hardness values obtained for fusion and heat affected zones are in line with the yield stress variation obtained zone wise.

A SPH model for the simulation of free surface granular flows in a dense regime

Smoothed Particle Hydrodynamics (SPH) is a particle-based Lagrangian numerical method, particularly suitable for modeling free surface flows of fluids with different constitutive equations. In this work, a SPH model is developed to solve flow problems of granular matter flowing in a dense regime, featuring a constitutive equation proposed by Pouliquen et al. (2006) [17] and Jop et al. (2006) [18].The problem of resolving the flow close to static conditions has been solved by introducing a weak compressibility for the grains: this leads to a third order algebraic equations to be solved for computing granular material density. A new viscosity regularisation method, suitable for any visco-plastic fluid with varying yield stress has been developed, in order to reproduce stopping flow and static conditions.Validation of the model has been performed by solving the simple uniform granular flow problem, for which the solution is known analytically and by reproducing a set of experimental data on granular columns collapses by Lube et al. (2004) [19].