Decrease In Yield Stress Research Articles

Prediction of notched strength for cylindrical composites pipes under tensile loading conditions

Abstract In this paper, an experimental study has been performed on thick, ±55° filament wound glass/epoxy tubes using quasi-static tests to examine their offshore applications. A new design for the experimental test is developed in which the end tabs and fixture system are made in a way that the stress concentration at the edges of the composite pipe can be reduced. Split disk uniaxial tensile test has been performed on composite pipes with 86 mm of the internal diameter and the thickness of 6.2 mm, in order to evaluate the mechanical properties and to quantify the effect of geometric non-linearities. These pipes can be subjected to different phenomena which can result in introducing a defect in the structure. Two types of pipe specimens are tested which include the composite with notch and the other without notches. The objective of the using a notched specimen is to examine the evolution of the damage and to study the mechanical properties as a function of this damage. The experimental results are described as the influence of the number and the size of notches on the mechanical behavior. These results have shown that the yield stress decreases and yield strain increase with increasing of the notch size and number. The evolution of damage showed that the presence of notches plays an unfavorable role in the integrity of the structure.

Effects of fumed silica weight fraction on rheological properties of magnetorheological polishing fluids

Magnetorheological (MR) polishing fluids with excellent rheological characteristics are efficient in material removal of workpiece. In this study, fumed silica (FS) was used as thixotropic agent in MR polishing fluid with ten different weight fractions to find the proper additive amount for good rheological and sedimentation properties. The rheological behavior of samples at several additive concentrations was determined by examining flow properties using a rotational rheometer connected to an external magneto-cell. Experimental results showed that, by adding FS to MR polishing fluids, apparent viscosity and yield stress exhibited an obvious increase, while the shear-thinning index decreased sharply. Model fitting showed that all the MR polishing fluids exhibited shear-thinning behavior and followed the Herschel-Bulkley model. Further investigations suggested that 0.5–0.6-wt% FS was the most promising additive proportion for proper rheological and sedimentation properties, while a further increase in FS weight fraction greatly increased apparent viscosity and decreased yield stress undesirably.

Improvement of room and high temperature tensile properties of NiAl-strengthened ferritic heat-resistant steels through Mo addition

In order to improve the room and high temperature strength–ductility balance of ferritic Fe-Al-Ni based alloys strengthened by the B2-type NiAl precipitates, Fe-Al-Ni-Cr-Mo alloys was developed. The microstructures and tensile properties of the alloys were examined focusing on Mo content. We found that the 2 at% or 4 at% Mo addition is effective in the improvement of the elongation of the Fe-Al-Ni based alloys at room temperature (RT) while maintaining high strength, which reflects the suppression of intergranular fracture by the strengthening of the bcc matrix by Mo addition. We also found that the RT tensile properties and deformation behavior of the alloys depend strongly on the volume fraction and the size of the NiAl precipitates. For instance, the yield stress decreases by the change in the primary slip direction from <111> to <001> and the formation of the Orowan loops with increasing the volume fraction and the mean diameter of the NiAl precipitates. In addition, the alloy with the appropriate Mo content exhibits a high yield stress above 660 MPa up to 923 K, owing to the solid-solution hardening by Mo addition and the low growth rate of the NiAl precipitates even at high temperatures. These findings indicate that the alloys have a great potential for steam turbines of advanced ultra‐supercritical thermal power plants.

Effect of total suspended solids and various treatment on rheological characteristics of municipal sludge

Rheological characteristics play an important role in the design and optimization of sludge treatment processes that are dependent on the total suspended solids (TSS). The present study systematically investigated the impact of various treatment processes on sludge rheology. The effect of TSS on rheological properties was comprehensively explored. Rheological tests were performed at (20 ± 0.1) °C, physicochemical parameters of the sludge were measured for compositional analysis. The composition of sludge is altered when subjected to both heat treatment and AD. All sludges exhibit shear-thinning behavior, the results of a t test (with significance level of 0.05) indicated that the Herschel–Bulkley model provides a valid description of sludge flow behaviors. The anaerobically digested sludge (ADS) is the least viscous while thermal hydrolyzed sludge (THS) shows slightly higher viscosity than that of fresh mixed sludge (FMS) at high shear rates. The yield stress and thixotropy decrease gradually as the treatment proceeds. Both infinite viscosity and yield stress of THS increases dramatically when the TSS exceeds 7.30%, hence 7.30% may serve as a critical level for THS to preliminarily determine whether it is economically viable to implement a thermal pretreatment. The TSS threshold for ADS is determined to be 7.12% given the sharp increase in infinite viscosity and yield stress, which are undesirable for dewatering. The thixotropy increases with increasing TSS, and TSS only has a slight influence on the thixotropic properties of THS relative to FMS, which demonstrates the advantage of thermal pretreatment for the purpose of achieving high solid sludge AD.

Measurements of the effects of pure and salt water absorption on the rate-dependent response of an epoxy matrix

Abstract The study reports the measured effects of water absorption on an epoxy resin. Epoxy samples were exposed to wet conditioning environments including pure water, NaCl-water solution, and pure water at boiling temperature, measuring absorption as a function of time. Vickers hardness and indentation creep tests were performed and the mechanical response of the material to uniaxial stress was also measured in both compression and tension, at imposed strain rates in the range 0.001–2500 s−1. It was found that the absorption of both pure and salt water caused decrease of stiffness, yield stress and hardness, but only mildly affected the sensitivity of the response to the imposed strain rate and the tensile ductility. Mechanical testing after re-drying of the samples revealed the permanent effects of water absorption.

Influence of primary α-phase volume fraction on the mechanical properties of Ti-6Al-4V alloy at different strain rates and temperatures

Bimodal microstructures with primary α-phase volume fractions ranging from 14.3% to 57.1% were gained in Ti-6Al-4V (Ti-64) alloy through annealed in two-phase region at various temperatures below the β-transus point. Then the influence of the primary α-phase volume fraction on the mechanical properties of Ti-64 were studied. The results show that, at room temperature and a strain rate of 10−3 s−1, the yield stress decreases but the fracture strain augments with added primary α-phase volume fraction. The equiaxed primary α-phase possesses stronger ability to coordinate plastic deformation, leading to the improvement of the ductile as well as degradation of the strength of Ti-64 with higher primary α-phase volume fraction. As the temperature goes up to 473 K, the quasi-static yield stress and ultimate strength decrease first and then increase with the incremental primary α-phase volume fraction, due to the interaction between the work hardening and the softening caused by the DRX and the growth of the primary α-phase. At room temperature and a strain rate of 3×103 s−1, the varying pattern of strength with the primary α-phase volume fraction resembles that at a quasi-static strain rate. However, the flow stress significantly increases but the strain-hardening rate decreases compared to those at quasi-static strain rate due to the competition between the strain rate hardening and the thermal softening during dynamic compression process.

Measurement of Local Mechanical Properties of T91 Steel Corroded by Molten Lead-Bismuth Eutectic Alloy via Micropillar Compression Test

ABSTRACTT91 ferritic/martensitic (F/M) steel is an expected structural material candidate for Gen IV liquid lead-bismuth cooled nuclear reactors. However, molten lead-bismuth eutectic alloy (mLBE) often causes liquid-metal embrittlement (LME) of F/M steels. Although prior austenite grain boundaries and martensite block boundaries were reported to be preferential sites for LME, the mechanism of LME in a T91/LBE couple is yet to be comprehensively understood. In this paper, the effect of mLBE on T91 steel was investigated using micropillar compression tests. mLBE corrosion was found to cause a significant decrease in yield stress. The micropillar made from mLBE-corroded specimen was significantly sheared along high angle boundaries. In addition, EDS analysis demonstrated the presence of Pb/Bi at these high angle boundaries.

The investigation of multi-walled carbon nanotubes dispersion and its influence on rheological properties of cement systems

The current research is devoted to the studying of multi-walled carbon nanotubes (MWCNT) dispersion and its influence on the rheological properties of cement pastes. The pH and electrical conductivity test, zeta potential measurements, particle size analysis were performed for characterization of MWCNT dispersion. The MWCNT dispersion were tested during 1 month and presented the decrease of pH level from 8.89 to 8.02, increase of electrical conductivity from 2.00 to 2.75 mS/cm. The particle size analysis did not reveal the significant changes during 1 month of storage. The rheological test of nanomodified cement pastes were performed for concentration of MWCNT dispersion in the range from 0 to 0.5% by weight of cement. The decrease of yield stress by 30.7% and increase of plastic viscosity by 29.6% were obtained for cement paste modified by MWCNT dispersion in dosage of 0.25% by weight of cement. The changes in flow behaviour were established for cement pastes tested in 5 min after mixing and were not observed for cement pastes tested after 120 min of mixing.

Rheological Properties of Cement Pastes with Multiwalled Carbon Nanotubes

The evaluation of rheological properties of cement systems is getting more relevant with growing interest to self‐consolidating concrete (SCC), high‐performance concrete (HPC) and ultrahigh‐performance concrete (UHPC). The rheology models are a perspective tool to predict and manage the properties of cement systems in the fresh and hardened state. The current research is focused on the rheological test of cement systems modified by multiwalled carbon nanotubes (MWCNT) dispersion with and without polycarboxylate ether (PCE). The content of dispersion with 1% concentration of MWCNT in cement pastes varied from 0.125 to 0.5% by weight of cement. The dosage of PCE was taken as 0.6% by weight of cement. The cement pastes were prepared based on Portland cement without mineral additives. The rheological test was carried out at 5, 30, 60, and 120 min after mixing of cement paste. The rheological test established that modification of cement pastes by MWCNT dispersion in dosage 0.25% leads to the decrease of yield stress by 30.7% and increase of plastic viscosity by 29.6%. The combined modification by PCE and MWCNT dispersion shows the decrease in plastic viscosity of cement pastes by 9.90% in dosage of MWCNT equal to 0.5% by weight of cement, reduction of water demand by 20% for the same workability, and decrease of yield stress till 0 Pa. It gives the ability to obtain the self‐compacting mixtures. The cement pastes with and without MWCNT dispersion revealed the shear‐thinning behavior during 120 min after mixing. The modification of cement pastes by PCE with and without MWCNT dispersion showed the shear‐thickening behavior which remains during 120 min after mixing.

Characterization of interfacial morphology of low temperature, low pressure Au–Au thermocompression bonding

Au–Au thermocompression bonding is a versatile technique of high interest for a variety of applications. We have investigated Au–Au bonding using sputter deposited Au films under conditions of low temperature (150–250 °C) and low bonding pressure (∼3 MPa) for short times (15 min). The combination of low temperature and short times is important for applications involving both hermetic sealing and packaging scaling. The initial surface roughness of the Au film was in the 3–5 nm range with peak-to-valley heights of 20–30 nm and a lateral correlation length of ∼400 nm. For samples bonded at 150 °C, the void morphology at the bonded interface was related to the initial surface roughness. The void morphology was different when bonding at the higher temperatures: the void length (along the bonded interface) decreased significantly but the void height (perpendicular to the interface) increased. These results can be understood in terms of a combination of increased surface Au diffusivity and decreased yield stress and elastic modulus with increased bonding temperature.

Optimization of Thermo-Mechanical Processing for Forging of Newly Developed Creep-Resistant Magnesium Alloy ABaX633

The compressive strength and creep resistance of cast Mg-6Al-3Ba-3Ca (ABaX633) alloy has been measured in the temperature range of 25 to 250 °C, and compared with that of its predecessor ABaX422. The alloy is stronger and more creep-resistant than ABaX422, and exhibits only a small decrease of yield stress with temperature. The higher strength of ABaX633 is attributed to a larger volume fraction of intermetallic particles (Al, Mg)2Ca and Mg21Al3Ba2 in its microstructure. Hot deformation mechanisms in ABaX633 have been characterized by developing a processing map in the temperature and strain rate ranges of 300 to 500 °C and 0.0003 to 10 s−1. The processing map exhibits two workability domains in the temperature and strain rate ranges of: (1) 380 to 475 °C and 0.0003 to 0.003 s−1, and (2) 480–500 °C and 0.003 to 0.5 s−1. The apparent activation energy values estimated in the above two domains (204 and 216 kJ/mol) are higher than that for lattice self-diffusion of Mg, which is attributed to the large back-stress that is caused by the intermetallic particles. Optimum condition for bulk working is 500 °C and 0.01 s−1 at which hot workability will be maximum. Flow instability is exhibited at lower temperatures and higher strain rates, as well as at higher temperatures and higher strain rates. The predictions of the processing map on the workability domains, as well as the instability regimes are fully validated by the forging of a rib-web (cup) shaped component under optimized conditions.

A Mathematical Formulation for Calculating Temperature Dependent Friction Coefficient Values: Application in Friction Stir Welding (FSW)

High rotational motion from the welding tool generates a significant amount of the heat during friction stir welding (FSW). Basically, during FSW the heat is mostly coming from the frictional force between the tool shoulder and the plates. Therefore, a precise calculation of the friction coefficient can increase the accuracy of the finite element analysis (FEA) of the process. However, researchers have applied constant values, and that causes a gap between the reality and the simulated model especially after the welding plunging step. In this study, a mathematical formulation is proposed in order to calculate the temperature dependent values of the friction coefficient and also to explore the influence of the temperature in the friction coefficient. To solve the governing equations of the process, the MATLAB®software is used. The results indicate that, from 25°C to the AA 6061-T6 melting point (580°C), the values of the friction coefficient fall steadily in a range of 0.207089 to 0.000582. Furthermore, the material shear stress and the material yield stress decrease consistently as the temperature rises. Consequently, the influence of the temperature in the contact input parameters and the material properties are discussed in detail and a good correlation with the published results is achieved.

Rheology of Al-Cu-Mg pre-alloyed powder in supersolidus liquid phase sintering process

ABSTRACTSintering process was performed on cylindrical consolidates in dry nitrogen atmosphere at various temperatures ranging from the solidus-to-liquidus temperatures for 30 min at a heating rate of 10°C min−1. Beam bending technique has been used to measure the macroscopic apparent viscosity of Al-Cu-Mg pre-alloyed powder (2024 Al alloy). Slumping test has been used to measure the yield strength at temperatures above solidus temperature. Wetting angle and capillary stress were calculated. Results showed that the rheological behaviour of this system follows as Bingham model. The yield stress varies from 0.37 kPa at 580°C to 0.3 kPa at 610°C on air atomised Al-Cu-Mg having 0.1% Sn. The correlation between variations of viscosity as function of temperature has been derived. The activation energy of viscous flow was estimated to 15.164 kJ mol−1. Capillary and yield stress decrease by increasing temperature. Maximum densification with minimum distortion is obtained at 600–610°C.

INFLUENCE OF HYDROGEN ON PLASTIC FLOW OF THE TITANIUM AND ITS ALLOYS

In this paper the effect of hydrogen as an alloying element on the yield stress in commercial titanium and Ti-6Al and Ti-6Al-4V alloys is investigated. The connection is made between the strain resistance and phase composition in the wide range of temperatures. It has been found that the hydrogen alloying of commercial titanium in the α-field is accompanied by the yield stress decrease. The role of volume phase ratio in the alloys softening in α+β-field has been shown. The hydrogen hardening parameters in α-field are defined. The paper materials are of a usage value for researchers and technologists, developing innovative technological processes of strained semi-finished titanium products manufacturing.

Unravelling the impact of lignin on cell wall mechanics: a comprehensive study on young poplar trees downregulated for CINNAMYL ALCOHOL DEHYDROGENASE (CAD).

Lignin engineering is a promising tool to reduce the energy input and the need of chemical pre-treatments for the efficient conversion of plant biomass into fermentable sugars for downstream applications. At the same time, lignin engineering can offer new insight into the structure-function relationships of plant cell walls by combined mechanical, structural and chemical analyses. Here, this comprehensive approach was applied to poplar trees (Populus tremula×Populus alba) downregulated for CINNAMYL ALCOHOL DEHYDROGENASE (CAD) in order to gain insight into the impact of lignin reduction on mechanical properties. The downregulation of CAD resulted in a significant decrease in both elastic modulus and yield stress. As wood density and cellulose microfibril angle (MFA) did not show any significant differences between the wild type and the transgenic lines, these structural features could be excluded as influencing factors. Fourier transform infrared spectroscopy (FTIR) and Raman imaging were performed to elucidate changes in the chemical composition directly on the mechanically tested tissue sections. Lignin content was identified as a mechanically relevant factor, as a correlation with a coefficient of determination (r²) of 0.65 between lignin absorbance (as an indicator of lignin content) and tensile stiffness was found. A comparison of the present results with those of previous investigations shows that the mechanical impact of lignin alteration under tensile stress depends on certain structural conditions, such as a high cellulose MFA, which emphasizes the complex relationship between the chemistry and mechanical properties in plant cell walls.

Effect of surfactants on the yield stress of cement paste

Although many surfactant molecules are available to produce stable aqueous foams, these molecules can strongly interact with cement grains. We focus therefore here on the interactions between various surfactants and cement grains in order to identify some selection criteria for surfactants dedicated to foam cement production. First, we check the surfactant ability to produce aqueous foam in synthetic cement pore solution. Then, we observe that most surfactants do adsorb on cement grains, inducing concentration-dependent effects on the yield stress of cement paste. Our results suggest that, for some of these surfactants, the yield stress increases significantly as inter-particle hydrophobic attractive forces develop at the surface of the grains. However, above a concentration threshold, those attractive forces sharply drop because of the formation of surface micelles acting as steric barriers between particles, which results in a strong decrease in yield stress

Physical, microbiological and rheological properties of probiotic yogurt supplemented with grape extract.

In this study, yogurt was supplemented with 1.5 and 3.0gL-1 of grape extract, inoculated culture containing Streptococcus thermophilus, Lactobacillus bulgaricus, Lactobacillus acidophilus and Bifidobacterium bb12 bifidum, fermented and stored at 4°C. Acid production, microbial growth, gel strength, syneresis, rheological and sensory properties were studied. An increase in grape extract concentration extended fermentation time. Bacterial strains were found in at least 109 CFU100g-1 of yogurt showing the possibility of probiotic yogurt production with grape extract. Gel strength decreased with increasing concentration of grape extract while syneresis increased. The addition of grape extract changed the dilatant behavior to a pseudoplastic behavior, decreased yield stress, whereas k values increased. Sensory attributes (color, flavor, taste, texture and appearance) didn't differ significantly.

On the rheological characteristics of GMZ bentonite suspension

At the interface where the buffer bentonite contacts with the surrounding rock of the HLW disposal repository, the bentonite is likely to extrude into the fractures under swelling pressure and hydrate completely, which will as a result bring about looser bentonite suspension that might face the risk of being brought away easily by ground water flow. In this paper, the rheological characteristics of GMZ bentonite suspensions under different water–solid ratios and different types of saline solutions in different concentrations were studied through a rheology experiment using a “Brookfield DV-III Ultra” rheometer. The yield stresses of these GMZ bentonite suspensions were found from the corresponding rheological curves obtained from the rheology experiment. Then, the equation estimating the critical flow velocity was derived based on the Stokes law, and the critical flow velocities corresponding to these yield stresses were calculated. The results show that the rheological curve moves lower with an increase in water–solid ratio, when the water–solid ratio is below 5.0, and meanwhile, the suspension shows a characteristic of Newtonian fluids under lower shear stresses and a characteristic of nonlinear Bingham fluids under higher shear stresses; but when water–solid ratio goes beyond 5.0, the suspension shows a characteristic of pseudo-plastic fluids; when NaCl concentration increases in bentonite suspension, bentonite suspension’s yield stress decreases firstly and then increases, while bentonite suspension’s yield stress decreases when CaCl2 concentration increases; the critical flow velocities range from 10−4 to 10−2 m/s under the conditions of this paper.

The coupled effects of oxygen defect and crystallographic orientation on the electromechanical properties of BaTiO 3 nanowires

Abstract Influence of oxygen vacancies on electromechanical properties of individual BaTiO 3 (BTO) nanowires (NWs) is investigated, using molecular dynamics simulations. The simulations were performed for defected 0–4% oxygen vacancy defects) tetragonal NWs with axial directions of [001] and [110]. Results show an increase in spontaneous polarization and piezoelectric constant of the individual NWs due to increasing oxygen vacancy concentration, and a decrease in yield stress and Young's modulus. It seems that in individual BTO NWs the softening effect of the oxygen vacancies overcomes the pinning effect and results in enhancement of piezoelectric constant and spontaneous polarization. Results also show that yield stress and Young's modulus of the NWs with axial direction of [001] are higher than those for NWs with axial direction of [110] while it is reverse for spontaneous polarization and piezoelectric constant.

Static recovery of an AlMg4.5Mn aluminium alloy during multi-pass hot-rolling

Abstract In multi-pass hot-rolling of aluminium alloys stored energy is created by the plastic deformation in the form of dislocations and sub-grains. The energy dissipates due to sub-grain growth and the annihilation or rearrangement of dislocations. Both mechanisms lead to a reduction of dislocation density and hence a decrease in yield stress, i.e. to recovery and softening. Reliable quantitative results on these aspects, experimental as well as theoretical, are of high technological importance. Literature provides many results on the hardening and softening behaviour of aluminium alloys during plastic deformation. Few quantitative investigations, however, deal with the static softening after hot deformation. In this contribution we share recent quantitative results on the static softening behaviour of an AlMg4.5Mn aluminium alloy and its implications for the industrial hot-rolling process. We show the static softening behaviour derived from stress relaxation tests on a Baehr DIL805 dilatometer. The results agree well with a microstructure model, which is based on thermally activated dislocation climb. The activation energy for static recovery, however, is found to be outside the range predicted by theory. The relaxation results together with additional thermomechanical characterization are used to calibrate a material model based on the Kocks-Mecking equation, which has been implemented in a commercial implicit Finite-Element software. Numerical results from a three-dimensional Finite-Element model of the industrial multi-pass hot-rolling process show how static recovery affects the distribution of dislocation density across the rolling ingot between passes.