Significant Increase In Yield Stress Research Articles

Effect of steel fiber shape and content on printability, microstructure and mechanical properties of 3D printable high strength cementitious materials

This study investigates the effects of steel fiber shape (straight and hooked-end) and content (0, 0.5, 1.0, and 1.5 vol%) on rheological properties, printability, mechanical performance and microstructure of 3D printable steel fiber reinforced high strength concrete (3DP-SFHSC). The results indicate that the increase of fiber content improves the mechanical behaviors of 3DP-SFHSC, but the extrudability suffers from reduction when the fiber content exceeded 1.0 vol% due to the significant increase in yield stress. The addition of 1.5 vol% hooked-end fibers enhances the compressive strength of 3DP-SFHSC by 8%, 25.7%, and 40.4% in the X, Y, and Z directions, respectively. Additionally, it also improves the tensile strength in the X direction by 37.67%. The printed specimens exhibit weaker mechanical properties compared to the cast specimens. On the one hand, the lower fiber-matrix compactness resulting from the absence of vibration during printing leads to increased porosity and weak bonding between the steel fibers and matrix. On the other hand, the inconsistent movement between fibers and mortar matrix during extrusion process may also cause the formation of gaps around fibers. Straight fibers show pronounced enhancements in buildability, compressive strength and tensile strength compared to hooked-end fibers at the same fiber volume fraction. Straight fibers align easily during the extrusion process, contributing to a matrix with lower porosity and smaller average pore size.

The Effect of Viscous Drag on the Maximum Residual Stresses Achievable in High-Yield-Strength Materials in Laser Shock Processing.

In this paper, the experimentally observed significant increase in yield stress for strain rates beyond 104 s-1 (viscous regime) is explicitly considered in laser shock processing (LSP) simulations. First, a detailed review of the most common high-strain-rate deformation models is presented, highlighting the expected strain rates in materials subject to LSP for a wide range of treatment conditions. Second, the abrupt yield stress increase presented beyond 104 s-1 is explicitly considered in the material model of a titanium alloy subject to LSP. A combined numerical-analytical approach is used to predict the time evolution of the plastic strain. Finally, extended areas are irradiated covering a squared area of 25 × 25 mm2 for numerical-experimental validation. The in-depth experimental residual stress profiles are obtained by means of the hole drilling method. Near-surface-temperature gradients are explicitly considered in simulations. In summary, the conventionally accepted strain rate range in LSP (106-107 s-1) is challenged in this paper. Results show that the conventional high-strain-rate hardening models widely used in LSP simulations (i.e., Johnson Cook model) clearly overestimate the induced compressive residual stresses. Additionally, pressure decay, whose importance is usually neglected, has been found to play a significant role in the total plastic strain achieved by LSP treatments.

A nano-twinned structure and tensile properties of a thermally cycled Fe-17Mn alloy

In the present study, the effect of thermal cycling on the microstructure and tensile properties of an Fe-17Mn (wt%) alloy were investigated. The thermal cycling was carried out until 10 times between a lower temperature (TL) of 25 °C and a upper temperature (TU) of 300 °C. A nano-twinned γ structure was observed at TU during thermal cycling through in-situ transmission electron microscopic observation. Many stacking faults, such as intrinsic I1, intrinsic I2, and twin-like T2 stacking faults, were observed in ε martensite at TL. The nano-twinned γ structure was formed by the stacking faults with a localized fcc structure in ε martensite during the ε → γ reverse transformation. Namely, half of the stacking faults with a localized fcc structure had the same crystallographic orientation as an initial γ austenitic phase, while the other half had a twin relationship with the γ austenite. The stacking faults with a twin relationship caused nano-twins during ε → γ reverse transformation. The volume fraction of nano-twins at TU was linearly proportional to that of ε martensite at TL, supporting the result that nano-twins in the γ phase resulted from preexisting stacking faults in ε martensite during ε → γ reverse transformation. Both the yield and ultimate tensile stresses of the γ austenite at TU increased with an increase in the number of thermal cycles, while the elongation decreased. The significant increase in yield stress at TU was due to the increase in dislocations and nano-twins introduced into γ austenite during thermal cycling.

A step closer to continuous buffer preparation from solids: Predicting powder compaction and how to prevent it

The preparation of buffer solutions used in the biopharmaceutical industry is typically performed manually by the addition of one or multiple buffering reagents to water. Recently, the adaptation of powder feeders for continuous solid feeding was demonstrated for continuous buffer preparation. However, the intrinsic characteristics of powders can change the stability of the process, due to the hygroscopic nature of some substances and humidity-induced caking and compaction behavior, but there is no simple and easy methodology available for predicting this behavior for buffer species. To predict which buffering reagents are suitable without special precautions and investigate their behavior, force displacement measurements were conducted with a customized rheometer over 18 h. While most of the eight investigated buffering reagents indicated uniform compaction, especially sodium acetate and dipotassium hydrogen phosphate (K2HPO4) showed a significant increase in yield stress after 2 h. Experiments conducted with a 3D printed miniaturized screw conveyor confirmed the increased yield stress measurements by visible compaction and failure of the feeding. By taking additional precautions and adjusting the design of the hopper, we demonstrated a highly linear profile of all buffering reagents over a duration of 12 and 24 h. We showed that force displacement measurements accurately predict the behavior of buffer components in continuous feeding devices for continuous buffer preparation and are a valuable tool to identify buffer components that need special precautions. Stable, precise feeding of all tested buffer components was demonstrated, highlighting the importance of identifying buffers that need a specialized setup with a rapid methodology.

Prediction of rheological behavior of self-compacting concrete by multi-variable regression and artificial neural networks

To control the rheological behavior of self compacting concrete, this study attempts to provide predictive models using a multi-variable regression model and artificial neural networks such as machine learning techniques on a database of 59 samples (average of 354 specimens), collated from literature. The statistical significance of the model coefficients and the coefficient of determination are adopted as criteria to check the performance of the model on test data.Interaction terms such as slump flow diameter, V-Funnel flow time, and L-Box ratio were inferred from literature to improve the efficacy of the predictive modeling. Two approaches for the prediction of yield stress and plastic viscosity are proposed and validated.With an increase in slump flow diameter and V-Funnel flow time, there was a significant increase in yield stress and plastic viscosity from 2.1 Pa and 18.2 Pa.s to 98.6 Pa and 198.8 Pa. s, respectively. A significant decrease in these rheological parameters was noted with an increase in L-Box ratio from 0.5 to 1. Thus, the comparison between the two numerical methods proposed shows that, for datasets with only one output, the multi-variable regression presents an important advantage. Thus, the regression model for the plastic viscosity represents values of R of the order of 0.955, 0.968, and 0.916 in correlation with slump flow diameter, V-Funnel flow time, and L-Box ratio, respectively.

High-strength ultrafine-grained titanium 99.99 manufactured by large strain plastic working

In this research, high-purity titanium (hp-Ti, 99.99 wt%) was subjected to a large strain via a cold plastic working process. To accumulate a relatively large plastic deformation in the workpiece, the hydrostatic extrusion (HE) technique was applied. The initial rod with a diameter of ∅50 mm was subjected to a multi-pass extrusion process, and, this way, rods with a diameter of ∅8 mm and ∅7 mm were obtained. In this paper, the results of an investigation of the structure and mechanical properties of the hp-Ti are presented. The size and shape of the grains of the as-received and extruded samples were examined, and an effective way of refining grain and strengthening hp-Ti using plastic working was demonstrated. Thanks to the process applied, an ultrafine-grained structure was obtained. In the transverse section, the average grain size determined by transmission electron microscopy was 117 nm on average. As a result of the extrusion, a significant increase in yield stress, tensile strength and microhardness was observed. Moreover, in this paper the overall potential of the HE technique was demonstrated. The results of this work confirm that it is possible to manufacture high-strength, ultrafine-grained high-purity titanium via cold plastic working.

PERFORMANCE CHARACTERISTICS OF COATINGS ON BASIS OF POLYVINYL ACETATE MODIFIED WITH MONTMORILLONITE

Water-dispersion materials based on poly (vinyl acetate) are widely used in industry as adhesive compositions and binders for polymer coatings due to their low cost, ecological safety and good performance characteristis. Drawbacks of such materials are insufficient water resistance and relatively low physical-mechanical properties. For improvement of performance characteristics of poly (vinyl acetate) based coatings it is proposed to modify the water-dispersion materials by incorporation of small amounts of sodium montmorillonite. In the article characteristics of model pigmented paint materials and coatings based on poly (vinyl acetate) dispersion modified with sodium montmorillonite are presented. Commercial water dispersion of PVA plasticized with dibutyl phatalate, natural sodium montmorillonite and titanium dioxide (pigment) were used for preparation of model paint materials. The obtained specimens of model paint materials are pseudoplastic fluids, incorporation of monmtorillonite leads to significant increase in yield stress. Materials modified with montmorillonite exhibit thixotropic properties. Addition of 1-2 % wt. montmorillonite to the material leads to increase in the Young modulus of dried films up to 8 times without decrease in tensile strength. The obtained effect is attributed to adhesion strengthening of the composite material. Incorporation of montmorillonite does not hinder the covering power of the paint material. Montmorillonite-modified coatings show increased hardness and reduced water absorption capacity (up to 2.5 times with addition of 2 % wt. montmorillonite). Reduction of water uptake of polymer material modified with montmorillonite can be attributed to reduction of free volume in the composite, formation of interface layers of polymer with different physico-phemical properties, and also to the decrease in the diffusion coefficient of water in the composite. The suggested approach to inprovement of the coating characteristics can be implemented without major expenditure due to low cost of natural sodium montmorillonite and simplicity of its incorporation in water-dispersion materials.

Effect of ECAP Prior to Aging on Microstructure, Precipitation Behaviour and Mechanical Properties of an Al-Cu-Mg Alloy

Microstructure, precipitation behaviour and mechanical properties of an Al-5.64Cu-0.33Mn-0.23Mg-0.14Zr-0.11Ti (in wt. %) alloy subjected to thermomechanical processing (TMP) involving equal-channel angular pressing (ECAP) at ambient temperature to total strains (ε) of ~1 and ~2 followed by aging at 180°C for 0-28 h have been investigated and compared with conventional aging at the same temperature (T6 state). TMP led to significant increase in yield stress (YS) and ultimate tensile strength (UTS) and decrease in elongation-to-fracture as compared to the peak-aged T6 state. The YS, UTS and δ values attained after ECAP to ε ~ 2 followed by peak ageing were ~510 MPa, ~540 MPa and ~7.6%, respectively. The changes in mechanical properties were related to microstructure evolution and precipitation behaviour. TMP conditions obtaining a high-strength state of the Al-Cu-Mg alloy are discussed.

Effect of rheology and dispersion degree on the regrinding of an iron ore concentrate

This investigation addressed the study of the effects of the slurry's rheological behavior and the state of aggregation and dispersion on wet ultrafine grinding of an iron ore concentrate. Regrinding tests were conducted under different conditions of dispersion (pH 7.3, 8.5 and 10.0) and with the addition of 300g/t of lime. The increase of pH from 7.3 to 10.0 increased the slurry dispersion degree, providing lower values of yield stress and apparent viscosity, and a reduction in specific energy consumption of 17.4%. So, sodium hydroxide, added at previous processing steps, has acted as a grinding aid. The addition of 300g/t of lime caused an increase of 27% in specific energy consumption, with significant increase in yield stress and consistency index of the fluid, especially for the condition of 18kWh/t.

Development of experimental method to characterize pressure-dependent yield criteria for polymeric foams

In addition to lightweight and moldable characteristics, polymeric foams possess an excellent energy-absorbing capability that can be used for a wide range of commercial applications, especially for crash protection in automobiles. The purpose of the present study is to develop an experimental methodology to characterize the pressure dependent yield behavior of energy-absorbing polymeric foams. A device was designed to perform a compression test in a triaxial stress state. For the test material, polyurethane foams of two different densities were used. The displacement of the specimen, the load applied to the specimen, and hydrostatic pressure applied to the specimen were measured and controlled. Stress–strain curves and yield stresses for different hydrostatic pressures were obtained. The experiments conducted in this study revealed that the polyurethane foams exhibited significant increase in yield stress with applied hydrostatic pressure or mean normal stress. Based on this observation, a yield criterion which included the effect of the stress invariant was established for the foams. The experimental constants obtained which constituted the pressure-dependent yield criterion were verified.

Identification of elastic-plastic material parameters from pyramidal indentation of thin films

The indentation experiment is a popular method for the investigation of mechanical properties of thin films. By application of conventional methods, the hardness and the stiffness of the film material can be determined by limiting the indentation depth to well below the film thickness so that the substrate effects can be eliminated. In this work a new method is proposed, which allows for a determination of the reduced modulus as well as the nonlinear hardening behaviour of both the film and substrate materials. To this end, comparable deep indentations are made on the film/substrate composite to obtain sufficient information on the mechanical properties of both materials. The inverse problem is solved by training neural networks on the basis of finite–element simulations using only the easily measurable hardness and stiffness behaviour as input data. It is shown that the neural networks are very robust against noise in the load and depth. The identification of the material parameters of aluminium films on different substrates results in a significant increase in yield stress and initial work–hardening rate for a reduction of the film thickness from 1.5 to 0.5 µm, while the elastic modulus and the extent of work hardening remain constant.

Size-related aspects of arm damage, tissue mechanics, and autotomy in the starfish Asterias rubens

Arm damage is a widely reported but superficially investigated aspect of the biology of the starfish Asterias rubens L. In the present study, the incidence of arm damage was surveyed in populations of A. rubens at two sites in the Firth of Clyde, Scotland, and three sites in Gullmarsfjorden, Sweden. The mean (±SD) incidence across all sites of individuals with basal arm damage (resulting from detachment at the basal autotomy plane) was 19.69 ± 8.86%, the incidence of those with distal arm damage (resulting from amputation at more distal levels) was 7.74 ± 10.01%. The mean incidence of arms with basal damage was 5.28 ± 4.12%, of those with distal damage 1.83 ± 2.45%. There was a significant negative correlation between size and the incidence of basal damage at all but one site, but no significant correlation between size and distal damage at any site. Mechanical tests on specimens of the aboral body wall from the basal region of the arm (which included the autotomy plane) and from a more distal region revealed that with increasing body size there was a significant increase in yield stress, ultimate stress and Young's modulus (stiffness) but no significant change in yield strain and ultimate strain. There was no significant difference between the relationships for basal and distal specimens. It is hypothesised that in larger individuals increased mechanical toughness replaces autotomy as an effective antipredator strategy. Using two methods to induce autotomy, a significant positive correlation between size and the delay between the onset of stimulation and arm detachment was found; this may represent a size-related decline in the efficiency of the autotomy mechanism through the relaxation of selection pressure. Since size is an unreliable indicator of age in A. rubens, the trends identified herein can be interpreted only tentatively as age-associated phenomena.

Deoxygenation-induced changes in sickle cell-sickle cell adhesion

The tendency for sickle cells to adhere to each other is increased in oxygenated sickle blood in parallel with cell density. The increased adherence of these cells occurred despite their reduced deformability and diminished ability to form rouleaux. Using a method developed in our laboratory, we measured the yield stress: a sensitive index of cell- cell adhesion of deoxygenated suspensions of sickle cells. Deoxygenation of whole sickle blood to 30 to 50 mm Hg caused a significant increase in yield stress of all sickle blood samples. Deoxygenation caused a significant increase in yield stress of both dense and light sickle cells. Deoxygenation-induced increases in yield stress occurred at higher oxygen tensions for dense (= 55 mm Hg) than for light sickle cells (< 45 mm Hg). The increase in yield stress on deoxygenation was correlated with hemoglobin polymerization as assessed morphologically by sickling or by changes in relative viscosity. Thus, deoxygenation-induced cell sticking must involve small areas of strong membrane adhesion because the changes in yield stress occurred despite a reduction in rouleaux formation and surface area of membrane contact. Sickle trait red blood cells also exhibited increased yield stress on deoxygenation but only under hypertonic conditions where sickling occurred. Thus, deoxygenation-induced cell adhesion did not require prior membrane damage because it occurred in sickle trait cells. No change in yield stress was seen when deoxygenated sickle cells were suspended in buffer, but the addition of physiologic amounts of fibrinogen to buffer restored the deoxygenation-induced increase in cell adhesion. We speculate that the increase in sticking among sickle cells on deoxygenation results from spicule formation and may involve interaction of fibrinogen and possibly other plasma proteins with the cell membrane.

Deoxygenation-Induced Changes in Sickle Cell–Sickle Cell Adhesion

The tendency for sickle cells to adhere to each other is increased in oxygenated sickle blood in parallel with cell density. The increased adherence of these cells occurred despite their reduced deformability and diminished ability to form rouleaux. Using a method developed in our laboratory, we measured the yield stress: a sensitive index of cell-cell adhesion of deoxygenated suspensions of sickle cells. Deoxygenation of whole sickle blood to 30 to 50 mm Hg caused a significant increase in yield stress of all sickle blood samples. Deoxygenation caused a significant increase in yield stress of both dense and light sickle cells. Deoxygenation-induced increases in yield stress occurred at higher oxygen tensions for dense (> 55 mm Hg) than for light sickle cells (< 45 mm Hg). The increase in yield stress on deoxygenation was correlated with hemoglobin polymerization as assessed morphologically by sickling or by changes in relative viscosity. Thus, deoxygenation-induced cell sticking must involve small areas of strong membrane adhesion because the changes in yield stress occurred despite a reduction in rouleaux formation and surface area of membrane contact. Sickle trait red blood cells also exhibited increased yield stress on deoxygenation but only under hypertonic conditions where sickling occurred. Thus, deoxygenation-induced cell adhesion did not require prior membrane damage because it occurred in sickle trait cells. No change in yield stress was seen when deoxygenated sickle cells were suspended in buffer, but the addition of physiologic amounts of fibrinogen to buffer restored the deoxygenation-induced increase in cell adhesion. We speculate that the increase in sticking among sickle cells on deoxygenation results from spicule formation and may involve interaction of fibrinogen and possibly other plasma proteins with the cell membrane.