Yield Stress Increment Research Articles

Effects of Static Strain Aging on Mechanical Performance of Ductile Cast Iron

EN-GJS-400-15U nodular cast iron intended to be used as load-bearing element in long-term geological disposal canisters containing spent nuclear fuel in Finland and Sweden was studied for static strain aging (SSA). Tensile test specimens manufactured from the nodular cast iron were pre-strained to 1%, 2% and 3% nominal plastic strains. The pre-strained specimens were aged at different temperatures ranging from room temperature to 400 °C for varying times. The aged specimens were tested with conventional tensile testing using constant cross-head speed of 0.016 mm/s. Additionally, four specimens were studied with digital image correlation (DIC) during the tensile testing to obtain full-field strain measurements. SSA resulted in elevated pronounced yield point in all the conditions, while the as-received material showed continuous yielding behavior. SSA reduced the elongation to fracture. DIC tests showed more localized yielding behavior in the SSA specimens. Over-aging effect was observed at 400 °C where increasing pre-strain did not increase the yield stress more. For 1-day aging time, the highest yield stress increment was found after aging at 200°C. The yield stress of the material was almost identical after aging in 100°C and 200°C.

Effect of recycled powder on the yield stress of cement paste with varied superplasticizers

The influence of superplasticizer on the yield stress of cement pastes with recycled powder (RP) was examined in the study. Four superplasticizers were used to obtain the similar fluidity by adjusting the dosage. The results show that the 10% RP decreases the yield stress of paste compared to the reference paste at the same fluidity, but 20% and 30% RP increases the yield stress, ranging from 11 to 599%. The superplasticizer with adsorptive group of phosphate-type minimizes the yield stress of paste than that of polycarboxylate -type, but it made a significant increment in yield stress as the incorporating of RP increased. Besides, the polycarboxylate superplasticizer with the higher molecular weight of side chain and charge density led to lower yield stress. Based on the Yodel model, the yield stress of paste with RP was analyzed by the polymer adsorption and particle packing density of particles to reveal the influence of RP with different superplasticizers on the colloidal interaction and contact network among the particles. The packing density of particles with recycled powder was a little higher than the reference paste, but the higher fraction of fine particles made a stronger PSD effect, which improved the particle contact interaction. On the other hand, due to the higher polymer adsorption of recycled powder than cement, especially for superplasticizer with phosphate group, the average surface coverage was increased, which extended the separation distance, so that colloidal interaction among particles was weaken.

Formulation and characterization of cleaner one-part novel fly ash/lime-based alkali-activated material

With the advanced development in sustainable cementitious materials, cleaner one-part alkali-activated materials (AAM) have shown a significant effect on increasing compressive strength and reduction in CO2 emissions. This study is aimed to prepare one-part AAM based on fly ash (FA) and hydrated lime (LM) as main precursors. The impact of LM content as replacement of FA and activator dosage was evaluated on fresh and hardened properties of one-part FA/LM-based alkali-activated mortar. Fresh characteristics of AAM were evaluated through workability (setting time, flow table) and rheology, while hardened properties were evaluated by compressive strength, flexural strength, water absorption, durability test, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) tests. The test results revealed that the addition of FA/LM-based AAM increases the reactivity leading to earlier hardening of AAM. FA/LM-based AAM has shown higher compressive strength of 41.5 MPa. The rheology parameters show the increment in yield stress and plastic viscosity by increasing the lime content in the AAM. The activator dosage of sodium hydroxide increases the setting time and durability of AAM. The enhancement in strength and rheological parameters can be assigned to the enhanced geopolymerization reactions, leading to the formation of denser and compacted gels.

Characterization of microstructure and hardening of SLM nickel-based alloy irradiated by He ions

For the application of additively manufactured nickel-based alloys in molten salt reactors (MSRs), it is crucial to evaluate their performance in nuclear environments, especially under He-induced damage. Therefore, in this study, the irradiation damage behaviour of selective laser melted (SLM) nickel-based GH3536 alloy was studied by irradiating it with He ions at various doses. Traditionally manufactured (TM) materials were used as reference materials to evaluate the resistance of SLM GH3536 to irradiation. Transmission electron microscopy (TEM) was performed to reveal the depth distribution of irradiation-induced He bubbles, as well as the characteristics of dislocation loops, using both on-zone scanning transmission electron microscopy (STEM) and rel-rod modes. Nanoindentation tests were performed on both alloys to investigate their irradiation hardening. The TEM graphs and quantitative analysis showed that, owing to the cellular sub-grain structures, the He bubbles in the SLM alloy exhibited a lower number density but larger size than those in the TM alloy. Additionally, the SLM alloy showed inferior He tolerance but better resistance to irradiation hardening, as revealed by the nanoindentation test results. Consistency between the experimental hardness increment and calculated yield stress increment showed that the irradiation hardening of the sample was mainly caused by the presence of irradiation-induced defects. A comparison between the on-zone STEM and rel-rod modes suggested that the rel-rod mode is more effective if only the effect of irradiation-induced defects on mechanical properties is studied. Furthermore, this study provides insight into the design of SLM nickel-based alloys for nuclear industry applications.

Quantitative Strengthening Evaluation of Powder Metallurgy Titanium Alloys with Substitutional Zr and Interstitial O Solutes via Homogenization Heat Treatment.

The decomposition behavior of ZrO2 particles and uniform distribution of Zr and O solutes were investigated by employing in situ scanning electron microscope-electron backscatter diffraction (SEM-EBSD) analysis and thermogravimetric-differential thermal analysis (TG-DTA) to optimize the process conditions in preparing Ti-Zr-O alloys from the pre-mixed pure Ti powder and ZrO2 particles. The extruded Ti-Zr-O alloys via homogenization and water-quenching treatment were found to have a uniform distribution of Zr and O solutes in the matrix and also showed a remarkable improvement in the mechanical properties, for example, the yield stress of Ti-3 wt.% ZrO2 sample (1144.5 MPa) is about 2.5 times more than the amount of yield stress of pure Ti (471.4 MPa). Furthermore, the oxygen solid-solution was dominant in the yield stress increment, and the experimental data agreed well with the calculation results estimated using the Hall-Petch equation and Labusch model.

The effects of nanosilica on the fresh and hardened properties of 3D printable mortars

This study presents the experimental results of an investigation on the effects of nanosilica (NS) on the material characteristics of printable mortars used for additive manufacturing. Printable cement mortars based on Ordinary Portland Cement, limestone filler and silica sand were modified with different dosages of nanosilica (from 2% to 6% by weight of binder) and its influence on their hydration, rheological, mechanical and transport properties was assessed. The study showed that NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. Moreover, an increment of yield stress, together with an increment in NS dosage, was found to have occurred. The incorporation of an optimal NS dosage results in a noticeable increase in the compressive strength and alteration of the pore structure as determined by the MIP measurements. Moreover, transport properties of the produced mortar are significantly improved due to incorporation of NS. In addition to the microstructure refinement, Micro-CT and scanning electron microscopy (SEM) studies revealed that 3D printed mortars exhibit pore anisotropy in accordance with the printing direction. However, incorporation of NS in the mixture resulted in improved buildability, thus decreasing pore anisotropy.

Phase-change heat storage backfill: Experimental study on rheological properties of backfill slurry with paraffin added

In phase-change heat storage backfill, pipeline transportation of slurry is one of the critical stages, using paraffin microcapsules as phase change material, the influence of paraffin percentage, slurry concentration and shear rate on rheological parameters was discussed and analyzed based on experimental test. Yield stress increases significantly especially paraffin percentage exceeding 5%, for slurry concentration of 68%, 70% and 72%, the increment of yield stress is 171.2 Pa, 353.3 Pa and 353.4 Pa respectively when paraffin percentage increases from 5% to 10%. Viscosity increases significantly with slurry concentration, the average increment is 0.054 Pa·s when slurry concentration increases from 68% to 72%. Shear stress increases with the increase of shear rate, paraffin percentage and slurry concentration. The average increment of shear stress is 32 Pa when shear rate increases from 50 s−1 to 70 s−1, shear stress increases significantly when paraffin percentage increases from 5% to 10%, especially reaching 10%, shear stress is larger, and the increment of shear stress caused by increasing concentration is also larger. So it is very important to control the dosage of heat storage material, adjust slurry proportion and determine pipeline parameters, which directly affect flow characteristics and flow resistance, in order to improving the transportation efficiency of backfill slurry.

Investigating the Effects of Mixing Time and Mixing Speed on Rheological Properties, Workability, and Mechanical Properties of Self-Consolidating Concretes

The mixing process performs a critical role in the concrete microstructure, which defines the final product’s quality. Optimizing mixing time and mixing speed during the mixing process can reduce the unit price and energy consumption without impacting product quality. Furthermore, the energy consumption to manufacture self-consolidating concretes (SCC) is more than that of conventional concretes due to their compositions and structures. Due to the fact that rheological and mechanical properties assess the performance of concrete, the effects of mixing energy on the mentioned properties of self-consolidating concretes are taken into consideration simultaneously in the current investigation. Accordingly, different mixtures contained polysaccharide-based viscosity modifying agent and limestone powder are made under different mixing speeds and mixing times. Drawing on the results of mixing time, the optimum time for mixing is 8 min in this study in order to improve the workability and to provide ideal levels of rheological features such as minimum yield stresses (static and dynamic). In the aspect of mixing speed, double increasing from 20 to 40 rpm rises the slump flow by 5%. Besides, accelerating mixing speed increases dynamic yield stress by 37% for 11 min mixed mixtures, which is more than that of other mixing times mixtures’ dynamic yield stress increment. In the aspect of mechanical properties, mixing time increment increases the concrete’s compressive strength. Nevertheless, other mechanical characteristic criteria do not significantly depend on mixing energy.

Transverse impact behavior of high-strength concrete filled normal-/high-strength square steel tube columns

High strength materials and dynamic loadings are two important research topics for building structures. In this study, high-strength concrete filled square steel tube columns (HSCFST) using S690 along with Q355 structural steel subjected to transverse impact loadings were experimentally tested by a drop hammer tester, and the impact force, deformation, and energy absorption of such specimens were obtained. Results showed that HSCFST has great impact resistance, showing high impact force plateau value and small deflection. The influence of steel strength, concrete strength, steel ratio, and impact energy on impact resistance was deeply analyzed. Compared with normal-strength materials, using high strength steel can improve its impact resistance but the contribution is not as great as the increment of yield stress, whilst using the high-strength concrete has limited effect. Besides, a refined finite element (FE) method by ABAQUS/Explicit employing the rate-dependent constitutive model for S690 was adopted to simulate the dynamic responses of HSCFST under impact loadings, performing a good agreement with tests. This study provides the basic experimental data of HSCFST subjected to transverse impact and develops a reasonable FE model to predict its impact resistant performance, contributing to the related studies of HSCFST components.

The effect of texture and grain size on improving the mechanical properties of Mg-Al-Zn alloys by friction stir processing

Friction stir processing (FSP) was used to achieve grain refinement on Mg-Al-Zn alloys, which also brought in significant texture modification. The different micro-texture characteristics were found to cause irregular micro-hardness distribution in FSPed region. The effects of texture and grain size were investigated by comparative analyses with strongly textured rolling sheet. Grain refinement improved both strength and elongation in condition of a basal texture while such led to an increment in yield stress and a drop in elongation and ultimate stress when the basal texture was modified by FSP.

Strengthening of Fe3Al Aluminides by One or Two Solute Elements

The compressive yield stress of Fe-26Al with additives Ti (0.5 to 4 at. pct), Cr (0.5 to 8 at. pct), Mo (0.5 to 4 at. pct), and V (0.5 to 8 at. pct) at 1073 K (800 °C) has been determined. The effect of the concentration of diverse solutes on the yield stress at 1073 K (800 °C) was compared, and the additivity of the effects of solutes was tested. The effects in iron aluminides with two solutes (V and Ti, Ti and Cr, V and Cr) are compared with those of a single solute V, Ti, and Cr. It is found that the additivity of yield stress increments is valid only for lower solute concentrations. When the amount of the solute atoms increases, the yield stress increment is substantially higher than the sum of the yield stress increments of single solutes. This behavior is related to the high-temperature order in iron aluminides.

The Role of Aging on Rheological Properties of Lime Putty

The role of aging on rheological properties of lime putties was investigated by rotational rheometry, scanning electron microscopy and particle size distribution analyzes. Disaggregation of large clusters during aging and resulting microstructural enrichment of micron-sized particles was found to be one of the possible reasons for the continuous increment of plasticity and yield stress of lime putties during long-term aging. The extent of thixotropic and rheopectic behavior is affected by both calcination process and microstructure development during aging.

Enhanced yield stress of magnetorheological fluids with dimer acid

A silicone oil-based magnetorheological fluid was prepared with carbonyl iron powder as magnetic particles and silicone oil as carrier. Dimer acid was employed as surfactant. The viscosity, yield stress, and flow behavior of MRFs under steady shearing flow were analyzed. It is found that the addition of dimer acid provoked a significant increment in field-induced yield stress, suggesting a strengthened chain microstructure was developed by the dimer acid modified particles.

Deformation microstructures, strengthening mechanisms, and electrical conductivity in a Cu–Cr–Zr alloy

The ultrafine-grained microstructures, mechanical properties and electrical conductivity of a Cu–0.87%Cr–0.06%Zr alloy subjected to multiple equal channel angular pressing (ECAP) at temperatures of 473–673K were investigated. The new ultrafine grains resulted from progressive increase in the misorientations of strain-induced low-angle boundaries during the multiple ECAP process. The development of ultrafine-grained microstructures is considered as a type of continuous dynamic recrystallization. The multiple ECAP process resulted in substantial strengthening of the alloy. The yield strength increased from 215MPa in the original peak aged condition to 480MPa and 535MPa after eight ECAP passes at 673K and 473K, respectively. The strengthening was attributed to the grain refinement and high dislocation densities evolved by large strain deformation. Modified Hall–Petch analysis indicated that the contribution of dislocation strengthening to the overall increment of yield stress (YS) through ECAP was higher than that of grain size strengthening. The formation of ultrafine grains containing high dislocation density leads to a small reduction in electrical conductivity from 80 to 70% IACS.

Modelling nanoscale grain size strengthening or weakening according to grain boundary thickness

The relationship of yield stress and grain size at nanoscale is modelled in this letter. The deformation behaviour of materials is divided into two regions: the grain rotation at the boundaries and deformation in the inner ordered region. It is found that the yield stress reaches its maximum when the grain size, d, is three times of the grain boundary thickness, t, corresponding to a 70% volume ratio of grain boundaries. When d>3t, the interface area increases with grain size reduction, leading to the yield stress increment. When d<3t, the interface area decreases with grain size reduction, leading to the yield stress decrement. The hardness has similar trend. The proposed model agrees well with the documented data in Cu, Fe, Pd and NiP alloys.

Improved Paint Baking Response of a Novel Al-Mg-Si Alloy Automotive Body Panels by Adding Zn Element and Pre-Ageing Matching

In the present work, the microstructure and properties of a novel Al-0.92Mg-0.78Si alloy with adding Zn element were investigated by electron back scattering diffraction (EBSD), transmission electron microscopy (TEM), high transmission electron microscopy (HREM), mechanical properties tests and Erichsen test. The alloy sheet was cold-rolled to about 1mm, then solution treated at 545 °C for 30mins, and immediately water quenched. After solution treatment, the alloy was pre-aged at 140 °C for 10mins. The average grain size of the alloy was about 20μm. The yield stress and elongation of pre-aged alloy was 137MPa and 30% respectively. The IE value of the alloy was 8.6mm. After paint baking at 170 °C for 30mins, the increment of yield stress was about 100MPa. The GP(II) zones were formed in the alloy during paint baking process through adding Zn element, which should play a very important role for improving paint hardening response significantly. GP(II) zones and η ́ phases were formed after artificial ageing at 170 °C for 10h.

The tunable mechanical property of water-filled carbon nanotubes under an electric field

The spring-induced compression of water-filled carbon nanotubes (CNTs) under an electric field is investigated by molecular dynamics simulations. Due to the incompressibility and polarity of water, the mechanical property of CNTs can be tuned through filling with water molecules and applying an electric field. To explore the variation of the mechanical property of water-filled CNTs, the effects of the CNT length, the filling density and the electric field intensity are examined. The simulation results indicate that the water filling and electric field can result in a slight change in the elastic property (the elastic modulus and Poisson's ratio) of water-filled CNTs. However, the yield stress and average post-buckling stress exhibit a significant response to the water density and electric field intensity. As compared to hollow CNTs, the increment in yield stress of the water-filled CNTs under an electric field of 2.0 V Å−1 is up to 35.29%, which is even higher than that resulting from metal filling. The findings from this study provide a valuable theoretical basis for designing and fabricating the controlling units at the nanoscale.

Tensile flow behavior of ultra low carbon, low carbon and micro alloyed steel sheets for auto application under low to intermediate strain rate

This paper is concerned with tensile characteristics of auto grade low carbon, ultra low carbon and micro alloyed steel sheets under low to intermediate strain rates ranging from 0.0007 to 250s−1. Experimental investigation reveals two important aspects of these steels under intermediate strain rate deformation. Firstly, the yield stress increases with strain rate in all these steels. Of course yield stress increment is higher for low carbon and ultra low carbon steel sheets. Secondly, the strain hardening rate drastically decreases with strain rate for low carbon and ultra low carbon steel sheets, whereas it remains steady for micro alloyed steel sheets. Based on these observations, a constitutive model has been proposed which predicts the strain rate sensitive flow behavior of all these grades within the strain rate range of automotive crash event.

S041023 酸素固溶強化純チタン粉末材の強化機構([S041-02]粉末成形とその評価(2))

In-process solid solution of oxygen atoms into pure titanium (Ti) powder via heat treatment under controlled atmosphere was employed to prepare powder metallurgy (P/M) extruded Ti materials. They showed extremely high tensile strength and elongation compared to P/M pure Ti material with no oxygen solid solution. The experimental data of yield stress increment corresponded well to the calculation results by using Labusch model generally used for analysis of the solid solute strengthening behavior. It was concluded that the main strengthening mechanism of P/M pure Ti materials was oxygen solid solution into Ti matrix.

Multi-Walled Carbon Nanotubes Reinforced Titanium Composites via Powder Metallurgy Process

Un-bundled multi-walled carbon nanotubes (MWCNTs) were coated on titanium powder surface by using zwitterionic surfactant solution and mixing process, and their CNT/Ti composite powders were consolidated into full-dense materials by spark plasma sintering and the following hot extrusion process in solid-state. Wrought titanium powder metallurgy (PM) composites containing CNTs revealed extreme increment of yield stress and tensile strength of 85% and 50%, respectively, compared to conventional PM titanium with no reinforcement, while they had enough high ductility. The mechanical improvement was mainly due to dispersion strengthening effect of CNTs and in-situ formed TiC fine particles and a small mount of carbon solid solution into Ti matrix. In addition, the control of grain growth was promoted by TiC dispersoids during SPS, and contributed to their strengthening behavior. Furthermore, high-temperature tensile strength less than 673K of PM pure titanium materials was also obviously improved by the pinning effect of TiC particles dispersed at grain boundaries. In this paper, microstructural and mechanical properties of powder metallurgy titanium composites would be introduced in detail.