Flow Stress Measurements Research Articles

Flow stress curves for 980MPa- and 1.5GPa-class ultra-high-strength steel sheets weakened under high-stress triaxiality

The weakening behavior of ultra-high-strength steel(UHSS) remains unknown, even though it is a requirement for the accurate prediction of strain localization and ductile fracture in automobile applications. In this study, we, therefore, revealed the stress–strain curves of UHSSs up to their ductile fracture under different stress triaxialities for the first time. Smooth and notched tiny round-bar specimens cut from UHSSs with 1.6 mm thickness were subjected to tensile tests, during which the forces and neck diameters of the specimens were measured. Notably, the evaluated flow stresses in the 1.5 GPa-class notched UHSS specimens exhibited weakening of up to 3%. This was in contrast to the 980 MPa-class UHSS specimens, which did not exhibit notch-induced weakening. The absence of weakening during stress measurements, as confirmed by synchrotron X-ray diffraction (XRD) analysis, suggested that weakening was caused by material damage. However, the weakening behavior did not follow the previously developed damage models based on microvoid formation. The volume fraction of the microvoids, as observed by X-ray computer tomography, was extremely small (at 0.2%) and cannot account for the 3% material weakening indicated by the flow stress measurement results. A new damage mechanism, associated with unobservable small lattice vacancies, was implied in UHSSs deformation under high stress triaxialities.

Compression temperature to activate kinking in pearlitic steel

The preferable condition for kinking in the compression of pearlitic steel is reported in this study. Spring steel (0.53 wt%C-1.48%Si-0.70%Mn-0.76%Cr–Fe) with a pearlitic structure was compressed at different temperatures ranging from ambient to 600 °C. The deformation microstructure clarified that the kinking occurred more oftenwhen the compression temperature was less than 400 °C. Three-dimensional reconstruction of the scanning electron microscopy images with focused-ion-beam serial sectioning revealed that the kinked structure comprised a band-like sheared area surrounded by the bent structures. The shear strain in the kinked structure is greater than the macroscopic strain applied by compression. A series of measurements of compressive flow stress (strain rate: 0.001/sec to 1/sec) elucidated negative strain rate sensitivity (m-value < 0) at ∼ 300 °C. Strain localization by kinking occurs preferentially at relatively lower temperature because a smaller strain rate sensitivity can promote local plastic instability.

On the Mechanical Response of Aluminum Alloy 6061-T6 under Extreme Strains and Strain Rates, and Rapid Heating

The flow stress of aluminum alloy 6061-T6 produced by conventional thermomechanical methods has been investigated under the application of strains up to 80% at a strain rate of the order of 50001/s, and temperatures ranging from ambient to near melting. The high strain and strain rate deformation was imposed using a Kolsky bar compression apparatus equipped with a fast pulse-current heating system to reach the target temperatures at high heating rates. The flow stress was measured under strains, strain rates, temperatures and heating rates that match thermomechanical conditions developed in the primary shear zone of a specially designed machining with chip pulling test method. Flow stress error is estimated by statistics obtained from experimental replicates. It is expected that these measurements will enable the formulation of realistic machining models for Al6061-T6. Temperature measurements were obtained using non-contact infrared (IR) full field imaging together with embedded micro-thermocouple (TC) point probing. The TC measurements were used to estimate the emissivity of the aluminum specimen via separate, in situ calibration tests, and to track temperature evolution during the compression, also via separate testing, not compromising the flow stress measurements. Type-K TC measurements were made using the separated junction principle by embedding the TC wires into two micromachined holes in the specimen. The temperature measurement technique is discussed in detail, and temperature uncertainties are estimated. The Kolsky bar and machining test with chip pulling will be used going forward to study the effect of heating time on the dynamic thermal softening behavior of Al6061-T6, which has been shown to be time-sensitive under quasi-static loading when temperatures exceed 200°C, due to Mg-Si precipitate growth.

Optimising compression testing for strain uniformity to facilitate microstructural assessment during recrystallisation

Predicting the kinetics of recrystallisation in metals, and recrystallised grain size distributions, is one the key approaches to controlling and refining grain size during metal processing, which typically increases strength and toughness/ductility. Recrystallisation prediction models and equations are supported by lab-based simulations that can systematically assess recrystallisation over a range of temperatures and strains for different materials and starting grain sizes. This work uses modelling and experimental verification to assess the different commonly used compression test sample geometries to determine strain uniformity and potential sources of error in microstructural assessment and proposes a modified geometry that increases the area of constant known strain. Whilst flow stress measurements in all samples showed good agreement. It has been shown that the new plane strain geometry offers a more consistent, homogeneous strain through the sample such that the large number of grains needed for accurate grain size distribution measurement can be readily achieved. Over double the area of ±10% of the target strain was achieved in the modified plane strain sample compared to a conventional uniaxial specimen, this area was also shown to be more conducive to metallographic assessment and offers in excess of 1500 grains of 250 μm to be assess per cross-sectional slice.

Correction of flow stress data due to non-homogeneous deformation and thermal conditions during hot compression testing of a polycrystalline nickel-base superalloy

Accurate flow stress data are essential for the design and optimization of thermo-mechanical processes for a wide range of metallic materials. Hot compression testing is generally used to establish the dependence of flow stress on temperature, strain and strain rate. Flow stress measurements have mostly used cylindrical specimens of different sizes. While the data has been corrected for friction and adiabatic heating, the measurements assume idealized uniform deformation in the test specimen when obtaining the flow stress data from the measured load and displacement. In this paper, it is shown that there is significant non-uniform deformation in the test specimen under practical testing conditions, and a methodology is developed to correct the flow stress data. This correction is generally more significant than the correction due to friction or adiabatic heating and is necessary to obtain more accurate flow stress data. The effect of specimen geometry is also investigated, and a geometry which results in more uniform strain distribution than a cylindrical specimen is used. Measurements are conducted for a prototype powder metallurgy nickel-base superalloy at a low strain rate over a deformation temperature range. Constitutive equations are constructed with the measured flow stress data. Finite element modeling of the tests using the corrected data provides a better agreement with the measured loads. The grain microstructures vary with the test parameters and are used to correlate the flow behavior.

The effect of Mn on the high temperature flow stress of Al–Mg–Si alloys

The demand for Al–Mg–Si extrusion alloys in the transportation sector is increasing rapidly. An important aspect of the process is high temperature extrusion and in particular, the breakthrough force for the extrusion press. The breakthrough force varies as a function of the extrusion ratio, extrusion speed and temperature, the complexity of the profile and the flow stress of the alloy. Further, Mn and Cr are often added to the alloy which form dispersoids during homogenization which precipitation strengthen the alloy at high temperature. This work presents a new approach to calculate the volume fraction of dispersoids using electron microprobe analysis, electrical resistivity measurements and transmission electron microscopy (TEM). The new method substantially increases the accuracy of the volume fraction determination compared with traditional image analysis or TEM since the volume of material sampled is much larger avoiding effects of local segregation. Using the volume fraction and dispersoid size measurements, a high temperature flow stress model based on dislocation theory has been developed. It is shown to describe the high temperature flow stress measurements to within 5% in 95% of the cases. The model was validated by independent experimentation where it predicted the flow stress within 5% accuracy. The flow stress model can be used independently based on experimental quantification of the dispersoids or as part of a through process model which includes a KWN based precipitation model for dispersoids and then implemented as constitutive equation for a FEM based model of the extrusion process.

Strain energy method to determine static recrystallization volume fraction of microalloyed steel

Double compression method is an effective method to determine static recrystallization volume fraction of materials because there is a marked difference in flow stress between two passes, which is caused by static recrystallization. And now offset method that is based on some specific flow stress is widely used. However, this method is not suitable to determine static recrystallization volume fraction for all the materials, especially for those alloyed steels. As the measurement of yield or flow stress is not very sensitive to a subtle change in precipitation, strain energy is the integration of the flow stress and so it is more sensitive to the subtle change and so this method could be better. This study aims to present strain energy method to determine static recrystallization volume fraction in details by performing uniaxial compression test on a microalloyed steel. And it was found that there were two points worth to note while performing this method: one is that if work hardening plays a dominant role during compression process, static recrystallization volume fraction can be determined by analyzing double compression curve and its extrapolation, on the contrary if dynamic softening plays the dominant role, the static recrystallization volume fraction has to be determined with the assistance of curve from single pass compression test.

Flow stress correction for hot compression of titanium alloys considering temperature gradient induced heterogeneous deformation

Uniaxial compression is widely employed to investigate the constitutive behavior and microstructural developments of metallic materials at elevated temperatures. However, the heat exchange between work-piece and anvils leads to temperature gradient and inhomogeneous deformation within the work-piece, which weakens the reliability of the testing method. The problem is more serious for titanium alloys due to the poor thermal conductivity and high sensitivity of deformation resistance on temperature. To quantify the effect of temperature gradient, a coupled thermo-electric-mechanical FE model is developed for hot compression of TA15 titanium alloy with a Gleeble thermal simulator. The precise temperature control is achieved via a negative feedback control algorithm. It is found that the temperature gradient prior to deformation is sensitive to the electrical contact conductance between specimen and anvils as well as the heat transfer between the anvil and anvil base. Bulging is sensitive to temperature gradient rather than interfacial friction. The measured flow stress is smaller than the true value at large strain for the titanium alloy, which is contrary to conventional perception. Thus, the traditional flow stress correction method degrades the accuracy of flow stress measurement. A correction procedure is developed based on inverse analysis of the FE simulation. The results can reduce the systematic error of flow stress measurement of titanium alloys with Gleeble system.

Flow Stress Measurement and Dynamic Response Analysis of Hot Compression Test Machine at High Strain Rates

Flow Stress Measurement and Dynamic Response Analysis of Hot Compression Test Machine at High Strain Rates

Constitutive flow behaviour of austenite at low temperatures and its influence on bainite transformation characteristics of ausformed medium-carbon steel

In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low temperature ausforming in the bay between ferrite and bainite C-curves at 500 °C in order to impart high dislocation densities in the austenite prior to phase transformation. Two experimental high-silicon, medium carbon steels were suitably designed and processed for this study, with one steel containing small additions of 0.3Mo and 0.03Nb. Flow stress measurements were made using single-hit compression tests in the temperature range 300–900 °C in steps of 100 °C at different strain rates in the range 0.1–10 s−1 on a Gleeble simulator. Samples ausformed at 500 °C were isothermally held for 1 h at different transformation temperatures in the range of 300–400 °C to complete the bainitic transformation. Influence of strain induced bainite transformation on flow stress was obvious at 0.01 s-1, particularly at 300 and 400 °C. Despite enhanced nucleation in fine-grained steel B containing Nb + Mo, growth of bainite sheaves was much slower. Dilatation behaviour was comparable for the two steels at <350 °C, but at higher temperatures, the effect of Nb + Mo on slower transformation kinetics was obvious. The microstructure of both steels showed extremely fine bainitic ferrite below 325 °C, but at higher temperatures, coarse bainite with M/A constituents and extensive martensite formed in steels without or with Nb + Mo constituents. A correlation between hardness data and retained austenite contents has been established in both the steels. The paper presents the first account of the flow stress and transformation behaviour including the influence of Nb + Mo alloying and the details concerning the carbon-enriched austenite retained at room temperature and hardness variation as a function of isothermal holding temperature.

Optimization of Hot Workability of Cu-Cr-Zr-Ti Alloy Through Development of a Processing Map

Abstract Owing to high thermal conductivity, copper-based alloys are used to fabricate heat transfer elements of rocket engines, such as combustion chambers, nozzle liners, and metal gaskets. One such alloy is the Copper-Chromium-Zirconium-Titanium (Cu-Cr-Zr-Ti) alloy, which possesses a good combination of thermal conductivity and strength at high temperatures. Thermomechanical processing is an important step in the realization of the various aforementioned heat transfer elements from this alloy. Hence, it is imperative to understand the hot deformation behavior of the alloy as a function of the process variables such as temperature and strain rate. An attempt is made to study the hot deformation behavior of the Cu-Cr-Zr-Ti alloy by subjecting it to uniaxial hot compression tests in the temperature range of 600°C–800°C and strain rate range of 0.001–10 s−1. The flow stress measurement tests were conducted up to a true strain of 0.5 using a Gleeble 3800 simulator. The nature of the flow curves obtained at the completion of the tests indicated the occurrence of dynamic recrystallization (DRX) at deformation temperatures greater than 700°C and with a strain rate below 1 s−1. The work hardening rate was plotted as a function of effective flow stress to assess the flow softening behavior due to DRX at different deformation conditions. A processing map delineating the stable and unstable regions during hot working is developed and validated by observing the microstructures in the corresponding domains. Optimum processing parameters (temperature of 750°C and strain rate of 0.1–10 s−1) for hot working of the alloy were proposed based on a contour map of efficiency of power dissipation. Two constitutive equations are also established: one using the Hyperbolic sine law as proposed by Sellars and McTegart and the other using Kocks Mecking analysis.

History Dependence of the Microstructure on Time-Dependent Deformation During In-Situ Cooling of a Nickel-Based Single-Crystal Superalloy

Time-dependent plastic deformation through stress relaxation and creep deformation during in-situ cooling of the as-cast single-crystal superalloy CMSX-4® has been studied via neutron diffraction, transmission electron microscopy, electro-thermal miniature testing, and analytical modeling across two temperature regimes. Between 1000 °C and 900 °C, stress relaxation prevails and gives rise to softening as evidenced by a decreased dislocation density and the presence of long segment stacking faults in γ phase. Lattice strains decrease in both the γ matrix and γ′ precipitate phases. A constitutive viscoplastic law derived from in-situ isothermal relaxation test under-estimates the equivalent plastic strain in the prediction of the stress and strain evolution during cooling in this case. It is thereby shown that the history dependence of the microstructure needs to be taken into account while deriving a constitutive law and which becomes even more relevant at high temperatures approaching the solvus. Higher temperature cooling experiments have also been carried out between 1300 °C and 1150 °C to measure the evolution of stress and plastic strain close to the γ′ solvus temperature. In-situ cooling of samples using ETMT shows that creep dominates during high-temperature deformation between 1300 °C and 1220 °C, but below a threshold temperature, typically 1220 °C work hardening begins to prevail from increasing γ′ fraction and resulting in a rapid increase in stress. The history dependence of prior accumulated deformation is also confirmed in the flow stress measurements using a single sample while cooling. The saturation stresses in the flow stress experiments show very good agreement with the stresses measured in the cooling experiments when viscoplastic deformation is dominant. This study demonstrates that experimentation during high-temperature deformation as well as the history dependence of the microstructure during cooling plays a key role in deriving an accurate viscoplastic constitutive law for the thermo-mechanical process during cooling from solidification.

Unified Hall-Petch description of nano-grain nickel hardness, flow stress and strain rate sensitivity measurements

It is shown that: (i) nano-grain nickel flow stress and hardness data at ambient temperature follow a Hall-Petch (H-P) relation over a wide range of grain size; and (ii) accompanying flow stress and strain rate sensitivity measurements follow an analogous H-P relationship for the reciprocal “activation volume”, (1/v*) = (1/A*b) where A* is activation area. Higher temperature flow stress measurements show a greater than expected reduction both in the H-P kε and in v*. The results are connected with smaller nano-grain size (&amp;lt; ∼20 nm) measurements exhibiting grain size weakening behavior that extends to larger grain size when tested at very low imposed strain rates.

Variability of 4D flow parameters when subjected to changes in MRI acquisition parameters using a realistic thoracic aortic phantom.

To assess the variability of peak flow, mean velocity, stroke volume, and wall shear stress measurements derived from 3D cine phase contrast (4D flow) sequences under different conditions of spatial and temporal resolutions. We performed controlled experiments using a thoracic aortic phantom. The phantom was connected to a pulsatile flow pump, which simulated nine physiological conditions. For each condition, 4D flow data were acquired with different spatial and temporal resolutions. The 2D cine phase contrast and 4D flow data with the highest available spatio-temporal resolution were considered as a reference for comparison purposes. When comparing 4D flow acquisitions (spatial and temporal resolution of 2.0 × 2.0 × 2.0 mm3 and 40 ms, respectively) with 2D phase-contrast flow acquisitions, the underestimation of peak flow, mean velocity, and stroke volume were 10.5, 10 and 5%, respectively. However, the calculated wall shear stress showed an underestimation larger than 70% for the former acquisition, with respect to 4D flow, with spatial and temporal resolution of 1.0 × 1.0 × 1.0 mm3 and 20 ms, respectively. Peak flow, mean velocity, and stroke volume from 4D flow data are more sensitive to changes of temporal than spatial resolution, as opposed to wall shear stress, which is more sensitive to changes in spatial resolution. Magn Reson Med 79:1882-1892, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Uncertainty in flow stress measurements using X-ray diffraction for sheet metals subjected to large plastic deformations.

X-ray diffraction techniques have been developed to measure flow stresses of polycrystalline sheet metal specimens subjected to large plastic deformation. The uncertainty in the measured stress based on this technique has not been quantified previously owing to the lack of an appropriate method. In this article, the propagation of four selected elements of experimental error is studied on the basis of the elasto-viscoplastic self-consistent modeling framework: (1) the counting statistics error; (2) the range of tilting angles in use; (3) the use of a finite number of tilting angles; and (4) the incomplete measurement of diffraction elastic constants. Uncertainties propagated to the diffraction stress are estimated by conducting virtual experiments based on the Monte Carlo method demonstrated for a rolled interstitial-free steel sheet. A systematic report on the quantitative uncertainty is provided. It is also demonstrated that the results of the Monte Carlo virtual experiments can be used to find an optimal number of tilting angles and diffraction elastic constant measurements to use without loss of quality.

Elevations in MR Measurements of Whole Brain and Regional Cerebral Blood Flow and Oxygen Extraction Fraction Suggest Cerebral Metabolic Stress in Children with Sickle Cell Disease Unaffected By Overt Stroke

Positron emission tomography (PET) studies have shown that the brain increases cerebral blood flow (CBF) and oxygen extraction fraction (OEF, the fraction of oxygen brain tissue extracts from blood) when oxygen delivery is compromised in adult ischemic stroke. Children with sickle cell disease (SCD) have higher CBF compared to healthy children, suggesting that autoregulatory mechanisms, compensating for compromised oxygen delivery, may underlie the pathophysiology of ischemic stroke in SCD. Until now, evaluation of cerebral oxygen metabolism in children with SCD has been limited to measurement of CBF due to the radiation risks of PET. We used a MR sequence that measures voxel-wise OEF to test our hypothesis that children with sickle cell disease will have elevated whole brain and regional OEF when compared to typically developing, sibling controls without SCD.Thirty-six participants, 8 controls and 28 with SCD (26 HbSS and 2 HbSB0), ages 5-21 years, were recruited from St. Louis Children's Hospital. Participants underwent brain MRI with measurement of CBF via pseudo-continuous arterial spin labeling and OEF via a novel processing of asymmetric spin echo sequence that measures tissue deoxyhemoglobin. CBF and OEF maps were individually co-registered to corresponding T1 images with FMRIB's Linear Image Registration Tool, and gray and white matter were segmented with FMRIB's Automated Segmentation Tool. Visual inspection identified a region of high OEF within the deep white matter of the frontal and parietal lobes in the majority of subjects (figure 1 a,b). OEF maps from control and SCD participants were coregistered and averaged into a single map, and then an OEF threshold of 47.5% was applied to demarcate this "high OEF region" (figure 1c). Hemoglobin (Hb) and hematocrit were obtained in SCD participants, while these values were assumed for the controls. Arterial oxygen content (CaO2) was calculated as 1.36 x Hb x SpO2. Comparisons were made with a Mann-Whitney U test or Student's t-test. Bivariate correlations were tested with Kendall's tau b. Bonferroni correction was used in determining significance. Multivariate linear regression modeling with block entry described covariates associated with CBF.The control and SCD cohorts did not differ in age, gender or SpO2. SCD participants demonstrated higher whole brain, gray matter and white matter CBF and OEF when compared to controls (table 1, figure 1a-b), but there was no difference in whole brain or segmented measures of CBF and OEF between SCD participants with structurally normal brain MRIs (n=16) and silent infarcts (n=12). SCD participants' OEF was higher within the "high OEF region" when compared to controls (table 1), but the regional OEF did not differ between SCD participants with structurally normal brain MRIs versus silent infarcts. Whole brain CBF negatively correlated with age (b = -0.554, p < 0.001), while whole brain OEF did not (b= 0.014, p = 0.921). Lower CaO2 correlated with higher whole brain CBF (b = -0.329, p < 0.016) and higher whole brain OEF (b = -0.587, p < 0.001). CaO2 remained a predictor (β = -0.38, p = 0.009) of CBF when controlling for age (β = -0.63, p < 0.001).We report the first whole brain, segmented and regional analysis of oxygen metabolism, including CBF and OEF, in a pediatric SCD cohort unaffected by overt stroke. Elevation in both CBF and OEF in response to low arterial oxygen content suggests children with SCD are chronically compensating to meet the brain's metabolic demands. OEF is more elevated in children with SCD when compared to healthy controls within the "high OEF region," which coincides with previously reported locations of silent infarcts. We propose that the "high OEF region" provides a tissue signature of vulnerable, metabolically stressed brain that is at risk for future stroke. [Display omitted] [Display omitted] DisclosuresFields:Neurophage: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Model for adhesion clutch explains biphasic relationship between actin flow and traction at the cell leading edge

Cell motility relies on the continuous reorganization of a dynamic actin–myosin–adhesion network at the leading edge of the cell, in order to generate protrusion at the leading edge and traction between the cell and its external environment. We analyze experimentally measured spatial distributions of actin flow, traction force, myosin density, and adhesion density in control and pharmacologically perturbed epithelial cells in order to develop a mechanical model of the actin–adhesion–myosin self-organization at the leading edge. A model in which the F-actin network is treated as a viscous gel, and adhesion clutch engagement is strengthened by myosin but weakened by actin flow, can explain the measured molecular distributions and correctly predict the spatial distributions of the actin flow and traction stress. We test the model by comparing its predictions with measurements of the actin flow and traction stress in cells with fast and slow actin polymerization rates. The model predicts how the location of the lamellipodium–lamellum boundary depends on the actin viscosity and adhesion strength. The model further predicts that the location of the lamellipodium–lamellum boundary is not very sensitive to the level of myosin contraction.

A physiologic flow phantom for the evaluation of 4D flow MRI in the left ventricle

Background 4D PCMR has the potential to help in understanding the direct relationship between fluid and structure interaction within the cardiovascular system, to allow for 3D visualization of blood flow patterns in the heart, and to make direct wall shear stress measurements. However, the wide dynamic range of velocities in the left ventricle (LV, range of 0-200 cm/s) can create difficulties when optimizing acquisition protocols, especially when attempting to measure complex flow features during filling and diastasis. Additionally, there is no reference standard method to validate velocity flow and wall shear stress measurements. Here we describe our experience with a custom-built MRI compatible physiologic left heart flow phantom which can be used to optimize acquisition parameters, and can be used in the lab to make velocity measurements with particle image velocimetry (PIV).

Measurement of Flow Stress for Pure Aluminum up to 10 in Strain

A stress-strain curve is generally measured by tensile testing or compression testing. In some metal forming processes, the maximum strain exceeds 5.0 (500%); however, the standard testing methods can only measure an stress-strain curve up to a strain of 1.0. Therefore, a new method was developed that enables us to measure a stress-strain curve for strains up to 10.0. High-pressure torsion was applied for the measurement. Using high-pressure torsion, it is possible to introduce larger strains than conventional methods. A stress-strain curve measurement was performed for pure aluminum. The stress-strain curve measured using the developed method was compared with that obtained using a compression test. It is confirmed that the flow stress was successfully measured up to a strain of 10.0. In addition, the results for the developed method correspond with that of the compression test up to a strain of 1.0.

塑性異方性を考慮した高張力鋼板と軟鋼板の変形抵抗測定

塑性異方性を考慮した高張力鋼板と軟鋼板の変形抵抗測定