Search NASA⌕ Search

SEARCH · Search NASA

Results for “FLOW STRESS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Mechanical annealing in the flow of supercooled metallic liquid

Flow induced structural evolution in a supercooled metallic liquid Vit106a (Zr{sub 58.5}Cu{sub 15.6}Al{sub 10.3}Ni{sub 12.8}Nb{sub 2.8}, at. %) was investigated via uni-axial compression combined with differential scanning calorimeter (DSC). Compression tests at strain rates covering the transition from Newtonian flow to non-Newtonian flow and at the same strain rate 2 × 10{sup −1} s{sup −1} to different strains were performed at the end of glass transition (T{sub g-end} = 703 K). The relaxation enthalpies measured by DSC indicate that the samples underwent non-Newtonian flow contain more free volume than the thermally annealed sample (703 K, 4 min), while the samples underwent Newtonian flow contain less, namely, the free volume of supercooled metallic liquids increases in non-Newtonian flow, while decreases in Newtonian flow. The oscillated variation of the relaxation enthalpies of the samples deformed at the same strain rate 2 × 10{sup −1} s{sup −1} to different strains confirms that the decrease of free volume was caused by flow stress, i.e., “mechanical annealing.” Micro-hardness tests were also performed to show a similar structural evolution tendency. Based on the obtained results, the stress-temperature scaling in the glass transition of metallic glasses are supported experimentally, as stress plays a role similar to temperature in the creation and annihilation of free volume. In addition, a widening perspective angle on the glass transition of metallic glasses by exploring the 3-dimensional stress-temperature-enthalpy phase diagram is presented. The implications of the observed mechanical annealing effect on the amorphous structure and the work-hardening mechanism of metallic glasses are elucidated based on atomic level stress model.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Flow and arrest in stressed granular materials

Flowing granular materials often abruptly arrest if not driven by sufficient applied stresses. Such abrupt cessation of motion can be economically expensive in industrial materials handling and processing, and is significantly consequential in intermittent geophysical phenomena such as landslides and earthquakes. Using discrete element simulations, we calculate states of steady flow and arrest for granular materials under the conditions of constant applied pressure and shear stress, which are also most relevant in practice. Here the material can dilate or compact, and flow or arrest, in response to the applied stress. Our simulations highlight that under external stress, the intrinsic response of granular materials is characterized by uniquely-defined steady states of flow or arrest, which are highly sensitive to interparticle friction. While the flowing states can be equivalently characterized by volume fraction, coordination number or internal stress ratio, to characterize the states of shear arrest, one needs to also consider the structural anisotropy in the contact network. Here, we highlight the role of dilation in the flow-arrest transition, and discuss our findings in the context of rheological transitions in granular materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Analysis of Hot- and Cold-Rolled Loads in Medium-Mn TRIP Steels

The purpose of this work is to investigate the hot- and cold-rolling requirements to produce third-generation advanced high-strength steels (AHSS). Therefore, five medium-Mn (10 to 14 wt pct Mn) alloys that exhibit transformation-induced plasticity (TRIP) were compared to a commercially produced grade of AISI 1018 using hot- and cold-rolling experiments. Experimental data collected from a STANAT instrumented rolling mill was utilized to measure force and torque during hot- and cold-rolling. Experimental data were processed by a 1D analytical model, based on Orowan model, to determine rolling pressure. It was determined that pressures required to hot-roll TRIP alloys are 1.4 to 1.8 times greater than pressures for rolling AISI 1018 steel. Cold rolling of the medium-Mn TRIP steels was found to be 1.5 to 2.8 times greater than the AISI 1018 steel. Mechanical and microstructural characterization was also performed and the variation in rolling pressure was related to the starting microstructural constituents, and alloys containing greater starting quantities of ε-martensite in the microstructure had higher flow stresses at equivalent rolling strains during cold rolling.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Study of Thermomechanical Behavior of Zirconium-0.3 Tin Alloy

Zirconium-based alloys are used in the manufacturing of nuclear fuel tubes. These tubes need to be clad with zirconium-tin (Zr-0.3 wt.% Sn) alloy to enhance the lives of tubes. Manufacturing of such clad tubes is a challenging task, due to work hardening and dynamic softening phenomenon. In this study, hot deformation behavior of Zr-0.3Sn alloy is investigated at different temperatures and strain rates. It is found that fully recrystallized microstructure can be obtained by working at around 900 °C and high strain rate of 10 s{sup −1}. Further, constitutive equations are developed for the entire domain of deformation range and fitted into standard FEM-based simulation model for extrusion. The simulation results are validated with experimental data. The difference in flow stresses values obtained from numerical and experimental methods was around 14%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Structural representation of additively manufactured 316L austenitic stainless steel

Three 316L stainless steel materials are studied and reported upon; wrought, as-built additively manufactured (AM), and heat-treated AM material. The AM material was produced from the laser engineered net shaping (LENS) process. Macroscopic uniaxial compression stress-strain curves were obtained for all three materials. The curves were similar for the wrought and heat-treated AM materials but the as-built AM material demonstrated approximately 1.7 times greater flow stress at any given level of strain than the other two materials. Electron-backscatter diffraction analysis of the materials also showed that the microstructures of the three materials differed; with complex grain morphology for the as-built AM material. The mean grain size of each of the three materials also differed. Furthermore, the initial dislocation density was also measured with neutron diffraction and line-profile analysis for both wrought and as-built AM materials with the density in the AM material approximately 2.5 times greater.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Shock Response of Commercial Purity Polycrystalline Magnesium Under Uniaxial Strain at Elevated Temperatures

Here, results of plate impact experiments designed to investigate the onset of incipient plasticity in commercial purity polycrystalline magnesium (99.9%) under weak uniaxial strain compression and elevated temperatures up to melt are presented. The dynamic stress at yield and post yield of magnesium, as inferred from the measured normal component of the particle velocity histories at the free (rear) surface of the target plate, are observed to decrease progressively with increasing test temperatures in the range from 23 to 500 °C. At (higher) test temperatures in the range 500–610 °C, the rate of decrease of dynamic stress with temperature at yield and post-yield in the sample is observed to weaken. At still higher test temperatures (617 and 630 °C), a dramatic increase in dynamic yield as well as flow stress is observed indicating a change in dominant mechanism of plastic deformation as the sample approaches the melt point of magnesium at strain rates of ~105/s. In addition to these measurements at the wavefront, the plateau region of the free surface particle velocity profiles indicates that the longitudinal (plastic) impedance of the magnesium samples decreases continuously as the sample temperatures are increased from room to 610 °C, and then reverses trend (indicating increasing material longitudinal impedance/ strength) as the sample temperatures are increased to 617 and 630 °C. Lastly, electron back scattered diffraction analysis of the as-received and annealed pre-test magnesium samples reveal grain coarsening as well as grain re-orientation to a different texture during the heating process of the samples.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Systematic Assessment of Various Stability/Instability Criteria in Predicting the Hot Deformation-Related Instabilities in Super-304H Stainless Steels

In this study, different existing stability/instability criteria (viz Prasad, Murty, Gegel, Alexander, Jonas, and Semiatin) are used to characterize the hot deformation instabilities of super-304H stainless steel. The instability maps are developed based on flow stress data acquired from uniaxial hot compression tests performed at various temperatures (1173-1423 K) and strain rates (0.001-10 s{sup −1}). The deformation instabilities are further validated through detailed microstructural observations. Flow localization and cracking have been identified as the signature of deformation instabilities in this alloy. The microcracks have been found to be associated with the grain boundary precipitation of coarse (~ 1- to 5-µm-sized) Nb-rich particles (mainly NbC). The instability criteria developed based on dynamic material modeling (i.e., Prasad’s and Murty’s criteria) could accurately predict the formation of microcracks in the entire hot deformation range employed in this study. Albeit the Gegel’s and Alexander’s stability criteria could correctly predict the flow localization, it has overestimated the deformation instabilities at high temperatures (> 1250 K) and low strain rates (≤ 0.1 s{sup −1}). In contrast, Jonas’ and Semiatin’s instability criteria have under-predicted the unstable domains in the studied alloy.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Computer-aided analysis and design of the shape rolling process for producing turbine engine airfoils

Mild steel (AISI 1018) was selected as model cold rolling material and Ti-6A1-4V and Inconel 718 were selected as typical hot rolling and cold rolling alloys, respectively. The flow stress and workability of these alloys were characterized and friction factor at the roll/workpiece interface was determined at their respective working conditions by conducting ring tests. Computer-aided mathematical models for predicting metal flow and stresses, and for simulating the shape rolling process were developed. These models utilized the upper bound and the slab methods of analysis, and were capable of predicting the lateral spread, roll separating force, roll torque, and local stresses, strains and strain rates. This computer-aided design system was also capable of simulating the actual rolling process, and thereby designing the roll pass schedule in rolling of an airfoil or a similar shape.

Lahoti, G. D.↗
NASA NTRS1978

Computer-aided analysis and design of the shape rolling process for producing turbine engine airfoils

Mild steel (AISI 1018) was selected as model cold-rolling material and Ti-6Al-4V and INCONEL 718 were selected as typical hot-rolling and cold-rolling alloys, respectively. The flow stress and workability of these alloys were characterized and friction factor at the roll/workpiece interface was determined at their respective working conditions by conducting ring tests. Computer-aided mathematical models for predicting metal flow and stresses, and for simulating the shape-rolling process were developed. These models utilize the upper-bound and the slab methods of analysis, and are capable of predicting the lateral spread, roll-separating force, roll torque and local stresses, strains and strain rates. This computer-aided design (CAD) system is also capable of simulating the actual rolling process and thereby designing roll-pass schedule in rolling of an airfoil or similar shape. The predictions from the CAD system were verified with respect to cold rolling of mild steel plates. The system is being applied to cold and hot isothermal rolling of an airfoil shape, and will be verified with respect to laboratory experiments under controlled conditions.

Lahoti, G. D.↗
DOE OSTI2018

Experimental Study on the Hot Deformation Characterization of Low-Carbon Nb-V-Ti Microalloyed Steel

X80 steel, as typical low-carbon Nb-V-Ti microalloyed steel, is getting more and more attention in oil and gas transmission pipeline manufacturing. In this paper, the hot deformation behavior of X80 steel has been investigated. Hot compression tests of the steel were conducted under different temperatures and strain rates. Based on the experimental data, the flow stress constitutive equations were established. It is found that the hot deformation activation energy of this steel is higher than C-Mn and low-carbon steel. Then, the kinetics model and grain size model of dynamic recrystallization were developed according to the flow curves and the optical microstructures. In addition, the processing maps were developed to analyze the workability of X80 steel at elevated temperature. The analysis results show that the optimum processing window is at the temperature of 1300-1473 K and the strain rate of 0.01-10 s{sup −1}. The microstructure observation indicates that the optimum processing parameters are applicable to the tested steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A Correction to the Stress–Strain Curve During Multistage Hot Deformation of 7150 Aluminum Alloy Using Instantaneous Friction Factors

Multistage stress–strain curve correction based on an instantaneous friction factor was studied for axisymmetric uniaxial hot compression of 7150 aluminum alloy. Experimental friction factors were calculated based on continuous isothermal axisymmetric uniaxial compression tests at various deformation parameters. Then, an instantaneous friction factor equation was fitted by mathematic analysis. After verification by comparing single-pass flow stress correction with traditional average friction factor correction, the instantaneous friction factor equation was applied to correct multistage stress–strain curves. The corrected results were reasonable and validated by multistage relative softening calculations. This research provides a broad potential for implementing axisymmetric uniaxial compression in multistage physical simulations and friction optimization in finite element analysis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Deformation Temperature on the Microstructure and Mechanical Properties of High-Strength Low-Alloy Steel During Hot Compression

The microstructure and mechanical properties of high-strength low-alloy steel were investigated at deformation temperatures of 800-1100 °C and strain rates of 0.1-10 s{sup −1} using an MMS-200 thermal mechanical simulator. The results indicated that the increased deformation processes observed between the starting and finishing temperatures during hot compression testing caused a polygonal ferrite transformation in the material. The polygonal ferrite grain sizes increased with increasing transformation temperatures and gradually grew larger at higher deformation temperatures. Widmanstätten ferrite and acicular ferrite were also formed at high temperatures from 1000-1100 °C, which accordingly led to an increase in Vickers microhardness. In addition, the flow stress in the material increased with an increase in the strain and a decrease in the deformation temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Physics-based Constitutive Model for the Hot Deformation of 2Cr11Mo1VNbN Martensitic Stainless Steel

The hot deformation behavior of 2Cr11Mo1VNbN martensitic stainless steel is investigated through isothermal compression tests between 1173 and 1423 K with strain rates of 0.005-5 s{sup −1}. Based on experimental results, the work hardening–dynamic recovery stage and dynamic softening stage are evaluated, and a couple-stage constitutive model is developed to describe the work hardening–dynamic recovery and dynamic softening behaviors. Phenomenological constitutive models, which consider temperature, strain and strain rate, are established. A comparison between the predicted and experimental flow stress values indicates that the established models accurately describe the hot deformation behaviors for the studied supercritical steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Cellular Automaton Modeling of Dynamic Recrystallization of Nimonic 80A Superalloy Based on Inhomogeneous Distribution of Dislocations Inside Grains

A modified cellular automaton (CA) model in the time and space scale is developed to simulate the dynamic recrystallization (DRX) mechanism of Nimonic 80A nickel-base superalloy during hot deformation. Some necessary thermo-mechanical parameters used as input data in CA approach such as work hardening and softening coefficient are fitted according to the previous hot compression experiments. Based on the experimental results, nucleation kinetics is modified as a function of Zener-Holloman parameter. A dislocation density model at transient recrystallization boundary is proposed, in which the dislocation density is a function of temperature, strain rate and annealed dislocation density. The simulated results are in good agreement with experimental results, proving that the modified CA model is capable of predicting both flow behavior and microstructural evolution of Nimonic 80A during hot deformation. According to the experimental and simulated results, the DRX behavior is discussed in detail. It demonstrates that the flow stress and microstructural evolution of DRX have a relationship with the deformation temperature and strain rate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

CTH Calculations of Richtmyer-Meshov Instability Growth

We have used the hydrodynamics code CTH to simulate Richtmyer-Meshkov (R-M) flow from the surfaces of shock-released samples of copper and tin with parallel sinusoidal grooves across a diameter. For copper, using strength and damage parameters from previous spall recompression experiments, we find good agreement with proton radiographs and velocimetry that show spike and bubble growth, but the ultimate lengths of the spikes are too large. Agreement with experiments can be improved by adding strength to the flow stress model in CTH. For tin, which melted upon shock release in the R-M studies, the strength model does not matter much, and good agreement was obtained there also.

Richtmyer-Meshov flow, spall, ejecta, flow stress ↗
DOE OSTI2023

Microscopic activated dynamics theory of the shear rheology and stress overshoot in ultradense glass-forming fluids and colloidal suspensions

We formulate a particle and force level, activated dynamics-based statistical mechanical theory for the continuous startup nonlinear shear rheology of ultradense glass-forming hard sphere fluids and colloidal suspensions in the context of the elastically collective nonlinear Langevin equation approach and a generalized Maxwell model constitutive equation. Activated structural relaxation is described as a coupled local-nonlocal event involving caging and longer range collective elasticity which controls the characteristic stress relaxation time. Theoretical predictions for the deformation-induced enhancement of mobility, the onset of relaxation acceleration at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady-state structural relaxation time and viscosity, a nonvanishing activation barrier in the shear thinning regime, an apparent Herschel–Buckley form of the shear rate dependence of the steady-state shear stress, exponential growth of different measures of a yield or flow stress with packing fraction, and reduced fragility and dynamic heterogeneity under deformation were previously shown to be in good agreement with experiments. The central new question we address here is the defining feature of the transient response—the stress overshoot. In contrast to the steady-state flow regime, understanding the transient response requires an explicit treatment of the coupled nonequilibrium evolution of structure, elastic modulus, and stress relaxation time. We formulate a new quantitative model for this aspect in a physically motivated and computationally tractable manner. Theoretical predictions for the stress overshoot are shown to be in good agreement with experimental observations in the metastable ultradense regime of hard sphere colloidal suspensions as a function of shear rate and packing fraction, and accounting for deformation-assisted activated motion appears to be crucial for both the transient and steady-state responses.

Mechanics↗
DOE OSTI2018

Extraordinary effect of the δ phase on the electrically-assisted deformation responses of a Ni-based superalloy

Highlights: • Dislocation-free ring is firstly found around the δ phase in the EA tension. • Remarkable reduction of flow stress with more second phase during the EA tension • Increase instead of a slight decrease of elongation with more second phase in EA tension • Local Joule heating effect is considered as the major mechanism. - Abstract: The effect of the δ phase (Ni{sub 3}Nb) on the electrically-assisted (EA) deformation responses, particularly stress, elongation and dislocation distribution, of the Ni-based superalloy is still unknown up to now, though it is clear as nonnegligible on the quasi-static and hot deformation processes. Therefore, the effect of the δ phase during the electrically-assisted tension were experimentally investigated and analyzed with comparison to that in the hot tension process. Generally, the δ phase existing in the Ni-based superalloy always causes the reduction of flow softening and elongation during cold and the initial stage of hot tension processes. What is distinct, the δ phase promotes the flow softening effect and elongation during the EA tension. In addition, the dislocation-free ring around the δ phase was firstly observed during the EA tension. The mechanism of these macro and micro extraordinary phenomena is concluded as that the δ phase causes serious lattice distortion and dislocation pile-up around itself, which leads to an extremely high temperature due to the local Joule heating effect. The faster elimination of dislocation pile-up in the local area because of the local higher temperature results in the dislocation-free ring phenomenon and the significant improvement of flow softening effect and elongation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Hot deformation behaviors and processing maps of B{sub 4}C/Al6061 neutron absorber composites

In this study, the hot deformation behaviors of 30 wt.% B{sub 4}C/Al6061 neutron absorber composites (NACs) have been investigated by conducting isothermal compression tests at temperatures ranging from 653 K to 803 K and strain rates from 0.01 to 10 s{sup −1}. It was found that, during hot compression, the B{sub 4}C/Al6061 NACs exhibited a steady flow characteristic which can be expressed by the Zener-Hollomon parameter as a hyperbolic-sine function of flow stress. High average activation energy (185.62 kJ/mol) of B{sub 4}C/Al6061 NACs is noted in current study owing to the high content of B{sub 4}C particle. The optimum hot working conditions for B{sub 4}C/Al6061 NACs are found to be 760–803 K/0.01–0.05 s{sup −1} based on processing map and microstructure evolution. Typical material instabilities are thought to be attributed to void formation, adiabatic shear bands (ASB), particle debonding, and matrix cracking. Finally, the effect of the plastic deformation zones (PDZs) on the microstructure evolution in this 30 wt.% B{sub 4}C/Al6061 composite is found to be very important. - Highlights: •The hot deformation behavior of the 30 wt.% B{sub 4}C/Al6061 NACs was first analyzed. •The 3D efficiency map and the instability map are developed. •The optimum hot working conditions were identified and validated by SEM and TEM. •The hot deformation schematic diagram of 30 wt.% B{sub 4}C/Al6061 NACs is developed.

36 MATERIALS SCIENCE↗