Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “strain”

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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

In-Situ SEM High Strain Rate Testing of Large Diameter Micropillars Followed by TEM and EBSD Postmortem Analysis

Background Dislocation dynamic simulations are intended as a tool to understand and predict the mechanical behavior of metallic materials, but its prediction has never been directly verified by experiments due to differences in specimen strain rate and size. Objective In this work, a comprehensive experimental framework is proposed to attempt direct comparison between experiments and discrete dislocation dynamics (DDD) modelling. Methods By integrating high-throughput sample fabrication and a customized testing apparatus, the sample size and strain rate typically employed in DDD simulations are explored experimentally. Constitutive properties such as stress-strain response are measured, and microstructural information is obtained from transmission electron microscopy (TEM) imaging, electron backscatter diffraction (EBSD), and TEM-based orientation mapping. Results Magnesium and copper were selected, as case studies, to demonstrate the newly developed experimental procedure. Measured stress-strain responses for Mg are consistent with those obtained with a miniaturized Hopkison bar experiments. Furthermore, by exploiting the validated workflow, the effect of strain rate on micropillar heterogeneous deformation and associated dislocation plasticity were revealed. Conclusion The work establishes a methodology for the systematic study of not only metals but also other materials and structures at the microscale and high strain rates.

42 ENGINEERING↗
DOE OSTI2018

Effects of Strain Rate on Tensile Deformation and Fracture Behavior of Directionally Solidified Superalloy DZ125L

The tensile deformation and fracture behavior of directionally solidified superalloy DZ125L were studied with a strain rate ranging from 10{sup −3} to 7 × 10{sup 2} s{sup −1} at room temperature. Results show that the strength of DZ125L alloy presents positive strain rate sensitivity in tested strain rate range which is mainly resulted from the increasing accumulation of dislocations in the matrix. The ductility of the alloy first increases and then it remains insensitive with the increase in strain rates. The enhancement of ductility is primarily due to the increasing amount of broken MC carbides and plastic deformation in the matrix at higher strain rates. The strain rates influence the amount of broken MC carbides and then affect the deformation behavior of the alloy. Based on observations and discussions of the lateral microstructures and dislocations, it is concluded that the MC carbides with sizes larger than 10 μm are easy broken in the early deformation stage and act as crack sources during the proceeding plastic deformation. The growth and connection of microcracks and the propagation of main crack lead to the fracture of the matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Determination of Strain Rate Sensitivity of Micro-struts Manufactured Using the Selective Laser Melting Method

Micro-lattice structures manufactured using the selective laser melting (SLM) process provides the opportunity to realize optimal cellular materials for impact energy absorption. In this paper, strain rate-dependent material properties are measured for stainless steel 316L SLM micro-lattice struts in the strain rate range of 10{sup −3} to 6000 s{sup −1}. At high strain rates, a novel version of the split Hopkinson Bar has been developed. Strain rate-dependent materials data have been used in Cowper–Symonds material model, and the scope and limit of this model in the context of SLM struts have been discussed. Strain rate material data and the Cowper–Symonds model have been applied to the finite element analysis of a micro-lattice block subjected to drop weight impact loading. The model output has been compared to experimental results, and it has been shown that the increase in crush stress due to impact loading is mainly the result of strain rate material behavior. Hence, a systematic methodology has been developed to investigate the impact energy absorption of a micro-lattice structure manufactured using additive layer manufacture (SLM). This methodology can be extended to other micro-lattice materials and configurations, and to other impact conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

An improved Armstrong–Frederick-Type Plasticity Model for Stable Cyclic Stress–Strain Responses Considering Nonproportional Hardening

This paper modified an Armstrong–Frederick-type plasticity model for investigating the stable cyclic deformation behavior of metallic materials with different sensitivity to nonproportional loadings. In the modified model, the nonproportionality factor and nonproportional cyclic hardening coefficient coupled with the Jiang–Sehitoglu incremental plasticity model were used to estimate the stable stress–strain responses of the two materials (1045HR steel and 304 stainless steel) under various tension–torsion strain paths. A new equation was proposed to calculate the nonproportionality factor on the basis of the minimum normal strain range. Procedures to determine the minimum normal strain range were presented for general multiaxial loadings. Then, the modified model requires only the cyclic strain hardening exponent and cyclic strength coefficient to determine the material constants. It is convenient for predicting the stable stress–strain responses of materials in engineering application. Comparisons showed that the modified model can reflect the effect of nonproportional cyclic hardening well.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Strain-Hardening Behavior of Dual-Phase Steel under Multistress States

The complexity and nonuniformity of microdeformation in high-strength dual-phase (DP) steels make it difficult to accurately predict the deformation processes in stamping. Here, in situ experiments under three kinds of stress states and a crystal plasticity finite element (CPFE) model were used to predict the strain-hardening behavior of DP steel. The microdeformation in various deformation stages was extracted from metallographs captured during in situ experiments to calculate the strain partitioning functions of each phase. Then, the multistress state mechanical behavior of DP steel was calculated based on the CPFE model combined with the strain partitioning functions of ferrite and martensite. The calculation results showed that (1) the CPFE model could accurately describe the load–stroke curves of the three experiments and that (2) the stress state had a significant effect on the strain partition ratio and the initial critical resolved shear stress of the material. In addition, the stress–strain curves of DP800 steel were predicted under six stress states; these curves did not coincide and had obvious deviations. Finally, the activity of the grain slip systems and the dislocation evolution were analyzed and discussed to predict the microdeformation and stress state effects on strain-hardening behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High Strain Rate Response of Cenosphere-Filled Aluminum Alloy Syntactic Foam

Deformation of metal foams under high rate of loading is a complex phenomenon due to the effects of various parameters involved therein. In the present investigation, cenosphere-filled aluminum alloy syntactic foam is studied under high rate of loading in comparison with their quasi-static behavior. The experiments, for high strain rates, are carried out using split Hopkinson pressure bar and full stress–strain curves of foam are developed under such rate of loadings. Foams with three different cenosphere sizes at three different high rates of loadings are investigated for their mechanical behavior. Compressive behavior and energy absorption capacity are reported considering the effect of high loading rates and cenosphere sizes. It is observed that, increase in loading rate results in higher strength of foams by an amount of 16-32%. Further, it is observed that energy absorption is improved with the increase in strain rates and cenosphere sizes and this improvement is observed in the range of 80-182%. Based on deformation modes and failure damage study, using SEM microstructure, deformation mechanism is observed to be almost independent of strain rate increment particularly at higher strain rates considered in this investigation. Moreover, there exists a limiting value of strain rate beyond which there is an insignificant increase in compressive strength of foams considered in the present investigation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Correlation of Tensile Deformation-Induced Strain in HSLA Steel with Residual Stress Distribution

During service, due to the dynamic loading rates encountered, structural components experience varying levels of plastic deformation which leads to changes in the mechanical behavior of materials. The dynamic loading leads to non-homogenous plastic deformation which changes the residual stress distribution. In this study, the magnitude and distribution of residual stress in tensile tested specimens of high-strength low-alloy steel were investigated using x-ray diffraction technique, by varying the deformation rate. Tensile testing was performed at four different strain rates, and residual stress measurement was done along the length of the fractured specimens. Distribution of residual stress was observed all along the length of the specimens which is attributed to the varying levels of plastic deformation. Measured residual stresses are correlated with elastic strain and plastic strain obtained from the tensile tests. Consistent decrease in the magnitude of residual stress with an increase in plastic strain is observed with decreasing strain rate at the necking region. Specimen with lower yield strength experienced higher plastic strains, hence lower elastic residual stresses. The reasons for the observed distribution of residual stress are discussed with XRD peak broadening associated with plastic deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Experimental Measurement and Analysis of Stress/Shock Wave Propagation Speed through Pre-strained Silicone Foam Pads under Lateral Confinement

Foam materials are increasingly utilized in aerospace, military, and transportation applications to mitigate blast or shock impact. When foam materials are subjected to an external high-speed impact, shock, or blast loading, an elastic wave or shock wave will form and propagate through the thickness of the foam materials. Here, silicone foam pads, which were confined laterally and pre-strained to different levels, were experimentally characterized and theoretically analyzed to understand their effects on wave propagation characteristics under impact loading. Depending on impact velocity, either an elastic strain wave or a shock wave would be generated in the silicone foam pad with different pre-strains. Above a certain impact velocity, a shock wave will be generated whereas, below this threshold impact velocity, an elastic strain wave will be generated. This threshold impact velocity depends on the pre-strain applied to the silicone foam pad. Equations are provided to estimate the wave propagation speed for either an elastic or a shock wave from the amount of pre-strain in the silicone foam pads and the impact velocity. These equations are expected to help improve silicone foam design and assembly processes for shock or blast mitigation applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Landau levels in uniaxially strained graphene: A geometrical approach

The effect of strain on the Landau levels (LLs) spectra in graphene is studied, using an effective Dirac-like Hamiltonian which includes the distortion in the Dirac cones, anisotropy and spatial-dependence of the Fermi velocity induced by the lattice change through a renormalized linear momentum. We propose a geometrical approach to obtain the electron’s wave-function and the LLs in graphene from the Sturm–Liouville theory, using the minimal substitution method. The coefficients of the renormalized linear momentum are fitted to the energy bands, which are obtained from a Density Functional Theory (DFT) calculation. In particular, we evaluate the case of Dirac cones with an ellipsoidal transversal section resulting from uniaxially strained graphene along the Arm-Chair (AC) and Zig-Zag (ZZ) directions. We found that uniaxial strain in graphene induces a contraction of the LLs spectra for both strain directions. Also, is evaluated the contribution of the tilting of Dirac cone axis resulting from the uniaxial deformations to the contraction of the LLs spectra. - Highlights: • The LLs in uniaxially strained graphene are found using a geometrical approach. • The energy of the LLs in function of the Dirac cone deformation is presented. • We found that uniaxial strain in graphene induces a contraction of the LLs spectra. • Contraction in LLs spectra depends on the geometrical parameters of the Dirac cone.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Influence of strain on SrFeO 3-δ oxidation, reduction, and water dissociation: Insights from ambient pressure X-ray photoelectron spectroscopy

The oxidation and reduction of metal oxides and their interaction with the environment play a critical role in their use for energy storage and conversion applications. The formation of surface adsorbates and their impact on oxide electronic structure can be challenging to probe experimentally, particularly for thin films which enable the study of epitaxial strain but have low surface areas. Here we present a detailed study using ambient pressure X-ray photoelectron spectroscopy of the reduction and oxidation of strained SrFeO 3-δ , induced by changes in temperature, oxygen partial pressure, and water vapor exposure. We find that in comparison to the oxidized lattice, application of tensile strain promotes the formation of oxygen vacancies at the surface, facilitating the subsequent incorporation of oxygen into the lattice in an oxygen environment. While tensile strain does promote the formation of hydroxyls from water dissociation, the kinetics of this process appear more sluggish than on less-strained surfaces, likely due to a competitive surface interaction with oxygen or bulk absorption of hydroxyl species. These findings relating strain, oxygen vacancies, and surface reactivity yield important insight into the surface functionality of oxides for energy conversion and storage applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Influence of strain-rate on the response of elastomeric architected materials

Architected materials have shown substantial promise in impact mitigation and protective applications, and there has accordingly been great interest in better characterizing their response at elevated strain rates due to impact. There remains ambiguity regarding the contribution of inertial and material responses to strain rate sensitivity, and, in particular, when these effects begin to gain dominance in the impact response of an architected material. The response of soft polymer architected materials as a function of strain rate, in particular, has been little investigated. We characterize the experimental impact response of four soft polymer architected lattice geometries across varying strain rates in the intermediate strain rate regime (∼10 3 s −1 ) using split-Hopkinson pressure bar loading and high speed video characterization of the resulting deformation fields. In conclusion, our results highlight the interplay of influence between constituent material, lattice geometry, length scale, and strain rate in determining the onset of significant inertia effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Deformation-induced spatiotemporal fluctuation, evolution and localization of strain fields in a bulk metallic glass

Deformation behavior and local strain evolutions upon loading and unloading of a bulk metallic glass (BMG) were systematically investigated by in situ digital image correlation (DIC). Distinct fluctuations and irreversible local strains were observed before the onset of macroscopic yielding. Statistical analysis shows that these fluctuations might be related to intrinsic structural heterogeneities, and that the evolution history and characteristics of local strain fields play an important role in the subsequent initiation of shear bands. Effects of sample size, pre-strain, and loading conditions were systematically analyzed in terms of the probability distributions of the resulting local strain fields. It is found that a higher degree of local shear strain heterogeneity corresponds to a more ductile stressestrain curve. Implications of these findings are discussed for the design of new materials.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Study on the mechanism of diametral cladding strain and mixed-oxide fuel element breaching in slow-ramp extended overpower transients

Cladding strain caused by fuel/cladding mechanical interaction (FCMI) was evaluated for mixed-oxide fuel elements subjected to 70–90% slow-ramp extended overpower transient tests in the experimental breeder reactor II. Calculated transient-induced cladding strains were correlated with cumulative damage fractions (CDFs) using cladding strength correlations. In a breached high-smeared density solid fuel element with low strength cladding, cladding thermal creep strain was significantly increased to approximately half the transient-induced cladding strain that was considered to be caused by the tertiary creep when the CDF was close to the breach criterion (=1.0), with the remaining strain due to instantaneous plastic deformation. In low-smeared density annular fuel elements, FCMI load was significantly mitigated and resulted in little cladding strain. The CDFs of the annular fuel elements were lower than 0.01 at the end of the overpower transient, indicating a substantial margin to breach. A substantial margin to breach was also maintained in a high-smeared density fuel element with high strength cladding.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2013

Study on grain boundary character and strain distribution of intergranular cracking in the CGHAZ of T23 steel

Intergranular reheat cracking in the coarse-grained heat-affected zone of T23 steel was produced by strain to fracture tests on a Gleeble 3500 thermal–mechanical simulator. Then the grain boundary character, as well as the strain distribution after reheat crack propagation, was studied by electron backscatter diffraction technique. The results showed that incoherent Σ3 boundaries were seldom found on the prior austenite grain boundaries. Therefore, only the type of random high-angle boundaries played a crucial role in the intergranular cracking. Microstructurally cavities and small cracks were preferentially initiated from high-angle grain boundaries. Low-angle grain boundaries and high-angle ones with misorientation angles less than 15° were more resistant to the cracking. More importantly, the fraction of high-angle grain boundaries increased with the plastic strain induced by both temperature gradient and stress in the coarse-grained heat-affected zone, which contributed to the crack initiation and propagation. Furthermore, the strain distributions in the vicinity of cavities and cracks revealed the accommodation processes of plastic deformation during stress relaxation. It also reflected the strength differences between grain interior and grain boundary at different heat-treated temperatures, which had a large influence on the cracking mechanism. - Highlights: • The coincidence site lattice boundaries play little role in the reheat cracking. • Cavity and crack occur at high-angle grain boundaries rather than low-angle ones. • The strain leads low-angle grain boundaries to transform to high-angle ones. • Strain distribution differs for cavity and crack zones at different temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

In situ X-ray diffraction strain-controlled study of Ti–Nb–Zr and Ti–Nb–Ta shape memory alloys: crystal lattice and transformation features

Phase and structure transformations in biomedical Ti–21.8Nb–6.0Zr (TNZ) and Ti–19.7Nb–5.8Ta (TNT) shape memory alloys (at.%) under and without load in the − 150 to 100 °S temperature range are studied in situ using an original tensile module for a low-temperature chamber of an X-ray diffractometer. Alpha″- and beta-phase lattice parameters, the crystallographic resource of recovery strain, phase and structure transformation sequences, and microstress appearance and disappearance are examined, compared and discussed. For both alloys, the crystallographic resource of recovery strain decreases with temperature increase to become 4.5% for TNZ and 2.5% for TNT alloy (at RT). Loading at low temperatures leads to additional α″-phase formation and reorientation. Heating under load, as compared to strain-free heating, affects the reverse transformation sequence of both alloys in different ways. For TNZ alloy, strain-free heating results in simultaneous ω→β and α″→β transformations, whereas during heating under stress, they are sequential: β + ω→α″ precedes α″→β. For TNT alloy, strain-free heating results in reverse α″→β transformation, whereas during heating under stress, α″→β transformation is preceded by α″-phase reorientation. - Highlights: • Comparative in situ XRD analysis of Ti–Nb–Zr(Ta) shape memory alloys is realized. • Lattice parameters of β- and α″-phases are calculated in the − 150 to + 100 °C range. • The higher the temperature, the lower the α″→β transformation strain. • Loading at low temperatures results in α″-phase formation and reorientation. • Transformation sequences upon heating with and without loading are different.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thickness-dependent optical properties in compressively strained BiFeO{sub 3}/LaAlO{sub 3} films grown by pulsed laser deposition

Graphical abstract: - Highlights: • BFO with various thicknesses was grown on LAO substrates by pulsed laser deposition. • The structure and compressive strains were clarified via Raman scattering. • The charge transfer excitation was blue shifted with increasing compressive strain. • The compressive strain affects the distortion of Fe{sup 3+} local environment and O 2p states. - Abstract: Bismuth ferrite (BiFeO{sub 3}) films with various thicknesses were epitaxially grown on LaAlO{sub 3} substrates by pulsed laser deposition. The X-ray diffraction and Raman scattering spectra reveal that the films were highly (11{sup ¯}1) oriented with the single phase. With increasing the thickness, the compressive strain decreases and the strain ratios between the film and bulk crystal are evaluated to be 1.75, 1.57, and 1. Moreover, the compressive strain induces band gap shrinkage from 2.7 to 2.65 eV, while the charge transfer transition energy increases from 3.5 to 4.1 eV. It could be due to the shift of O 2p states and the variation of local Fe{sup 3+} crystal field.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Graphene-elastomer nanocomposites based flexible piezoresistive sensors for strain and pressure detection

Highlights: • Fabricating flexible piezoresistive sensors based on PDMS/GNPs nanocomposites. • Positive strain sensitive capacity occurs during increasing pressure-loading process but before the threshold of forces loading. • Tunable strain and pressure sensitivity are achieved by controlling GNPs incorporation, and improved to 140 and 260 at 5 wt% GNPs respectively. • Superior dynamic correspondence with applied pressure and relatively high repeatability. • Application of the GNPs/PDMS piezoresistive sensors as detecting the finger motions. - Abstract: Strain and pressure sensitive flexible piezoresistive sensors have been fabricated based on nanocomposites of graphene nanoplatelets (GNPs) and Polydimethylsiloxane (PDMS) elastomer. The strain detective sensors show positive piezoresistive characteristic under diverse levels of static strain up to 20% due to the flexibility and stretchability of PDMS elastomer. Tunable strain sensitivity of GNPs/PDMS piezoresistive sensors can be achieved by controlling GNPs incorporation into PDMS elastomer, which will be improved to 140 at 5 wt% GNPs concentration. In addition, pressure sensitive response, which exists when the pressure loading is less than threshold pressure, also can be observed in GNPs/PDMS piezoresistive sensors. The GNPs network-elastomer nanocomposite based pressure sensors show tunable piezoresistivity in the ranges of ∼260 under increasingly higher GNPs concentrations. During the cyclic pressure loading, the pressure sensitive behavior of GNPs/PDMS nanocomposites manifests a superior dynamic correspondence with applied pressure while the resistance response can retain at a relatively high level under multiple loading-unloading cycles. Since the GNPs/PDMS piezoresistive sensors can not only respond to subtle bending but also distinguish them accurately for detecting the finger motions, it will make the GNPs/PDMS nanocomposite a good candidate for artificial skin and wearable sensor applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗