1,721,026 research outputs found
High-temperature superelasticity of Ni50.6Ti24.4Hf25.0 shape memory alloy
Full text linkChanges in transformation temperatures and high-temperature mechanical behavior of the new Ni50.6Ti34.4Hf25.0 alloy were determined after selected aging treatments. Differential scanning calorimetry was used to compare the homogenized, solution-treated and aged specimens for a wide combination of aging temperatures and times. Isothermal deformation experiments were conducted measuring reversible transformation strains with digital image correlation. Extraordinary superelasticity was found at test temperatures up to 300 °C for the specimen aged at 500 °C for 4 h
Molecular dynamics modeling of NiTi superelasticity in presence of nanoprecipitates
No full textThe presence of nano-sized coherent precipitates is well known to have crucial impact on the mechanical behaviors of a broad class of superelastic alloys. As a representative material, the pseudoelasticity of austenitic NiTi alloy in presence of a lenticular coherent Ni4Ti3 precipitate is investigated using atomic scale simulations. We predict the local stress gradient at the matrix-precipitate interface induced by inter-lattice atomic disregistry. The calculated stress distribution conforms to the latest high resolution electron microscopy measurements in the literature. Due to the presence of the local disturbance fields, the preference for activating different martensitic variants, given the uni-directionality thereof, is influenced substantially. The resultant constitutive attributes are thus observed to undergo adjustments in terms of reduced transformation stress, strain and hysteresis in general agreement with experimental literature
Crystal Plasticity Simulations of Haynes 230, an Analysis of Single Crystal and Polycrystalline Experiments
No full textThe behavior of a Ni-based superalloy, Haynes 230, was investigated at macro and micro scale level by means of a Crystal Plasticity (CP) model implemented in an open source Finite Element code, Warp3D. Single Crystal and polycrystalline specimens have been experimentally characterized with Digital Image Correlation (DIC) to identify the local strain field evolution. The results of single crystal’s tensile tests were used to obtain an estimation of the constitutive model parameters. Then a polycrystalline model, reproducing a tensile test with loading/unloading steps, was created starting from the microstructural data obtained with EBSD (electron back-scatter diffraction), which allowed the identification of grains geometry and orientations. The polycrystalline simulations were used to verify the prediction of the CP model over the experiment. The results of this study show that the comparison between experiments and numerical analysis is in good agreement on both global and local scale levels
Multi-scale crack closure measurements with digital image correlation on Haynes 230
Full text linkAn experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN –∆Keff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects
Finite element simulations of fatigue crack growth and closure
No full textA modified elastic-plastic finite element program was used to simulate fatigue crack growth and closure. The study has identified some of the factors that contribute to crack closure. It has provided an evaluation of stress, strain and displacement fields near crack tips and their contribution to crack closure. The thesis contains three parts:In the first part, the mechanism of crack closure under plane strain condition was explained. It is identified that material ahead of the crack tip contracts in the transverse direction, and this mechanism provides residual material on crack surfaces to cause crack closure. Stress-strain history and material displacements as the crack advances are presented to support this model.In the second part, a distinction is drawn between residual stresses in the absence of crack closure and those due to plasticity induced closure. A new crack tip parameter, S\sb{\rm tt}, is defined as the applied stress level corresponded to the development of tensile stresses immediately ahead of crack tips. The results demonstrated the importance of this parameter, as the stresses ahead of crack tips could remain compressive even when the crack surfaces opened.The notch effect on crack closure is studied in the third part of the thesis. Crack opening levels were obtained for crack growing from notches. A set of closure prediction equations was proposed to determine crack growth rate from notches for variable notch shape, applied maximum load level, R ratio and crack length from notch roots on different materials. The model has applied to a steel and an aluminum alloy, and the prediction of crack growth rate from notches were very satisfactory.Made available in DSpace on 2011-05-07T13:42:31Z (GMT). No. of bitstreams: 2
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Previous issue date: 1992Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:58:12Z
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Modelling of thermo-mechanical fatigue behavior in superalloys
No full textThermo-mechanical fatigue (TMF) and isothermal fatigue (IF) behavior of coated and uncoated Mar-M247 and, uncoated Mar-M246 was studied. The Mar-M247 was coated with Alpak-S1. The coating consists of a manganese and Aluminium powder slurry that is applied by painting, dipping, or spraying with an air brush.Thermo-mechanical experiments for coated Mar-M247 were conducted under T\sb{\rm min} = 500\sp\circC and T\sb{\rm max} = 871\sp\circC conditions. Both out-of-phase and in-phase strain-temperature phasing conditions were considered. Isothermal experiments were conducted at 500\sp\circC and 871\sp\circC. Comparison with the uncoated Mar-M247 showed that fatigue lives of coated material could be lower than the uncoated material and the difference was as much as four folds in some cases. For uncoated Mar-M247, only thermo-mechanical out-of-phase experiments were conducted. The temperature limit considered was T\sb{\rm min} = 500\sp\circC and T\sb{\rm max} = 1038\sp\circC. SEM, Microprobe and Auger spectroscopy were used to gain insight into oxidation effects at high temperature.An experimental program for Mar-M246 was also carried out. Due to the coarse grained structure, the stress-strain response indicates variation from specimen to specimen in some cases. The effect of this variation on life does not appear significant, except at smaller strain ranges.A numerical method is also developed to estimate the stress field due to a surface inhomogeneity in an elastic half space. The technique is based on Eshelby's equivalent inclusion method. The results were used in developing a life prediction methodology for the coated superalloys.Made available in DSpace on 2011-05-07T13:30:24Z (GMT). No. of bitstreams: 2
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Previous issue date: 1992Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:55:37Z
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An investigation into the mechanisms of cone bore growth in railroad bearings
No full textThe mechanisms of cone bore growth in railroad bearings have been investigated. Systematic studies of (1) stress-induced transformation of retained austenite to martensite, (2) thermal-induced transformation of retained austenite to bainite, and (3) low temperature creep have been conducted. Both carburized and uncarburized 4320 steels were tested. The retained austenite contents (A\sb{\rm ret}) of the two carburized steels were 35% and 14%.Stress-induces transformation was studied by conducting monotonic and cyclic tests at temperatures ranging from 22\sp\circC to 150\sp\circC. Stress-induced transformation was only observed in the A\sb{\rm ret} = 35% steel at temperatures below 60\sp\circC when a tensile stress was applied. Under a uniaxial tensile stress, the transformation strain was anisotropic; the axial transformation strain exceeded the diametral.Thermal-induced transformation occurred primarily between 80\sp\circC and 200\sp\circC only in the A\sb{\rm ret} = 35% steel. The transformation rate increased with temperature and with positive stress. Unlike stress-induced transformation, thermal-induced transformation also occurred under a compressive stress. The anisotropy of the transformation strain was dependent on stress direction and magnitude.Creep strains of at least 0.05% at stresses near the 0.001% offset yield strength were measured at 70\sp\circC and above in all treatments of the steel. The creep rates obeyed a logarithmic creep law and increased with temperature and stress magnitude.The fatigue life of the A\sb{\rm ret} = 35% steel was a factor of 10 greater than the other steels. A compressive mean stress developed from the volume increase associated with stress-induced transformation within the first four cycles.The Three Ring Model was developed to simulate a cone as an assemblage of the outer case, core, and inner case. The model simulated the dimensional changes at the bore during both service and recondition treatment after service. Results indicated that both creep and thermal-induced transformation can cause bore growth during service. However, only creep can explain the recovery of the residual stresses during the recondition treatment.Made available in DSpace on 2011-05-07T12:12:29Z (GMT). No. of bitstreams: 2
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Previous issue date: 1991Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:38:06Z
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The fatigue crack growth behavior of superelastic nickel titanium
Full text linkUnderstanding of fatigue crack growth phenomena in shape memory alloys under superelastic conditions continues to elude the scientific community: The stress induced martensite phase causes deviations of stress intensity and state at the tip from the classical handbook solutions widely relied on for crack growth characterization. In this work, novel methods of stress intensity factor extraction based on in situ and ex situ Digital Image Correlation are implemented to characterize the effective fatigue damage tolerance of NiTi single crystals under superelastic conditions. It is found that, due to crack tip stress induced martensite transformation, NiTi exhibits shielding (reduction from theoretical stress intensity factor calculations) of about 30\% on average. The stress intensity reduction levels from the superelastic austenite, stable martensite and stable austenite are also compared. It is found that the stable austenite-martensite crystal structures exhibit almost identical K reduction, attributed to plastic flow, while the superelastic austenite structure, exhibiting higher reduction levels, demonstrates the shielding effect of the stress induced transformation.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2021-12-01The student, Jad Yaacoub, accepted the attached license on 2019-12-11 at 12:19.The student, Jad Yaacoub, submitted this Thesis for approval on 2019-12-11 at 12:23.This Thesis was approved for publication on 2019-12-12 at 09:19.DSpace SAF Submission Ingestion Package generated from Vireo submission #14789 on 2020-02-28 at 17:24:23Made available in DSpace on 2020-03-02T22:18:19Z (GMT). No. of bitstreams: 2
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An investigation into thermomechanical fatigue of metal matrix composites
No full textExperimental and theoretical approaches are used to characterize the thermomechanical deformation behavior of metal matrix composites. Experiments on unreinforced and SiC particulate reinforced Al 2xxx-T4 have been conducted under several mechanical strain-temperature phasing conditions. Based on stress range, substantial improvements in fatigue life have been observed. However, based on strain range, the effect of reinforcement on fatigue lives differs depending on the mechanical strain-temperature phasing, temperature, and strain rate. Several deformation mechanisms of unreinforced and reinforced Al 2xxx-T4 have been identified, including void formation, crack initiation, intergranular/transgranular crack growth, oxide penetration at the crack tips, crack deflection due to particle interference, and mean stress effects.Theoretical approaches include the development of a general micromechanistic constitutive equation, based on Eshelby's equivalent inclusion theory, and a life prediction methodology for metal matrix composites. Synergistic effects of particulate reinforcement on high temperature thermomechanical behavior are studied. The constitutive model provides insight into the internal stress-strain behavior, including effective and hydrostatic stresses, of both the matrix and the reinforcement developed during cyclic loading conditions. The deformation behavior of the constituents is used to develop an experimentally based micromechanistic life prediction model. The damage caused by internal stresses, oxidation, creep, and fatigue mechanisms as a function of reinforcement volume fraction is quantified for wide range of loading conditions.Made available in DSpace on 2011-05-07T11:59:18Z (GMT). No. of bitstreams: 2
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Previous issue date: 1992Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:35:11Z
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Modeling of twinning as a deformation mechanism in iron (bcc) crystals and iron-based alloys
Full text linkWe develop an analytical expression for twin nucleation stress in bcc metals considering GPFE
(generalized planar fault energy) and the dislocations bounding the twin nucleus. We minimize
the total energy to predict the twinning stress relying only on parameters that are obtained
through atomistic calculations, thus excluding the need for any empirical constants. We validate
the present approach by means of precise measurements of onset of twinning in bcc Fe50Cr
single crystals showing excellent agreement. The experimental observations of the three slip
systems resulting in the twin formation was demonstrated via EBSD and TEM techniques along
with DIC (Digital Image Correlation).
In addition, the present work is geared towards understanding the twin-slip and twin-twin
interactions in alpha -Fe crystals using Molecular Dynamics (MD) technique. Five types of twintwin
and twin-slip intersections have been analyzed, namely , , , ,
, and the magnitude of the residual dislocation left at the twin boundary for each type of
intersection was identified. Further, the role of the residual dislocations in affecting the
magnitude of the critical stress required for twin migration has been established for each
intersection type. We are able to investigate the Schmid factor criteria for slip and twin
nucleation under tensile and compressive loading orientations, and the results obtained are in
close agreement with the theoretical critical resolved shear stress (CRSS) of the activated
systems.
Furthermore, the results obtained through MD simulations and experiments utilizing EBSD and
DIC are used to investigate the effect of the residual dislocations on the energetics of twin-slip
and twin-twin interactions. An analytical expression is developed based on the geometrical
parameters such as twin width and length, and the magnitude of the residual dislocation that
quantifies the critical stress required for twin migration. Energy analysis based on the total
elastic energies of the interacting dislocations and the associated fault energies reveal a strong
dependence of the twin migration stress as a function of the magnitude of the residual
dislocation. A higher magnitude of the residual dislocation causes an increase in the twin-migration stress and makes the transmission of slip/twin more difficult through the coherent twin boundary (CTB).Item withdrawn by Mark Zulauf ([email protected]) on 2013-04-23T14:29:14Z
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Release Date: 2015-05-28 14:21:22 UTC
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