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M D Ediger - One of the best experts on this subject based on the ideXlab platform.
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measurement of segmental mobility during constant strain Rate Deformation of a poly methyl methacrylate glass
Macromolecules, 2014Co-Authors: Benjamin Bending, Kelly Christison, Josh Ricci, M D EdigerAbstract:We describe an apparatus for performing constant strain Rate Deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at Tg – 19 K, for local strain Rates between 3.7 × 10–5 and 1.2 × 10–4 s–1. In these experiments, the mobility initially increases in the preyield regime, by a factor of 40–160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain Rates lead to higher mobility in the postyield regime, and for the range of strain Rates investigated, mobility and strain Rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that Deformation leads to a narrowing of the distribution of segmental relaxation times. ...
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measurement of segmental mobility during constant strain Rate Deformation of a poly methyl methacrylate glass
arXiv: Materials Science, 2013Co-Authors: Benjamin Bending, Kelly Christison, Josh Ricci, M D EdigerAbstract:We describe an apparatus for performing constant strain Rate Deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at Tg - 19 K, for local strain Rates between 3.7x10-5/s and 1.2x10-4/s. In these experiments, the mobility initially increases in the pre-yield regime, by a factor of 40 to 160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain Rates lead to higher mobility in the post-yield regime and, for the range of strain Rates investigated, mobility and strain Rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that Deformation leads to a narrowing of the distribution of segmental relaxation times. These last three observations are consistent with previously reported constant stress experiments on PMMA glasses. The experimental features reported here are compared to computer simulations and theoretical models.
M A Meyers - One of the best experts on this subject based on the ideXlab platform.
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spontaneous and forced shear localization in high strain Rate Deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1999Co-Authors: Y J Chen, M A Meyers, V F NesterenkoAbstract:High-strain-Rate shear localization was induced in tantalum by (a) lowering the Deformation temperature or (b) subjecting it to high strains by dynamic Deformation (up to ot 0.8) or (c) pre-shocking (at oeff 0.22) and then deforming it. Although at ambient temperature the Deformation of tantalum is macroscopically uniform to high strains (ot$ 0.8), at 77 K shear localization under the same loading condition was developed at a critical strain of 0.2 to 0.3. This higher propensity to shear localization at low temperatures is a direct consequence of the combination of lower heat capacity and higher Rate of thermal softening. At the three temperatures investigated (77, 190 and 298 K), localization occurs at strains significantly higher than the instability strains (the maxima of the adiabatic stress‐strain curves for these three temperatures). The thicknesses of the forced localization regions and shear bands were found to be a function of temperature, and decreased with decreasing temperature (at the same strain) in accord with the equation proposed by Y. Bai et al. (Y. Bai, C. Cheng, S. Yu, Acta Mechanica Sinica 2 (1986) 1). Shock Deformation of tantalum enhances its predisposition to subsequent shear localization, and this was demonstRated by subjecting shocked and unshocked specimens to high strain, high strain Rate Deformation through the collapse of a thick-walled cylinder assembly. © 1999 Elsevier Science S.A. All rights reserved.
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effect of strain Rate on plastic flow and failure in polycrystalline tungsten
Acta Materialia, 1998Co-Authors: T Dummer, Guruswami Ravichandran, J C Lasalvia, M A MeyersAbstract:Polycrystalline tungsten (less than 100 p.p.m. impurities) was subjected to different heat treatments to yield different grain morphologies and tested at quasi-static (3×10^(−3)) and dynamic (10^3–4×10^3/s) strain Rates. Three mechanisms of Deformation were identified and evaluated: slip, twinning, and intergranular cracking. Whereas plastic flow by slip has considerable strain-Rate sensitivity in tungsten (which is found to be well represented by the Mechanical Threshold Stress constitutive equation) the cohesive strength of the grain boundaries was found to decrease with heat treatment temperature, but was insensitive to strain-Rate changes. Low-strain-Rate Deformation yielded limited damage at strains as high as 0.25, whereas high-strain-Rate Deformation led to catastrophic failure at strains between 0.05 and 0.10. Slip and grain-boundary decohesion being competing Deformation mechanisms, the material undergoes a ductile-to-brittle transition as the strain Rate is increased from 10^(−3) to 10^3/s. Two failure modes are identified: debonding initiated by shear along a grain-boundary facet (similar to the wing-crack mechanism) and debonding initiated at voids. The interactions between microcracks and twins are characterized, and there is both evidence of fracture initiation at twins (intergranular cracks), and twin initiation at cracks (transgranular cracks). Calculations based on existing wing-crack models enable the estimation of the grain-boundary cohesive energies.
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shear localization and chemical reaction in high strain high strain Rate Deformation of ti si powder mixtures
Acta Materialia, 1998Co-Authors: H C Chen, J C Lasalvia, V F Nesterenko, M A MeyersAbstract:Ti-Si mixtures were subjected to high-strain-Rate Deformation at a pressure below the threshold for shock-wave initiation. Whereas the collapse of interparticle pores did not initiate reaction, regions of localized macro-Deformation initiated reaction inside shear bands at suAciently high strains (g010), and propagation of the reaction through the entire specimen at higher strains (g020-40). This study demon- stRates that temperature increases in shear localization regions can initiate chemical reaction inside a reac- tive powder mixture. The shear band spacing was 00.6-1 mm. Thermodynamic and kinetic calculations yield the reaction Rate outside the shear bands, in the homogeneously deformed material, which has a
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shear localization and recrystallization in high strain high strain Rate Deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1997Co-Authors: V F Nesterenko, M A Meyers, J C Lasalvia, Y J Chen, M P Bondar, Ya L LukyanovAbstract:Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain Rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-geneRated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved geneRated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (eeff 2.5, T > 1000 K). Whereas the post-Deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the Deformation time is not sufficient to geneRate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (eeff > 1). Residual tensile ‘hoop’ stresses are geneRated near the central hole region upon unloading; this resulted in ductile fracturing along shear localization bands.
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shear localization in high strain Rate Deformation of granular alumina
Acta Materialia, 1996Co-Authors: V F Nesterenko, M A Meyers, H C ChenAbstract:Abstract Dynamic Deformation of densified granular alumina of two different particle sizes was investigated by the radial symmetric collapse of a thick-walled cylinder. The densified granular alumina was used to model the flow in ballistic impact and penetration of fragmented ceramic armor. Shear localization was a well developed Deformation mode at an overall radial strain of ∼0.2–0.4 and strain Rate of 10 4 s −1 . The following qualitative features of shear bands were established: • Shear bands have clear boundaries and their thickness does not depend on the initial particle size and has a typical value ∼10 μm. • The structure of the shear bands was dependent on initial particle size, suggesting differences in the mechanisms of flow. For the ∼4 μm alumina, comminution (break-up) and softening of particles were observed. For the ∼0.4 μm particles, a peculiar structure consisting of a central crack with two lateral cracks was formed. • Distributions of shear bands and displacement magnitudes were dependent on initial particle size. The observed differences in powder behavior are associated with different mechanisms of powder repacking. For large particles (∼4 μm), additional hardening resulting from microfracture and subsequent repacking of different size particles in the powder takes place. The small-sized (∼0.4 μm) ceramic does not go through the particle fracturing stage and the hardening is due to “classical” repacking.
Sunghak Lee - One of the best experts on this subject based on the ideXlab platform.
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understanding of adiabatic shear band evolution during high strain Rate Deformation in high strength armor steel
Journal of Alloys and Compounds, 2020Co-Authors: Selim Kim, Dae Woong Kim, Hyung Keun Park, Sung Suk Hong, Hong Kyu Kim, Hyoung Seop Kim, Seok Su Sohn, Sunghak LeeAbstract:Abstract The microstructural evolution and formation mechanism of adiabatic shear band (ASB) in a high-strength armor steel were investigated using a laboratory-scale split Hopkinson pressure bar, and the results were correlated with the actual ballistic impact behavior. The interrupted dynamic compressive test results reveal that a deformed ASB (dASB) starts to form right after the stress collapse and it develops into a transformed ASB (tASB). In the ballistic impact, wide tASBs form mostly at the perfoRated surface, and narrower tASBs are branched from the tASB. Very fine equiaxed grains of ∼190 nm in the tASB developed during the dynamic compression indicates that the dynamic recrystallization occurs even in 86.5 μs, and then the grains grow up to 260 nm in 9.5 μs. Rotational dynamic recrystallization mechanism and grain-growth Rate model were proposed based on the calculation of temperature rise from a thermo-elasto-plastic finite element method, which provide a reasonable explanation for the formation and growth of fine equiaxed grains during both the dynamic compression and ballistic impact. A linkage of equiaxed subgrains and elongated parent subgrains demonstRates that the equiaxed subgrains did not evolve from nucleation and growth processes but from the sub-boundary rotation. Based on the underlying formation mechanisms and kinetics of ASBs, this study would suggest a reliable method to interpret the ASB formation and associated fracture mechanism during the ballistic impact.
Benjamin Bending - One of the best experts on this subject based on the ideXlab platform.
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measurement of segmental mobility during constant strain Rate Deformation of a poly methyl methacrylate glass
Macromolecules, 2014Co-Authors: Benjamin Bending, Kelly Christison, Josh Ricci, M D EdigerAbstract:We describe an apparatus for performing constant strain Rate Deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at Tg – 19 K, for local strain Rates between 3.7 × 10–5 and 1.2 × 10–4 s–1. In these experiments, the mobility initially increases in the preyield regime, by a factor of 40–160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain Rates lead to higher mobility in the postyield regime, and for the range of strain Rates investigated, mobility and strain Rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that Deformation leads to a narrowing of the distribution of segmental relaxation times. ...
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measurement of segmental mobility during constant strain Rate Deformation of a poly methyl methacrylate glass
arXiv: Materials Science, 2013Co-Authors: Benjamin Bending, Kelly Christison, Josh Ricci, M D EdigerAbstract:We describe an apparatus for performing constant strain Rate Deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at Tg - 19 K, for local strain Rates between 3.7x10-5/s and 1.2x10-4/s. In these experiments, the mobility initially increases in the pre-yield regime, by a factor of 40 to 160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain Rates lead to higher mobility in the post-yield regime and, for the range of strain Rates investigated, mobility and strain Rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that Deformation leads to a narrowing of the distribution of segmental relaxation times. These last three observations are consistent with previously reported constant stress experiments on PMMA glasses. The experimental features reported here are compared to computer simulations and theoretical models.
V F Nesterenko - One of the best experts on this subject based on the ideXlab platform.
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path dependent high strain strain Rate Deformation of polymer toroidal elements
Journal of Applied Physics, 2014Co-Authors: Chienwei Lee, V F NesterenkoAbstract:The dynamic behavior of toroidal elements (o-rings) is investigated at the range of global engineering strains up to 0.7 and strain Rates about 100 s−1. It was observed that the corresponding average dynamic stiffness of rubber toroidal elements increases up to 3 times in comparison with their quasistatic compression. The viscoelastic dynamic model using linear strain-Rate dependence and Hertz damped model did not satisfactory agree with experimental data in investigated range of strains and strain-Rates. In order to reflect experimental results, a modified viscoelastic model with power-law strain-Rate dependence was proposed. Path dependent Deformation of o-rings with different levels of pre-compression was investigated under dynamic loading conditions. It was found that dynamic response of pre-compressed o-rings at the initial strain range of 0.04–0.25 is similar to the behavior of uncompressed o-rings, but further increasing pre-compression to 0.4 and 0.5 results in different force-strain curves demonstrating memory effect. This phenomenon is explained using a model incorporating dependence of dynamic force on initial pre-compression introducing critical level of dynamic strain, after which memory of initial pre-compression fades. This model predicts that force history of weakly compressed o-rings (initial strain 4%) on the stage of loading represents an envelope for all other data in agreement with experiments. In all cases, the dynamic behavior was characterized by stiffer force-displacement curves in comparison with quasistatic compression of o-rings.
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spontaneous and forced shear localization in high strain Rate Deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1999Co-Authors: Y J Chen, M A Meyers, V F NesterenkoAbstract:High-strain-Rate shear localization was induced in tantalum by (a) lowering the Deformation temperature or (b) subjecting it to high strains by dynamic Deformation (up to ot 0.8) or (c) pre-shocking (at oeff 0.22) and then deforming it. Although at ambient temperature the Deformation of tantalum is macroscopically uniform to high strains (ot$ 0.8), at 77 K shear localization under the same loading condition was developed at a critical strain of 0.2 to 0.3. This higher propensity to shear localization at low temperatures is a direct consequence of the combination of lower heat capacity and higher Rate of thermal softening. At the three temperatures investigated (77, 190 and 298 K), localization occurs at strains significantly higher than the instability strains (the maxima of the adiabatic stress‐strain curves for these three temperatures). The thicknesses of the forced localization regions and shear bands were found to be a function of temperature, and decreased with decreasing temperature (at the same strain) in accord with the equation proposed by Y. Bai et al. (Y. Bai, C. Cheng, S. Yu, Acta Mechanica Sinica 2 (1986) 1). Shock Deformation of tantalum enhances its predisposition to subsequent shear localization, and this was demonstRated by subjecting shocked and unshocked specimens to high strain, high strain Rate Deformation through the collapse of a thick-walled cylinder assembly. © 1999 Elsevier Science S.A. All rights reserved.
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shear localization and chemical reaction in high strain high strain Rate Deformation of ti si powder mixtures
Acta Materialia, 1998Co-Authors: H C Chen, J C Lasalvia, V F Nesterenko, M A MeyersAbstract:Ti-Si mixtures were subjected to high-strain-Rate Deformation at a pressure below the threshold for shock-wave initiation. Whereas the collapse of interparticle pores did not initiate reaction, regions of localized macro-Deformation initiated reaction inside shear bands at suAciently high strains (g010), and propagation of the reaction through the entire specimen at higher strains (g020-40). This study demon- stRates that temperature increases in shear localization regions can initiate chemical reaction inside a reac- tive powder mixture. The shear band spacing was 00.6-1 mm. Thermodynamic and kinetic calculations yield the reaction Rate outside the shear bands, in the homogeneously deformed material, which has a
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shear localization and recrystallization in high strain high strain Rate Deformation of tantalum
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1997Co-Authors: V F Nesterenko, M A Meyers, J C Lasalvia, Y J Chen, M P Bondar, Ya L LukyanovAbstract:Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain Rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-geneRated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved geneRated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (eeff 2.5, T > 1000 K). Whereas the post-Deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the Deformation time is not sufficient to geneRate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (eeff > 1). Residual tensile ‘hoop’ stresses are geneRated near the central hole region upon unloading; this resulted in ductile fracturing along shear localization bands.
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shear localization in high strain Rate Deformation of granular alumina
Acta Materialia, 1996Co-Authors: V F Nesterenko, M A Meyers, H C ChenAbstract:Abstract Dynamic Deformation of densified granular alumina of two different particle sizes was investigated by the radial symmetric collapse of a thick-walled cylinder. The densified granular alumina was used to model the flow in ballistic impact and penetration of fragmented ceramic armor. Shear localization was a well developed Deformation mode at an overall radial strain of ∼0.2–0.4 and strain Rate of 10 4 s −1 . The following qualitative features of shear bands were established: • Shear bands have clear boundaries and their thickness does not depend on the initial particle size and has a typical value ∼10 μm. • The structure of the shear bands was dependent on initial particle size, suggesting differences in the mechanisms of flow. For the ∼4 μm alumina, comminution (break-up) and softening of particles were observed. For the ∼0.4 μm particles, a peculiar structure consisting of a central crack with two lateral cracks was formed. • Distributions of shear bands and displacement magnitudes were dependent on initial particle size. The observed differences in powder behavior are associated with different mechanisms of powder repacking. For large particles (∼4 μm), additional hardening resulting from microfracture and subsequent repacking of different size particles in the powder takes place. The small-sized (∼0.4 μm) ceramic does not go through the particle fracturing stage and the hardening is due to “classical” repacking.
