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T W Wright - One of the best experts on this subject based on the ideXlab platform.
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the dynamic growth of a single void in a viscoplastic material under transient Hydrostatic Loading
Journal of The Mechanics and Physics of Solids, 2003Co-Authors: K T Ramesh, T W WrightAbstract:We have examined the problem of the dynamic growth of a single spherical void in an elastic-viscoplastic medium, with a view towards addressing a number of problems that arise during the dynamic failure of metals. Particular attention is paid to inertial, thermal and rate-dependent effects, which have not previously been thoroughly studied in a combined setting. It is shown that the critical stress for unstable growth of the void in the quasistatic case is strongly affected by the thermal softening of the material (in adiabatic calculations). Thermal softening has the effect of lowering the critical stress, and has a stronger influence at high strain hardening exponents. It is shown that the thermally diffusive case for quasistatic void growth in rate-dependent materials is strongly affected by the initial void size, because of the length scale introduced by the thermal diffusion. The effects of inertia are quantified, and it is demonstrated that inertial effects are small in the early stages of void growth and are strongly dependent on the initial size of the void and the rate of Loading. Under supercritical Loading for the inertial problem, voids of all sizes achieve a constant absolute void growth rate in the long term. Inertia first impedes, but finally promotes dynamic void growth under a subcritical Loading. For dynamic void growth, the effect of rate-hardening is to reduce the rate of void growth in comparison to the rate-independent case, and to reduce the final relative void growth achieved.
Djimedo Kondo - One of the best experts on this subject based on the ideXlab platform.
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3d fem formulations of limit analysis methods for porous pressure sensitive materials
International Journal for Numerical Methods in Engineering, 2013Co-Authors: Franck Pastor, Djimedo Kondo, Joseph PastorAbstract:SUMMARY The first purpose of this paper is the numerical formulation of the three general limit analysis methods for problems involving pressure-sensitive materials, that is, the static, classic, and mixed kinematic methods applied to problems with Drucker–Prager, Mises–Schleicher, or Green materials. In each case, quadratic or rotated quadratic cone programming is considered to solve the final optimization problems, leading to original and efficient numerical formulations. As a second purpose, the resulting codes are applied to non-classic 3D problems, that is, the Gurson-like hollow sphere problem with these materials as matrices. To this end are first presented the 3D finite element implementations of the static and kinematic classic methods of limit analysis together with a mixed method formulated to give also a purely kinematic result. Discontinuous stress and velocity fields are included in the analysis. The static and the two kinematic approaches are compared afterwards in the Hydrostatic Loading case whose exact solution is known for the three cases of matrix. Then, the static and the mixed approaches are used to assess the available approximate criteria for porous Drucker–Prager, Mises–Schleicher, and Green materials. Copyright © 2013 John Wiley & Sons, Ltd.
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Plastic limit state of the hollow sphere model with non-associated Drucker-Prager material under isotropic Loading
Computational Materials Science, 2012Co-Authors: Long Cheng, Géry De Saxcé, Yun Jia, Abdelbacet Oueslati, Djimedo KondoAbstract:The paper is devoted to the determination of plastic limit state of a hollow sphere with a Drucker-Prager matrix and subjected to Hydrostatic Loading. There are two possible plastic regimes corresponding respectively to the tensile and compressive stresses. For the associated case (dilation angle equal to the friction angle), the collapse is complete (the whole sphere is plastified) with a unique regime. For the non-associated cases, we consider weaker solutions (partial collapse and regime change). Nevertheless, we show the collapse is complete and exhibits a single regime. Consequently, the collapse stress field and the limit load do not depend on the value of the dilation angle. This theoretical result is confirmed by numerical simulations.
Donald J Weidner - One of the best experts on this subject based on the ideXlab platform.
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high energy x ray focusing and applications to pair distribution function investigation of pt and au nanoparticles at high pressures
Scientific Reports, 2016Co-Authors: Xinguo Hong, Thomas S. Duffy, Lars Ehm, Zhong Zhong, Sanjit Ghose, Donald J WeidnerAbstract:We report development of micro-focusing optics for high-energy x-rays by combining a sagittally bent Laue crystal monchromator with Kirkpatrick-Baez (K–B) X-ray focusing mirrors. The optical system is able to provide a clean, high-flux X-ray beam suitable for pair distribution function (PDF) measurements at high pressure using a diamond anvil cell (DAC). A focused beam of moderate size (10–15 μm) has been achieved at energies of 66 and 81 keV. PDF data for nanocrystalline platinum (n-Pt) were collected at 12.5 GPa with a single 5 s X-ray exposure, showing that the in-situ compression, decompression and relaxation behavior of samples in the DAC can be investigated with this technique. PDFs of n-Pt and nano Au (n-Au) under quasi-Hydrostatic Loading to as high as 71 GPa indicate the existence of substantial reduction of grain or domain size for Pt and Au nanoparticles at pressures below 10 GPa. The coupling of sagittally bent Laue crystals with K–B mirrors provides a useful means to focus high-energy synchrotron X-rays from a bending magnet or wiggler source.
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pressure induced stiffness of au nanoparticles to 71 gpa under quasi Hydrostatic Loading
Journal of Physics: Condensed Matter, 2015Co-Authors: Xinguo Hong, Thomas S. Duffy, Donald J WeidnerAbstract:The compressibility of nanocrystalline gold (n-Au, 20 nm) has been studied by x-ray total scattering using high-energy monochromatic x-rays in the diamond anvil cell under quasi-Hydrostatic conditions up to 71 GPa. The bulk modulus, K0, of the n-Au obtained from fitting to a Vinet equation of state is ~196(3) GPa, which is about 17% higher than for the corresponding bulk materials (K0: 167 GPa). At low pressures (<7 GPa), the compression behavior of n-Au shows little difference from that of bulk Au. With increasing pressure, the compressive behavior of n-Au gradually deviates from the equation of state (EOS) of bulk gold. Analysis of the pair distribution function, peak broadening and Rietveld refinement reveals that the microstructure of n-Au is nearly a single-grain/domain at ambient conditions, but undergoes substantial pressure-induced reduction in grain size until 10 GPa. The results indicate that the nature of the internal microstructure in n-Au is associated with the observed EOS difference from bulk Au at high pressure. Full-pattern analysis confirms that significant changes in grain size, stacking faults, grain orientation and texture occur in n-Au at high pressure. We have observed direct experimental evidence of a transition in compressional mechanism for n-Au at ~20 GPa, i.e. from a deformation dominated by nucleation and motion of lattice dislocations (dislocation-mediated) to a prominent grain boundary mediated response to external pressure. The internal microstructure inside the nanoparticle (nanocrystallinity) plays a critical role for the macro-mechanical properties of nano-Au.
Arun Shukla - One of the best experts on this subject based on the ideXlab platform.
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response of an underwater cylindrical composite shell to a proximal implosion
Journal of The Mechanics and Physics of Solids, 2021Co-Authors: Shyamal Kishore, Prathmesh Naik Parrikar, Arun ShuklaAbstract:Abstract An experimental investigation is conducted to study the dynamic underwater response of a cylindrical composite shell under near critical Hydrostatic pressure, to the implosion of another shell in proximity. A primary cylindrical composite shell is imploded in proximity to a secondary shell which is similar in all respects except for the secondary shell having a smaller length. Length differences of 10% and 20% are chosen to simulate variations in collapse pressures occurring in shells from real life manufacturing defects and/or degradation during operational use. The response of the secondary shell is investigated to understand if and how its collapse occurs in addition to studying the Fluid-Structure Interaction (FSI) phenomenon. The pair of shells are subjected to underwater Hydrostatic Loading using a large pressure vessel suitable for high-speed photography in conjunction with 3D Digital Image Correlation (DIC). 3D-DIC is employed to obtain full-field displacement measurements of both the shells, and local dynamic pressure histories are also simultaneously recorded. The primary shell always imploded first, causing a dynamic Loading on the secondary shell. In cases of implosion of the secondary shell, although the transient radial deformations occurred in mode 2, the failure itself occurred with a localized failure of the shell walls. It is observed that there exists an inner critical stand-off distance for the secondary shell to fail catastrophically upon the implosion of the primary shell, and an outer critical stand-off distance beyond which the secondary shell does not implode. A critical stand-off distance is found to exist only in the case of the 10% smaller secondary shell length. If the secondary shell stand-off distance is more than the outer critical distance or when length of the secondary shell is 20% smaller, the secondary shell responds with bending and breathing modes and no visible damage is recorded. When the stand-off distance is in between the inner and the outer critical distances, the relative orientation of the incipient modal shapes of the two shells is the factor governing the collapse of the secondary shell. A method is also developed to decouple full-field 3D-DIC measurements into bending and breathing deformation measurements.
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underwater mechanical behavior of closed cell pvc foams under Hydrostatic Loading through 3d dic technique
Polymer Testing, 2019Co-Authors: Koray Senol, Arun ShuklaAbstract:Abstract The underwater constitutive behavior of poly vinyl chloride foams with varying densities was investigated in this study. The experiments were conducted in an optically clear acrylic tube, which allowed for visualization of the specimen and the application of 3D Digital Image Correlation. A series of calibration experiments was conducted to investigate the applicability of the Digital Image Correlation technique for measuring the deformation of objects underwater inside of a curved acrylic tube of considerable thickness. The results of the calibration experiments demonstrated that a submerged object located in the middle of the acrylic tube appears magnified in the radial direction. This apparent magnification was taken into account during the analysis of the deformation for all underwater experiments. The Hydrostatic Loading was achieved by fitting the acrylic tube with a nylon piston, and compressing the piston with an Intron testing machine. Hydrostatic load of up to 5 MPa was achieved during quasi-static compression of the piston. The load applied by the Instron machine was coupled with the Digital Image Correlation data to analyze the constitutive behavior of the PVC foams. The hydraulic crush pressure, bulk modulus, and energy stored up to densification strain were determined for each foam density.
Joseph Pastor - One of the best experts on this subject based on the ideXlab platform.
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3d fem formulations of limit analysis methods for porous pressure sensitive materials
International Journal for Numerical Methods in Engineering, 2013Co-Authors: Franck Pastor, Djimedo Kondo, Joseph PastorAbstract:SUMMARY The first purpose of this paper is the numerical formulation of the three general limit analysis methods for problems involving pressure-sensitive materials, that is, the static, classic, and mixed kinematic methods applied to problems with Drucker–Prager, Mises–Schleicher, or Green materials. In each case, quadratic or rotated quadratic cone programming is considered to solve the final optimization problems, leading to original and efficient numerical formulations. As a second purpose, the resulting codes are applied to non-classic 3D problems, that is, the Gurson-like hollow sphere problem with these materials as matrices. To this end are first presented the 3D finite element implementations of the static and kinematic classic methods of limit analysis together with a mixed method formulated to give also a purely kinematic result. Discontinuous stress and velocity fields are included in the analysis. The static and the two kinematic approaches are compared afterwards in the Hydrostatic Loading case whose exact solution is known for the three cases of matrix. Then, the static and the mixed approaches are used to assess the available approximate criteria for porous Drucker–Prager, Mises–Schleicher, and Green materials. Copyright © 2013 John Wiley & Sons, Ltd.
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Hollow sphere models, conic programming and third stress invariant
European Journal of Mechanics - A Solids, 2011Co-Authors: Philippe Thoré, Franck Pastor, Joseph PastorAbstract:In this paper the hollow sphere model is investigated within the framework of limit analysis (LA), using the classical two-part velocity field, i.e., the exact solution for Hydrostatic Loading plus a linear solution, in both cases of von Mises and Drucker–Prager matrices. We use the kinematic LA approach in a quasi-analytical approach by imposing the plastic admissibility (PA) condition (Drucker–Prager matrix) or upper bounding the dissipated power (von Mises matrix) in a sufficiently high number of distributed points, thanks to conic programming formulations. Then we analyze the “porous Drucker–Prager” case to confirm that the so-called UBM (Upper Bound Model) approach of Guo et al. (2008) is only an estimate, although a good one in fact. Moreover, it is shown that the mean stress axis should not be a strict axis of symmetry for the macroscopic criterion. Then, considering the “Porous von Mises” case, we obtain that the real criterion is not only lower than the Gurson criterion, but probably non-symmetric with respect to the mean stress axis, more than in the Drucker–Prager case. We finally use ad hoc updated 3D-FEM LA codes to confirm the previous results and to evaluate the entire influence of the third stress invariant: the classical (Σm, Σeqv) formulation of the Gurson criterion clearly overestimates the real, non-symmetric solution of the hollow sphere model, at least for porosities of the same order of magnitude as the value used in this work.
