Yehuda Partom - Academia.edu (original) (raw)
Papers by Yehuda Partom
Journal de physique, Sep 1, 1994
RCsumC-Nous prCsentous une validation/calibration d'un modde de matCriau pour I'alumina AD995. No... more RCsumC-Nous prCsentous une validation/calibration d'un modde de matCriau pour I'alumina AD995. Nous utilisons les resultats de cinq exptriences d'impact de plaques symetrique menCes par Grady [I], dCsignCes CE 56 6 60. Notre modCle de mat6riau est semblable 5 celeu employ6 par Johnson et Holmquist [5]. I1 a une courbe de rupture fragile pour le mat6riau sans endommagement, et une courbe d'ecoulement pour le matCriau ruin6 (granuleux). La rCsponse iriscoplastique de matCriau endommag6 aux contraintes de cisaillement, au de la courbe d'Ccoulement quasistatique, est maxwellienne. Nous calibrons la rksponse viscoplastique pour correspondre au test CE 58, puis v6rifious la validit6 du modCle de matkriau, en pr6disant les rksultats pour les quatre autres tests. Un asez bon accord est obtenu. Abstract-We present a validatiodcalibration of a material model for AD995 alumina. We use five of Grady's symmetric impact test data designated CE56 to CE60. Our material model is similar to that employed by Johnson and Holmquist [5]. It has a fracture surface for the intact material, and a flow surface for the fractured (granularlike) material. The viscoplastic response of the fractured material to shear stresses beyond the quasistatic yield surface is Maxwellian. We calibrate the viscoplastic response to match test CE58, and then check the validity of the material model by predicting the results for the four other tests. Agreement is quite good.
Journal of Energetic Materials, Aug 16, 2012
Shock initiation of homogeneous explosives has been a subject of research since the 1960s, with n... more Shock initiation of homogeneous explosives has been a subject of research since the 1960s, with neat and sensitized nitromethane as the main materials for experiments. A shock initiation model of homogeneous explosives was established in the early 1960s. It involves a thermal explosion event at the shock entrance boundary, which develops into a superdetonation that overtakes the initial shock. In recent years, Sheffield and his group, using accurate experimental tools, were able to observe details of buildup of the superdetonation. There are many papers on modeling shock initiation of heterogeneous explosives, but there are only a few papers on modeling shock initiation of homogeneous explosives. In this article, bulk reaction reactive flow equations are used to model homogeneous shock initiation in an attempt to reproduce experimental data of Sheffield and his group. It was possible to reproduce the main features of the shock initiation process, including thermal explosion, superdetonation, input shock overtake, overdriven detonation after overtake, and the beginning of decay toward Chapman-Jouget (CJ) detonation. The time to overtake (TTO) as function of input pressure was also calculated and compared to the experimental TTO.
Journal of Physics D, Jul 14, 1982
ABSTRACT Commercial Manganin gauges were used in a series of planar impact experiments in order t... more ABSTRACT Commercial Manganin gauges were used in a series of planar impact experiments in order to determine the Hugoniot elastic limits (HEL) of differently heat-treated 2024 aluminium samples. The gauges were used in an indirect configuration by which they were embedded in a soft backing material (polymethylmethacrylate (PMMA) in this case) at the back surface of the specimen. The resulting HEL values were compared with the corresponding yield stress values from uniaxial tension tests. It is found that the simple elastic relation between yield in uniaxial strain and uniaxial stress is obeyed. An additional measurement of the HEL of a pressurised sample also conforms with this simple relation.
Journal of Applied Physics, Oct 15, 1985
Nucleation and Atmospheric Aerosols, 2020
Previously we presented a model (which we call PORT) with rate dependent pore closing/opening, fo... more Previously we presented a model (which we call PORT) with rate dependent pore closing/opening, for which we assumed that all pores close/open with the same dynamics [1]. Here we upgrade PORT to take into account different dynamics as function of pore size. We represent different pore sizes by their volume (v), and we have k discrete pore sizes. We therefore call this model VK. For the i th pore size we have ni, i=1,k pores per unit mass. We're not aware of information on pore size distributions of porous materials. We therefore assume that pore sizes are initially distributed with a lognormal distribution. Similar to PORT, we define quasistatic pore closure curves that depend on pore volume v, and we compute the rate of pore closure with a linear overstress equation relative to these curves. From the values of v and vdot (rate of change of v) we then compute (for each cell and for each time) the overall porosity j and its rate of change jdot. Finally we compute Pdot and Tdot (P=pressure and T=temperature) in the same way as in PORT, using the equation of state of the porous material. To show how our VK model works we apply it to a simple 1D problem. A 20GPa sustained pressure pulse enters a porous aluminum target. We show histories of pressure, temperature and porosity at several locations into the target. We compare these curves with the ones obtained for k=1 (as in PORT).
Nucleation and Atmospheric Aerosols, 2020
Using a Split Hopkinson Tension Bar (SHTB) to test ductile materials with high strain to failure ... more Using a Split Hopkinson Tension Bar (SHTB) to test ductile materials with high strain to failure (of the order of ~50%), poses some challenges. 1) Interpretation of SHTB tests is not straight forward due to neck formation, which causes non-uniform stress and strain distributions along the specimen. 2) Neck location varies with specimen geometry and loading conditions, and it is not clear if this may influence the strain to failure. 3) To cause failure of a long specimen requires a long loading pulse, and this may be practically limited by the maximum striker's length possible for a given system. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address problems 1 and 2 by using full numerical simulations (including the striker, the bars and the specimen) to predict the test results. In this way we are able to calibrate the strength model, taking into account necking, neck location and plastic heating.
Epj Web of Conferences, 2018
This work presents a methodology to calibrate a strength model for ductile metals, based on dynam... more This work presents a methodology to calibrate a strength model for ductile metals, based on dynamic tension tests of relatively long Dog-Bone specimens conducted on a Split Hopkinson Tension Bar (SHTB). We address the main difficulties involved in conducting and interpreting such tests, namely the duration of the loading pulse needed to deform long specimens and the non-uniform stress and strain distributions along the specimen due to neck formation. The first issue is addressed by using the waves' reflections within the output bar, as explained below. When the first loading (tension) wave does not cause failure of the specimen, a reflected compression wave travels from the specimen's bar end to the free bar's end. Upon reaching the free end this latter compression wave is reflected again as a second tension wave, which travels back along the bar until it reaches the specimen and loads it the second time. This enables further deformation of the specimen, practically doubling the loading pulse duration without changing the striker's length. The second issue is addresses by using full numerical simulations of the experimental setup, including the striker, the bars and the specimen. This way, the full dynamic behaviour of the specimen is taken into account, eliminating the need to consider specimen equilibrium and taking into account the current strain rate in the specimen as it deforms. Hence, model calibration can be done from the very start of plastic deformation and without the need to keep the strain rate constant during deformation. As a result, it is possible to reliably calibrate the strength model considering necking and neck location, as well as plastic heating which is a significant factor in the plastic deformation of ductile metals.
Journal of Energetic Materials, Jul 1, 2012
ABSTRACT In a previous article a new equation of state (EOS) for detonation products called adiab... more ABSTRACT In a previous article a new equation of state (EOS) for detonation products called adiabatic-gamma(V) was presented [1]. Briefly, adiabatic-gamma(V) is a Gruneisen EOS referred to a principal isentrope, which is specified in terms of the adiabatic gamma γ(V). The γ(V) function is piecewise linear with an arbitrary number of nodes n ≥ 3. The nodes of γ(V) are calibrated from Chapman-Jouguet (CJ) conditions and from expanding cylinder test data. In an expanding cylinder test the products release along the principal isentrope. It is therefore possible to calibrate the Gruneisen parameter Γ from such a test only at the CJ point and at large V, and in between we assume a linear variation. Here we show that in a plate push test with a gap between the explosive and the plate the products release along a higher isentrope (in the PV plane), which makes it possible to calibrate Γ(V). We define Γ(V) as a piecewise linear curve with nodes at the same values of V as γ(V). We demonstrate a procedure for calibrating Γ(V) by matching a target velocity history u(t), to be obtained from a plate push test with a gap. Like the procedure for calibrating γ(V) demonstrated in [1], this, too, is a recursive procedure.
Propellants, Explosives, Pyrotechnics, Dec 1, 1997
The impact of a shaped charge jet, created by a stationary charge onto a moving target, creates a... more The impact of a shaped charge jet, created by a stationary charge onto a moving target, creates an elongated cut on the target face. The length of the cut, together with additional data about the charge and target configuration, enables the calculation of the jet tail velocity. It was found that this velocity increases with the stand‐off distance between the charge and the target. A possible cause for this acceleration is that the jet material is under tension due to the velocity gradient along it and the stress throughout it equals the dynamic flow stress of the material. This stress pulls on the jet material towards the center of gravity and tends to reduce the velocity gradient, i.e. decelerate the tip and accelerate the tail. 1D simulations show that this mechanism results in a constant acceleration of the tail, and that the acceleration depends on the flow stress of the jet material, hence on its temperature. Based on the experimental evidence and on the simulations, it is postulated that in addition to the velocity gradient along the jet, there are temperature and strength gradients along it, as well.
Journal of Energetic Materials, Oct 15, 2010
Many equations of state (EOS) for detonation products have been proposed and used. Some of them a... more Many equations of state (EOS) for detonation products have been proposed and used. Some of them are in analytical form and some in tabular form. The most popular is the Jones-Wilkins-Lee (JWL) EOS. One of the main parameters of a product's EOS is the so-called adiabatic gamma along its main isentrope (γs). For JWL EOSs γs(V) varies in a nonmonotonic way. Going down from the CJ point along the main isentrope, it first increases to create a hump, and then, as V goes to infinity, gamma decreases to perfect gas-like behavior with gamma around 1.3. But according to Davis [1], γs(V) should decrease monotonically with V. Accordingly, in this article we investigate the following: (1) Is the hump in γs(V) necessary? and (2) Is it possible to construct a product's EOS with a monotonic γs(V) that is consistent with experimental data? We find that (1) it is possible to construct a product's EOS without a hump in γs(V); and (2) without a hump in γs(V) there are not enough degrees of freedom to reproduce cylinder test data.
Journal of physics, May 7, 2014
ABSTRACT Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a r... more ABSTRACT Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a rate stick, we observe that a partially reacted layer (PRL) is formed near the boundary. We are not aware that such a PRL has been observed in tests, and this is why we regarded it in the past as a numerical artifact. Assuming that such an artefact may be caused by the finite rise time of the detonation shock, we showed in [1] how it can be eliminated by delaying the outward boundary motion for a length of time comparable with the shock rise time. Here we revisit the PRL problem. We first show that it is not a numerical artifact but a real phenomenon. We do this by repeating the reactive flow run with a finer mesh. By looking at the PRL structure, we see that doubling the resolution affects the PRL only slightly. We then conjecture that the PRL formation has to do with the finite duration of the reaction process (or the finite extent of the reaction zone). By the time the boundary rarefaction reaches a cell near the boundary, it may be only partially reacted, and its reaction may therefore be cut off. To establish our conjecture we show how the PRL structure changes with the reaction duration.
Journal of Applied Physics, Aug 1, 1982
Epj Web of Conferences, 2018
Ductile materials (mainly metals) exhibit a sharp upturn of stress at strain rates around 10 3 to... more Ductile materials (mainly metals) exhibit a sharp upturn of stress at strain rates around 10 3 to 10 4 /s, which is not specific to a certain type of material. It is important to consider stress high rate upturn when dealing with high rate loading, such as shock loading and unloading. Using classical strength models, usually calibrated at not so high rates, may lead to errors with high rate loading and not so high pressures. Here we model high rate upturn on the macroscale. We assume that the upturn mechanism is also responsible for the 4 th power law mechanism put forward by Swegle and Grady. In the past we calibrated our overstress dynamic viscoplasticity model for aluminium from 4 th power law data. Here we use this calibration to predict the high rate stress upturn.
Journal of Applied Physics, Jul 1, 1979
The kinematics and equations of motion of an equivalent single-degree-of-freedom system for a mov... more The kinematics and equations of motion of an equivalent single-degree-of-freedom system for a moving dislocation in a modified Frenkel-Kontorova linear-chain model are derived and then used in conjunction with computer simulation runs. The average drag force on a moving dislocation at zero temperature (lattice drag force) is computed for a range of dislocation velocities. At a velocity comparable with the loss-free velocity, obtained by Earmme and Weiner from a steady-state solution, we obtain a minimum value for the lattice drag force. For velocities above the loss-free velocity the results agree with those obtained fom steady-state solutions by Earmme and Weiner. Below the loss-free velocity the lattice drag force is relatively small. It goes through a maximum which is of the order of 10−4 times the interatomic spring constant k1 of the model.
Journal of Applied Mechanics, Sep 1, 1972
A large deformation elastic-viscoplastic theory is formulated which considers both elastic and in... more A large deformation elastic-viscoplastic theory is formulated which considers both elastic and inelastic deformations to be present at all stages of loading and unloading. The theory does not require the assumption of a yield criterion or the prior determination of whether the material is loading or unloading. The theory is based on relating the essential parameters to state variables; the particular constitutive relations are motivated by the equations of “dislocation dynamics.” In the present formulation, thermal effects and properties representing the worked state of the material such as strain hardening are not considered. A numerical scheme for calculating deformations is developed and applied to a thick-walled spherical shell under internal pressure. Various numerical examples are presented.
DYMAT 2009 - 9th International Conferences on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, 2009
ABSTRACT This work presents 2D numerical simulations of shear-band formation in collapsing thick ... more ABSTRACT This work presents 2D numerical simulations of shear-band formation in collapsing thick walled cylinder experiments with 304L stainless steel. We use a simple shear-failure model which incorporates a positive feedback mechanism. Both the global behavior and the shear band evolution are examined. The calculated global behavior compares well with the experimental results. The calculated shear-bands follow the patterns of self organization demonstrated in experiments, with a good quantitative agreement with the observed final spatial configuration. The calculations reveal a clear spacing between initiation sites at the inner surface of the cylinder. The evolving shear-bands, having a width of several mesh elements in which strength decreases to zero, develop outwards in spiral paths while maintaining an angle of 45 degrees to the radial direction. Interactions between shear-bands, either by direct contact or through relief waves, result in competitive growth, eventually leading to a typical distribution of lengths and spacing. The spacing at the initiation stage and at the matured developed stage is quantitatively compared with existing analytical models.
Bulletin of the American Physical Society, Jun 15, 2015
Journal of physics, May 7, 2014
ABSTRACT Long rod penetration tests show a scaling effect that cannot be explained by rate depend... more ABSTRACT Long rod penetration tests show a scaling effect that cannot be explained by rate dependent strength. We propose here that this scaling effect may be explained by rate dependent failure. We start by revisiting the well known result that long rod penetration efficiency depends on strain to failure of both projectile and target materials. We then make the strain to failure depend on strain rate, using the overstress concept. In this way the effective strain to failure increases with strain rate. As strain rate increases with decreasing scale, we get that penetration efficiency decreases with decreasing scale, as observed in tests. In the paper we show results of hydrocode runs that demonstrate the relation between strain rate sensitivity of strain to failure and the scaling effect in long rod penetration.
Journal of physics, May 7, 2014
View the article online for updates and enhancements. Related content High temperature and dynami... more View the article online for updates and enhancements. Related content High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process S Winter, F Schmitz, T Clausmeyer et al.-A different viewpoint on adiabatic shear localization D Rittel-Local shear texture formation in adiabatic shear bands by high rate compression of high manganese TRIP steels
Journal de physique, Sep 1, 2003
An Explosive Reactive Annor (ERA) cassette is a sandwich of an explosive layer between two steel ... more An Explosive Reactive Annor (ERA) cassette is a sandwich of an explosive layer between two steel plates. To assess the performance of an ERA cassette against a shaped charge metal jet, we need to estimate the velocity and shape of the moving plates interacting with the jet. This is usually donc with the classical Gumey model or by computer simulation. The Gurney model assumes that the accelerated plates are rigid, and is able to predict their final velocity quite accurately. But tests and computer simulations show that because of their plasticity, the plates become curved as they accelerate. In this paper we revisit Gurney's derivation and extend it to include the plasticity of the plates. We assume that the plates are rigid-plastic and we model the symmetric case as well as the asymmetric case. To check the model we perform computer simulations with AUTODYN for the same two problems. We find a fair agreement between the model predictions and the computer simulation results.
Journal de physique, Sep 1, 1994
RCsumC-Nous prCsentous une validation/calibration d'un modde de matCriau pour I'alumina AD995. No... more RCsumC-Nous prCsentous une validation/calibration d'un modde de matCriau pour I'alumina AD995. Nous utilisons les resultats de cinq exptriences d'impact de plaques symetrique menCes par Grady [I], dCsignCes CE 56 6 60. Notre modCle de mat6riau est semblable 5 celeu employ6 par Johnson et Holmquist [5]. I1 a une courbe de rupture fragile pour le mat6riau sans endommagement, et une courbe d'ecoulement pour le matCriau ruin6 (granuleux). La rCsponse iriscoplastique de matCriau endommag6 aux contraintes de cisaillement, au de la courbe d'Ccoulement quasistatique, est maxwellienne. Nous calibrons la rksponse viscoplastique pour correspondre au test CE 58, puis v6rifious la validit6 du modCle de matkriau, en pr6disant les rksultats pour les quatre autres tests. Un asez bon accord est obtenu. Abstract-We present a validatiodcalibration of a material model for AD995 alumina. We use five of Grady's symmetric impact test data designated CE56 to CE60. Our material model is similar to that employed by Johnson and Holmquist [5]. It has a fracture surface for the intact material, and a flow surface for the fractured (granularlike) material. The viscoplastic response of the fractured material to shear stresses beyond the quasistatic yield surface is Maxwellian. We calibrate the viscoplastic response to match test CE58, and then check the validity of the material model by predicting the results for the four other tests. Agreement is quite good.
Journal of Energetic Materials, Aug 16, 2012
Shock initiation of homogeneous explosives has been a subject of research since the 1960s, with n... more Shock initiation of homogeneous explosives has been a subject of research since the 1960s, with neat and sensitized nitromethane as the main materials for experiments. A shock initiation model of homogeneous explosives was established in the early 1960s. It involves a thermal explosion event at the shock entrance boundary, which develops into a superdetonation that overtakes the initial shock. In recent years, Sheffield and his group, using accurate experimental tools, were able to observe details of buildup of the superdetonation. There are many papers on modeling shock initiation of heterogeneous explosives, but there are only a few papers on modeling shock initiation of homogeneous explosives. In this article, bulk reaction reactive flow equations are used to model homogeneous shock initiation in an attempt to reproduce experimental data of Sheffield and his group. It was possible to reproduce the main features of the shock initiation process, including thermal explosion, superdetonation, input shock overtake, overdriven detonation after overtake, and the beginning of decay toward Chapman-Jouget (CJ) detonation. The time to overtake (TTO) as function of input pressure was also calculated and compared to the experimental TTO.
Journal of Physics D, Jul 14, 1982
ABSTRACT Commercial Manganin gauges were used in a series of planar impact experiments in order t... more ABSTRACT Commercial Manganin gauges were used in a series of planar impact experiments in order to determine the Hugoniot elastic limits (HEL) of differently heat-treated 2024 aluminium samples. The gauges were used in an indirect configuration by which they were embedded in a soft backing material (polymethylmethacrylate (PMMA) in this case) at the back surface of the specimen. The resulting HEL values were compared with the corresponding yield stress values from uniaxial tension tests. It is found that the simple elastic relation between yield in uniaxial strain and uniaxial stress is obeyed. An additional measurement of the HEL of a pressurised sample also conforms with this simple relation.
Journal of Applied Physics, Oct 15, 1985
Nucleation and Atmospheric Aerosols, 2020
Previously we presented a model (which we call PORT) with rate dependent pore closing/opening, fo... more Previously we presented a model (which we call PORT) with rate dependent pore closing/opening, for which we assumed that all pores close/open with the same dynamics [1]. Here we upgrade PORT to take into account different dynamics as function of pore size. We represent different pore sizes by their volume (v), and we have k discrete pore sizes. We therefore call this model VK. For the i th pore size we have ni, i=1,k pores per unit mass. We're not aware of information on pore size distributions of porous materials. We therefore assume that pore sizes are initially distributed with a lognormal distribution. Similar to PORT, we define quasistatic pore closure curves that depend on pore volume v, and we compute the rate of pore closure with a linear overstress equation relative to these curves. From the values of v and vdot (rate of change of v) we then compute (for each cell and for each time) the overall porosity j and its rate of change jdot. Finally we compute Pdot and Tdot (P=pressure and T=temperature) in the same way as in PORT, using the equation of state of the porous material. To show how our VK model works we apply it to a simple 1D problem. A 20GPa sustained pressure pulse enters a porous aluminum target. We show histories of pressure, temperature and porosity at several locations into the target. We compare these curves with the ones obtained for k=1 (as in PORT).
Nucleation and Atmospheric Aerosols, 2020
Using a Split Hopkinson Tension Bar (SHTB) to test ductile materials with high strain to failure ... more Using a Split Hopkinson Tension Bar (SHTB) to test ductile materials with high strain to failure (of the order of ~50%), poses some challenges. 1) Interpretation of SHTB tests is not straight forward due to neck formation, which causes non-uniform stress and strain distributions along the specimen. 2) Neck location varies with specimen geometry and loading conditions, and it is not clear if this may influence the strain to failure. 3) To cause failure of a long specimen requires a long loading pulse, and this may be practically limited by the maximum striker's length possible for a given system. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address problems 1 and 2 by using full numerical simulations (including the striker, the bars and the specimen) to predict the test results. In this way we are able to calibrate the strength model, taking into account necking, neck location and plastic heating.
Epj Web of Conferences, 2018
This work presents a methodology to calibrate a strength model for ductile metals, based on dynam... more This work presents a methodology to calibrate a strength model for ductile metals, based on dynamic tension tests of relatively long Dog-Bone specimens conducted on a Split Hopkinson Tension Bar (SHTB). We address the main difficulties involved in conducting and interpreting such tests, namely the duration of the loading pulse needed to deform long specimens and the non-uniform stress and strain distributions along the specimen due to neck formation. The first issue is addressed by using the waves' reflections within the output bar, as explained below. When the first loading (tension) wave does not cause failure of the specimen, a reflected compression wave travels from the specimen's bar end to the free bar's end. Upon reaching the free end this latter compression wave is reflected again as a second tension wave, which travels back along the bar until it reaches the specimen and loads it the second time. This enables further deformation of the specimen, practically doubling the loading pulse duration without changing the striker's length. The second issue is addresses by using full numerical simulations of the experimental setup, including the striker, the bars and the specimen. This way, the full dynamic behaviour of the specimen is taken into account, eliminating the need to consider specimen equilibrium and taking into account the current strain rate in the specimen as it deforms. Hence, model calibration can be done from the very start of plastic deformation and without the need to keep the strain rate constant during deformation. As a result, it is possible to reliably calibrate the strength model considering necking and neck location, as well as plastic heating which is a significant factor in the plastic deformation of ductile metals.
Journal of Energetic Materials, Jul 1, 2012
ABSTRACT In a previous article a new equation of state (EOS) for detonation products called adiab... more ABSTRACT In a previous article a new equation of state (EOS) for detonation products called adiabatic-gamma(V) was presented [1]. Briefly, adiabatic-gamma(V) is a Gruneisen EOS referred to a principal isentrope, which is specified in terms of the adiabatic gamma γ(V). The γ(V) function is piecewise linear with an arbitrary number of nodes n ≥ 3. The nodes of γ(V) are calibrated from Chapman-Jouguet (CJ) conditions and from expanding cylinder test data. In an expanding cylinder test the products release along the principal isentrope. It is therefore possible to calibrate the Gruneisen parameter Γ from such a test only at the CJ point and at large V, and in between we assume a linear variation. Here we show that in a plate push test with a gap between the explosive and the plate the products release along a higher isentrope (in the PV plane), which makes it possible to calibrate Γ(V). We define Γ(V) as a piecewise linear curve with nodes at the same values of V as γ(V). We demonstrate a procedure for calibrating Γ(V) by matching a target velocity history u(t), to be obtained from a plate push test with a gap. Like the procedure for calibrating γ(V) demonstrated in [1], this, too, is a recursive procedure.
Propellants, Explosives, Pyrotechnics, Dec 1, 1997
The impact of a shaped charge jet, created by a stationary charge onto a moving target, creates a... more The impact of a shaped charge jet, created by a stationary charge onto a moving target, creates an elongated cut on the target face. The length of the cut, together with additional data about the charge and target configuration, enables the calculation of the jet tail velocity. It was found that this velocity increases with the stand‐off distance between the charge and the target. A possible cause for this acceleration is that the jet material is under tension due to the velocity gradient along it and the stress throughout it equals the dynamic flow stress of the material. This stress pulls on the jet material towards the center of gravity and tends to reduce the velocity gradient, i.e. decelerate the tip and accelerate the tail. 1D simulations show that this mechanism results in a constant acceleration of the tail, and that the acceleration depends on the flow stress of the jet material, hence on its temperature. Based on the experimental evidence and on the simulations, it is postulated that in addition to the velocity gradient along the jet, there are temperature and strength gradients along it, as well.
Journal of Energetic Materials, Oct 15, 2010
Many equations of state (EOS) for detonation products have been proposed and used. Some of them a... more Many equations of state (EOS) for detonation products have been proposed and used. Some of them are in analytical form and some in tabular form. The most popular is the Jones-Wilkins-Lee (JWL) EOS. One of the main parameters of a product's EOS is the so-called adiabatic gamma along its main isentrope (γs). For JWL EOSs γs(V) varies in a nonmonotonic way. Going down from the CJ point along the main isentrope, it first increases to create a hump, and then, as V goes to infinity, gamma decreases to perfect gas-like behavior with gamma around 1.3. But according to Davis [1], γs(V) should decrease monotonically with V. Accordingly, in this article we investigate the following: (1) Is the hump in γs(V) necessary? and (2) Is it possible to construct a product's EOS with a monotonic γs(V) that is consistent with experimental data? We find that (1) it is possible to construct a product's EOS without a hump in γs(V); and (2) without a hump in γs(V) there are not enough degrees of freedom to reproduce cylinder test data.
Journal of physics, May 7, 2014
ABSTRACT Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a r... more ABSTRACT Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a rate stick, we observe that a partially reacted layer (PRL) is formed near the boundary. We are not aware that such a PRL has been observed in tests, and this is why we regarded it in the past as a numerical artifact. Assuming that such an artefact may be caused by the finite rise time of the detonation shock, we showed in [1] how it can be eliminated by delaying the outward boundary motion for a length of time comparable with the shock rise time. Here we revisit the PRL problem. We first show that it is not a numerical artifact but a real phenomenon. We do this by repeating the reactive flow run with a finer mesh. By looking at the PRL structure, we see that doubling the resolution affects the PRL only slightly. We then conjecture that the PRL formation has to do with the finite duration of the reaction process (or the finite extent of the reaction zone). By the time the boundary rarefaction reaches a cell near the boundary, it may be only partially reacted, and its reaction may therefore be cut off. To establish our conjecture we show how the PRL structure changes with the reaction duration.
Journal of Applied Physics, Aug 1, 1982
Epj Web of Conferences, 2018
Ductile materials (mainly metals) exhibit a sharp upturn of stress at strain rates around 10 3 to... more Ductile materials (mainly metals) exhibit a sharp upturn of stress at strain rates around 10 3 to 10 4 /s, which is not specific to a certain type of material. It is important to consider stress high rate upturn when dealing with high rate loading, such as shock loading and unloading. Using classical strength models, usually calibrated at not so high rates, may lead to errors with high rate loading and not so high pressures. Here we model high rate upturn on the macroscale. We assume that the upturn mechanism is also responsible for the 4 th power law mechanism put forward by Swegle and Grady. In the past we calibrated our overstress dynamic viscoplasticity model for aluminium from 4 th power law data. Here we use this calibration to predict the high rate stress upturn.
Journal of Applied Physics, Jul 1, 1979
The kinematics and equations of motion of an equivalent single-degree-of-freedom system for a mov... more The kinematics and equations of motion of an equivalent single-degree-of-freedom system for a moving dislocation in a modified Frenkel-Kontorova linear-chain model are derived and then used in conjunction with computer simulation runs. The average drag force on a moving dislocation at zero temperature (lattice drag force) is computed for a range of dislocation velocities. At a velocity comparable with the loss-free velocity, obtained by Earmme and Weiner from a steady-state solution, we obtain a minimum value for the lattice drag force. For velocities above the loss-free velocity the results agree with those obtained fom steady-state solutions by Earmme and Weiner. Below the loss-free velocity the lattice drag force is relatively small. It goes through a maximum which is of the order of 10−4 times the interatomic spring constant k1 of the model.
Journal of Applied Mechanics, Sep 1, 1972
A large deformation elastic-viscoplastic theory is formulated which considers both elastic and in... more A large deformation elastic-viscoplastic theory is formulated which considers both elastic and inelastic deformations to be present at all stages of loading and unloading. The theory does not require the assumption of a yield criterion or the prior determination of whether the material is loading or unloading. The theory is based on relating the essential parameters to state variables; the particular constitutive relations are motivated by the equations of “dislocation dynamics.” In the present formulation, thermal effects and properties representing the worked state of the material such as strain hardening are not considered. A numerical scheme for calculating deformations is developed and applied to a thick-walled spherical shell under internal pressure. Various numerical examples are presented.
DYMAT 2009 - 9th International Conferences on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, 2009
ABSTRACT This work presents 2D numerical simulations of shear-band formation in collapsing thick ... more ABSTRACT This work presents 2D numerical simulations of shear-band formation in collapsing thick walled cylinder experiments with 304L stainless steel. We use a simple shear-failure model which incorporates a positive feedback mechanism. Both the global behavior and the shear band evolution are examined. The calculated global behavior compares well with the experimental results. The calculated shear-bands follow the patterns of self organization demonstrated in experiments, with a good quantitative agreement with the observed final spatial configuration. The calculations reveal a clear spacing between initiation sites at the inner surface of the cylinder. The evolving shear-bands, having a width of several mesh elements in which strength decreases to zero, develop outwards in spiral paths while maintaining an angle of 45 degrees to the radial direction. Interactions between shear-bands, either by direct contact or through relief waves, result in competitive growth, eventually leading to a typical distribution of lengths and spacing. The spacing at the initiation stage and at the matured developed stage is quantitatively compared with existing analytical models.
Bulletin of the American Physical Society, Jun 15, 2015
Journal of physics, May 7, 2014
ABSTRACT Long rod penetration tests show a scaling effect that cannot be explained by rate depend... more ABSTRACT Long rod penetration tests show a scaling effect that cannot be explained by rate dependent strength. We propose here that this scaling effect may be explained by rate dependent failure. We start by revisiting the well known result that long rod penetration efficiency depends on strain to failure of both projectile and target materials. We then make the strain to failure depend on strain rate, using the overstress concept. In this way the effective strain to failure increases with strain rate. As strain rate increases with decreasing scale, we get that penetration efficiency decreases with decreasing scale, as observed in tests. In the paper we show results of hydrocode runs that demonstrate the relation between strain rate sensitivity of strain to failure and the scaling effect in long rod penetration.
Journal of physics, May 7, 2014
View the article online for updates and enhancements. Related content High temperature and dynami... more View the article online for updates and enhancements. Related content High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process S Winter, F Schmitz, T Clausmeyer et al.-A different viewpoint on adiabatic shear localization D Rittel-Local shear texture formation in adiabatic shear bands by high rate compression of high manganese TRIP steels
Journal de physique, Sep 1, 2003
An Explosive Reactive Annor (ERA) cassette is a sandwich of an explosive layer between two steel ... more An Explosive Reactive Annor (ERA) cassette is a sandwich of an explosive layer between two steel plates. To assess the performance of an ERA cassette against a shaped charge metal jet, we need to estimate the velocity and shape of the moving plates interacting with the jet. This is usually donc with the classical Gumey model or by computer simulation. The Gurney model assumes that the accelerated plates are rigid, and is able to predict their final velocity quite accurately. But tests and computer simulations show that because of their plasticity, the plates become curved as they accelerate. In this paper we revisit Gurney's derivation and extend it to include the plasticity of the plates. We assume that the plates are rigid-plastic and we model the symmetric case as well as the asymmetric case. To check the model we perform computer simulations with AUTODYN for the same two problems. We find a fair agreement between the model predictions and the computer simulation results.