Shree Krishna - Academia.edu (original) (raw)
Papers by Shree Krishna
Mechanics of Materials, 2011
In this paper we present a micromechanics-based model for neutron-irradiated single and polycryst... more In this paper we present a micromechanics-based model for neutron-irradiated single and polycrystalline BCC molybdenum which is capable of representing not only the effects of radiation hardening, yield drop and non-zero stress offset from the unirradiated stress–strain curves, but also the unique “radiation softening” effect observed in Mo at low to intermediate homologous temperatures (0.05 ⩽ T/Tm ⩽ 0.2) (Li et al., 2008) and low radiation doses. Specifically, a single smooth viscoplastic potential has been developed in which the critical resolved shear stress is decomposed into thermal and athermal components that overcome short range and long range barriers, respectively. The evolution of the athermal part is dependent on dislocation and defect densities, whereas the thermal part is modeled to be a function of temperature only. Impediment of dislocation motion due to defects results in hardening while defect annihilation due to dislocation motion accounts for yield drop and stress offset. Radiation softening is explained by invoking a critical temperature (Tc), with increase in radiation dose below which the thermal part of the flow stress undergoes a reduction due to increase in mobile point defects in the dislocation core area, whereas the athermal part increases. Beyond the critical temperature, however, thermal activation is sufficient for dislocation motion and the thermal component disappears. We argue that for low radiation doses, this critical temperature decreases with increase in radiation dose, resulting in a temperature range over which the flow stress actually drops below its value corresponding to the unirradiated condition. Polycrystalline response has been simulated based on a Taylor type homogenization scheme. The model is validated with experimental data for a range of temperatures and strain rates with increasing radiation dose.
International Journal of Plasticity, 2008
Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of load... more Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of loading non-proportionality on ratcheting responses of duplex stainless steel. In order to further explore their new observation, a set of experiments was conducted on stainless steel (SS) 304L under various biaxial stress-controlled non-proportional histories. This new set of data reiterated Aubin and her coworkers’ observation and illustrated many new responses critical to model development and validation. Two recent and different classes of cyclic plasticity models, the modified Chaboche model proposed by Bari and Hassan and the version of the multi-mechanism model proposed by Taleb and Cailletaud, are evaluated in terms of their simulations of the SS304L non-proportional ratcheting responses. A modeling scheme for non-proportional ratcheting responses using the kinematic hardening rule parameters in addition to the conventionally used isotropic hardening rule parameter (yield surface size change) in the modified Chaboche model is evaluated. Strengths and weaknesses of the models in simulating the non-proportional ratcheting responses are identified. Further improvements of these models needed for improving the non-proportional ratcheting simulations are suggested in the paper.
International Journal of Plasticity, 2009
A recent study by Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonpropo... more A recent study by Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] demonstrated that some of the nonproportional ratcheting responses under stress-controlled loading histories cannot be simulated reasonably by two recent cyclic plasticity models. Two major drawbacks of the models identified were: (i) the stainless steel 304 demonstrated cyclic hardening under strain-controlled loading whereas cyclic softening under stress-controlled loading, which depends on the strain-range and which the existing models cannot describe; (ii) the change in biaxial ratcheting responses due to the change in the degree of nonproportionality were not simulated well by the models. Motivated by these findings, two modified cyclic plasticity models are evaluated in predicting a broad set of cyclic and ratcheting response of stainless steel 304. The experimental responses used in evaluating the modified models included both proportional (uniaxial) and nonproportional (biaxial) loading responses from Hassan and Kyriakides [Hassan, T., Kyriakides, S., 1994a. Ratcheting of cyclically hardening and softening materials. Part I: uniaxial behavior. Int. J. Plasticity, 10, 149–184; Hassan, T., Kyriakides, S., 1994b. Ratcheting of cyclically hardening and softening materials. Part II: multiaxial behavior. Int. J. Plasticity, 10, 185–212.] and Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] The first model studied is a macro-scale, phenomenological, constitutive model originally proposed by Chaboche et al. [Chaboche, J.L., Dang-Van, K., Cordier, G., 1979. Modelization of the strain memory effect on the cyclic hardening of 316 stainless steel. In: Proceedings of the Fifth International Conference on SMiRT, Div. L, Berlin, Germany, L11/3.]. This model was systematically modified for incorporating strain-range dependent cyclic hardening–softening, and proportional and nonproportional loading memory parameters. The second model evaluated is a polycrystalline model originally proposed by Cailletaud [Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. Int. J. Plasticity, 8, 55–73.] based on crystalline slip mechanisms. These two models are scrutinized against simulating hysteresis loop shape, cyclic hardening–softening, cross-effect, cyclic relaxation, subsequent cyclic softening and finally a broad set of ratcheting responses under uniaxial and biaxial loading histories. The modeling features which improved simulations for these responses are elaborated in the paper. In addition, a novel technique for simulating both the monotonic and cyclic responses with one set of model parameters is developed and validated.
Several features of cyclic plasticity, e.g. cyclic hardening/softening, ratcheting, relaxation, a... more Several features of cyclic plasticity, e.g. cyclic hardening/softening, ratcheting, relaxation, and their dependence on strain range, nonproportionality of loading, time, and temperature determine the stress-strain responses of materials under cyclic loading. Numerous efforts have been made in the past decades to characterize and model these responses. Many of these responses can be simulated reasonably by the existing constitutive models, but the same models would fail in simulating the structural responses, local stress-strain or global deformation. One of the reasons for this deficiency is that the constitutive models are not robust enough to simulate the cyclic plasticity responses when they interact with each other. This deficiency can be understood better or resolved by developing and validating constitutive models against a broad set of experimental responses and two or more of the responses interacting with each other. This dissertation develops a unified constitutive model by studying the cyclic plasticity features in an integrated manner and validating the model by simulating a broad set of proportional and nonproportional cyclic plasticity responses. The study demonstrates the drawbacks of the existing nonlinear kinematic hardening model originally developed by Chaboche and then develop and incorporate novel ideas into the model for improving its cyclic response simulations. The Chaboche model is modified by incorporating strain-range dependent cyclic hardening/softening through the kinematic hardening rule parameters, in addition to the conventional method of using only the isotropic hardening parameters. The nonproportional loading memory parameters of Tanaka and of Benallal and Marquis are incorporated to study the influence of nonproportionality. The model is assessed by simulating hysteresis loop shape, cyclic hardening-softening, cross-effect, cyclic relaxation, subsequent cyclic softening, and finally a series of ratcheting responses under uniaxial and biaxial loading responses. Next, it is demonstrated that the hysteresis loop shape and width can be improved by incorporation of time dependence (visco-effect) and a novel modeling scheme of backstress shift. Overall, this dissertation demonstrates a methodical and systematic development of a constitutive model for simulating a broad set of low-cycle fatigue responses.
Philosophical Magazine, 2010
Philosophical Magazine Vol. 90, No. 30, 28 October 2010, 40134025 ... Dislocation and defect den... more Philosophical Magazine Vol. 90, No. 30, 28 October 2010, 40134025 ... Dislocation and defect density-based micromechanical modeling of the mechanical behavior of fcc metals under neutron irradiation ... Shree Krishna, Amir Zamiri and Suvranu De* ... Department of Mechanical, ...
The paper presents a rate-independent dislocation growth and defect annihilation mechanism to cap... more The paper presents a rate-independent dislocation growth and defect annihilation mechanism to capture the pre-and post-yield material behavior of FCC metals subjected to different doses of neutron radiation. Based on observation from molecular dynamics simulation and TEM experiments, the developed model is capable of capturing the salient features of irradiation induced hardening including increase in yield stress followed by yield drop and non-zero stress offset from the unirradiated stress-strain curve. The key contribution is a model for the critical resolved slip resistance that depends on both dislocation and defect densities which are governed by evolution equations based on physical observations. The result is an orientation-dependent nonhomogeneous deformation model which accounts for defect annihilation on active slip planes. Results for both single and polycrystalline simulations of OFHC copper are presented and are observed to be in reasonably good agreement with experimental data. Extension of the model to other FCC metals is straightforward and is currently being developed for BCC metals giving its way for generation of new materials.
Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of load... more Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of loading non-proportionality on ratcheting responses of duplex stainless steel. In order to further explore their new observation, a set of experiments was conducted on stainless steel (SS) 304L under various biaxial stress-controlled non-proportional histories. This new set of data reiterated Aubin and her coworkers' observation and illustrated many new responses critical to model development and validation. Two recent and different classes of cyclic plasticity models, the modified Chaboche model proposed by Bari and Hassan and the version of the multi-mechanism model proposed by Taleb and Cailletaud, are evaluated in terms of their simulations of the SS304L non-proportional ratcheting responses. A modeling scheme for non-proportional ratcheting responses using the kinematic hardening rule parameters in addition to the conventionally used isotropic hardening rule parameter (yield surface size change) in the modified Chaboche model is evaluated. Strengths and weaknesses of the models in simulating the non-proportional ratcheting responses are identified. Further improvements of these models needed for improving the non-proportional ratcheting simulations are suggested in the paper.
In this paper we present a micromechanics-based model for neutron-irradiated single and polycryst... more In this paper we present a micromechanics-based model for neutron-irradiated single and polycrystalline BCC molybdenum which is capable of representing not only the effects of radiation hardening, yield drop and non-zero stress offset from the unirradiated stressstrain curves, but also the unique ''radiation softening'' effect observed in Mo at low to intermediate homologous temperatures (0.05 6 T/T m 6 0.2) and low radiation doses. Specifically, a single smooth viscoplastic potential has been developed in which the critical resolved shear stress is decomposed into thermal and athermal components that overcome short range and long range barriers, respectively. The evolution of the athermal part is dependent on dislocation and defect densities, whereas the thermal part is modeled to be a function of temperature only. Impediment of dislocation motion due to defects results in hardening while defect annihilation due to dislocation motion accounts for yield drop and stress offset. Radiation softening is explained by invoking a critical temperature (T c ), with increase in radiation dose below which the thermal part of the flow stress undergoes a reduction due to increase in mobile point defects in the dislocation core area, whereas the athermal part increases. Beyond the critical temperature, however, thermal activation is sufficient for dislocation motion and the thermal component disappears. We argue that for low radiation doses, this critical temperature decreases with increase in radiation dose, resulting in a temperature range over which the flow stress actually drops below its value corresponding to the unirradiated condition. Polycrystalline response has been simulated based on a Taylor type homogenization scheme. The model is validated with experimental data for a range of temperatures and strain rates with increasing radiation dose.
International Journal of Plasticity journal homepage: www.elsevier.com/locate/ijplas recent cycli... more International Journal of Plasticity journal homepage: www.elsevier.com/locate/ijplas recent cyclic plasticity models. Int. J. Plasticity, 24, 1863-1889 The first model studied is a macro-scale, phenomenological, constitutive model originally proposed by . Modelization of the strain memory effect on the cyclic hardening of 316 stainless steel. In: Proceedings of the Fifth International Conference on SMiRT, Div. L, Berlin, Germany, L11/3.]. This model was systematically modified for incorporating strain-range dependent cyclic hardening-softening, and proportional and nonproportional loading memory parameters. The second model evaluated is a polycrystalline model originally proposed by Cailletaud . A micromechanical approach to inelastic behavior of metals. Int. J. Plasticity, 8, 55-73.] based on crystalline slip mechanisms. These two models are scrutinized against simulating hysteresis loop shape, cyclic hardening-softening, cross-effect, cyclic relaxation, subsequent cyclic softening and finally a broad set of ratcheting responses under uniaxial and biaxial loading histories. The modeling features which improved simulations for these responses are elaborated in the paper. In addition, a novel technique for simulating both the monotonic and cyclic responses with one set of model parameters is developed and validated.
Mechanics of Materials, 2011
In this paper we present a micromechanics-based model for neutron-irradiated single and polycryst... more In this paper we present a micromechanics-based model for neutron-irradiated single and polycrystalline BCC molybdenum which is capable of representing not only the effects of radiation hardening, yield drop and non-zero stress offset from the unirradiated stress–strain curves, but also the unique “radiation softening” effect observed in Mo at low to intermediate homologous temperatures (0.05 ⩽ T/Tm ⩽ 0.2) (Li et al., 2008) and low radiation doses. Specifically, a single smooth viscoplastic potential has been developed in which the critical resolved shear stress is decomposed into thermal and athermal components that overcome short range and long range barriers, respectively. The evolution of the athermal part is dependent on dislocation and defect densities, whereas the thermal part is modeled to be a function of temperature only. Impediment of dislocation motion due to defects results in hardening while defect annihilation due to dislocation motion accounts for yield drop and stress offset. Radiation softening is explained by invoking a critical temperature (Tc), with increase in radiation dose below which the thermal part of the flow stress undergoes a reduction due to increase in mobile point defects in the dislocation core area, whereas the athermal part increases. Beyond the critical temperature, however, thermal activation is sufficient for dislocation motion and the thermal component disappears. We argue that for low radiation doses, this critical temperature decreases with increase in radiation dose, resulting in a temperature range over which the flow stress actually drops below its value corresponding to the unirradiated condition. Polycrystalline response has been simulated based on a Taylor type homogenization scheme. The model is validated with experimental data for a range of temperatures and strain rates with increasing radiation dose.
International Journal of Plasticity, 2008
Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of load... more Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of loading non-proportionality on ratcheting responses of duplex stainless steel. In order to further explore their new observation, a set of experiments was conducted on stainless steel (SS) 304L under various biaxial stress-controlled non-proportional histories. This new set of data reiterated Aubin and her coworkers’ observation and illustrated many new responses critical to model development and validation. Two recent and different classes of cyclic plasticity models, the modified Chaboche model proposed by Bari and Hassan and the version of the multi-mechanism model proposed by Taleb and Cailletaud, are evaluated in terms of their simulations of the SS304L non-proportional ratcheting responses. A modeling scheme for non-proportional ratcheting responses using the kinematic hardening rule parameters in addition to the conventionally used isotropic hardening rule parameter (yield surface size change) in the modified Chaboche model is evaluated. Strengths and weaknesses of the models in simulating the non-proportional ratcheting responses are identified. Further improvements of these models needed for improving the non-proportional ratcheting simulations are suggested in the paper.
International Journal of Plasticity, 2009
A recent study by Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonpropo... more A recent study by Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] demonstrated that some of the nonproportional ratcheting responses under stress-controlled loading histories cannot be simulated reasonably by two recent cyclic plasticity models. Two major drawbacks of the models identified were: (i) the stainless steel 304 demonstrated cyclic hardening under strain-controlled loading whereas cyclic softening under stress-controlled loading, which depends on the strain-range and which the existing models cannot describe; (ii) the change in biaxial ratcheting responses due to the change in the degree of nonproportionality were not simulated well by the models. Motivated by these findings, two modified cyclic plasticity models are evaluated in predicting a broad set of cyclic and ratcheting response of stainless steel 304. The experimental responses used in evaluating the modified models included both proportional (uniaxial) and nonproportional (biaxial) loading responses from Hassan and Kyriakides [Hassan, T., Kyriakides, S., 1994a. Ratcheting of cyclically hardening and softening materials. Part I: uniaxial behavior. Int. J. Plasticity, 10, 149–184; Hassan, T., Kyriakides, S., 1994b. Ratcheting of cyclically hardening and softening materials. Part II: multiaxial behavior. Int. J. Plasticity, 10, 185–212.] and Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] The first model studied is a macro-scale, phenomenological, constitutive model originally proposed by Chaboche et al. [Chaboche, J.L., Dang-Van, K., Cordier, G., 1979. Modelization of the strain memory effect on the cyclic hardening of 316 stainless steel. In: Proceedings of the Fifth International Conference on SMiRT, Div. L, Berlin, Germany, L11/3.]. This model was systematically modified for incorporating strain-range dependent cyclic hardening–softening, and proportional and nonproportional loading memory parameters. The second model evaluated is a polycrystalline model originally proposed by Cailletaud [Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. Int. J. Plasticity, 8, 55–73.] based on crystalline slip mechanisms. These two models are scrutinized against simulating hysteresis loop shape, cyclic hardening–softening, cross-effect, cyclic relaxation, subsequent cyclic softening and finally a broad set of ratcheting responses under uniaxial and biaxial loading histories. The modeling features which improved simulations for these responses are elaborated in the paper. In addition, a novel technique for simulating both the monotonic and cyclic responses with one set of model parameters is developed and validated.
Several features of cyclic plasticity, e.g. cyclic hardening/softening, ratcheting, relaxation, a... more Several features of cyclic plasticity, e.g. cyclic hardening/softening, ratcheting, relaxation, and their dependence on strain range, nonproportionality of loading, time, and temperature determine the stress-strain responses of materials under cyclic loading. Numerous efforts have been made in the past decades to characterize and model these responses. Many of these responses can be simulated reasonably by the existing constitutive models, but the same models would fail in simulating the structural responses, local stress-strain or global deformation. One of the reasons for this deficiency is that the constitutive models are not robust enough to simulate the cyclic plasticity responses when they interact with each other. This deficiency can be understood better or resolved by developing and validating constitutive models against a broad set of experimental responses and two or more of the responses interacting with each other. This dissertation develops a unified constitutive model by studying the cyclic plasticity features in an integrated manner and validating the model by simulating a broad set of proportional and nonproportional cyclic plasticity responses. The study demonstrates the drawbacks of the existing nonlinear kinematic hardening model originally developed by Chaboche and then develop and incorporate novel ideas into the model for improving its cyclic response simulations. The Chaboche model is modified by incorporating strain-range dependent cyclic hardening/softening through the kinematic hardening rule parameters, in addition to the conventional method of using only the isotropic hardening parameters. The nonproportional loading memory parameters of Tanaka and of Benallal and Marquis are incorporated to study the influence of nonproportionality. The model is assessed by simulating hysteresis loop shape, cyclic hardening-softening, cross-effect, cyclic relaxation, subsequent cyclic softening, and finally a series of ratcheting responses under uniaxial and biaxial loading responses. Next, it is demonstrated that the hysteresis loop shape and width can be improved by incorporation of time dependence (visco-effect) and a novel modeling scheme of backstress shift. Overall, this dissertation demonstrates a methodical and systematic development of a constitutive model for simulating a broad set of low-cycle fatigue responses.
Philosophical Magazine, 2010
Philosophical Magazine Vol. 90, No. 30, 28 October 2010, 40134025 ... Dislocation and defect den... more Philosophical Magazine Vol. 90, No. 30, 28 October 2010, 40134025 ... Dislocation and defect density-based micromechanical modeling of the mechanical behavior of fcc metals under neutron irradiation ... Shree Krishna, Amir Zamiri and Suvranu De* ... Department of Mechanical, ...
The paper presents a rate-independent dislocation growth and defect annihilation mechanism to cap... more The paper presents a rate-independent dislocation growth and defect annihilation mechanism to capture the pre-and post-yield material behavior of FCC metals subjected to different doses of neutron radiation. Based on observation from molecular dynamics simulation and TEM experiments, the developed model is capable of capturing the salient features of irradiation induced hardening including increase in yield stress followed by yield drop and non-zero stress offset from the unirradiated stress-strain curve. The key contribution is a model for the critical resolved slip resistance that depends on both dislocation and defect densities which are governed by evolution equations based on physical observations. The result is an orientation-dependent nonhomogeneous deformation model which accounts for defect annihilation on active slip planes. Results for both single and polycrystalline simulations of OFHC copper are presented and are observed to be in reasonably good agreement with experimental data. Extension of the model to other FCC metals is straightforward and is currently being developed for BCC metals giving its way for generation of new materials.
Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of load... more Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of loading non-proportionality on ratcheting responses of duplex stainless steel. In order to further explore their new observation, a set of experiments was conducted on stainless steel (SS) 304L under various biaxial stress-controlled non-proportional histories. This new set of data reiterated Aubin and her coworkers' observation and illustrated many new responses critical to model development and validation. Two recent and different classes of cyclic plasticity models, the modified Chaboche model proposed by Bari and Hassan and the version of the multi-mechanism model proposed by Taleb and Cailletaud, are evaluated in terms of their simulations of the SS304L non-proportional ratcheting responses. A modeling scheme for non-proportional ratcheting responses using the kinematic hardening rule parameters in addition to the conventionally used isotropic hardening rule parameter (yield surface size change) in the modified Chaboche model is evaluated. Strengths and weaknesses of the models in simulating the non-proportional ratcheting responses are identified. Further improvements of these models needed for improving the non-proportional ratcheting simulations are suggested in the paper.
In this paper we present a micromechanics-based model for neutron-irradiated single and polycryst... more In this paper we present a micromechanics-based model for neutron-irradiated single and polycrystalline BCC molybdenum which is capable of representing not only the effects of radiation hardening, yield drop and non-zero stress offset from the unirradiated stressstrain curves, but also the unique ''radiation softening'' effect observed in Mo at low to intermediate homologous temperatures (0.05 6 T/T m 6 0.2) and low radiation doses. Specifically, a single smooth viscoplastic potential has been developed in which the critical resolved shear stress is decomposed into thermal and athermal components that overcome short range and long range barriers, respectively. The evolution of the athermal part is dependent on dislocation and defect densities, whereas the thermal part is modeled to be a function of temperature only. Impediment of dislocation motion due to defects results in hardening while defect annihilation due to dislocation motion accounts for yield drop and stress offset. Radiation softening is explained by invoking a critical temperature (T c ), with increase in radiation dose below which the thermal part of the flow stress undergoes a reduction due to increase in mobile point defects in the dislocation core area, whereas the athermal part increases. Beyond the critical temperature, however, thermal activation is sufficient for dislocation motion and the thermal component disappears. We argue that for low radiation doses, this critical temperature decreases with increase in radiation dose, resulting in a temperature range over which the flow stress actually drops below its value corresponding to the unirradiated condition. Polycrystalline response has been simulated based on a Taylor type homogenization scheme. The model is validated with experimental data for a range of temperatures and strain rates with increasing radiation dose.
International Journal of Plasticity journal homepage: www.elsevier.com/locate/ijplas recent cycli... more International Journal of Plasticity journal homepage: www.elsevier.com/locate/ijplas recent cyclic plasticity models. Int. J. Plasticity, 24, 1863-1889 The first model studied is a macro-scale, phenomenological, constitutive model originally proposed by . Modelization of the strain memory effect on the cyclic hardening of 316 stainless steel. In: Proceedings of the Fifth International Conference on SMiRT, Div. L, Berlin, Germany, L11/3.]. This model was systematically modified for incorporating strain-range dependent cyclic hardening-softening, and proportional and nonproportional loading memory parameters. The second model evaluated is a polycrystalline model originally proposed by Cailletaud . A micromechanical approach to inelastic behavior of metals. Int. J. Plasticity, 8, 55-73.] based on crystalline slip mechanisms. These two models are scrutinized against simulating hysteresis loop shape, cyclic hardening-softening, cross-effect, cyclic relaxation, subsequent cyclic softening and finally a broad set of ratcheting responses under uniaxial and biaxial loading histories. The modeling features which improved simulations for these responses are elaborated in the paper. In addition, a novel technique for simulating both the monotonic and cyclic responses with one set of model parameters is developed and validated.