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Egidio Rizzi - One of the best experts on this subject based on the ideXlab platform.

  • an enhanced johnson cook strength model for splitting Strain rate and temperature effects on lower yield stress and Plastic flow
    Computational Materials Science, 2016
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    Abstract This paper introduces a new ‘strength model’, named Split Johnson–Cook (SJC). The model is a generalization of classical Johnson–Cook (JC) and provides a much improved coherence for the Plastic material description. Specifically, the new model tackles the issue that the effects of Equivalent Plastic Strain rate and temperature shall not be taken as equal for each Equivalent Plastic Strain, avoiding then heavy modeling errors on the lower yield stress and on the subsequent Plastic flow. The salient features of the original JC model are shortly reviewed first, paying specific attention to possible modeling incoherencies. Two main shortcoming issues are framed and discussed. Further, a review on several modifications of the JC model from the literature is outlined. Then, the new SJC model is introduced in such a framework and thoroughly described. A comprehensive discussion on its calibration strategies follows, by developing three alternative calibration approaches. The new model is then applied to the material description of three real material cases (a structural steel, a commercially pure metal and a stainless steel), by considering literature sets of hardening functions recorded at different Equivalent Plastic Strain rates and temperatures. SJC predicted trends are checked against experimental data, for each calibration strategy, by evaluating the material prediction on both lower yield stress and Plastic flow. Obtained results are also compared to those provided by plain JC. The SJC model shows the capability to remarkably improve the material description, as compared to plain JC. Moreover, the fact of presenting a form very similar to that of the original JC model allows to possibly reusing some of the JC material parameters, which may be already known from available calibrations. Also, the SJC model keeps the same computational appeal of the original JC model and need of experimental data toward calibration, while heaviness of calibration and computational weight remain almost unchanged.

  • on the calibration strategies of the johnson cook strength model discussion and applications to experimental data
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    Abstract The present paper aims at assessing the various procedures adoptable for calibrating the parameters of the so-called Johnson–Cook strength model, expressing the deviatoric behavior of elastoPlastic materials, with particular reference to the description of High Strain Rate (HSR) phenomena. The procedures rely on input experimental data corresponding to a set of hardening functions recorded at different Equivalent Plastic Strain rates and temperatures. After a brief review of the main characteristics of the Johnson–Cook strength model, five different calibration strategies are framed and widely described. The assessment is implemented through a systematic application of each calibration strategy to three different real material cases, i.e. a DH-36 structural steel, a commercially pure niobium and an AL-6XN stainless steel. Experimental data available in the literature are considered. Results are presented in terms of plots showing the predicted Johnson–Cook hardening functions against the experimental trends, together with tables describing the fitting problematics which arise in each case, by assessing both lower yield stress and overall Plastic flow introduced errors. The consequences determined by each calibration approach are then carefully compared and evaluated. A discussion on the positive and negative aspects of each strategy is presented and some suggestions on how to choose the best calibration approach are outlined, by considering the available experimental data and the objectives of the following modeling process. The proposed considerations should provide a useful guideline in the process of determining the best Johnson–Cook parameters in each specific situation in which the model is going to be adopted. A last section introduces some considerations about the calibration of the Johnson–Cook strength model through experimental data different from those consisting in a set of hardening functions relative to different Equivalent Plastic Strain rates and temperatures. In particular, the opportunity of using experimental data coming from Taylor impact tests is assessed, together with an evaluation of the possibility of using other less popular and somehow innovative ways for obtaining the Johnson–Cook strength model parameters.

  • on the constitutive modeling of Strain rate and temperature dependent materials part ii an enhanced johnson cook strength model for splitting Strain rate and temperature effects on lower yield stress and Plastic flow
    2013
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    This work deals with the constitutive modeling of Strain rate and temperature dependent elastoPlastic materials, by considering theoretical, experimental and computational aspects. Present Part II aims at introducing and discussing a new empiric strength model, named here Split Johnson-Cook model. The model is formulated as a generalization of the Johnson-Cook strength model, previously discussed in Part I. The aims are those of improving the original Johnson-Cook hardening function, in order to mitigate shortcomings such as the fact that the Equivalent Plastic Strain, the Equivalent Plastic Strain rate and the temperature effects on the yield stress are totally independent from each other. In particular, the new model tackles the issue that the effects of the Equivalent Plastic Strain rate and of the temperature need to be assumed as equal for each Equivalent Plastic Strain, a factor which may lead to heavy modeling errors for the prediction of either the lower yield stress or the subsequent Plastic flow. Two main issues of the original Johnson-Cook model are framed and discussed, together with a commented review of several modifications of the model proposed in the literature. After that, the new Split Johnson-Cook strength model is introduced and thoroughly described. A comprehensive discussion on its calibration strategies follows. Through a reasoned approach, three different calibration approaches are presented and widely described. The new model is then applied to the same three real material cases already considered in Part I for the original Johnson-Cook model, i.e. the determination of the Split Johnson-Cook parameters for a structural steel, a commercially pure metal and a stainless steel, by relying on experimental data taken from the literature, consisting in a set of hardening functions at different Equivalent Plastic Strain rates and temperatures. Results are presented in terms of plots showing the predicted Split Johnson-Cook hardening functions against the experimental trends, considering each calibration strategy, together with tables reporting the fitting problematics which arise in each case, by assessing both lower yield stress and Plastic flow introduced errors. The obtained results are also checked against the results provided by the original Johnson-Cook model, by relying on the results achieved in Part I. The replacement of the original Johnson-Cook model with the new model appears to almost exclusively introduce positive consequences. The Split Johnson-Cook model shows the capability to remarkably improve the fitting to experimental data for the three considered material cases, both for the lower yield stress and for the overall Plastic flow predictions, comparing to the original Johnson-Cook model predictions. Also, the fact of presenting a form very similar to that of the original Johnson-Cook model allows for further interesting options, such as the possibility to substitute one or more of the Split Johnson-Cook model lower yield stress and Plastic flow Strain rate and temperature terms with some of the substitutive terms proposed in the literature. Furthermore, having a form very similar to that of the original Johnson-Cook model allows to partially reuse some of the material parameters of the original Johnson-Cook model, that may be already known from previous calibrations. Also, the Split Johnson-Cook model is conceived in such a way as to be capable of maintaining the same computational appeal of the original Johnson-Cook model. The need of experimental data for calibrating the model, the heaviness of calibration and the computational burden remain almost unchanged, comparing to the original Johnson-Cook model. Negative implications, if really any, appear to be very limited.

Thomas Bohlke - One of the best experts on this subject based on the ideXlab platform.

  • Equivalent Plastic Strain gradient Plasticity with grain boundary hardening and comparison to discrete dislocation dynamics
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2015
    Co-Authors: Eric Bayerschen, D Weygand, Stephan Wulfinghoff, Markus Stricker, Thomas Bohlke
    Abstract:

    The gradient crystal Plasticity framework of Wulfinghoff et al. (Wulfinghoff et al. 2013 Int. J. Plasticity 51, 33–46. (doi:10.1016/j.ijplas.2013.07.001)), incorporating an Equivalent Plastic Strain γ eq and grain boundary (GB) yielding, is extended with GB hardening. By comparison to averaged results from many discrete dislocation dynamics (DDD) simulations of an aluminium-type tricrystal under tensile loading, the new hardening parameter of the continuum model is calibrated. Although the GBs in the discrete simulations are impenetrable, an infinite GB yield strength, corresponding to microhard GB conditions, is not applicable in the continuum model. A combination of a finite GB yield strength with an isotropic bulk Voce hardening relation alone also fails to model the Plastic Strain profiles obtained by DDD. Instead, a finite GB yield strength in combination with GB hardening depending on the Equivalent Plastic Strain at the GBs is shown to give a better agreement to DDD results. The differences in the Plastic Strain profiles obtained in DDD simulations by using different orientations of the central grain could not be captured. This indicates that the misorientation-dependent elastic interaction of dislocations reaching over the GBs should also be included in the continuum model.

  • a gradient Plasticity grain boundary yield theory
    International Journal of Plasticity, 2013
    Co-Authors: Stephan Wulfinghoff, Eric Bayerschen, Thomas Bohlke
    Abstract:

    Abstract A Strain gradient crystal Plasticity theory is presented that accounts for the resistance of grain boundaries against Plastic flow based on an interface yield condition. This theory incorporates the previously presented numerically efficient visco-Plastic treatment by the gradient of an Equivalent Plastic Strain ∇ γ eq in Wulfinghoff and Bohlke (2012) . The finite element implementation is discussed and the three-dimensional numerical model is fitted to experimental data of polycrystalline copper micro-tensile tests. The size dependent yield strength is reproduced notably well.

  • Equivalent Plastic Strain gradient crystal Plasticity enhanced power law subroutine
    Gamm-mitteilungen, 2013
    Co-Authors: Stephan Wulfinghoff, Thomas Bohlke
    Abstract:

    A gradient crystal Plasticity theory is presented including a defect energy based on the gradient of an Equivalent Plastic Strain measure. Preserving the single crystal slip kinematics, the gradient hardening contribution models dislocation long range interactions adding a back-stress term to the flow rule, similar to other gradient crystal Plasticity theories. Owing to a reduced number of four nodal degrees of freedom the finite element implementation can handle systems consisting of an increased number of grains (compared to other theories) without elaborate or costly computer systems. Emphasis is put on the enhancement of the power law material subroutine. The associated implicit Euler scheme is optimized based on an improved starting value for the Newton scheme. Three-dimensional simulations illustrate that the proposed algorithm facilitates significantly larger time steps compared to the standard Newton scheme. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

  • Equivalent Plastic Strain gradient enhancement of single crystal Plasticity theory and numerics
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2012
    Co-Authors: Stephan Wulfinghoff, Thomas Bohlke
    Abstract:

    We propose a visco-Plastic Strain gradient Plasticity theory for single crystals. The gradient enhancement is based on an Equivalent Plastic Strain measure. Two physically Equivalent variational settings for the problem are discussed: a direct formulation and an alternative version with an additional micromorphic-like field variable, which is coupled to the Equivalent Plastic Strain by a Lagrange multiplier. The alternative formulation implies a significant reduction of nodal degrees of freedom. The local algorithm and element stiffness matrices of the finite-element discretization are discussed. Numerical examples illustrate the advantages of the alternative formulation in three-dimensional simulations of oligo-crystals. By means of the suggested formulation, complex boundary value problems of the proposed Plastic Strain gradient theory can be solved numerically very efficiently.

Huiji Shi - One of the best experts on this subject based on the ideXlab platform.

  • the effect of crystal orientation on fretting fatigue crack formation in ni based single crystal superalloys in situ sem observation and crystal Plasticity finite element simulation
    Tribology International, 2018
    Co-Authors: Qinan Han, Wenhui Qiu, Huiji Shi
    Abstract:

    Abstract The effect of crystal orientation on fretting fatigue crack formation behaviors of Ni-based single crystal (NBSX) superalloys is investigated in this paper. Differences of slip lines and crack formation patterns between two crystal orientations are revealed by in-situ fretting fatigue experiments. The cracks are found to initiate along the direction of slip lines for both crystal orientations, which indicates the crack formation has close relations with crystallographic slip. Crystal Plastic finite element method (CPFEM) simulation is implemented and shows the difference of slip system activations between two crystal orientations. Equivalent Plastic Strain is proved effective to predict crack formation sites and directions. The predicted slip lines and cracks obtained from CPFEM simulations are in good agreements with the experimental observations.

  • in situ sem observation and crystal Plasticity finite element simulation of fretting fatigue crack formation in ni base single crystal superalloys
    Tribology International, 2016
    Co-Authors: Qinan Han, Wenhui Qiu, Yibo Shang, Huiji Shi
    Abstract:

    Abstract Fretting fatigue behavior of Ni-base single crystal (NBSX) superalloys was investigated based on in-situ scanning electron microscope (SEM) observations. Evolutions of slip lines and initiations of cracks were observed. Slip lines were found on specimen free surface along three typical directions. Short cracks were observed to initiate along the slip line. Crystal Plasticity constitutive model considering cyclic hardening effect was employed. Crystal Plasticity finite element method (CPFEM) simulation showed the activations of crystallographic slip systems at contact region. The slip plane with the maximum Plastic slip was determined as dominate slip plane. Equivalent Plastic Strain was proved effective to predict crack initiation. The simulation results including predicted slip lines, crack initiation locations and orientations were in good agreements with observations.

Luca Gambirasio - One of the best experts on this subject based on the ideXlab platform.

  • an enhanced johnson cook strength model for splitting Strain rate and temperature effects on lower yield stress and Plastic flow
    Computational Materials Science, 2016
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    Abstract This paper introduces a new ‘strength model’, named Split Johnson–Cook (SJC). The model is a generalization of classical Johnson–Cook (JC) and provides a much improved coherence for the Plastic material description. Specifically, the new model tackles the issue that the effects of Equivalent Plastic Strain rate and temperature shall not be taken as equal for each Equivalent Plastic Strain, avoiding then heavy modeling errors on the lower yield stress and on the subsequent Plastic flow. The salient features of the original JC model are shortly reviewed first, paying specific attention to possible modeling incoherencies. Two main shortcoming issues are framed and discussed. Further, a review on several modifications of the JC model from the literature is outlined. Then, the new SJC model is introduced in such a framework and thoroughly described. A comprehensive discussion on its calibration strategies follows, by developing three alternative calibration approaches. The new model is then applied to the material description of three real material cases (a structural steel, a commercially pure metal and a stainless steel), by considering literature sets of hardening functions recorded at different Equivalent Plastic Strain rates and temperatures. SJC predicted trends are checked against experimental data, for each calibration strategy, by evaluating the material prediction on both lower yield stress and Plastic flow. Obtained results are also compared to those provided by plain JC. The SJC model shows the capability to remarkably improve the material description, as compared to plain JC. Moreover, the fact of presenting a form very similar to that of the original JC model allows to possibly reusing some of the JC material parameters, which may be already known from available calibrations. Also, the SJC model keeps the same computational appeal of the original JC model and need of experimental data toward calibration, while heaviness of calibration and computational weight remain almost unchanged.

  • on the calibration strategies of the johnson cook strength model discussion and applications to experimental data
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    Abstract The present paper aims at assessing the various procedures adoptable for calibrating the parameters of the so-called Johnson–Cook strength model, expressing the deviatoric behavior of elastoPlastic materials, with particular reference to the description of High Strain Rate (HSR) phenomena. The procedures rely on input experimental data corresponding to a set of hardening functions recorded at different Equivalent Plastic Strain rates and temperatures. After a brief review of the main characteristics of the Johnson–Cook strength model, five different calibration strategies are framed and widely described. The assessment is implemented through a systematic application of each calibration strategy to three different real material cases, i.e. a DH-36 structural steel, a commercially pure niobium and an AL-6XN stainless steel. Experimental data available in the literature are considered. Results are presented in terms of plots showing the predicted Johnson–Cook hardening functions against the experimental trends, together with tables describing the fitting problematics which arise in each case, by assessing both lower yield stress and overall Plastic flow introduced errors. The consequences determined by each calibration approach are then carefully compared and evaluated. A discussion on the positive and negative aspects of each strategy is presented and some suggestions on how to choose the best calibration approach are outlined, by considering the available experimental data and the objectives of the following modeling process. The proposed considerations should provide a useful guideline in the process of determining the best Johnson–Cook parameters in each specific situation in which the model is going to be adopted. A last section introduces some considerations about the calibration of the Johnson–Cook strength model through experimental data different from those consisting in a set of hardening functions relative to different Equivalent Plastic Strain rates and temperatures. In particular, the opportunity of using experimental data coming from Taylor impact tests is assessed, together with an evaluation of the possibility of using other less popular and somehow innovative ways for obtaining the Johnson–Cook strength model parameters.

  • on the constitutive modeling of Strain rate and temperature dependent materials part ii an enhanced johnson cook strength model for splitting Strain rate and temperature effects on lower yield stress and Plastic flow
    2013
    Co-Authors: Luca Gambirasio, Egidio Rizzi
    Abstract:

    This work deals with the constitutive modeling of Strain rate and temperature dependent elastoPlastic materials, by considering theoretical, experimental and computational aspects. Present Part II aims at introducing and discussing a new empiric strength model, named here Split Johnson-Cook model. The model is formulated as a generalization of the Johnson-Cook strength model, previously discussed in Part I. The aims are those of improving the original Johnson-Cook hardening function, in order to mitigate shortcomings such as the fact that the Equivalent Plastic Strain, the Equivalent Plastic Strain rate and the temperature effects on the yield stress are totally independent from each other. In particular, the new model tackles the issue that the effects of the Equivalent Plastic Strain rate and of the temperature need to be assumed as equal for each Equivalent Plastic Strain, a factor which may lead to heavy modeling errors for the prediction of either the lower yield stress or the subsequent Plastic flow. Two main issues of the original Johnson-Cook model are framed and discussed, together with a commented review of several modifications of the model proposed in the literature. After that, the new Split Johnson-Cook strength model is introduced and thoroughly described. A comprehensive discussion on its calibration strategies follows. Through a reasoned approach, three different calibration approaches are presented and widely described. The new model is then applied to the same three real material cases already considered in Part I for the original Johnson-Cook model, i.e. the determination of the Split Johnson-Cook parameters for a structural steel, a commercially pure metal and a stainless steel, by relying on experimental data taken from the literature, consisting in a set of hardening functions at different Equivalent Plastic Strain rates and temperatures. Results are presented in terms of plots showing the predicted Split Johnson-Cook hardening functions against the experimental trends, considering each calibration strategy, together with tables reporting the fitting problematics which arise in each case, by assessing both lower yield stress and Plastic flow introduced errors. The obtained results are also checked against the results provided by the original Johnson-Cook model, by relying on the results achieved in Part I. The replacement of the original Johnson-Cook model with the new model appears to almost exclusively introduce positive consequences. The Split Johnson-Cook model shows the capability to remarkably improve the fitting to experimental data for the three considered material cases, both for the lower yield stress and for the overall Plastic flow predictions, comparing to the original Johnson-Cook model predictions. Also, the fact of presenting a form very similar to that of the original Johnson-Cook model allows for further interesting options, such as the possibility to substitute one or more of the Split Johnson-Cook model lower yield stress and Plastic flow Strain rate and temperature terms with some of the substitutive terms proposed in the literature. Furthermore, having a form very similar to that of the original Johnson-Cook model allows to partially reuse some of the material parameters of the original Johnson-Cook model, that may be already known from previous calibrations. Also, the Split Johnson-Cook model is conceived in such a way as to be capable of maintaining the same computational appeal of the original Johnson-Cook model. The need of experimental data for calibrating the model, the heaviness of calibration and the computational burden remain almost unchanged, comparing to the original Johnson-Cook model. Negative implications, if really any, appear to be very limited.

Qinan Han - One of the best experts on this subject based on the ideXlab platform.

  • the effect of crystal orientation on fretting fatigue crack formation in ni based single crystal superalloys in situ sem observation and crystal Plasticity finite element simulation
    Tribology International, 2018
    Co-Authors: Qinan Han, Wenhui Qiu, Huiji Shi
    Abstract:

    Abstract The effect of crystal orientation on fretting fatigue crack formation behaviors of Ni-based single crystal (NBSX) superalloys is investigated in this paper. Differences of slip lines and crack formation patterns between two crystal orientations are revealed by in-situ fretting fatigue experiments. The cracks are found to initiate along the direction of slip lines for both crystal orientations, which indicates the crack formation has close relations with crystallographic slip. Crystal Plastic finite element method (CPFEM) simulation is implemented and shows the difference of slip system activations between two crystal orientations. Equivalent Plastic Strain is proved effective to predict crack formation sites and directions. The predicted slip lines and cracks obtained from CPFEM simulations are in good agreements with the experimental observations.

  • in situ sem observation and crystal Plasticity finite element simulation of fretting fatigue crack formation in ni base single crystal superalloys
    Tribology International, 2016
    Co-Authors: Qinan Han, Wenhui Qiu, Yibo Shang, Huiji Shi
    Abstract:

    Abstract Fretting fatigue behavior of Ni-base single crystal (NBSX) superalloys was investigated based on in-situ scanning electron microscope (SEM) observations. Evolutions of slip lines and initiations of cracks were observed. Slip lines were found on specimen free surface along three typical directions. Short cracks were observed to initiate along the slip line. Crystal Plasticity constitutive model considering cyclic hardening effect was employed. Crystal Plasticity finite element method (CPFEM) simulation showed the activations of crystallographic slip systems at contact region. The slip plane with the maximum Plastic slip was determined as dominate slip plane. Equivalent Plastic Strain was proved effective to predict crack initiation. The simulation results including predicted slip lines, crack initiation locations and orientations were in good agreements with observations.